Processing and Characterization of Silicon Carbide (6H...

ISRN KTH/EKT/FR-02/1-SE

Processing and Characterization of Silicon Carbide (6H- and

4H-SiC) Contacts for High Power and High Temperature

Device Applications

by Sang-Kwon Lee

Ph.D Dissertation

KTH, Royal Institute of Technology Department of Microelectronics and Information Technology

Device Technology Laboratory

Stockholm 2002

Processing and Characterization of Silicon Carbide (6H- and 4H-SiC) Contacts for High Power and High Temperature Device Applications A dissertation submitted to Kungliga Tekniska Högskolan, Stockholm, Sweden, in partial fulfillment of the requirements for the degree of Teknisk Doktor (Ph.D.). 2002 Sang-Kwon Lee KTH (Kungliga Tekniska Högskolan) Royal Institute of Technology Department of Microelectronics and Information Technology Electrum 229, S-164 40, Kista-Stockholm SWEDEN ISRN KTH/EKT/FR-02/1-SE ISSN 1650-8599 TRITA-EKT Forskningsrapprt 2002:1 Printed in 250 copies by Kista Snabbtryck AB, Kista 2002

To Mejeong for her never-ending support

Lee, Sang-Kwon : Processing and Characterization of Silicon Carbide (6H- and 4H-SiC) Contacts for High Power and High Temperature Device Applications, ISRN KTH/EKT/FR-02/1-SE, KTH, Royal Institute of Technology, Department of Microelectronics and Information Technology, Stockholm, 2002

Abstract

Silicon carbide is a promising wide bandgap semiconductor material for high-temperature, high-power, and high-frequency device applications. However, there are still a number of factors that are limiting the device performance. Among them, one of the most important and critical factors is the formation of low resistivity Ohmic contacts and high-temperature stable Schottky diodes on silicon carbide. In this thesis, different metals (TiW, Ti, TiC, Al, and Ni) and different deposition techniques (sputtering and evaporation) were suggested and investigated for this purpose. Both electrical and material characterizations were performed using various techniques, such as I-V, C-V, RBS, XRD, XPS, LEED, SEM, AFM, and SIMS. For the Schottky contacts to n- and p-type 4H-SiC, sputtered TiW Schottky contacts had excellent rectifying behavior after annealing at 500 oC in vacuum with a thermally stable ideality factor of 1.06 and 1.08 for n- and p-type, respectively. It was also observed that the SBH for p-type SiC (φBp) strongly depends on the choice the metal with a linear relationship φBp = 4.51 – 0.58φm, indicating no strong Fermi-level pinning. Finally, the behavior of Schottky diodes was investigated by incorporation of size-selected Au nano-particles in Ti Schottky contacts on silicon carbide. The reduction of the SBH is explained by using a simple dipole layer approach, with enhanced electric field at the interface due to the small size of the circular patch (Au nano-particles) and large difference of the barrier height between two metals (Ti and Au) on both n- and p-SiC. For the Ohmic contacts, titanium carbide (TiC) was used as contacts to both n- and p-type 4H-SiC epilayers as well as on Al implanted layers. The TiC contacts were epitaxially deposited using a co-evaporation method with an e-beam Ti source and a Knudsen cell for C60, in a UHV system at low substrate temperature (500 oC). In addition, we extensively investigated sputtered TiW (weight ratio 30:70) as well as evaporated Ni Ohmic contacts on both n- and p-type epilayers of SiC. The best Ohmic contacts to n-type SiC are annealed Ni (> 950oC) with the specific contact resistance of ≈ 8×10-6 Ωcm2 with doping concentration of 1.1 ×10-19 cm-3 while annealed TiW and TiC contacts are the preferred contacts to p-type SiC. From long-term reliability tests at high temperature (500 oC or 600 oC) in vacuum and oxidizing (20% O2/N2) ambient, TiW contacts with a platinum capping layer (Pt/Ti/TiW) had stable specific contact resistances for > 300 hours. Keywords : silicon carbide, Ohmic and Schottky contacts, co-evaporation, current-voltage, capacitance-voltage measurement, power devices, nano-particles, Schottky barrier height lowering, and TLM structures.

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Table of Contents

Appended Papers ......................................................................................................... iii

Summary of appended papers ................................................................................... vii

Acknowledgements ...................................................................................................... ix

Used Acronyms............................................................................................................ xii

1. INTRODUCTION................................................................................................. 1

2. PROPERTIES OF SIC......................................................................................... 5

2.1 SIC MATERIAL PROPERTIES .............................................................................. 5 2.1.1 Crystal structure ............................................................................................ 5 2.1.2 Polytypes of SiC ............................................................................................. 5

2.2 SIC ELECTRONIC PROPERTIES .......................................................................... 7 2.2.1 Density of States (DOS) and Energy bandgap (Eg) .................................... 7 2.2.2 Bandgap narrowing .................................................................................... 8 2.2.3 Incomplete ionization.................................................................................. 9 2.2.4 Carrier recombination .............................................................................. 10 2.2.5 Impact Ionization ...................................................................................... 10 2.2.6 Mobility ..................................................................................................... 11

3. METAL-SEMICONDUCTOR JUNCTIONS .................................................. 15

3.1 CURRENT TRANSPORT MECHANISM ................................................................ 15 3.1.1 Schottky barrier formation........................................................................ 15 3.1.2 Current transport mechanism ................................................................... 16

3.2 OHMIC CONTACTS .......................................................................................... 18 3.3 SCHOTTKY DIODE PERFORMANCE .................................................................. 20

3.3.1 Specific on-resistance ............................................................................... 20 3.3.2 Forward voltage drop ............................................................................... 21 3.3.3 Breakdown voltage and reverse leakage current...................................... 21 3.3.4 Edge termination for high breakdown voltage ......................................... 23 3.3.5 Schottky barrier lowering ......................................................................... 25

3.4 OTHER RECTIFIERS ......................................................................................... 26 3.4.1 Junction barrier Schottky (JBS) diodes .................................................... 26 3.4.2 Merged P-i-N / Schottky (MPS) diodes..................................................... 27 3.4.3 DMT (Dual metal trench) diodes.............................................................. 28 3.4.4 TMBS (Trench MOS Barrier Schottky) diodes ......................................... 28

4. FABRICATION PROCESS............................................................................... 29

4.1 PROCESS DESCRIPTION ................................................................................... 29 4.1.1 Wafer preparation and surface cleaning .................................................. 29 4.1.2 Etching process ......................................................................................... 30

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4.1.3 Deposition Techniques.............................................................................. 32 4.1.4 Ion implantation........................................................................................ 35 4.1.5 Annealing .................................................................................................. 36

4.2 TEST STRUCTURES FOR OHMIC CONTACTS ..................................................... 37 4.2.1 Kuphal structure ....................................................................................... 37 4.2.2 Two-terminal contact resistance methods ................................................ 38 4.2.3 3-contacts, two-terminal methods............................................................. 39 4.2.4 Linear transmission line method (LTLM) ................................................. 40 4.2.5 Circular transmission line method (CTLM) ............................................. 42 4.2.6 Four-terminal contact resistance method................................................. 43 4.2.7 Six-terminal contact resistance method .................................................... 44 4.2.8 Comparison of each measurement technique ........................................... 45

5. CHARACTERIZATION AND RESULTS....................................................... 47

5.1 MATERIAL CHARACTERIZATION..................................................................... 47 5.1.1 X-ray Diffraction (XRD) ........................................................................... 48 5.1.2 Secondary Ion Mass Spectrometry (SIMS) ............................................... 52 5.1.3 Rutherford Backscattering Spectrometry (RBS) ....................................... 53 5.1.4 Transmission Electron Microscopy (TEM)............................................... 55 5.1.5 Atomic Force Microscopy (AFM) & Optical microscopy ........................ 55

5.2 ELECTRICAL CHARACTERIZATION OF SCHOTTKY CONTACTS ......................... 57 5.2.1 Measurement techniques........................................................................... 57 5.2.2 A review of the Schottky contacts (Paper I, IV, VIII)................................ 61 5.2.3 The relationship between metal work function and barrier height .............. (Paper IV) ................................................................................................. 62 5.2.4 Reduction of the Schottky barrier height (Paper VIII) ............................. 65

5.3 SPECIFIC CONTACT RESISTANCE MEASUREMENTS........................................... 70 5.3.1 TiC and Ti on n- and p-SiC (Paper II, III, V) ........................................... 71 5.3.2 Ni and TiW (30:70) contacts on n- and p-SiC (Paper V, VI, VII)............. 74 5.3.3 Microscopic mapping of specific contact resistance (Paper VI, VII) ....... 76

5.4 LONG-TERM RELIABILITY TESTS AT HIGH TEMPERATURE ............................... 80 5.4.1 In vacuum.................................................................................................. 80 5.4.2 In oxidizing ambient.................................................................................. 81

6. CONCLUSIONS AND FUTURE WORK........................................................ 83

REFERENCES............................................................................................................ 85

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Appended Papers I. Schottky diode formation and characterization of titanium tungsten to n-

and p-type 4H silicon carbide. S.-K. Lee, C.-M. Zetterling, M. Östling, J. Appl. Phys. 87(11), 8039 (2000).

II. Low resistivity Ohmic titanium carbide contacts to n- and p-type 4H-Silicon

carbide S.-K. Lee, C.-M. Zetterling, M. Östling, J.-P. Palmquist, H. Högberg, and U. Jansson, Solid State Electronics 44(7), 1179 (2000).

III. Electrical characterization of TiC Ohmic contacts to aluminum implanted 4H-SiC

S.-K. Lee, C.-M. Zetterling, M. Östling, J.-O. Palmquist, H. Högberg, and U. Jansson, Appl. Phys. Lett. 77(19), 1478 (2000).

IV. Schottky barrier height dependence on the metal work function for p-type

4H-SiC S.-K. Lee, C.-M. Zetterling, and M. Östling, J. Electron. Mater. 30(3), 242 (2001).

V. Low resistivity Ohmic contacts to silicon carbide for high temperature

device applications S.-K. Lee, C.-M. Zetterling, M. Östling, J.-P. Palmquist, and U. Jansson, Microelectronic Engineering, 60(1-2), 261 (2002).

VI. Ohmic contact formation on inductively coupled plasma (ICP) etched 4H-

silicon carbide using sputtered titanium tungsten S.-K. Lee, S.-M. Koo, C.-M. Zetterling, and M. Östling, to be published in J. Electron. Mater. (May 2002).

VII. The microscopic specific contact resistance mapping and long term reliability on 4H-silicon carbide using sputtered titanium tungsten contacts for high temperature device applications

S.-K. Lee, C.-M. Zetterling, and M. Östling, to be published in J. Appl. Phys. (June 2002).

VIII. Reduction of the Schottky barrier height on silicon carbide using Au nano-

particles S.-K. Lee, C.-M. Zetterling, M. Östling, I. Åberg, M. H. Magnusson, K. Deppert, L.-E. Wernersson, L. Samuelson, and A. Litwin, to be published in Solid-State Electron (June 2002).

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Related work not included in the thesis

Journal papers and proceedings 1. Dry etching and metallization schemes in a GaN/SiC heterojuction device

process. E. Danielsson, C.-M. Zetterling, S.-K. Lee, M. Östling, , K. Linthicum, D.B. Thomson, O.-H. Nam, and R.F. Davis, Mater. Sci. Forum. 338-342, 1049 (2000).

2. The formation and characterization of epitaxial titanium carbide contacts to

4H-SiC. S.-K. Lee, E. Danielsson, C.-M. Zetterling, M. Östling, J.-P. Palmquist, H. Högberg, and U. Jansson, Mat. Res. Soc. Symp. Proc. 622, T6.9 (2000).

3. TiW (Titanium tungsten) for Ohmic and Schottky contacts to 4H-SiC S.-K. Lee, C.-M. Zetterling, and M. Östling, Mat. Res. Soc. Symp. Proc. 640,

H7.2 (2000). 4. Low damage Schottky diode formation on inductively coupled plasmas

etched 4H-silicon carbide E. Danielsson, S.-K. Lee, C.-M. Zetterling, and M. Östling, J. Electron. Mater. 30(3), 247 (2001).

5. Electrical characterization of titanium-based Ohmic contacts to 4H-SiC for

high-power and high-temperature operation S.-K. Lee, C.-M. Zetterling, M. Östling, and B. M. Moon, to be published in J. Korean Phys. Soc. (April 2002).

6. Influence of the trenching effect on the characterization of buried-gate SiC

junction field-effect transistors S.-M. Koo, S.-K. Lee, C.-M. Zetterling, M. Östling, U. Forsberg, and E. Janzen, to be published in Mater. Sci. Forum (April 2002, presented at ICSCRM 2001).

7. Reduction of the barrier height and enhancement of tunneling current of titanium contacts using embedded Au nano-particles on 4H- and 6H-silicon carbide S.-K. Lee, C.-M. Zetterling, M. Östling, I. Åberg, M. H. Magnusson, K. Deppert, L.-E. Wernersson, L. Samuelson, and A. Litwin, to be published in Mater. Sci. Forum (April 2002, presented at ICSCRM 2001).

8. Electrical characterization of metal-oxide-semiconductor capacitors on plasma-etch damaged silicon carbide S,-M. Koo, S.-K. Lee, C.-M. Zetterling, and M. Östling, to be published in Solid-State Electron (June 2002).

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Conference presentation (Oral, Poster, and invited talks)

1. Schottky barrier height dependence on the metal work function for p-type

4H-SiC S.-K. Lee, C.-M. Zetterling, M. Östling, presented at TMS Electron. Mater. Conf

(EMC), Denver, Colorado, U.S.A. June (2000). 2. Low resistivity Ohmic contacts to silicon carbide for high temperature

device applications S.-K. Lee, C.-M. Zetterling, and M. Östling, presented at European workshop on

Materials for Advanced Metallization (MAM), Sigtuna, Sweden, March (2001). 3. Contact resistance on III-V (InP, InGaAs, and InGaAsP) semiconductors H. Kosmaz, C. Zaring, S.-K. Lee, and M. Östling, presented at European

workshop on Materials for Advanced Metallization (MAM), Sigtuna, Sweden, March (2001).

4. Metallization schemes for combined unipolar / bipolar SiC process U. Zimmermann, E. Danielsson, S.-K. Lee, C.-M. Zetterling, and A. Hallen,

presented at European workshop on Materials for Advanced Metallization (MAM), Sigtuna, Sweden, March (2001).

5. Ohmic contact formation on inductively coupled plasma (ICP) etched 4H-

silicon carbide using sputtered titanium tungsten S.-K. Lee, S.-M. Koo, C.-M. Zetterling, and M. Östling, presented at Electron.

Mater. Conf. (EMC), Indiana, U.S.A. June (2001).

6. Electrical characterization of Ohmic contacts to 4H-silicon carbide for high power and high temperature operation

S.-K. Lee, C.-M. Zetterling, and M. Östling, presented at the 8th Korean Conf. on Semiconductor (KCS), Seoul, Korea, Feb. (2001).

7. Measurement on linear TLM structures with TiW/Ti/Pt contacts for

corrosive and high temperature applications L. Uneus, S.-K. Lee, C.-M. Zetterling, L.-G. Ekedahl, I. Lundström, M. Östling, and A. Lloyd Spetz, presented at Inter. Conf. on Silicon Carbide and Related Mater. (ICSCRM), Tsukuba, Japan, Oct (2001).

8. Recent advances and issues in SiC process and device technologies M. Östling, S.-M. Koo, S.-K. Lee, E. Danielsson, and C.-M. Zetterling, presented

at the Solid State and Integrated Circuit Technology (ICSICT), Shanghai, China , Oct. (2001).

9. Metal-oxide-semiconductor structures in inductively coupled plasma etch

damaged 6H- and 4H-SiC S.-M. Koo, S.-K. Lee, C.-M. Zetterling, and M. Östling, presented at the semiconductor interface specialists conference (SISC), Washington D.C. U.S.A, Dec. (2001).

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10. SiC device technology for high voltage and RF power applications M. Östling, S.-M. Koo, S.-K. Lee, E. Danielsson, M. Domej, and C.-M. Zetterling, presented at 23rd International Conference on Microelectronics, Nis, Yugoslavia, May 12-15 (2002).

11. Comparison study of Ohmic contacts in oxidizing ambient at high temperature for gas sensor applications S.-K. Lee, L. Uneus, S.-M. Koo, C.-M. Zetterling, L.-G. Ekedahl, I. Lundström, A. Lloyd Spetz, and M. Östling, to be presented in TMS Electronic Material Conference (EMC), Santa Barbara, U.S.A, June (2002).

12. Influence of size-selected aerosol Au nano-particles in titanium Schottky contacts on silicon carbide S.-K. Lee, C.-M. Zetterling, M. Östling, I. Åberg, M.-H. Magnusson, K. Deppert, L.-E. Wernersson, L. Samuelson, and A. Litwin, to be presented in 7th International Conference on Nanometer-scale Science and Technology (Nano-7), Malmö, Sweden, June (2002).

Not related work not included in the thesis Part of MS thesis work 1. Positron Annihilation study of phase transitions in Ethane physisorbed on

grafoil P. C. Jain, S.-K. Lee, N. Hozhabri, and S. C. Sharma, Phys. Rev. B 49(4), 2821 (1994).

2. Phase transition in Physisorbed Ethane investigated by positron annihilation

spectroscopy P. C. Jain, S.-K. Lee, N. Hozhabri, and S. C. Sharma, Phys. Rev. B 60(3), 2057 (1999).

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Summary of appended papers The appended papers are in chronological order and divided in two parts; Schottky and Ohmic contacts to silicon carbide. In the beginning of the study the characterization of metal-silicon carbide junctions and low resistivity Ohmic contacts with various titanium-based metals (Ti, TiW, and TiC) was performed. Later on new concepts for Schottky diodes and the performance of Ohmic contacts at high-temperature were approached. Normally the first author is in charge of preparing manuscripts and following up the publication in the appended papers. Paper I. This paper presents the Schottky diode formation and characterization of titanium tungsten (TiW) to silicon carbide: The main achievements are the first characterization of TiW to both n- and p-type 4H-SiC with thermally stable ideality factor and Schottky barrier height for complementary device applications. The author performed all the processing, material and electrical characterizations, and all analysis including manuscript writing and following up. Paper II. This paper describes the results from a study of low resistivity titanium carbide (TiC) Ohmic contacts to both highly doped n+- and p+-type epilayers of 4H-silicon carbide using co-evaporation. The author performed all processing after the deposition of TiC, most of the electrical characterization, and took part in the material characterization, and most analysis including writing the manuscript. Paper III. This paper investigates a novel combination of epitaxially grown TiC and PECVD grown silicon nitride sacrificial layer to achieve lower specific contact resistance on Al-implanted epilayers of 4H-SiC. The author performed all processing after the deposition of TiC and ion implantation, most of the electrical characterization, and took part in the material characterization, and most analysis including writing the manuscript Paper IV. This paper tried to answer the question of which metals to select on silicon carbide using the relationship between Schottky barrier height and metal work functions. The author performed all the processing, material and electrical characterization, all analysis, and writing the manuscript. Paper V. This paper presents the investigation of both low resistivity TiC and TiW Ohmic contacts to silicon carbide. The long-term stability test of TiW is also performed at high temperature. The author performed most of the processing, all electrical characterization, and most of the analysis including writing the manuscript. Paper VI. This paper describes the investigation of Ohmic contact formation on ICP etch-damaged surface. For this study, sputtered TiW contacts were used as Ohmic contacts. The author suggested the study, performed all the processing, performed the analysis, and wrote the manuscript. Paper VII. The investigation of the first microscopic mapping of specific contact resistance is presented in this paper, where the long-term reliability tests on silicon

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carbide using sputtered TiW and a capping layer were also performed. The author performed all processing, all of the electrical and material characterization, and all analysis including writing the manuscript. Paper VIII. This paper presents the new approach of SiC Schottky diodes using aerosol deposited and embedded Au nano-particles, where the reduction of Schottky barrier height is also discussed with a model of a dipole layer. The author performed all the processing (except aerosol deposition of Au nano-particles), material and electrical characterization, all analysis including computer simulation, and writing the manuscript.

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Acknowledgements At this moment, when I am finishing my Ph.D dissertation, I realize that I have to thank a lot of people whom I knew and met in my life. I will borrow this space for acknowledgement even though I feel it is not enough space for them! I also perceived that this is not the end of study in my life. And, I know what kind of new life and difficulties I have to solve and figure out are waiting for me in near future. I also believe that I will do as before I did with some curiosity and confidence.

First of all, I would like to thank my academic supervisor Professor Mikael

Östling, who is head of our department, Microelectronics and Information Technology (IMIT), without his guidance and continuous help this dissertation would never have been finished. Prof. Östling accepted me as a Ph.D student in the former Device Technology Lab. (EKT) at KTH (Royal Institute of Technology) in December of 1998. Prof. Östling also introduced many opportunities to study the field of wide bandgap semiconductor materials and collaborate with other Swedish research groups at inorganic chemistry department in Uppsala University, at Solid State Physics in Lund University, at S-SENCE in Linköping University, and with the ABB SiC research groups. Prof. Östling encouraged me to attend and present my works at conferences. I will not forget the 8th Korean Conference on Semiconductor (KCS) in Feb. 2001 because it was my first visit to Korea since I started my Ph.D in Sweden. It also gave me many chances to meet Korean researchers in the field of silicon carbide and other wide bandgap semiconductor materials.

Many thanks go to my supervisor, Dr. Carl-Mikael Zetterling, who is called “Bellman” by people in EKT and IMIT, who is a leader of the SiC research groups in EKT at KTH, and who gave me massive advise throughout this thesis. I have learned many things from his courses and also Dr. Zetterling taught me how to write scientific papers. Dr. Zetterling spent quite many hours to correct my manuscripts and my research plan. Thank you so much, Bellman!

Thanks to my colleague, Dr. Erik Danielsson. Erik showed me processing and electric characterization of SiC in the cleanroom as well as the measurement room. Erik was happy to share his experiences with me throughout the SiC projects. Thank you Erik! Good Luck! I also thank Uwe Zimmermann for his help in the cleanroom and for high-voltage measurements in the FTE measurement room. I have to acknowledge Sang-Mo Koo, who was my last office mate and Korean HUBAE at KTH. Sang-Mo is the kindest man I have ever met. We spent many hours in the office, the measurement room, and mainly the cleanroom together. We also spent quite many times at PUB. I could not forget the time we had in last ICSCRM 2001 conference in Japan, in Korea, and also in Sweden. I wish that you would do much better than I did and good luck to you and your family!

I also thank all the member of SiC group, Dr. Martin Domej, Fanny Dahlqvist, and Wei Liu as well as other members in EKT, Dr. Shi-li Zhang, Dr. Yong-Bin Wang, Dr. Per-Erik Hellberg, Dr. Johan Pejnefors, Ann-Chatrin Lindgren, Erik Haralson, Johan Seger, Erdal Suvar, Stefan Persson, Dongping Wu, and Christian Isheden.

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Special thanks go to Dr. Gunnar Malm for his help during the Ph.D. Gunnar will do his defense at the end of April this year just before me. Congratulation, Gunnar!

Many thanks also go to Dr. Anders Hallen, Dr. Andrej Kuznetsov, Dr. Margareta Linnarsson, Mikael Jargelius, Martin Janson, John Österman, Paulius Grivickas, and Antonio Martinez in the Solid State Electronics Laboratory for their technological help, measurements, and fruitful discussion. Special thanks to Leonardo Hillkirk, we had quite many discussions not related to our own projects until last night. Leo, it was a nice time. Please keep in touch.

I thank other Korean HUBAE-DUL at KTH, Jung-Hyuk Koh and new-comer Jang-Yong Kim. Good luck!

During my Ph.D study I also met several special people, Dr. Henry Bleichner, Dr. Mark Irwin, and Sofia Hatzikonstantinidou from ABB Corporate Research AB in Kista, Sweden. They supported me overwhelmingly such as providing expensive wafers ($$$) and helping with the processes including ion implantation by Henry at ABB cleanroom. I have learned many things like mass-production and industrial point of view from the corporation with them. Specially I thank Sofia for her help, I wish that you will have good luck in your own new business.

And the co-authors for the TiC papers, Dr. Ulf Jansson, Dr. Hans Högberg, and Jens-Petter Palmquist in Uppsala are also acknowledged.

I would like to thank nano-particle (aerosol) and device people, Prof. Lars Samuelson, Dr. Knut Deppert, Dr. Lars-Erik Wernersson, Dr. Martin H. Magnusson, Dr. Andrej Litwin (also in Ericsson Microelectronics AB), and Ingvar Åberg in Solid State Physics at Lund University. They introduced me to the world of nanometer-size devices. Among them, I could not forget the endless discussion with Lars-Erik and Ingvar (now in electrical engineering department at MIT) to improve our manuscript. Thanks guys! Keep in touch as well.

Dr. Lars Uneus and Dr. Anita Lloyd Spetz at S-SENCE in Linköping University are also acknowledged for their high-temperature measurements in oxygen ambient. Dr. Spetz was also my opponent for my licentiate thesis defense on Oct. 23, 2000. Thanks for the nice discussion during at that time.

I have to say “thank you” to Dr. Eun-Dong Kim and Dr. Nam-Kyun Kim at Korea Electrontechnology Research Institute (KERI) in Korea for their warm hospitality during my visit in Changwon in 2001.

I’d like to thank my former professors, Professor Suresh C. Sharma, who was my first supervisor for my master period in the Physics department at the University of Texas at Arlington in U.S.A, Professor Byung-Moo Moon at Korea University in Korea, who introduced me to KTH in Sweden, and Professor K.V. Rao for support during the first 6 months stay in Sweden. Thanks also to my past office mates, Hubert and Linwei and our department secretary, Zandra Lundberg.

I’d like to thank all of the family in Korea. Very special thanks to my parents as

well as to my mother-in-law for their kind support in Seoul, Korea. I believe they will be happy about my graduation, specially my father. I dedicate my Ph.D to my father. I wish you, all my parents and mother-in-law, a long life without any health trouble. Many many thanks to other my-side and my wife-side family members, specially my sisters-in-law (called EONNEE-DUL) in Korea for their encouragements all the time. Thanks EONNEE-DUL (usually I call them like my wife does).

Finally, I could not forget the great support of my wife, Me-Jeong during the stay

in Sweden. Thank you so much, Me-Jeong! Without your strong support (so called

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NAE-JO in a good sense or in a wide meaning you said). Yes, I know, it was greatly helpful to keep my study and finish my final Ph.D. work. In addition, everybody in our family knows well you did a good support to me and GeeHee. Finally I thank my daughter Geehee. I love you.

This work was supported by the Swedish Foundation for Strategic Research

(SSF) in one of the research projects in SiCEP (Silicon Carbide Electronics Program). Thank you all. _________________________ Sang-Kwon Lee Stockholm March 22, 2002

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Used Acronyms ⋅ AFM : Atomic force microscopy ⋅ CTLM : Circular Transmission Line Method ⋅ C-V : Capacitance-Voltage ⋅ DMT : Dual Metal Trench ⋅ ECR : Electron Cyclotron Resonance ⋅ FM : Figure of Merit ⋅ FMR : Floating Metal Ring ⋅ ICP : Inductively Coupled Plasma ⋅ IMPATT : IMPact ionization Avalanche Transit Time diode ⋅ I-V : Current-Voltage ⋅ JBS : Junction Barrier Schottky ⋅ JFET : Junction Field-Effect Transistor . LEED : Low-Energy Electron Diffraction ⋅ LOCOS : LOCal Oxidation of Silicon ⋅ LPCVD : Low Pressure Chemical Vapor Deposition ⋅ LTLM : Linear Transmission Line Method ⋅ MEMS : MicroElectroMechanical Systems ⋅ MESFET : MEtal-Semiconductor Field-Effect Transistor ⋅ MISiCFET : Metal-Insulator Silicon Carbide Field Effect Transistor ⋅ MOSFET : Metal-Oxide Semiconductor Field-Effect Transistor ⋅ MPS : Merged P-i-N/Schottky ⋅ MS : Metal-Semiconductor ⋅ PECVD : Plasma Enhanced Chemical Vapor Deposition ⋅ PVD : Physical Vapor Deposition ⋅ RBS : Rutherford Backscattering Spectrometry ⋅ RESP : REsistive Schottky barrier field Plate ⋅ RIE : Reactive Ion Etching ⋅ RTA : Rapid Thermal Annealing ⋅ SBH : Schottky Barrier Height ⋅ SEM : Scanning Electron Microscopy ⋅ SiC : Silicon Carbide ⋅ SIMS : Secondary Ion Mass Spectrometry . TEOS : TetraEthylOrthoSilicate, Si(OC2H5)4 ⋅ TEM : Transmission Electron Microscopy ⋅ TFE : Thermionic Field Emission ⋅ TiC : Titanium Carbide ⋅ TiW : Titanium tungsten ⋅ TLM : Transmission Line Method ⋅ TMBS : Trench Metal-oxide-semiconductor (MOS) Barrier Schottky ⋅ UHV : Ultra High Vacuum ⋅ XPS : X-ray Photoelectron Spectroscopy ⋅ XRD : X-Ray Diffraction

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S

1. Introduction

ilicon carbide (SiC) has received remarkable attention during the last decade as a promising device material for high temperature, high frequency, and high power

device applications due to its high thermal conductivity and high critical field for breakdown. It exhibits higher values of thermal conductivity (3-13 times), critical electric field (4-20 times), and saturated carrier velocity (2-2.5 times) compared to the conventional semiconductor materials such as silicon and gallium arsenide [1-3]. Table 1-1 shows the comparison of physical properties of SiC and other semiconductor materials. These favorable properties of SiC are desirable for efficient high power device operation[2, 3]. It is also an attractive material for high temperature operating (> 650 oC) gas sensors as well as solid-state transducers such as pressure sensors and accelerometers for automotive and space industry applications using microelectromechanical systems (MEMS)[4, 5]. Table 1-1. Comparison of electrical / mechanical properties for various semiconductors.

Si GaAs 3C-SiC 6H-SiC 4H-SiC GaN Eg (eV) 1.1 1.4 2.4 3.0 3.26 3.4

Ec(MV/cm) 0.3 0.4 1.2 2.5 2.2 3.3

vsat (107 cm/s) 1 2.0 2.0 2 2 2.5

µn(cm2/Vs) 1350 8500 900 370 720 1000

µp(cm2/Vs) 480 400 40 80 120 30

εr 11.8 12.8 9.7 10 10 8.9

λ (W/cmK) 1.5 0.5 5 5 5 1.3

lattice constant (Å)

a=5.43 5.65 4.36 a=3.08 c=15.12

a=3.08 c=10.08

a=3.19 c=5.19

ρ (g/cm3) 2.3 5.3 3.2 3.2 3.2 6.1

In order to qualify the advantages of semiconductors for various applications the various FMs (figure of merit) have been used[6-9]. The analysis of these FM will help in quantifying the benefits of using wide bandgap semiconductors for making unipolar power devices. Chow and Tyagi[10] reported a critical evaluation of the performance capabilities of various wide-bandgap semiconductors for high-power and high- frequency unipolar electron devices using seven different FMs such as JM, KM, BM, BHFM, QF1,2, and 3. Johnson's[6] and Keyes'[7] figures of merit are basically used for comparison purposes like high frequency and power evaluation. The other FMs are more critical for power device performance. Table 1-2 summarizes the various figures

Chapter 1 Introduction Sang-Kwon Lee

- 2 -

of merit for various semiconductors. For high-frequency device applications, SiC devices can provide much higher speed compared to the Si devices due to the higher saturated drift velocity vsat (influence the delay time τ = W/ vsat) and lower permittivity εr (capacitance C ∝ εr). Apart from these advantages, there are two more benefits that SiC-based electronics offer in the areas of high-temperature and high-power device operation.

Table 1-2. various figures of merit for various semiconductors.

Material JMa (Ecvsat/π)2

KMb λ(vsat/εr)

1/2 QF1c λσA

g QF2d

λσAEC QF3(BM)e σA = εrµEC

3 BHFMf µEC

2

Si 1 1 1 1 1 1 GaAs 7 0.5 36 48 16 11 GaN 756 1.6 644 7089 744 90 3C-SiC 278 5.2 117 468 35 11 4H-SiC 278 5.1 594 4357 178 29 6H-SiC 215 5.1 448 3734 134 19

all values are normalized with respect to Si a Johnson's FM (JM) for the basic limit on the device performance (high power and frequency). b Keyes's FM is based on the switching speed of the transistor. c Quality factor 1 (thermal FM) for heat sink material and the active device area in power device d Quality factor 2 is based on perfect heat sinks. e Quality factor 3 is based on no assumptions about the sink materials or geometry. f Baliga FM for evaluation of high frequency application g σA is QF3 (Baliga FM) Beyond on favorable properties of SiC, the full performance of SiC devices is limited by the material quality itself and the fabrication of high temperature stable Schottky contacts and low resistivity Ohmic contacts. Generally, contacts between metals and semiconductors play a major role in all classes of devices. Contacts are used in controlling certain device functions, as well as providing means for communication between the active devices and the outside world. In view of the major role of these device applications, the nature and essential parameters, which affects the properties of contacts, should be addressed and studied in detail. Lowering the Schottky barrier height (φB) or increasing the doping concentration can reduce the specific contact resistance (ρC) since the most systematic way to compare different metallization technologies with respect to the specific contact resistance is by measuring itself [11]. Indeed, unlike the actual contact resistance, this parameter (ρC) is unaffected by the current crowding and geometry-independent characteristic of the metal-semiconductor interface. However, it is hard to achieve Ohmic contacts with low (<10-6 Ωcm2) specific contact resistance due to the relatively large Schottky barrier height (0.9∼1.8 eV and 1.3∼2.0 eV for n- and p-type, respectively, see chapter 2). Lower specific contact resistance is obtained to n-type 4H-SiC and 6H-SiC (∼10-4 to 10-6 Ωcm2) than to p-type 4H- and 6H-SiC (10-3 to 10-5 Ωcm2)[12-14]. According to calculation of the specific contact resistance ρC in case of a barrier height of 0.3 eV, a doping level of 1 × 1019 cm-3, for n-type 6H-SiC using a general thermionic field emission (TFE) theory[11], a specific ρC of 1 × 10-6 Ωcm2 can be achieved (see chapter 2). The theoretical and experimental values are quite consistent with each other except for p-type SiC. Currently the investigation on thermally stable Schottky contacts and low resistivity Ohmic contacts is still required for high-power

Processing and characterization of silicon carbide contacts for high power and high temperature device applications

- 3 -

and high-temperature device applications even though there are a plenty of publications in this topic. In addition, a few publications and works are reported on long-term reliability tests at elevated temperature (> 600oC) and on the issue of distribution of the specific contact resistance on SiC wafer for stable performance of devices at high temperature and high power operation. The answers for the questions: what kinds of phases are created at the interface between metal and silicon carbide, what kind of phase makes Ohmic and low resistivity contacts to SiC, what kind of metal and metal-stacks can be applied for high-power as well as high-temperature device applications, what kind of metal is compatible with the fabrication processes, and what is the mechanism of metal-semiconductor junctions will be described in this thesis. This thesis is focused on the electrical characterization of Ohmic as well as Schottky contacts to n- and p-type 4H- and 6H-silicon carbide. P-type Ohmic and Schottky contacts are much more highlighted because of its importance and urgent need for device applications. A background to silicon carbide is given in Chapter 2. Chapter 3 covers the basics of the current-transport mechanism and Schottky diode performance. Chapter 4 describes the experimental technique and test structures for the measurement of specific contact resistances. The characterization and results are presented in Chapter 5. Chapter 6 contains concluding remarks and suggested future investigations.

- 5 -

A

2. Properties of SiC

s we realized in chapter 1, silicon carbide is a wide bandgap semiconductor, which has many advantages compared to the conventional semiconductors. In

this chapter, the material properties of SiC is focused. Some of the important parameters for device simulation and calculation are also presented with useful values, which are based on the recent literature and commercially available simulators. 2.1 SiC material properties 2.1.1 Crystal structure

The common semiconductors occur in the diamond crystal structure (Si and Ge), the zincblende crystal structure and the wurtzite crystal structure (for example, GaAs and other III-V compound semiconductors) even though there is a large number of different crystal structures possible in nature [15]. Silicon carbide has several stable crystal structures. 2.1.2 Polytypes of SiC

SiC has equal parts silicon and carbon, both of which are group IV elements. The distance between neighboring silicon (a) or carbon atom is approximately 3.08 Å for all polytypes. The carbon atom is situated at the center of mass of the tetragonal structure outlined by the four neighboring Si atoms (see Figure 2-1). The distance between the C atom and each of the Si atoms is approximately 2.52 Å. The height of the unit cell, called c, varies between the different polytypes. Therefore, the ratio of c/a differs from polytype to polytype. This ratio is 1.641, 3.271, and 4.908 for the 2H, 4H, and 6H-SiC polytypes, respectively (see also Table 1-1). The polytype is a variation of crystalline material in which the stacking order of planes in the unit cell is different. Each SiC bilayer, while maintaining the tetrahedral binding scheme of the crystal, can be situated in one of three possible positions with respect to the lattice (A, B, or C). The bonding between Si and C atoms in adjacent bilayer planes is either of a Zinc-blende (Cubic) or Wurtzite (Hexagonal) nature depending on the stacking order[16]. As shown in Figure 2-2, if the stacking is ABCABC...the cubic polytype commonly abbreviated as 3C-SiC, is realized.

Si atom

C atom a

a

Figure 2-1. The tetragonal bonding of a carbon atom with the four nearest silicon.

Chapter 2 Properties of SiC Sang-Kwon Lee

- 6 -

The purely Wurtzite ABAB... stacking sequence is called 2H-SiC. The 4H-SiC (ABAC...) and 6H-SiC (ABCACB...) are also shown in Figure 2-2[17]. These two types of SiC are the most common hexagonal polytypes [18]. 4H-SiC consists of equal amounts cubic and hexagonal bonds, while 6H-SiC is two-thirds cubic. All the polytypes of SiC are referred to in a hexagonal coordinate system consisting of three a-plane coordinates a1, a2, and a3, and a c-axis coordinate. The c-axis is the direction of the stacking of hexagonally close packed layers, and the three a-plane axes are all in the plane perpendicular to c-axis (see Figure 2-3), with 120-degree angle between a-planes. Commercially available SiC bulk material is generally cut and polished 3∼8 degrees off-axis towards <11 2 0> for avoiding the growth of 3C inclusions in the epitaxial layers of 4H, called step-controlled epitaxy by Matusunami et al. [19]. Two different faces perpendicular to the c-axis (Si 0001 and C 000 1 ) exist in commonly used SiC. SiC with the silicon face is commonly used for device applications since the quality of epitaxial growth is better than that on the carbon face.

Figure 2-2. The staking sequence of common 3C-, 2H-, 4H-, and 6H-silicon carbide (after ref. [17]).

Figure 2-3. The Miller indices describing the hexagonal structure.


- 7 -

2.2 SiC electronic properties Physical models, which also include most important parameters, are presented based on the recent literature and the corresponding parameters for the Silvaco ATLAS [20] physical device simulator. The comparison of electrical and structural properties for various semiconductors can be found from a Ph.D. thesis[21]. 2.2.1 Density of States (DOS) and Energy bandgap (Eg)

The intrinsic carrier concentration ni in a semiconductor is a fundamental parameter and high operating temperature limit. It has also been found that the formation of current filaments (mesoplasmas) can be related thermal runway when the intrinsic concentration becomes comparable to that of the background concentration. The relationship between ni and temperature and energy bandgap is given by [22]

−=

kT2

EexpNNn g

VCi (2-1)

where NC and NV is the effective density of states in the conduction and valence band states, respectively given by

( ) )300(N300TkTm2

2TN C

23

23

2

*e

C

=

=

h (2-2)

( ) )300(N300TkTm2

2TN v

23

23

2

*h

V

=

=

h (2-3)

where me

* and mh* is 0.76 m0 and 1.20 m0, respectively[23]. Using equations 2-2 and 2-

3, the NC and NV for 4H-SiC equal 1.66×1019 cm-3 and 3.19×1019 cm-3, respectively at room temperature (300K). The temperature dependent energy bandgap is given by

β+

−β+

⋅α+=TT

300300

)300(E)T(E22

gg (2-4)

The effective density of states in the conduction and valence band as well as the energy bandgap at room temperature (300K) for different semiconductors are summarized in Table 2-1 [24].

Table 2-1. Calculated parameters for different semiconductors at 300K. 4H-SiC Si Ge GaAs

NC (300) cm-3 1.66 × 1019 2.89 × 1019 1.04 × 1019 4.7 × 1017 NV (300) cm-3 3.19 × 1019 1.04 × 1019 6.00 × 1018 7.0 × 1018 Eg (300) eV 3.26 1.08 0.66 1.42

α 3.3 × 10-4 4.73 × 10-4 4.77 × 10-4 5.41 × 10-4 β 0 636 235 204


- 8 -

From equations 2-1, 2-2, 2-3, and 2-4, the intrinsic carrier concentration as a function of the temperature for different semiconductors having a different energy bandgap based on the calculation (see Table 2-1) was plotted in Figure 2-4. For room temperature (300K), ni is equal to be approximately 7 × 10-7 cm-3 for 4H-SiC. As shown in Figure 2-4, the wider bandgap and thereby lower intrinsic carrier concentration allows SiC to maintain semiconducting behavior at much higher temperature than conventional Si and Ge semiconductors. 2.2.2 Bandgap narrowing

The bandgap narrowing can be induced both by doping and by carrier injection. For the ATLAS[20] simulations of SiC devices, a model that is valid for Si is used since there are few models available for the bandgap narrowing effect of SiC. The bandgap narrowing (∆Eg) is given by

+

⋅+

⋅⋅=∆ −

21

2

17173

g 5.0101N

ln101N

ln109E (2-5)

However, Persson et al. [25, 26] theoretically investigated the energy bandgap narrowing for 3C, 4H-, and 6H-SiC, where the valence band and conduction band change are extracted as a function of ionized dopant concentration or carrier concentration. This model [25, 26] can be described by equation 2-6

l1

0x

k1

0xx N

BN

AE

Ξ+

Ξ=∆ (2-6)

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0108

1010

1012

1014

1016

1018

1020

300 K

4H-SiC

Ge

GaAs

Si

Intri

nsic

car

rier

conc

entra

tion

n i (cm

-3)

1000/T (1000/K)

Figure 2-4. Intrinsic concentrations for various semiconductors as a function of the temperature.


- 9 -

where Ξ is the controlling property of the bandgap narrowing, and A, B, No, k, and l are the fitting parameters where k and l are positive integers. 2.2.3 Incomplete ionization

One of the disadvantages with wide-bandgap semiconductors is that the dopant ionization levels are quite deep. Hence, the dopants are not fully ionized even at higher temperature. The carrier concentration N+

D,A (i.e. the number of ionized donors or acceptors) can be calculated with the following equations [3, 27],

++−

=+

kTE

NN

g

kTE

NN

g

NNAD

VC

ADc

AD

VC

ADc

ADAD,

,

,

21

,

,

,

,,

exp2

exp411

(2-7)

23

23

19, 300

10513.2

×=

V

CVC m

mK

TN (2-8)

where gC is the spin degeneracy (in this case 2 for donors and 4 for acceptors), NC(NV) is the density of states given by equation 2-8 with the effective density of states masses

mC or mV (=1× m0) for electrons and holes, respectively, and ED and EA are the donors and acceptors levels . Using equations 2-7 and 2-8, the calculated ionization aluminum and nitrogen in 6H-SiC at 300K and 800K is shown in Figure 2-5. According to the calculation, a doping concentration of 1020 cm-3 for aluminum at 300K would only result in a carrier concentration of 5 × 1017 cm-3. However, at these doping

1013 1014 1015 1016 1017 1018 1019 1020 10211013

1014

1015

1016

1017

1018

1019

1020

1021

n-type p-type

6H-SiCE

D=100meV

EA=200meV

300K

800K

Car

rier

conc

entr

atio

n (c

m-3)

Doping NDor N

A (cm-3)

Figure 2-5. Calculated ionization of aluminum and nitrogen in 6H-SiC at 300K and 800K using equations 2-7 and 2-8.


- 10 -

concentrations 1% of the atoms have been replaced, and the bands are degenerate, resulting in much higher free carrier concentration. Therefore, the equations 2-7 and 2-8 are not quite valid for higher doping concentration than about 1019 cm-3. 2.2.4 Carrier recombination

SRH Recombination

Schottky, Read, and Hall (SRH) described the recombination process with the phonon transitions by the way of defects or traps. The SRH recombination rate is given by

+τ+

+τ

−=

−−

kT

EE

ienkT

EE

iep

2ie

SRHTrapiiTrap

enpenn

nnpR (2-9)

where ETrap-Ei is the distance between the trap level and the intrinsic level, and τn, and τp are the lifetime of electrons and holes at low injection, respectively. Commonly used values of carrier lifetimes are in the range of 0.1 ∼ 2 µs for n-type 4H-SiC [21] since the reported lifetimes in the literature are varying with a big difference.

Auger Recombination

Auger recombination, which is an important model parameter for high-power device design, occurs both at high doping level and in the high injection regime due to the direct band-to-band recombination between an electron and a hole across the forbidden gap, accompanied by the transfer of energy to other free electron or hole [28]. It is given by equation 2-10, where Cn,p are the Auger coefficient for electrons and holes, and their sum is extracted from the measurement in the high-level injection regime.

[ ] [ ]2ienpAuger nnpnCpCR −⋅+= (2-10)

The values we used for Cn and Cp is 5×10-31 and 2×10-31 cm-6s-1, respectively for n-type 4H-SiC with a doping concentration of 1×1018 cm-3 at room temperature [28]. 2.2.5 Impact Ionization

The maximum electric field (critical field; EC) and the blocking voltage (VB) are determined by the impact ionization rate αn and αp for electrons and holes respectively. SiC shows lower impact ionization rate of electrons than that of holes, which is the opposite compared to conventional Si. The ionization integral equation is given by[29, 30]

( ) 1)()(exp)(0

=

−∫ ∫ dxdxxxx

W W

xpnn ααα (2-11)


- 11 -

−

=

Ref

D

Ref CR,CR

10log41

1NN

EE

q

J

q

JG

p

p

n

nII

rr

αα +=

pn,

,

pn,

3001

300

refp,n,

AD

maxp,n,minp,n,pn,

α

γ

α µµµ

++

+

=

T

NNN

Tpn

−=

−=

)(exp)(

)(exp)(

xE

bax

xEb

ax

ppp

nnn

α

α

(2-12)

The critical field with an impurity concentration of 1015<ND<1018 can be calculated by (2-13) It is known that, for 6H-SiC, the critical field parallel to the c-axis (E) is half the critical field perpendicular to the c-axis (E⊥) and the temperature coefficient of the breakdown voltage is negative for E, while it is positive for E⊥. There are no measurement results on the anisotropic critical field in 4H-SiC. The total impact ionization rate is expressed as (2-14)

The specific impact ionization parameter for 4H-SiC can be found in the literature [21, 31]. 2.2.6 Mobility

The Arora model [32] is used for the doping dependence of the mobility. Another important model is the drift velocity saturation, which is implemented as a decrease in the mobility at high electric fields. The low field mobility (Arora model) is given by [23, 33, 34] (2-15) where ND and NA are the density of donors and acceptors respectively. The mobility is more anisotropic for 6H-SiC than it is for 4H-SiC. Schaffer et al. [34] presented the results from measurements of electron and hole majority carrier mobilities in both 4H- and 6H-SiC as functions of temperature, doping, and directions in epitaxilly grown crystals. According to their measurements, the electron mobility perpendicular to the c-axis ([11 2 0] for 4H- and [1 1 00] for 6H-SiC) is 0.83 times for 4H-SiC and 4.8 times for 6H-SiC at above 200K than parallel to the c-axis ([0001])[23, 34]. For 4H-SiC, it is independent of the temperature. The parameter values for different materials (Si, 6H- and 4H-SiC) are summarized in Table 2-2 below. Figure 2-6 and 2-7 shows the low field mobilities of µn and µp as a function of the doping concentration at different temperatures (300K, 450K, and 600K) for 4H-SiC, respectively. At high electric fields,


- 12 -

ββµ

µµ

1

sat

0pn,

0pn,

pn,

1

)(

+

=

v

E

E

the carrier drift velocity (v) saturates due to the increase of the optical phonon scattering and reaches the saturation velocity (vsat). There is no model reported for this effect in 4H-SiC yet. The high field mobility can be described as (2-16) For SiC, the parameter for high field mobility i.e. β=2 and νsat=2×107 cm/s (see also Table 1-1) is usually chosen.

Table 2-2. Mobility for different semiconductor materials (Ref. [23, 33, 34]).

6H-SiC 4H-SiC Si ⊥ to c-axis

µ[1 1 00] to c-axis µ[0001]

⊥ to c-axis µ[11 2 0]

to c-axis µ[0001]

µn,min cm2/Vs 92 0 0 0 0 µn,max cm2/Vs 1268 415 87 947 1136 Nn,ref cm-3 1.3×1017 1.1×1018 1.94×1017 γn 0.91 0.59 - 0.61 αn -2.42 -3 - -2.15 -2.40 µp,min cm2/Vs 52 6.8 - 15.9 - µp,max cm2/Vs 453 99 - 124 - Np,ref cm-3 1.9×1017 2.1×1019 - 1.76×1019 - γp 0.63 0.31 - 0.34 - αp -2.2 -3 - -2.15 -

1015 1016 1017 1018 1019 1020

0

200

400

600

800

1000

1200

Perpendicular to c-axis

Parallel to c-axis

300 K 300 K 450 K 600 K

Ele

ctro

n M

obili

ty (

cm2 /V

s)

ND (cm-3)

Figure 2-6. Low field electron mobility (Arora model) as a function of the doping concentration at different temperatures (300 K, 450 K, 600 K) in 4H-SiC (after ref. [23, 33, 34]).


- 13 -

1015 1016 1017 1018 1019 10200

20

40

60

80

100

120

140

300 K 450 K 600 K

Hol

e M

obili

ty (

cm2 /V

s)

NA (cm-3)

Figure 2-7. Low field hole mobility (Arora model) as a function of the doping concentration at different temperatures (300 K, 450 K, 600 K) in 4H-SiC (after ref. [23, 33, 34]).

- 15 -

M

3. Metal-Semiconductor junctions

etal-semiconductor contacts have many applications such as the gate electrode of metal semiconductor field-effect transistors (MESFET), the source and

drain contacts in metal oxide semiconductor field-effect transistors (MOSFET), and the electrode for impact ionization avalanche transit time (IMPATT) oscillators[24]. In this chapter, we will describe the basics of metal-semiconductor junctions, the key factors for both Ohmic and Schottky contacts to semiconductor, and some examples of metal-semiconductor devices. 3.1 Current transport mechanism 3.1.1 Schottky barrier formation

When a metal and a semiconductor are brought in contact, the respective Fermi-levels must coincide in thermal equilibrium as shown in Figure 3-1 (b). There are two limiting cases such as the ideal case (referred to as Schottky-Mott limit[35]) and a practical case (known as the Bardeen limit[36]) to describe the relationship between a metal and a semiconductor. Figure 3-1 shows the energy band diagram for the ideal case (Schottky-Mott limit) with the absence of surface states.

Figure 3-1. The formation of a barrier between the metal and the semiconductor when (a) neutral and isolated and (b) in perfect contact without any oxide between them (Schottky-Mott limit).

Metal

Semiconductor n-type

qφm

EF

qχ qφs

Vacuum level

EF

EC

EV

(a)

Metal

qφB

qVbi=qφm-qφs

EF

EF

EC

EV Semiconductor

(b)

Chapter 3 Metal-Semiconductor junctions Sang-Kwon Lee

- 16 -

In this case the barrier height for n-type semiconductor can simply be determined to be the difference between the metal work function (φm) and electron affinity (χS) of the semiconductor; ( )smBn qq χ−φ=φ (3-1) For a given semiconductor and a metal, the sum of the barrier height on n- and p-type semiconductor is expected to be equal to the energy bandgap ( ) gBpBn Eq =φ+φ (3-2) This relationship for Schottky-Mott limit implies that the control of the barrier height is achieved by the choice of the metal. The second limiting case is the Bardeen limit[36] where a large density of states is present at the semiconductor to metal interface. In the Bardeen limit the barrier height φB is completely independent of the metal work function φm in contrast to the Schottky-Mott limit and the Fermi level is said to be pinned by the high density of interface states. 3.1.2 Current transport mechanism

Figure 3-2 shows four basic transport processes for n-type semiconductors under forward bias [11]. The four processes are a) emission of electrons from the semiconductor over the top of the barrier into the metal, b) quantum mechanical tunneling through the barrier, c) recombination in the space-charge region, and d) recombination in the neutral region (called hole injection). For the lowly doped semiconductor the current flows as a result of thermionic emission (TE)[37] as shown

(a) Low ND (TE) (b) Intermediate ND (TFE) (c) High ND (FE)

Figure 3-3. Energy band diagram for (a) low doped, (b) intermediate doped, and (c) high doped n-type semiconductor. The arrow indicates the electron flow.

Figure 3-2. Current transport processes in a forward-biased Schottky barrier.

Metal

EF EC

EV

Semiconductor

a

b

c d


- 17 -

in Figure 3-3 (a) with electrons thermally excited over the barrier. In the intermediate doping range, thermionic field emission (TFE)[38] dominates as shown in Figure 3-3 (b). For high doping, the barrier is sufficiently narrow at or near the bottom of the conduction band for the electrons to tunnel directly, known as field emission (FE). The three regimes can be distinguished by considering the characteristic energy E00 defined by[38]

[ ]eVN101.183em

N4pqh

E D11

s*tun

D00

−×== (3-3)

where ND is the doping concentration, m*

tun (≈ 0.25 m0 for 6H-SiC[10]) is the effective tunneling mass , and εs (≈ 10 ε0) is the dielectric constant for SiC. Using equation 3-3, a plot of characteristic energy E00 as a function of doping density is shown in Figure 3-4. A comparison of E00 to the thermal energy kT shows that when thermionic emission dominates kT » E00, for thermionic-field emission kT ≈ E00, and for field emission kT « E00. For simplicity, the range of each regime can be chosen by E00 ≤ 0.2 kT for TE, 0.2 kT < E00 < 5kT for TFE, and E00 ≥ 5 kT as shown in Figure 3-4. For 6H-SiC with a tunneling effective mass of 0.25 m0, this corresponds approximately to TE for ND ≤ 1.86 × 1017 cm-3, TFE for 1.86 × 1017 < ND < 1.15 × 1020, and FE for ND ≥ 1.15 × 1020.

1016 1017 1018 1019 1020 102110-3

10-2

10-1

100

6H-SiC (m*

tun~ 0.25m

0)

ND~1.1 x 1020cm-3(E

00~ 0.127)

1.9 x 1017cm-3(E00

~ 0.005)

FETE TFE

E00

kT

E00

, kT

(eV

)

Doping density (cm-3)

Figure 3-4. E00 and thermal energy kT as a function of the doping density for 6H-SiC with m*

tun/m0=0.25, T=300K.


- 18 -

3.2 Ohmic contacts The specific contact resistance (ρC) is given by

0V

C JV

→

∂∂

=ρ (3-4)

and, can be calculated by using the following equations for each current transport process[10].

ϕ

=ρkT

expTqA

k B*C for TE (3-5)

( )

−

+ϕ

+ϕ=ρ

kTU

EU

expkTE

cothkTE

coshUpqA

Ek f

0

fB0000

fB*

00C for TFE (3-6)

( )

1

f00

B22

22*

00

B2*

C cUE

expqkcTcqA

Eexp

ckTkTsinqTA

−

−

ϕ−−

ϕ−π

π=ρ for FE (3-7)

where A* is the modified Richardson’s constant, Uf is the Fermi level with respect to the band edge, and E0 and c are given by

ϕ=

=

f

B

00

00000 U

4ln

2E1

c,kTE

cothEE (3-8)

The theoretical specific contact resistance for different doping concentration can be calculated using equation 3-5 to 3-7. For example, Schottky barrier height for a 0.3 eV and surface concentration of 1 × 1019 cm-3, the specific contact resistance is 1.0 × 10-6 Ωcm2 using equations 3-3, 3-4 and 3-7. The plot of the calculated specific contact resistance versus doping and barrier height is shown in Figure 3-5. Since the first successful single transistor was built, device fabrication technology has grown at a tremendous rate and device size is becoming smaller and smaller. Here, the nanometer-size contacts are pointed out briefly due to its importance in future research. Electronic properties of metallic nanometer-size contacts (or clusters or particles) on semiconductors rely on the metal comprising the cluster, the size of cluster or contacts, the semiconductor substrates, and the fabrication process and techniques. Until now, metal/semiconductor nanostructures fall into three broad categories: (1) single electron tunneling (SET) devices, (2) nanoscale Schottky barriers, and (3) Ohmic contacts. When the size of a contact (particle or cluster) is reduced to a few nanometers, an additional mechanism is required to understand and explain the new effects due to the size. When we are working on the nanometer scale, the capacitance of the structure can be low enough so that the single electron charge energy e2/2C can be large compared to


- 19 -

the thermal energy KBT which is about 26 mV at 300K. For asymmetric structures, this effect is observed as steps in an I-V curve and some oscillation in dI/dV and is known as a Coulomb staircase. Also the tunneling probability is very small for voltages smaller that e/2C, resulting in an energy gap in this voltage range. This phenomenon is referred to as Coulomb blockage. Andres et al. [39] reported that a 1 nm diameter Au cluster on Au (111) substrate showed the Coulomb blockade effects at room temperature. But single electron tunneling effects at room temperature can not be observed in a cluster larger than ∼ 3 nm in diameter [40]. Lee et al. [40] reported the formation of characterization of nanometer size Ohmic contacts to n-type GaAs substrates. They also determined the specific contact resistance (ρC=1×10-6 Ωcm2) from I-V data of ultra high vacuum scanning tunneling microscopy (UHV STM) in the Au cluster/xylyl dithiol/GaAs substrates. This nanometer size is required to further research.

0 1x10-9 2x10-9 3x10-910-9

10-7

1x10-5

10-3

10-1

101

103 1019 1018 10171020

ΦB=0.2 eV

ΦB=0.4 eV

ΦB=0.6 eV

ΦB=0.8 eVΦ

B=1.0 eV

ρ C (

Ωcm

2 )

1/sqrt(ND) cm3/2

Figure 3-5. calculated specific contact resistance (ρC) versus doping concentration for barrier height from 0.2 to 1.2 eV. The calculation used Thermionic field emission (TFE) for doping concentration from 1016 to 1018 cm-3 and field emission (FE) for doping concentration from 1018 to 1021 cm-3.


- 20 -

3.3 Schottky diode performance Schottky diodes are of interest for high-power devices because they are majority carrier devices and consequently have very fast switching times and no reverse recovery current. Here, some important properties of Schottky barrier diodes for power rectifiers will be described. 3.3.1 Specific on-resistance

For high-power device application, the specific on-resistance should be as low as possible. It is shown in equation 3-9, where W is the thickness of epilayer and substrate, VB is the breakdown voltage.

substrateDnlayerEpiDn

3Cn

2B

Dnspon

NqµW

NqµW

Ee4V

NqµW

R

+

=

µ==

−

−

(3-9)

Using equation 3-9, the specific on-resistance versus the breakdown voltage for different semiconductors such as Si, 4H-SiC, and GaN are shown in Figure 3-6.The straight line (theoretical line) for Si, 4H-SiC, and GaN is calculated assuming that the specific on-resistance of the substrate is less than ≈ 10-7 Ωcm2. The contribution of the specific on-resistance for the substrate is also plotted in the same figure for the 4H-SiC. As shown in Figure 3-6, SiC has large advantage for high power application in comparison to Si. In order to design 1kV devices the specific on-resistance should theoretically be lower than 10-4 Ωcm2.

100 101 102 103 104 10510-10

10-8

10-6

1x10-4

10-2

100

102

104

with contribution of Substrate

Theoretical line (without contribution of Substrate)

Ron-sp

=1x10-5Ωcm2

Si4H-SiC

GaNRon-sp

=1x10-3Ωcm2

Spe

cific

on-

resi

stan

ce R

on-s

p(Ωcm

2 )

Breakdown voltage (V)

Figure 3-6. Specific on-resistance Ron-sp versus the breakdown voltage VB for Si, 4H-SiC and GaN.


- 21 -

3.3.2 Forward voltage drop

The forward voltage drop is given by

( ) FsponB

25

FsponB2*F

F

JRT685.0ln)T(1061.8

JRTA

Jln

qkT

V

−−−

−

+φ+⋅×=

+ηφ+

η

= (3-10)

where η is the ideality factor, k is the Boltzman's constant, JF is the forward current density, and A* (≈ 146 Ak-2cm-2)[41] is the Richardson's constant. The forward voltage drop (VF) is a function of the temperature, Schottky barrier height, and specific on-resistance given by equation 3-9. From equation 3-10, if the specific on-resistance is

negligible and the logarithmic term is nearly constant and negative, the forward voltage drop decreases linearly with increasing temperature as shown in Figure 3-7. The results for a TiW Schottky diode (Paper I) with measured specific on-resistance (in our case ≈ 4.3 mΩcm2) were plotted in the same Figure 3-7. 3.3.3 Breakdown voltage and reverse leakage current

The breakdown voltage depends on the critical field, epilayer doping and thickness, and edge termination. The breakdown is given by[30]

0 50 100 150 200 250 3000.0

0.4

0.8

1.2

1.6

2.0

Theoretical lineΦ

Bn=1.22 eV

Ron

~ 4.3 mΩcm2

Ron

negligible

ΦBn

=1.22 eV (for TiW) from Paper I

ΦBn

=1.0 eV

For

war

d V

olta

ge D

rop

(V)

Temperature (oC)Figure 3-7. Forward voltage drop of a Schottky rectifier as a function of the temperature and Schottky barrier height. The figure also includes the experimental results of TiW Schottky diodes to 4H-SiC.


- 22 -

×=

ε=

NE

1077.2

qN2E

V

2C6

D

2CS

B

(3-11)

where critical electric field with doping dependence can be determined experimentally for 4H-SiC[31]

−

×=

16D

6

C

10N

log41

1

1049.2E (3-12)

From Figure 3-8 shows that for a given epilayer thickness, a decrease in epilayer doping does not necessarily increase the breakdown voltage since the decrease in doping may correspondingly decrease the critical field. The reverse leakage current is affected by Schottky barrier height, temperature, and image force barrier height lowering. First of all, the reverse leakage current density (JL) without the contribution of the image force lowering can be determined to be

φ−

−=kTq

expTAJ B2*L (3-13)

Using equation 3-13, a plot of the leakage current of a Schottky rectifier as a function of the temperature and Schottky barrier height is shown in Figure 3-9. As shown in Figure 3-9, the leakage current density of the Schottky rectifier increases rapidly with the temperature.

1014 1015 1016 1017 1018101

102

103

104

105

103

104

105

106

107

50µm

20µm

100µm

5µm

Si for EC=(4010xN

D

1/8)

Bre

akdo

wn

Vol

tage

(V

)

Background Doping (cm-3)

4H-SiC

4H-SiC

Max

imum

Ele

ctric

fiel

d (V

/cm

)

Figure 3-8. Breakdown voltage and maximum electric field versus epilayer doping for 4H-SiC and Si for punch through and non-punch through epilayer thickness.


- 23 -

3.3.4 Edge termination for high breakdown voltage

For high voltage Schottky diodes to reach the ideal parallel plane breakdown voltage, it is necessary to have an edge termination around the periphery of the diodes to reduce the electric field crowding at the diode edge. Several techniques of edge terminations have been shown to reduce the electric field crowding, resulting in higher breakdown voltage. In this section, we will describe these techniques. Bhatnagar et al.[42] reported a study on floating metal rings (FMR, see Figure 3-10) and resistive Schottky barrier field plate (RESP, Figure 3-10) for 6H-SiC Schottky barrier diodes. For the FMR termination, simulations indicate a breakdown voltage of 600 V for 3-ring termination with a ring spacing of 0.8 µm. Experimentally it is determined to be >400 V compared to a breakdown voltage of 220 V for un-terminated diodes with 10 µm n- epi-layer. For the RESP termination, a breakdown voltage of 500 V was achieved with a length of 75 µm and a sheet resistance of 1 MΩ/£ for the TiOx. Ueno et al. [43] reported p-epi guard ring (see Figure 3-11) formed by a local oxidation process (LOCOS). With this ring-shaped p-n junction guard ring, they achieved a breakdown voltage of 600 V, which is about 70 % of the ideal value of the p-n junction diodes. Alok and Baliga [44] investigated an edge termination with a resistive layer created by high dose argon (Ar+) ion implantation, resulting in close to the ideal parallel plane breakdown voltage (see Figure 3.11). Other groups [45, 46] also investigated similar edge termination using boron (B+) implantation (see Figure 3-12) for higher resistive regions at the edge to improve the reverse breakdown voltage. Itoh et al. [46] reported the employment of B+ implantation (energy of 30 keV and a dose of 1.0 × 1015 cm-2 at room temperature) for edge termination lead to an increase in the breakdown voltage close to the theoretical voltage without increasing leakage current. With this structure the measured breakdown voltage was up to 1750 V. Saxena et al. [47] also reported improved breakdown voltage with oxide field ring termination.

300 350 400 450 500 550 6000

10

20

30

40

50

ΦBn

=1.2 eV

A* (=146 A/K2cm2)

ΦBn

=0.8 eV

ΦBn

=1.0 eV

Leak

age

curr

ent d

ensi

ty (

mA

/cm

2 )

Temperature (K)

Figure 3-9. Reverse leakage current versus temperature and Schottky barrier heights.


- 24 -

N- epi (10 µm), 2×1016 cm-3

N+ SiC (300 µm)2×1018 cm-3

Floating metal rings

Schottky

Al-1% Si

N- epi (10 µm), 2×1016 cm-3

N+ SiC (300 µm)2×1018 cm-3

Schottky

Al-1% Si

TiOx RESPAl (1.5 µm)Pt (100 nm)Ti (5 – 8 nm)

N- epi (5 µm), 2×1016 cm-3

N+ SiC, 2×1018 cm-3

Schottky

Ni

p-epiAl-Ti

6H-SiC n+

300 µm

Al (1µm)

Ti (0.2 µm)

Ar+ implantation

N+ SiC, 200 µm

Schottky

Ni

B+

(1.0×1015 cm-2, 30 keV)Ti

10 µm100 µm

4H-, 6H-SiC n+

Schottky(Ni or Ti)

Ni

Field oxide(3000 Å)

Figure 3-10. Schematic Schottky diodes with FMR and RESP termination (after ref. [42]).

Figure 3-11. Schematic Schottky diodes with p-epi guard ring formed by LOCOS process (after ref. [43]) and edge termination by Ar+ implantation (after ref. [44]).

Figure 3-12. Schematic cross-section of Ti/4H-SiC Schottky diodes with B+ edge termination (after ref. [46]) and field oxide termination (after ref. [47]).


- 25 -

3.3.5 Schottky barrier lowering

Under reverse bias voltage, there is a reduction of the Schottky barrier height due to the image force lowering. When an electron approach a metal, the requirement that the electric field must be perpendicular to the surface enables the electric field to be calculated as if there is a positive charge of magnitude q located at the mirror-image of the electron with respect to the surface of the metal. The barrier height reduction due to the image force lowering is given by[11]

,4qE

S

mB πε

=φ∆ (3-14)

where Em is the maximum electric field given by

( )biRS

Dm VV

qN2E +

ε= (3-15)

where VR is the reverse bias voltage and Vbi is the built-in voltage for SiC. Finally, equation 3-13, the leakage current density including the image force barrier height lowering can be given by

φ∆

φ−−=

kTexp

kTexpTAJ BB2*

L (3-16)

Figure 3-13 shows the calculated Schottky barrier height reduction due to the image force lowering effect as a function of the reverse bias voltage up to 10kV.

101 102 103 1040.00

0.05

0.10

0.15

0.20

0.25

at 300 K

ND=1.0 x 1016cm-3 for 4H-SiC

∆φΒ (

eV)

Reverse bias voltage (V)

Figure 3-13. The calculated Schottky barrier height reduction at room temperature due to the image force lowering versus reverse bias voltage.


- 26 -

3.4 Other rectifiers In order to achieve better reverse blocking characteristics while maintaining Schottky-like forward conduction characteristics several modern rectifiers such as JBS, MPS, DMT, and TMBS have been developed and presented in the literature. The most common advantages and disadvantages of each device are presented in this section.[48, 49]. 3.4.1 Junction barrier Schottky (JBS) diodes

The junction barrier Schottky diode is a device which has the advantage of a low forward voltage drop while keeping a low leakage current at high blocking voltage. It is normally a Schottky structure with a normally implanted P+ grid into its drift region [30]. The schematic structure of a JBS diode is shown in Figure 3-14. The unipolar current flows through the conductive channels under the Schottky metal with a voltage drop that is normally determined by the Schottky barrier height like the Schottky diode in forward mode. In reverse conduction mode the p+n junctions become reverse biased and the depletion layers spread into the channel and pinch-off the Schottky barrier. The spacing, width, and thickness of the p+ grid are important design factors to optimize its performance. The total resistance which is a large contribution of an increasing voltage drop in JBS is the sum of the resistance in grid (Rgrid), in drift (Rdrift), and at backside Ohmic contacts (Rcathode) as shown in Figure 3-14. The contribution of backside Ohmic contacts to the voltage drop might be small and can be ignored. Because the contact resistance of annealed n-type Ni contacts is in the range of below 10-5 Ωcm2, the voltage drop is in the range of ≈ 1mV at 100 A/cm2 [50]. Therefore, the total on-resistance (Ron) and the voltage drop (VF) in JBS diode are given by equations 3-17 and 3-18, where s is the space between p+ grid, w is its width, x is its depth, tepi is the thickness of the epi-layer, and d is the junction depletion width from the p+ grid region [51].

Figure 3-14. Schematic JBS diode structure and its equivalent circuit.

N+ Substrate

N- epi-layer

P+ P+ P+ P+ P+

Schottky metal (Anode)

Ohmic contacts (Cathode) Rcathode

Rdrift

Rgrid


- 27 -

grid

dn

drift

dn

epi

on d2sws

lnd2s

wsNq2w

x

Nq2w

xtR

−+

⋅

++

⋅

µ

−+

µ

−−= (3-17)

( )( ) FonBn2*

FF JR

TAJ

d2sws

lnqkT

V +ηφ+

⋅

−+η

= (3-18)

Recently Dahlquist et al. [52] successfully demonstrated junction barrier Schottky diodes with a blocking voltage of 2.8 kV and a forward drop of 1.8V at 100 A/cm2 and low on-resistance of 8 mΩcm2 at room temperature. Asano et al. [53] fabricated high-temperature and high-voltage JBS diodes using 4H-SiC, which have high breakdown voltage of 3.7 – 3.9 kV and the specific on-resistance of 31.4 – 40.2 mΩcm2. They also reported a very fast recovery time of 9.7 ns, which is about 10 times faster than a Si high-speed diodes. 3.4.2 Merged P-i-N / Schottky (MPS) diodes

The MPS [30] is an attractive approach to reducing the switching losses in high voltage power rectification without increasing the on-state voltage drop. Figure 3-15 shows the schematic view of the MPS structure. The MPS is a similar approach as JBS rectifier. However, the operating mode of two rectifiers is different. In the MPS rectifier, the P-N junction becomes forward biased in the on-state, unlike the case of the JBS, because the drift region has a very high resistance due to its design for supporting high voltage during the reverse blocking. Reverse leakage current and breakdown voltage can be achieved by employing this MPS even though the performance of the reverse recovery behavior of the diodes does not reduce.

Figure 3-15. Schematic structure of MPS rectifier.

Figure 3-16. Device structure of Ti/Ni dual metal-trench rectifier.

Cathode

N- epi-layer

N+ Substrate

P+ P+

Anode

4H-SiC substrate (n+)

Ohmic contact

4H-SiC (epi) n-

Ni Ti


- 28 -

3.4.3 DMT (Dual metal trench) diodes

Recently, Schoen et al.[54] reported the DMT diode utilizing metals with two different barrier heights to achieve similar performance as conventional Schottky diode (see Figure 3-16). The DMT rectifier has advantages such as a simple structure (self-aligned structure), simple process flow, and does not need an ion implantation process. The DMT device had Ti deposited to achieve a low barrier height on top of the mesa followed by conformal deposition of Ni over the entire device to achieve high barrier height at the bottom of the trenches. The forward and reverse current reported for this device was between Ti and Ni Schottky barrier diode. In particular, a forward voltage drop close to that of Ti SBD but with leakage current two order of magnitude lower than that of Ti SBD and about a factor of two higher than Ni SBD. The DMT diode can be applied for applications such as lower voltage drop and lower leakage current. 3.4.4 TMBS (Trench MOS Barrier Schottky) diodes

Khemka et al. [55] reported that embedding a UMOS trench like grid instead of a pn junction grid as in the JBS/MPS rectifier yields a structure known as the TMBS rectifiers shown in Figure 3-17. The forward and reverse characteristics of a poly-silicon planarized Ni-TMBS in 4H-SiC for two different Schottky areas (40% or 57%) were compared to that of simultaneously fabricated Ni SBD and a pin diode. A forward voltage drop of 1.75 V at 60 A/cm2 and the reverse leakage current density of 6 × 10-6 A/cm2 were obtained from this TMBS rectifiers.

Figure 3-17. Schematic cross-section of a trench MOS barrier Schottky (TMBS) rectifier.

Oxide

N+ Substrate

N epi (n-)

Schottky diode metal

Cathode

- 29 -

S

4. Fabrication Process

ilicon carbide processing is similar to conventional Si and GaAs processing even though it has quite different material properties. The largest difference is that there

are no wet etching methods for SiC at room temperature, and that much higher temperatures are required to get thermal oxide and to activate implanted dopants in SiC than those in conventional semiconductors. In this chapter, the most commonly used processes are described. In addition, a summary of test structures for measuring the specific contact resistances is included. 4.1 Process description 4.1.1 Wafer preparation and surface cleaning

Wafer Preparation The starting wafers for the experiments were 4H- and 6H-SiC with Si-face orientation (0001) from CREE Research Inc.[18]. Standard high doped substrates (1∼2 × 1018 cm-3) were selected with a 4 µm-thick lowly doped epitaxial layer (1015 ∼ 1016 cm-3) for Schottky contacts and around 1 µm-thick highly doped epitaxial layers (1019 ∼ 1020 cm-

3) grown by chemical vapor deposition for Ohmic contacts. Some wafers from Linköping university and Acreo AB were also used for the experiment. Normally both nitrogen (n-type) and aluminum (p-type) are used for the dopants. The wafers we used were 1 ½ inch and 2 inch in diameter. For experiments, they are cut into segments (1 × 1 cm2) with a diamond cutting saw.

Table 4-1. Chemicals used for cleaning of SiC wafers and removing SiO2.

Chemical / Mixture Temp./Time Comments H2O:NH4OH:H2O2 (5:1:1) 75oC / 5min RCA SC1 H2O:HCl:H2O2 (5:1:1) 75oC / 5min RCA SC2 H2SO4:H2O2 (2.5:1) 100oC/5min Seven-up H2O:HF:CH3CH(OH)CH3(100:3:1) 25oC/100s IMEC HCl:HNO3 (3:1) 50oC/5min aqua regia HF:H2O(1:10) 25oC Dilute HF HF:NH4F (1:7) 25oC BHF BHF+NH4OH 25oC pH-modified BHF (pH 12)

Chapter 4 Fabrication process Sang-Kwon Lee

- 30 -

Surface cleaning The wafers were first directly degreased in acetone, propanol, and DI water for 2 min each. Some standard chemical cleaning[3] is listed in Table 4-1. Two different chemical cleaning recipes, so called "Seven-up" and "IMEC", were used prior to the oxidation and metal deposition (See Table 4-1). 4.1.2 Etching process

Wet etching Chemical etching has been a key technology in the fabrication of devices using conventional silicon as well as silicon carbide material. Unfortunately, wet etching of single crystal SiC by single acids at room temperature is impossible [56]. It can be wet etched by molten salt fluxes at high temperature, for instance in solution of NaOH/KOH at 480 oC [57], hot gases, electrochemical process or plasma etching. As a result, plasma based etching is today the common way to etch SiC. However, wet etching of both metals and dielectric materials has been used frequently for many of the device processes in SiC. Table 4-2 shows various etchants for insulators and metals[22, 58].

Table 4-2. Etchants for insulators and metals.

Material Etchant Composition Etch rate

Si3N4 Buffered HF (HF:NH4F = 1:7) H3PO4

5 Å/min 100 Å/min

Al H3PO4:CH3COOH:HNO3:H2O (4:4:1:1)

350 Å/min at 35oC in ultrasonic bath

TiW NH3:H2O2 (1:5) H2O2

600 ∼ 4000 Å/min 45 Å/min

TiC NH3:H2O2 (1:5) 300 Å/min Au KI:I2:H2O (4g:1g:40ml) 3500 Å/min Ni HCl:HNO3:H2O (4:1:5)

H3PO4:CH3COOH:HNO3:H2O (4:4:1:1)

1500 Å/min (at 65oC) 500 Å/min at 35∼ 40oC in ultrasonic bath

Ti HF(5%):H2O(1:2) 5000 Å/min Mo H3PO4:CH3COOH:HNO3:H2O

(5:4:2:150) 5000 Å/min

Pt HNO3:HCl:H2O (1:7:8) 400∼500Å/min at 85oC Pd HCl:HNO3:CH3COOH(1:10:10) 1000Å/min

Dry etching Dry etching has been used to etch and pattern SiC since conventional wet chemical wet etching is not straightforward due to the chemical inertness of SiC and due to the high bond energies existing between silicon and carbon. Many publications on plasma based etching of SiC in reactive ion etching (RIE)[5, 59], electron cyclotron resonance (ECR)[60], and inductively coupled plasma (ICP) [61-64] were reported. Among these , ICP is preferable since RIE increases mask erosion at high ion energies and residual lattice damage in the semiconductor [61] and these plasma chemistries cause micro-masking problem when


- 31 -

aluminum is used as an etch mask [59] [21]. However, ICP provides the potential of achieving excellent anisotropy, low surface damage, smooth morphology, and even high etch rates for SiC[65]. ICP etching was used on 4H-SiC using a fluorine-based chemistry (SF6/Ar/O2) throughout the thesis. ICP etching is a high-density plasma technique where the plasma is formed (see Figure 4-1) in a dielectric vessel encircled by an inductive coil into which rf power is applied. Anisotropic profiles are obtained by using low-pressure conditions to minimize ion scattering and lateral etching. Gas switching between low O2 or Ar and high O2 percentage was tested to improve selectivity to the Al mask. Generally in order to define the mesa structure for the TLM (transmission line method) ICP was performed in a mixture of SF6 (21 sccm) and Ar (9 sccm) at 600 W rf forward power and 5.0 mTorr base pressure with 0.25 µm thick e-beam evaporated Al mask. The etch rate was around 0.16 µm/min with 30W of platen power. We also reported that the performance of Schottky diodes (Ti/n-4H-SiC) have been shown to change significantly due to ICP etch induced damage and we proved that the etch damage can remove by high temperature passivation [64] since dry etching damage is created in the near-surface region of the semiconductor by the bombardment of energetic ions and it can have significant impact on the electrical performance of fabricated devices. In paper VI, we proved that low power (30W) ICP etching process did not affect the formation of Ohmic contacts and we did not observe any difference between the un-etched and the 30W etched sample from TLM measurement, having the same value of the ρC when medium platen power (60W) ICP etching showed significant influence on the Ohmic contact formation. The trenching effect is known to occur for most dry etching conditions [66], caused by the deflection of ions on the sidewall inducing enhanced ion bombardment at the bottom. Our current results show that junction field-effect transistors (JFET) fabricated with Ni metal mask show a trenching profile (> 0.2 µm, see Figure 4-2) after dry etch, where 30W platen power, 600W coil power, a mixture of SF6 and Ar using ICP were used, in the channel groove region and also showed static induction transistor (SIT)-like characteristics in the sub-threshold region of current-voltage curves [67].

Chamber

Vacuum pump

Gas distribution (SF6, Ar, O2)

2 MHz Power supply

∼13.56 MHz rf source

Power electrode

Figure 4-1. Inductively coupled plasma etching equipment.


- 32 -

4.1.3 Deposition Techniques

Several methods exist for metal deposition in device fabrication. In this section, techniques we used throughout the thesis, such as sputtering and co-evaporation including e-beam evaporation are described. In this thesis, most pure metals, such as Au, Ti, Ni, Al etc. were deposited by e-beam evaporation shown in Figure 4-3 (a). Alloy metal (TiW) was sputtered by DC magnetron sputtering system in our cleanroom. It is known that sputtering can be performed for deposition of the compound and alloy metals. We also used co-evaporation for the deposition of titanium carbide (TiC). TiC is a promising material for low resistivity n- and p-type metallization on highly doped silicon carbide using a co-evaporation method as shown in Paper II and III. This co-evaporation set-up had a limited deposition area, normally we used 1×1cm2, slow deposition rate, the substrate temperature, and lack of reproducibility.

Figure 4-2. Scanning electron microscopy image of (a) the trench on the bottom of the sidewall and (b) trenching corner after using angled oxide mask which was wet-etched by BHF (after ref. [67]).

θ θSiO2

S = tan θ / tan θSiO2

(a) (b)

(a) (b) Figure 4-3. (a) Typical e-beam source evaporation system and (b) cross-sectional view of the DC sputtering system.

Substrate

Vacuum pumps

Chamber

Power supply

+ -1kV

10kV

Source

e- B

Substrate

Target

Load lock system

Gas distribution

Heater

Vacuum pumps

CHAMBER

Plasma

Power supply


- 33 -

Sputtering Sputtering is a physical vapor deposition (PVD) process involving the removal of material from a solid cathode. This is accomplished by bombarding the cathode with positive ions emitted from a rare gas discharge[58]. When ions with high kinetic energy are incident on the cathode, the subsequent collisions knock loose, or sputter, atoms from the material, which could not be deposited using the thermal evaporation techniques of that time. A typical sputter system[22][69] is shown in Figure 4-3 (b). The deposition of titanium tungsten (TiW, weight ratio 30:70) was performed in a DC magnetron sputter under deposition condition of 3kW power, 120 ∼ 200 oC substrate temperature, 50 ∼ 69 sccm of Ar gas flow, 5 × 10-7 Torr base pressure, and 5 × 10-3 Torr deposition pressure. TiW has been used extensively as a diffusion barrier for Al metallization in conventional Si process technology and fuse structures in programmable log device. The addition of 5- 30 weight % titanium provides improved adhesion between metals and semiconductor, corrosion, barrier, and contact resistance properties. TiW has low resistivity, can be wet etched, and is very inert with respect to reaction with SiC under high temperature due to its high melting point (1660 oC for Ti and 3410 oC for W). The deposition rate of the TiW metal, the thickness, and the metal resistivity were 500 ∼ 600 Å/min, 1500 ∼ 2500 Å, and 85 – 88 µΩcm. The sputtered TiW can easily be wet-etched using a mixture of 25% NH3 : H2O2(1:5) with an etch rate of 600 ∼ 4000 Å/min depending on the sample pre-treatment (Paper I, V, VI, VII). So far we have considered two methods of metal deposition. These are the more widely used metallization techniques. To conclude this section, we highlight the advantages that the sputtering method offers. Because sputter deposition can be used to deposit refractory materials, which are useful metals for long-term reliability, since such materials are often difficult to evaporate, and hence sputtering may be the practical way for their deposition on semiconductors. Co-evaporation

For the growth of titanium carbide (Paper II and III) contacts, a co-evaporation technique in a UHV chamber comprising an electron beam evaporator for Ti and a Knudsen effusion cell for C60 was used for both Ohmic and Schottky contact studies. The UHV system for TiC deposition consists of one deposition chamber connected to two different analysis chambers via an in vacuo transfer system[69]. The commercial metal evaporator works by directing an electron beam towards the tip of the metal rod (5-6 mm in diameter) to be evaporated shown in Figure 4-4. The metal flux can be regulated by control by the electron current (50 ~ 100mA) with a fixed bias voltage (700V). The C60 from a Knudsen cell shown in Figure 4-4 was used as a carbon source for making a TiC films. The carbon fluxes were controlled by tuning of the cell temperature from 440 oC to 500 oC. In this thesis (Paper II, III) cell temperature ranging from 450 oC to 460 oC were applied. The deposition rate was around 35Å/min for TiC films. The substrate temperature for TiC and Ti deposition was 500 oC and 300 oC, respectively. As mentioned in beginning of this section, the co-evaporation method is still under development with a lack of applicability even though it was best deposition method for TiC metals compared to the other methods. PECVD & LPCVD

Low temperature (300 oC) plasma enhanced chemical vapor deposition (PECVD) was used to deposit a 1400 Å Silicon nitride layer (Si3N4) on 4H-SiC. The Si3N4 was used as


- 34 -

a sacrificial layer prior to the ion implantation (See Paper III). Si3N4 layers can also be deposited by a low pressure chemical vapor deposition (LPCVD) process with intermediate temperature (750 oC). The difference between LPCVD and PECVD was that the LPCVD films are of stoichiometric composition with high density (2.9 to 3.1 g/cm3) whereas the films deposited by PECVD are not stoichiometric and have a lower density (2.4 to 2.8 g/cm3). In the PECVD process, silicon nitride is formed either by reacting silane and ammonia in an argon plasma or by reacting silane in a nitrogen discharge. The reactions are as follows : SiH4 + NH3 SiNH + 3H2 (4-1) 2SiH4 + N2 2SiNH + 3H2 (4-2) Oxides also can be deposited by both PECVD and LPCVD at higher temperature. The quality of these PECVD or LPCVD oxide can be improved by following up with high temperature annealing (> 1000 oC for 1 hour)[3]. LPCVD deposited tetra ethyl orthosilicate (TEOS) was also used as a dry etch mask for defining the mesa structures. The deposition conditions for silicon nitride using PECVD are shown in Table 4-3.

Table 4-3. The deposition condition for Si3N4 using PECVD.

Gas Flow SiH4(5%) + He (58 sccm), NH3 (1.4 sccm), N2 (268 sccm), Ar (500 sccm)

Parameters Pressure (600 mTorr), rf power (15W), Temperature (300oC), Deposition rate (60Å/min)

300 oC

700 V/50-100 mA

Deposition chamber (UHV)

Knudsen cell

Ti metal rod

e-beam

Vacuum pump

440 oC – 500 oC

5V/10A

C60

heater

sample

Figure 4-4. Schematic view of the co-evaporation system for TiC deposition.


- 35 -

4.1.4 Ion implantation

Ion implantation is the introduction of energetic, charged particles into a substrate. The practical use of ion implantation in semiconductor technology has been mainly to change the electrical properties of the substrate. Typical ion energies are usually between 30 and 300 keV, and ion doses vary from 1011 to 1016 ions/cm2. Figure 4-5 shows a typical ion implantation system [22]. Implantation of dopants into SiC surface has been recognized as crucial means of selective area doping since thermal diffusion is not feasible due to slow diffusion rates of most dopants in SiC. Normally N (Nitrogen) and Al (Aluminum) are used frequently for doping in SiC due to their low activation energy in SiC (∼ 80 meV for N and 240 meV for Al in 6H-SiC) compared to other impurities of the same type [70]. For n-type implantation, it has been demonstrated that nitrogen gives sufficiently good n-type doping with low sheet resistance (less than 1kΩ/£) as well as reasonable n+-p diodes characteristics [71, 72]. On the contrary, attempts at obtaining good p-type SiC by implantation, using either Al or B ions, have encountered difficulties such as high sheet resistance and low activation efficiency. A high temperature anneal is usually necessary to reduce the damage caused by high-energy ions during implantation [70, 73]. Normally post ion implantation annealing is performed at temperatures around 1500 oC to 1800 oC for 10 ∼ 30 min, in a CVD furnace with Ar ambient and with a small introduction of silane to avoid pitting of the surface. Ion implantation can be used for making highly doped Ohmic contact regions. Recently, Zhao et al.[74] showed the possibilities of achieving a low specific contact resistance of 10-5 Ωcm2 for Al contacts on C-Al co-implanted 6H-SiC. Ion implantation is an important technology for device applications and quite promising solutions with many advantages for formation of low resistivity Ohmic contacts to SiC exist. In order to determine the ion implantation condition TRIM[75] simulation was used. Figure 4-5 shows typical depth profile from TRIM simulation using a Si3N4 layer on the silicon carbide surface with energy of 180keV and different doses, where the measured depth profile of Al in SiC using SIMS is also included.

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81014

1015

1016

1017

1018

1019

1020

1021

SIMS profile (3 x 1014) 180 keV (3 x 1014) 180 keV (1 x 1015)

Dose (3 x 1014 ions / cm2)Energy (180 keV)Annealed at 1700 oC, 30min

Con

cent

ratio

n (a

tom

s / c

m3 )

Depth (µm)

Figure 4-5. The depth profile generated by TRIM simulation and SIMS using Si3N4 sacrificial layers for different doses with energy of 180 keV.


- 36 -

4.1.5 Annealing

In order to make the deposited metal contact Ohmic it is not enough to deposit them on highly doped SiC epilayer because most of metals have a high Schottky barrier height. Therefore, the best suggested way is to anneal and sinter the metal contacts at high temperature in inert gas ambient. The required temperature to make the contacts Ohmic for most of the metals was at least > 900 oC. In this thesis, both Ohmic and Schottky contacts were annealed at 500 ∼ 980 oC in Ar or in 10% H2/Ar using rapid thermal annealing (RTA). Some of the samples were annealed at the same temperature in a low-pressure vacuum chamber. Long-term reliability tests (Paper V and VII) were also performed at 500 ∼600 oC in a vacuum chamber.


- 37 -

4.2 Test structures for Ohmic contacts For an Ohmic contact, the parameter of interest is not the barrier height, but rather the resistance of the linear pat of the current-voltage characteristics. This parameter is important for metal-semiconductor metallization as well as scaling-down effects of MOS transistors in modern VLSI circuits. For example, in the limit case, known as the electrically long contact, the contact resistance RC ≈ρC/LTW increases as K (scaling factor), when the lithographic dimensions of the integrated circuit are scaled down by a factor of K[76]. This means that the contact resistance or specific contact resistance also becomes an essential parameter in the design of modern integrated circuits as seen in conventional Si world. Ohmic metal-semiconductor contacts are defined as a metal-semiconductor interface whose voltage drop is small, ideally zero, compared to the active region of the devices. The parameter which characterizes an Ohmic contact is its specific contact resistance or contact resistivity, ρc, and is commonly used to compare the quality of each Ohmic contact, usually in unit of Ωcm2. In this section, we will illustrate various measurement techniques and corresponding test structures. 4.2.1 Kuphal structure

A four-in-line circular structure was suggested by Kuphal [77]. It requires only one mask process to fabricate and is the simplest test structure to measure the specific contact resistance. Beyond these advantages, these simple test structures tend to overestimate the specific contact resistance compared to the other test structures. A schematic view of the test structures is shown in Figure 4-6. The total resistance between two contacts consists of the contact resistance (2Rc), the spreading resistance (2Rsp), and the sheet resistance of the epitaxial layer of the semiconductor. As shown in Figure 4-6, a current Iad is applied between contact a and d, and the voltages Vab and Vbc are measured, finally the specific contact resistance can be calculated by

Figure 4-6. Schematic view of Kuphal test structure (after ref. [77]).

Rs

RcRc

Rsp Rsp

ab c

d

I

epi-layer

s s s


- 38 -

(4-3) where s is the spacing between the contacts. Rsp can be neglected if the ratio between the contact area and the epilayer thickness is small. 4.2.2 Two-terminal contact resistance methods

This method is the simplest, earliest, and also with questionable accuracy if not properly executed [78]. These subdivide into two-element structure and multi-element structures that are widely known as contact strings or contact chains. The contact string technologies are considered to be a coarse measurement method that is not very useful for detailed evaluations of contact resistance. It is used, however, as a process monitor. Figure 4-7 shows 2-terminal test structure (a) and contact string (b).

To confine the current flow, the region on which the contact is located must be isolated from the remainder of the substrate. This is done by either confining the implanted or diffused region by planar techniques or by etching the region surrounding the island, leaving in as a mesa. The contact resistance RC is

2

RRW

dR

RWd

ST

C

++

ρ−

= (4-4)

where RT is the total resistance, ρS is the sheet resistance of the n-layer, Rd is the resistance due to current crowding under the current, and RW is a contact width correction if Z<W.

(a) (b) Figure 4-7. A lateral two-terminal contact resistance structure (a) in cross section and top view and (b) contact string (after ref. [84]).

−

−−=ρ2ln2

21

ds3

lnVRV

IA

bcspabad

c


- 39 -

4.2.3 3-contacts, two-terminal methods

From Figure 4-8 (a), the total resistance and contact resistance are given by

( )( )21

21T12TCC

2S2T

C1S

1T

dd2dRdR

R,finallyR2Wd

R

R2Wd

R

−−

=+ρ

=

+ρ

= (4-5)

As shown in equation 4-5 this structure does not have the ambiguities of the simpler two-terminal structure, because neither the bulk resistance nor the layer sheet resistance need be known. This structure only allows the contact resistance to be determined. The specific contact resistance, which is quite useful and practical parameter, cannot be directly extracted from the two resistance measurements. Murrmann and Widmann [79] used a simple transmission line model (TLM) considering both the semiconductor sheet resistance and the contact resistance to take current crowding into account and to be able to extract the specific contact resistance ρC. Berger [80] extended this method. When current flows from the semiconductor to metal, it sees the resistance ρC and ρs, shown in Figure 4-8 (b). The potential distribution under the contact is determined by both ρC and ρs according to [80]

( )[ ]

−ρρ=

T

TCs

LL

sinhZ

L/xLcoshI)x(V (4-6)

where L is the contact length, Z is the contact width, and I is the current flowing into the contact. It is obvious that the voltage is the highest near the contact edge x=0 and drops nearly exponentially with distance x as shown in Figure 4-9 (a). The 1/e distance of the voltage curve is defined as the transfer length LT (= SC ρρ ). Figure 4-9 (b) also shows transfer length as a function of the specific contact resistance. The transfer length is on the order of 1µm or less for such contacts. Using equation 4-6, the contact resistance can be written by

(a) (b) Figure 4-8. (a) Test structure and (b) the equivalent circuit of the metal-semiconductor with the current choosing the path of least resistance (after ref. [84]).


- 40 -

(4-7) There are two limiting cases to simplifications of equation 4-7, that is, for L≤ 0.5LT, coth(L/LT) ≈ LT/L and simply, RC ≈ ρC/LZ. For L ≥ 1.5LT. coth(L/LT) ≈ 1 and finally,

ZL

RT

CC

ρ= (4-8)

4.2.4 Linear transmission line method (LTLM)

The transmission line method (TLM) was originally proposed by Shockley [81]. The TLM structure is very much like 3-contacts (two-terminal methods) shown in Figure 4-3, but consists of more than three contacts. In this thesis, only this linear TLM method was used to extract the specific contact resistance (Paper II, III, V, VI, and VII). From equation 4-5 through 4-8 for contacts with L ≥ 1.5 LT the total resistance between any two contacts can be given by

( )TS

CS

T L2dZ

R2Zd

R +ρ

≈+ρ

= (4-9)

The total resistance is plotted as a function of spacing d as shown in Figure 4-10 (a). From Figure 4-10 three parameters including sheet resistance (ρS) under the contacts

ρ=

ρρ==

TT

C

T

CSC L

Lcoth

ZLLL

cothZI

VR

0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

1.0

10-6Ωcm210-7Ωcm2

ρC=10-4Ωcm2

10-5Ωcm2

V(x

)

X (µm)10-8 10-7 10-6 1x10-5 1x10-4 10-3 10-2

10-6

1x10-5

1x10-4

10-3

10-2

1000300

100

ρs=10 Ω/square

L T(cm

)

ρC(Ωcm2)

(a) (b) Figure 4-9. (a) Normalized potential under a contact versus x as a function of ρC

(where L & Z =100µm, ρs = 100 Ω/£ using equation 4-6) and (b) shows the transfer length versus specific contact resistance and sheet resistance.


- 41 -

(from the slope), contact resistance (RC), and the transfer length (LT) (from the intercept) can be extracted. Finally, from equation 4-8 the specific contact resistance (ρC) is determined. The TLM method has its own problems even if it is used a great deal in the practical study. The x-axis from Figure 4-10 (at RT=0) giving LT is sometimes not very distinct, leading to incorrect ρC values. This problem is due to the uncertainty of the sheet resistance under the contacts. The sheet resistance from TLM methods includes the sheet resistance both under the contacts and between contacts. Reeves and Harrison [82] suggested that the sheet resistance under the contacts might differ from that of between contacts due to the alloying effects of contact formation. They modified equations 4-6 and 4-7, that is

ρ=

TcTc

CCf L

Lcoth

ZLR and

ρρ

+ρ

≈+ρ

= TcS

SCCC

ST L2d

ZR2

Zd

R (4-10)

where ρSC is the sheet resistance under the contact and LTc=(ρC /ρSC )1/2. The slope still gives the sheet resistance (ρS/Z) and the intercept at d=0 gives 2RC. However, the intercept at RT=0 now gives 2LTc(ρSC /ρS) instead of 2LT in the simple TLM method and the parameter ρC can not be extracted from this method because the parameter ρSC is

Substrate (p+ or n+ 4H- or 6H-SiC)

I (current)

n+ or p+ epilayer

Metal

5 µm 10 µm 15 µm 20 µm 25 µm

(a)

0 d

2LT

2Rc

RT

slope = Rs/Z

Figure 4-10. (a) A plot of total resistance versus spacing d (from 5 µm to 25 µm) and (b) schematic view of TLM structure.

(b)


- 42 -

( )( )2C1CB2

'1C0CA1

RRRR

RRRR

++=++=

still unknown. Therefore, they combined the TLM method (determining Rc) and the end resistance method (determining Rce shown in equation 4-11). From equations 4-10, 4-11, and 4-12, the parameters, LTc and ρC, are extracted. This modified TLM method can determine the contact resistance and the specific contact resistance in addition to the sheet resistance between and under the contacts as well.

ρ

=

ρρ=

TCTC

C

TC

CSCce

LL

sinhZLLL

sinhZR (4-11)

=

TC

c

ce

LL

cosh

1RR

(4-12)

In our case, we did not see any large difference between LTLM and modified methods because there is not too much alloying effect (reaction between SiC and metals). Therefore, we have used LTLM for our Ohmic contact measurements through this thesis. 4.2.5 Circular transmission line method (CTLM)

The circular transmission line method (CTLM) was proposed by Reeves [83]. It eliminates the necessity of mesa isolation of the contact pattern, thus simplifying test structure fabrication. It can also avoid the problem of linear TLM, inconsistency of the width of pads and mesa structures due to the etching process. Circular contact pattern

and cross-sectional view of circular TLM are shown in Figure 4-11 and 4-12, respectively. Since the contacts are circularly symmetrical, the pattern of current flow between two contacts should be circularly symmetrical when the contacts are equi-potential surfaces. The resistance between the inner two contacts and the outer two contacts, are called R1 and R2, respectively. From the Figure 4-11, the resistances measured between the inner two contacts, R1 and between outer two contacts, R2 are given by (4-13)

Figure 4-11. Schematic view of the circular patterns for specific contact resistance measurement (after ref. [83]).

r0

r1

r2

C0

C1 C2


- 43 -

Using two measurements between the inner and outer pads and the circular transmission line model, the specific contact resistance (contact resistivity, ρc) can be determined by the directly following equations. (4-14) (4-15) and (4-16) where RSK is the series resistance element under the contacts.

4.2.6 Four-terminal contact resistance method, Kelvin structures

As discussed in previous section of TLM, the contact resistance RC is often a small fraction of the total measured resistance RT. This problem can be solved by using the test structure as illustrated in Figure 4-13. The four-terminal contact resistance method is also known as the Kelvin test structure or the cross-bridge Kelvin structure [84]. In principle, this method also allows the specific contact resistance to be measured without being affected by the underlying semiconductor or the contacting metal parasitic contribution. Figure 4-13 (a) shows the equivalent circuit of the Kelvin structure and (b) the principle of the Kelvin structure method, respectively. Since the internal resistance of the voltmeter is much larger than the resistance of the device under test, RX, or the connecting wire, the current I and the voltage drop in the resistor R1 and R2 can be

Figure 4-12. Cross-section view of the circular transmission line method (after ref. [83]).

r2 r1 r0

RC0 R'C1 RC1 RC2

RA RB

x=0

( ) ∆⋅⋅

⋅

−⋅

=ρ 2

020

'1

e11

'2

ec rRrr

logRrr

log

( )( ) ( )

( ) ( )

+

⋅φ⋅α

π

=∆'

11'11

'11

'11

2

0

r,rDr,rC

r,rBr,rAr

2

c

SKRρ

=α


- 44 -

neglected. Hence the voltage V34 (=V3-V4) measured by the voltmeter coincides with the voltage drop across the resistance RX=V34/I. RX can present to RC given by

CCC34

C AR,I

VR ⋅=ρ= (4-17)

4.2.7 Six-terminal contact resistance method

In this case two more contacts provide additional measurement options and additional information which is not available with the conventional 4-terminal Kelvin structure. This method is shown in Figure 4-14. For the conventional Kelvin structure contact resistance measurement, the current is forced between contacts 1 and 3 as shown in Figure 4-14, and the voltage is measured between contacts 2 and 4. (RC=V24/I and

Figure 4-14. Six-terminal Kelvin test structure (after ref. [84]).

Diffusion

1

3 4

5

2

Contact

6

Metal

I

V34

R2

R1

Rx

(a) (b) Figure 4-13. (a) equivalent circuit of Kelvin resistance test structure and (b) schematic view of four-terminal contact resistance test structure (after ref. [84]).


- 45 -

ρC=RCAC). This analysis for conventional Kelvin structure is one-dimensional case. However, modified six-terminal Kelvin structure as shown in Figure 4-14 can provide the contact resistance, the specific contact resistance, the contact end resistance, the contact front resistance, and the sheet resistance under the contact to be determined. In order to measure the contact end resistance (Rce=V54/I) the current is forced between contacts 1 and 3, and the voltage is sensed across contacts 4 and 5. The sheet resistance of the epilayer under the contact can be determined from the end resistance using equation 4-11. Then the front resistance can be calculated using equation 4-10. 4.2.8 Comparison of each measurement technique

In this thesis, only the linear TLM (LTLM) method was used to extract the specific contact resistance (Paper II, III, V, VI, and VII). Table 4-4 shows a comparison among the measurement techniques. We can measure the specific contact resistance of >10-6 Ωcm2, which is low enough to apply to the high-power device applications (≈voltage drop of 0.1 mV at 100 A/cm2). But the Kuphal structure can be useful, it can extract the specific resistance of >10-4 Ωcm2 and is easy to fabricate (1 mask step) even though it overestimate the specific contact resistance.

Table 4-4. A comparison of each measurement technique.

Kuphal method TLM Kelvin method

Indirect method Indirect method Direct method Simple structure Simple structure Complex 1 mask (Contact)

2 mask steps (Contact, Mesa)

3 mask steps (Contact, Metal, Diffusion)

Measurement limit ≈ 10-4 Ωcm2 (overestimates ρC)

Measurement limit ≈ 10-6 Ωcm2

Measurement limit ≈ 10-8 Ωcm2

- 47 -

B

5. Characterization and results

oth Schottky and Ohmic contacts should be evaluated by material and electrical characterizations when they have been manufactured. Blanket pieces (samples)

are prepared simultaneously for additional material characterization. Various material techniques such as XRD, RBS, LEED, SIMS, AFM, and TEM are introduced to understand the contact properties. For material characterization, we mostly stress the solid-state reaction on Ti/, Ni/, TiC/, and TiW/4H-SiC. We will try to answer the question : what kinds of phases are created at the interface and what kind of phase makes Ohmic and low resistivity contacts. These are also related to the answers for the electrical characterization for both Schottky and Ohmic contacts to silicon carbide. The main results for Schottky diodes, specific contact resistance, and microscopic mapping of specific contact resistance are included with short introduction of the measurement techniques. Finally, the results and discussion for the long-term reliability tests at high temperature and different ambients (in vacuum and oxidizing ambient) are described at the end of chapter. 5.1 Material characterization The samples, as-deposited and annealed at high-temperature, were characterized by 2.4 MeV 4He+ Rutherford backscattering spectrometry (RBS) for depth profiles of composition and solid-state reaction after high-temperature annealing, X-ray diffraction (XRD) for phase identification, and atomic force microscopy (AFM) for the surface morphology. The ion implanted Al depth profiles were also analyzed by secondary ion mass spectrometry (SIMS). For both good and reproducible Schottky and Ohmic contacts, it is essential to study the solid-state reaction between the metal and SiC. There are materials that do not react (Ag, Au etc.), materials that form the silicide but

Figure 5-1. Ternary phase diagrams of Ni/SiC (a) and Ti/SiC (b), where T1 and T2

denote Ti3SiC2 and Ti0.6Si0.34C0.05, respectively.

Si

CNi

NiSi2

NiSiNi2Si

Ni5Si2Ni3Si

SiC

T=850oC(a)

Si

CTi

TiSi2

TiSiTi5Si4

Ti3Si

TiCx

SiC

T=1000 oC

T2

T1

Ti5Si3

Chapter 5 Characterization and results Sang-Kwon Lee

- 48 -

not the carbide (Co, Ni, Pd, Pt etc.), materials that form both silicide and carbide (Cr, Fe, Mn, W etc.), and finally materials that forms silicides, carbides, and a ternary phase (Mo, Ta, Ti, Zr etc.)[85]. In this section, we will focus on the phase formation, related to the specific contact resistance and Schottky barrier height. The ternary phase diagrams of the two different systems (Ni/SiC and Ti/SiC) are shown in Figure 5-1 (a) and (b)[85]. It can be known that in the case of Ni reaction on SiC, the only stable silicide phase is the Ni2Si (See sub-section below). The carbon present in the consumed silicon carbide layer should precipitate. The Ti/SiC system has a ternary phase (Ti-Si-C), carbide phase (TiC), and silicide phases (Ti2Si, TiSi2 etc.) after high-temperature annealing. 5.1.1 X-ray Diffraction (XRD)

XRD measurements were performed in the θ-2θ geometry using a Siemens D 5000 powder diffractometer (CuKα1 radiation) to identify the phases formed on the SiC substrate before and after high-temperature annealing at > 950oC. For the measurements, the samples were tilted 3.5 ∼ 8-degree off-axis to find the exact phase of metals on SiC (0001) and finally the oblique tilt (± 0.5 degree) to reduce the single crystal SiC substrate signal. Titanium (Ti) The x-ray spectrum of a Ti film deposited on 4H-SiC for as-deposited and annealed at 950oC are shown in Figure 5-2 (Paper II, IV,VIII). The spectra for as-deposited Ti/SiC indicate that Ti films are highly textured on silicon carbide with 001-type reflection from Figure 5-2. After high-temperature annealing at 950 oC, the phase at the interface was changed into two different phases (see also Figure 5-2), such as Ti5Si3 and TiC, which is in good agreement with previous results from Porter et al. [86]. These two phases (titanium silicide and titanium carbide) have a much lower resistivity than that of Ti only.

3 0 3 5 4 0 4 5 5 0

TiC

(111

)

Ti 5S

i 3(002

)

Ti 5S

i 3(030

)

Ti 5S

i 3(121

) +

TiC

(00

2)

A s - d e p o s ite d

Ti5S

i 3(012

)

SiC

(004

)

Ti (0

02)

T i/S iC ( 9 5 0 o C )

Inte

nsity

(A.U

)

2 Θ ( d e g )

Figure 5-2. x-ray spectra of Ti/SiC(4H-SiC) for as-deposited and annealed at 950 oC.


- 49 -

Nickel (Ni) For Ni, the reaction at the interface at high-temperature annealing only proceeds to the formation of the Ni2Si phase (see Figure 5-3) as seen from our X-ray diffraction measurements. For as-deposited Ni, the x-ray spectra show that it is highly oriented with the (111) orientation on silicon carbide (Paper IV and VII).

30 35 40 45 500

10

20

30

40

50

Ni 2S

i

Ni 2S

i

Ni 2S

i

Ni (

111)

Ni/SiC annealed Ni/SiC as-dep.

In

tens

ity (

A.U

)

2Θ(Deg)

Figure 5-3. X-ray spectra of Ni metal deposited on 4H-silicon carbide (0001) for as deposited and after anneal at 950 oC.

30 35 40 45

100

200

SiC

(00

4)

Au

(111

)

Inte

nsity

(A

.U)

2Θ(Deg)

Figure 5-4. X-ray diffraction spectra of as-deposited Au on 4H-SiC.


- 50 -

Gold (Au) and Ti/Al/TiW The XRD spectra show that Au metals are textured on silicon carbide with 111-type reflection (see Figure 5-4) . Figure 5-5 shows the x-ray spectra for as-deposited and 950 oC annealed multiple-stack contacts (Ti/Al/TiW). It indicates that as-deposited films have strong Ti and Al peaks and the new compound phases (aluminum titanium, such as AlTi or AlTi3) are created after annealing at 950 oC (Paper IV and VII).

Titanium carbide (TiC) X-ray diffraction spectra of typical TiC films are shown in Figure 5-6. The diffractogram shows only reflections of the 111-type from the films suggesting a

Figure 5-5. X-ray diffraction spectra for as-deposited Ti/Al/TiW contacts, indicating no strong peak for tungsten (W).

30 40 50 60 70 800

10

20

30

40

As-dep Annealed

SiC

(00

4)

Al (

111)

Al (

122)

(00

3)

Al (

013)

Ti (

002)

Al (

112)

In

tens

ity (

A.U

)

2Θ(Deg)

Figure 5-6. X-ray diffractogram of a 900Å thick blanket TiC0.7 film deposited on 4H-SiC with clear hexagonal LEED pattern.


- 51 -

highly textured or an epitaxial growth with the relationship TiC(111)//4H-SiC(0001). This was also confirmed by low-energy electron diffraction (LEED) that showed a clear hexagonal pattern indicating epitaxial growth of the film (see inset of Figure 5-6). TiC can be formed with two stacking sequences, ABCABC and ACBACB, which are equivalent to an 180o rotation about the [111] direction in the TiC film. These domains will give rise to identical and overlapping LEED patterns (Paper II, III, V).

30 35 40 45 500

500

1000

1500

2000

TiW(@RT) TiW(@120oC)

Ti (

002)

SIC

(004

)

Inte

nsity

(A

.U)

2 θ

Figure 5-7. (a) X-ray diffraction spectra of a 1000 Å-thick blanket sample of as-deposited TiW on 4H-silicon carbide.

30 35 40 45 50

50

100

150

200

(Ti,W

)C1-

x (00

2)

(Ti,W

)Si 2 (

111)

. (00

3)

SIC

(004

)

Inte

nsity

(A

.U)

2 θ

7.7 deg. 7.9 deg. 8.1 deg.

Figure 5-7. (b) X-ray diffraction spectra of a 1000 Å thick blanket sample of 950 oC annealed TiW on 4H-silicon carbide.


- 52 -

Titanium tungsten (TiW) Ti30W70 was deposited by sputtering from a compound TiW target (nominally 30:70 weight percent ratio). The x-ray diffraction spectra of as-deposited TiW films with different substrate temperatures are shown in Figure 5-7 (a). X-ray results show that the Ti-W alloys of all compositions are β-Ti rich with a weak bcc solid solution of W, indicating that the substrate heating transforms the β-Ti phase to the α-Ti phase. Babcock et al.[87] observed similar behavior with Ti8W2 after anneal at 500oC using XRD measurements. Figure 5-7 (b) shows the XRD spectra for the TiW film after annealing at 950oC. This result indicates that polycrystalline (Ti,W)Si2 and (Ti,W)C1-x phases form due to high temperature annealing. We suggest that these silicide and carbide phases with (Ti,W) make the TiW films exhibit low resistivity at the interface , which is finally changed to Ohmic behavior from the previously rectifying with high resistance (Paper I, V, Vi, VII). 5.1.2 Secondary Ion Mass Spectrometry (SIMS)

SIMS is an analytical technique based on the fact that under particle bombardment of a target, atoms and molecules are ejected[88]. SIMS data can be recorded as mass spectra, depth profiles and ion images. Depth profiling is the primary mode of detection in semiconductor analysis where the secondary ion intensity is recorded as a function of sputtering time. Commonly used primary sputtering ions are O-, O2

+, Ne+, Ar+, Kr+, Xe+ and Cs+ with typical impact energies in the range of 1∼20 keV[89]. Figure 5-8 shows the depth profile of SIMS of Al implanted 4H-SiC after post-implantation annealing at 1700 oC for 30 min compared with results of the TRIM simulation (Paper III). Both TRIM simulation and SIMS depth profile show that the surface Al peak concentration is around 2 × 1019 cm-3. From SIMS it was found that 60% of the dose remains close to the surface (≈ 1.8 × 1014 cm-2). This Al dose corresponds to a theoretical sheet

0.0 0.2 0.4 0.6 0.8 1.0 1.21015

1016

1017

1018

1019

1020

C

once

ntra

tion

(ato

ms

/ cm

3 )

SIMS Profile (Al Implanted) TRIM simulation results

Depth (µm)

Figure 5-8. The depth profile after Al implantation using SIMS and TRIM. The SIMS results show the depth profile after high temperature annealing (1700 oC, 30 min in a CVD furnace. The TiC was removed prior to the SIMS measurements.


- 53 -

resistance of about 0.5 kΩ/£ if completely activated, using a hole-mobility of 70 cm2/Vs for the calculation[23, 90]. 5.1.3 Rutherford Backscattering Spectrometry (RBS)

The RBS technique normally uses 1 to 3 MeV ions (usually 4He ions) to analyze the surface and the outer 0.5 to 3.0 µm of semiconductors and other materials[91]. In this thesis RBS measurements (2.4 MeV 4He+ ions) were performed to investigate the TiW (Paper I, V, VI) and TiC (Paper II, III, V) film thickness, composition ratio, and interface reaction between metals and the silicon carbide substrates. For the TiW Schottky contacts shown in Figure 5-9, the atomic composition ratio (Ti:W) and thickness was determined to be Ti:W (0.58:0.42) and 1250Å, respectively. In addition,

we obtained no detectable reaction between TiW and SiC substrate after annealing at 500oC in a vacuum chamber (Paper I). In order to investigate the TiW film thickness, the composition ratio, and the interface reaction between TiW and SiC, Rutherford backscattering measurements were carried out with a 1000 Å-thick blanket sample, which was deposited simultaneously with the other samples. The RBS spectra for samples both the as-deposited and annealed sample at 950 oC are shown in Figure 5-10 (Paper VI, VII). After annealing, the intensity of the Ti and W peaks decreases. This implies that there is a reaction between W and Ti and SiC at the interface. These results are in good agreement with our previous XRD results after annealing at 950 oC (see Figure 5-7 b). The RUMP simulator [92] shows the atomic composition ratio of as-deposited TiW was 0.61 and 0.39 for Ti and W, respectively. This corresponds to the nominal weight ratio of the TiW target (30:70 in weight and 62:38 in atomic ratio). From the Figure 5-11 for TiC Ohmic contacts, RBS spectra indicated around 10% oxygen at the surface after 950 oC 180 s RTA. The presence of oxygen after RTA was also supported by XPS depth profiles, which shows the region of this film, contained around 15% oxygen and then decreased to around 1% at the interface. RBS and XPS

0.5 1.0 1.5 2.0 2.50

2000

4000

6000

8000

10000

12000

14000

16000

W

CSi Ti

As-deposited Annealed at 500 oC, 30 min

Yie

ld (

arb.

uni

ts)

Backscattering Energy (MeV)

Figure 5-9. RBS spectra for as-deposited and 500 oC annealed TiW Schottky contacts.


- 54 -

analysis showed that high temperature annealing caused the oxidation of the TiC contacts and finally resulted in an increase of the specific contact resistance for n-type TiC Ohmic contacts on epilayer and implanted 4H-SiC (Paper II, III, V).

0.5 1.0 1.5 2.0 2.50

5

10

15

20

Ti(0.61)W(0.39)

As-deposited 950oC annealed

Si W

Ti

Yie

ld (A

.U)

Energy (MeV)

Figure 5-10. RBS spectra for as-deposited and 950 oC annealed TiW Schottky contacts.

0.4 0.8 1.2 1.6 2.0 2.40

1000

2000

3000

4000

5000

O

C

Si

Ti

Annealed (950 oC RTA)

RBS raw data RUMP simulation

RUMP simulationComposition : Ti

0.51C

0.39O

0.10

Thickness : 900 Å

Yie

ld (

arb.

uni

ts)

Backscattering Energy (MeV)

Figure 5-11. RBS spectra for 950 oC annealed TiC Ohmic contacts to 4H-SiC. The solid line indicates the simulation results using the RUMP simulator.


- 55 -

5.1.4 Transmission Electron Microscopy (TEM)

TEM is used to examine cross-sections of the films to determine the phases and sharpness of the interface. As mentioned in section 5.1.1, Ti/SiC creates new phases, such as titanium silicide (Ti5Si3) and titanium carbide (TiC) at the interface after high-temperature annealing. Porter et al. [86] have shown using high-resolution transmission electron microscopy that around 10-15 nm of cubic TiC1-x and an orthorhombic Ti5Si3 layer is formed at the interface after annealing Ti/6H-SiC at 700 oC for 20 – 60 min. For the system of Ni/SiC the TEM view of Ni/SiC after annealing at 950oC shows that the δ-Ni2Si phase is formed by high temperature annealing[85], which is in good agreement with the observation by X-ray diffraction.

TEM analysis of an as-deposited TiC film on 4H-SiC is shown in Figure 5-12. The TiC exhibits a columnar microstructure, where the individual columns have an average width of about 200 ∼ 2500Å. The step repetition length on 4H-SiC (0001) substrate with an 8 degree cut-off angle can be estimated to be about 70Å. The fact that the column width is significantly larger than the repetition length suggests that the simple model of domain formation by nucleation at surface steps may be incorrect (Paper II). 5.1.5 Atomic Force Microscopy (AFM) & Optical microscopy

The purpose of the AFM measurement is to monitor the surface morphology. In Paper VI, we investigated Ohmic contact formation on ICP etched 4H-silicon carbide using AFM. The roughness of samples such as un-etched, 30W etched, 60 W etched with sacrificial oxidation, and 60 W etched samples, was 14∼16Å, 24∼27Å, 23∼28Å, and 44∼80Å, respectively. Figure 5-13 shows typical AFM images of SiC surface of un-etched sample and 60-W of platen power etched samples. From the roughness measurements, a sacrificial oxidation (1250 oC, 1hr) seems to improve and recover the roughness to the same extent as that of a 30W etched sample (Paper VI). In addition,

TiC

4H-SiC 50 nm

Figure 5-12. The cross-section view of TiC/4H-SiC using TEM (Paper II).


- 56 -

optical microscopy view of nickel (Ni) and titanium tungsten (TiW) contacts after high-temperature annealing (950 oC) in a vacuum chamber (see also Section 4.1.5) is shown in Figure 5-14(a) and (b), respectively. As shown in Figure 5-14 (a), the surface of Ni is quite rough even after low-temperature annealing (650 oC) due to the creation of a new phase (δ-Ni2Si, nickel silicide) after high-temperature annealing (see also section 5.1.1)[93]. In view of wire bonding and packaging, and long-term reliability, Ni contacts have disadvantages, causing ultimate device failure via contact degradation and wire bond failure under the exposure of high-power and high-temperature operation even though they initially have extremely low specific contact resistance. In order to keep away from this kind of problems, we introduced sputtered TiW contacts as shown in Figure 5-14 (b), indicating no surface changes even after high-temperature annealing (950 oC). In Paper VII, we suggested this TiW with a capping layer to be the best candidate for long-term reliability tests.

(a) (b) Figure 5-14. Optical microscope view of (a) Ni contacts and (b) TiW contacts after 950oC annealing in a vacuum chamber for 30 min.

(a) (b) Figure 5-13. AFM images of the silicon carbide surface of (a) un-etched sample and (b) 60-W etched sample.


- 57 -

5.2 Electrical characterization of Schottky contacts 5.2.1 Measurement techniques

The Schottky barrier height (φB) and ideality factor (η) are essential paramters to characterize Schottky diodes. In general, there are four methods such as the current-voltage (I-V) measurement, the capacitance-voltage (C-V) measurement, photoelectric measurement, and activation energy measurement for evaluating the barrier height of metal-semiconductor contacts.[11] In this section, we will describe briefly how they work. Capacitance-voltage measurement When a small ac voltage (normally Vp-p ≈ 30 ∼ 50 mV) is superimposed upon a dc bias, charges of one sign are induced on the metal surface and charges of opposite sign in the semiconductor. If the Schottky diode is nearly ideal and the semiconductor has a uniform donor concentration the differential capacitance C under reverse bias Vr is given for non-generate semiconductor by (5-1) where A is the area of the contact and ξ is the difference in energy between the Fermi level and the bottom of the conduction band in the bulk semiconductor (n-type). Equation 5-1 can be written in the following form

21

rb

21

sD

qkT

V2

qNAC

−

−+−⋅

= ξφ

ε

-2 -1 0 1 2 30

1x1021

2x1021

3x1021

4x1021

5x1021

Intercept

VI=1.87

φBp

=1.49 eV (Au)V

I=1.30

φBp

=2.07 eV (Ti)

φBp

=1.56 eV (Ni)V

I=1.36

1/C

2 (F

-2)

p-type 4H-SiC

Reverse Voltage VR (V)

Figure 5-15. 1/C2 versus reverse bias voltage for Ti, Ni, and Au Schottky contacts to p-type 4H-SiC in the frequency of 100 kHz at room temperature. The area of the diodes is 1.26 × 10-3 cm2.


- 58 -

(5-2) If the barrier height (φb) is independent of Vr, a plot of 1/C2 versus Vr should give a straight line with an intercept –VI on the horizontal axis equal to –(φb-ζ-kT/q). Hence, the barrier height is given by (5-3) where VI is the voltage intercept and ∆φ, given in equation 3-16, is the image force lowering of the Schottky barrier. From the slope of this plot, we can also extract the doping concentration, given in equation 5-4, of the epilayer. Figure 5-15 shows typical 1/C2 versus reverse voltage for Ti, Au, and Ni Schottky contacts to p-type 4H-SiC. (5-4) Current-voltage measurement According to the thermionic-emission theory, the current density (J)-forward voltage (V) characteristics is given by

−+−⋅

=−

qkT

VqNA

2C rb

sD2

2 ξφε

φξφ ∆−++=q

kTVIbn

( ) 2

r

2

s

D

AdV

C1dq

2N

⋅

−

=ε

0.0 0.5 1.0 1.5 2.0 2.5 3.010-12

1x10-10

1x10-8

1x10-6

1x10-4

1x10-2

1x100

1x102

300200

10025 oC

Au/4H-SiCTemp Φ

B η

25 1.75 1.30100 1.84 1.20200 1.77 1.10300 1.78 1.05

Ti/4H-SiCTemp Φ

B η

25 1.12 1.03100 1.11 1.05200 1.12 1.06300 1.14 1.06

Ti Schottky Au Schottky

n-type 4H-SiC

Cur

rent

(A

)

Forward Voltage (VF)

Figure 5-16. The current (log I) versus forward bias voltage (VF) for Ti (titanium) and Au (gold) Schottky diodes to n-type 4H-SiC.


- 59 -

(5-5) where JS is the saturation current density, η is the ideality factor, A* is effective Richardson's constant (146 Acm-2K-2), and φe is the effective barrier height (=φb-∆φ). The barrier height and ideality factor are obtained from the following equation 5-6 (5-6) The ideality factor (η) and the saturation current density (Js) can be extracted from the experimentally obtained forward current density-voltage (ln J-V) characteristics. Typical current (log I) versus forward bias voltage (VF) for Ti and Au Schottky contacts to n-type 4H-SiC is shown in Figure 5-16. From Figure 5-16, the Schottky barrier height for titanium metal contacts to n-type 4H-SiC was 1.11∼ 1.14 eV in the range of 25 to 300 oC with stable ideality factor of 1.03 ∼1.06. Photoelectric measurement When a monochromatic light is incident upon a metal surface (see Figure 5-17), photo-current (Y) is generated. The photo-current per absorbed photon Y as a function of the photon energy, hν, is given by (5-7) where hν0 is the barrier height qφBn, Es is the sum of hν0 and x is defined as h(ν-ν0)/kT. equation 5-7 can be rewritten to

−=

−

=

kTq

expTAJwhere1kT

qVexpJJ e2*

S

,

S

φη

( )

∂

∂=

=

JlnV

kTq

JTA

lnq

kT

,S

2*

B ηφ

0xfor4

ee

62x

hET

Yx2

x22

S

2

≥

⋅⋅⋅+−−+

−≈

−−π

ν

A

Metal

Semiconductor

Ohmic contact

hν(Back illumination)hν

(Front illumination)

Figure 5-17. Schematic set-up for photoelectric measurement (see ref. [24]).

0 1 2 30

2

4

6

8

10

(Y)1/

2 (A

rbitr

ary

Uni

t)

Photon energy hυ (ev)

Intercept (=qφBn)

Figure 5-18. A plot at square of photo current vs photon energy. The extrapolated values are the corresponding barrier heights (see ref. [24).


- 60 -

for hν-qφBn > 3kT (5-8) When the square root of the photo response (Y) is plotted as a function of the photo energy (see Figure 5-18), a straight line should be obtained. The extrapolated value on the photo energy axis directly yields the barrier height (qφB). Activation energy measurement The advantage of this method is that no assumption of electrically active area is required. Using active area (Ae), we can obtain (5-9) where q(φB-VF) is the activation energy. Thus for a given forward bias voltage VF, the slope of a plot of ln(IF/T2) versus 1/T yields the barrier height and the intercept at 1/T=0 yields the product of the active area Ae and the effective Richardson constant A*. Summary I-V and C-V measurements have been the main methods to characterize the Schottky diodes to silicon carbide. These two methods are convenient for evaluating the Schottky barrier height of metal-semiconductor contacts, which is an essential factor to characterize Schottky diodes. C-V measurements were mainly used for extracting the doping concentration of the epilayer as shown in equation 5-4. The Schottky barrier heights from C-V measurements are slightly higher than those from I-V measurements as shown in Paper I. Therefore, in order to characterize the Schottky diodes the best electrical characterization is I-V measurements among various measurements as described above.

( )2BnqhY φν −≈

( ) ( ) kTVqAAlnTI

ln FB*

e2F −−=

φ


- 61 -

5.2.2 A review of the Schottky contacts (Paper I, IV, VIII)

During recent years, the interest in 4H-SiC for device applications has increased because of its higher electron mobility and wider band gap than the 6H polytype. There are a few reports on the metal/4H-SiC systems compared to those of on 3C-SiC and 6H-SiC system. In this section, some of the earlier works related to the Schottky contacts to only 4H-SiC are summarized in Table 5-1.

Table 5-1. Schottky barrier height of metals using I-V, C-V and other methods on n- and

p-type 4H-SiC.

Barrier Height (eV)

n or p

Metals

I-V C-V

η face Comments Refs.

0.80 - 1.15 as-deposited (20 oC)

Ti

0.85 - 1.10 122 oC 1.30 - 1.21 20 oC 1.40 - 1.12 122 oC

n

Ni

1.50 - 1.12

Si-

255 oC

[45]

1.73 1.85 Si- Au 1.80 2.10 C- 1.62 1.75 Si- Ni 1.60 1.90 C- 0.95 1.17 Si- Ti 1.16 1.30

1.02 ∼ 1.20

C-

1.81 (IPE) Si- Au 2.07 (IPE) C- 1.69 (IPE) Si- Ni 1.87 (IPE) C- 1.09 (IPE) Si-

n

Ti 1.25 (IPE)

C-

IPE (Internal Photoemission Spectroscopy)

[41]

1.22 1.23 1.05 as-deposited n 1.18 1.19 1.10 500 oC, 30 min 1.41 2.11 3.11 as-deposited p

TiW

1.91 1.66 1.08

Si-

500 oC, 30 min

Paper I

Ni 1.31 1.56 1.29 Au 1.35 1.49 1.08

p

Ti 1.94 2.07 1.07

Si- Paper IV

0.91 ∼0.94

1.17 ∼1.22

as-deposited n Ti

0.99 ∼1.04

- -

1.03 ∼1.04

Si-

500 oC annealed

[94]

1.6 2 as-deposited n Ni 1.9

- 1.1

Si- 500 oC annealed

[95]

Ni 1.59 - 1.05 n Pt 1.39 - 1.01

Si- [47]


- 62 -

5.2.3 The relationship between metal work function and barrier height

(Paper IV)

Itoh et al.[41] reported Schottky barrier height of several metals (Ti, Ni, and Au) to n-type 4H-SiC using I-V, C-V and IPE measurements (also see Table 5-1). They also proved that the barrier height depends on the metal work function with evidence of no strong Fermi-level, and with a linear relationship with slope (S≡φBn/φm), referred to the index of interface behavior, of 0.7 for Si-face of n-type 4H-SiC between the barrier height (φBn) for n-type 4H-SiC and metal work function. This value agrees with works of Waldrop et al.[96] for n-type 6H-SiC. In view of a basic understanding of metal-semiconductor interfaces, Schottky barriers on p-type 4H-SiC are also of interest. Unfortunately, there are only few publications on the subject of Schottky barrier on p-type 4H-SiC. In this section, we will describe our investigation of Schottky diodes of several metals to p-type 4H-SiC (Si-face) using I-V and C-V characteristics. We also suggest that there be a strong relationship between barrier height and metal work function and that it will reach the Schottky-Mott limit with these results (Paper IV). Here, highly doped p-type 4H-SiC (0001) substrate (1018 cm-3) with 4 µm thick lightly doped p-type epilayer (1.3×1016 cm-3) was used. The Schottky diodes (1500 Å thick) were fabricated by e-beam evaporation of metals (Ti, Au, and Ni). The metal work function (φm) of a metal is defined by the amount of energy required to raise an electron from the Fermi level to a state of rest out side the surface of metals (called vacuum level), and it consists of two parts; the volume contribution and the surface contribution, which indicates that any of surface modification in the surface electron charge distribution (adsorption of gas on surface and different crystallographic faces of the same crystals) will lead to a change in work function [11]. Some results also show that there is 0.78 eV difference between the work function of the tungsten (110) and (111) faces due the difference of the surface contribution. For example, there is 0.81 eV difference between the metal work function of the (111) plane and the (331) plane of platinum. In order to get the exact barrier height on a metal-semiconductor, the exact crystallographic faces are important. The XRD spectra (see also Section 5.1.1) show that the metals are highly textured on SiC with 111,001, and 111-type reflections for Au, Ti, and Ni, respectively. From the XRD results, we have the crystallographic face dependent work function of each metal; Au (5.31 eV) and Ni (5.35 eV) for 111. For Ti (4.33 eV) the work function for the polycrystalline phase was used instead of 001 because of no available reference data [97]. The Schottky barrier height was determined by C-V and I-V measurements. Figure 5-15 shows the plot of the square of the inverse of the capacitance per unit area as a function of reverse voltage for Ni, Ti, and Au on p-type 4H-SiC at the frequency of 100 kHz at room temperature. The barrier height was 1.56, 1.49, and 2.07 eV for Ni, Au, and Ti, respectively. The current-voltage characteristics of Au on p-type 4H-SiC in the range of 24 oC to 300 oC are shown in Figure 5-19. All of the contacts show good rectifying behavior with stable ideality factor of 1.07, 1.23, and 1.06 for Ti, Ni, and Au, respectively, in the range of 24 oC to 300 oC. The barrier heights from C-V characteristics were 0.13 ∼ 0.25 eV higher than those from C-V measurements. It could be explained by additional capacitance at the interface due to a thin oxide during the sample preparation. The barrier heights of each metal for n-type and p-type 4H-SiC using C-V and I-V measurements as a function of the metal work function at room temperature are plotted in Figure 5-20 and Figure 5-21, respectively. From the both Figure 5-20 and 5-21, the slope S (S≡dφBn,p/dφm) for n-type and p-type was 0.64 – 0.73 (C-V, I-V) and 0.54 – 0.61 (C-V, I-V), respectively.


- 63 -

These results indicate that the Schottky barrier height strongly depends on the metal work function even though there is partial Fermi level pinning. Figure 5-22 summarizes the barrier height from C-V and I-V for n- and p-type 4H-SiC as a function of the metal work function. From the Schottky-Mott limit [11], the sum of the Schottky barrier

0 1 2 3 4 510-10

10-8

10-6

1x10-4

10-2

100

102

300 0C (φBn

:1.51 eV, η:1.05)

200 0C (φBn

:1.42 eV, η:1.06)

100 0C (φBn

:1.40 eV, η:1.05)

24 0C (φBn

:1.35 eV, η:1.08)

Au (p-type 4H-SiC)

Cur

rent

Den

sity

J (

A/c

m2 )


Figure 5-19. Current density (log J) versus forward voltage (VF) for Au Schottky diodes to p-type 4H-SiC with measurement temperature from 24 to 300 oC.

4.0 4.5 5.0 5.5 6.00.5

1.0

1.5

2.0

2.5

ΦB-n

=0.64 Φm-0.58

ΦB-n

=0.73 ΦM-2.21

NiAuTi

4H-SiC (n-type) I-V C-V Fitting line

Bar

rier

Hei

ght Φ

Bn (

eV)

Metal work function Φm (eV)

4.0 4.5 5.0 5.5 6.01.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

At room temperature

C-V measurement I-V measurement

P-type 4H-SiC

φBp

=4.42 - 0.54 φm

φBp

=4.58 - 0.61 φm

Ti NiAu

Sch

ottk

y B

arrie

r H

eigh

t φB

p (eV

)

Metal work function φm (eV)

Figure 5-21. Schottky barrier height of Ni, Ti, and Au to p-type 4H-SiC using I-V and C-V characteristics as a function of each metal work function.

Figure 5-20. Schottky barrier height of Ni, Ti, and Au to n-type 4H-SiC using I-V and C-V characteristics as a function of each metal work function (after ref.[41]).


- 64 -

height for n- and p-type semiconductor should be given by (φBn+φBp ≈ Eg-4H-SiC). From Figure 5-22, the sum of the SBH is very close to the energy band gap of 4H-SiC, indicating that our results satisfy the Schottky-Mott model without strong Fermi-level pinning. It also indicates that n-type Schottky barrier diodes have weaker Fermi-level pinning compared to p-type Schottky diodes. In addition, we plotted the various metals as a function of the metal work function in Figure 5-23. It shows there is a great deal of scatter in the experimental data for a given metal compared to our data on 4H-SiC. As pointed out above the surface contribution caused by different sample preparation and surface quality are important factors to have good Schottky barrier diodes without strong Fermi level pinning.

4.0 4.5 5.0 5.5 6.0 6.50.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

ΦB-n

=0.39 Φm-0.60

PtPdNiAuTi

4H-SiC (n-type) I-V C-V IPE, BEEM Average Fitting line

Bar

rier

Hei

ght Φ

Bn (

eV)

Metal work function Φm (eV)

Figure 5-23. Schottky barrier heights of various metals on n-type 4H-SiC as a function of the metal work function. Data were taken from Table 5-1.

4.0 4.5 5.0 5.5 6.00.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

(Eg=φ

Bp-φ

Bn)

Egat 24 oC (4H-SiC)

φBp

=4.51 - 0.58 φm (p-type) at 24oC

φBn

=0.67φm - 1.85 (n-type) at room temp.

Ti NiAu

Sch

ottk

y B

arrie

r H

eigh

t φB (

eV)

Metal work function φm (eV)

Figure 5-22. Schottky barrier heights of Ni, Ti, and Au to both n- and p-type type 4H-SiC as a function of the metal work function (see ref. Paper IV and [41]).


- 65 -

5.2.4 Reduction of the Schottky barrier height (Paper VIII)

A lower barrier height of the contacts was observed by the incorporation of size-selected Au nano-particles in Ti Schottky contacts on silicon carbide (Paper VIII). For this study, the contacts were formed by first depositing Au aerosol nano-particles with a diameter of 20 nm and with a density of 90 ∼ 100 µm-2 on the SiC surface (see Figure 5-24) [98]. After deposition of the Au aerosol, the samples were loaded into the evaporation chamber to deposit additional Ti (200 nm) on the Au nano-particles (see Figure 5-25). The reason we selected Au and Ti for our study as Schottky metals is that they have a large barrier height difference. Finally, the Schottky diodes (50 to 1000 µm diameter) were patterned using a standard photo-lithographic process and wet-etched using solutions of H2O : HF 5% (2:1) and KI : I2 : H2O (4:1:10) for Ti and Au, respectively. Then large area Al backside Ohmic contacts (250 nm thick) were evaporated onto the backside of the SiC with a photo-resist mask in order to prevent the contamination of the front side metals. Diodes were finally annealed with a 10% H2/Ar ambient at 350 ∼ 550oC for 60 s using rapid thermal annealing (RTA). I-V as well as C-V measurements were performed on different diodes for different temperatures up to 300oC The SBHs were extracted from both I-V and C-V measurements for comparison at different temperature ranges

Figure 5-24. A picture of the aerosol machine and schematic view of aerosol apparatus (See after ref. [98])

Figure 5-25. Schematic view of Ti Schottky contacts with embedded Au nano-particles on SiC. The thickness of the epilayer is 4 µm.

4H- or 6H-SiC (Substrate) n- or p- epilayer

n- or p- epilayer 4 µm

0.2 µm

200-400 µm

Au nano-particles

Ti

Backside Ohmic


- 66 -

Table 5-2. Summary of the Schottky barrier height and ideality factor as a function of the measurement temperature for particle-free control Ti Schottky contacts and Ti Schottky contacts with embedded Au nano-particles to n- and p-type 4H- and 6H-SiC using current-voltage measurements.

Measurement Temperature (oC) Samples 25 50 75 100 125 150 175 200 250 300

φBp 1.66 x 1.71 x 1.77 x x x x x Nanoa

η 1.41 x 1.37 x 1.33 x x x x x φBp 1.71 x 1.76 x 1.75 x x x x x

p

Tib η 1.33 x 1.31 x 1.35 x x x x x

φBn 0.93 0.93 x 0.93 x 0.93 x 0.94 0.95 0.95 Nanoa

η 1.04 1.03 x 1.03 x 1.04 x 1.03 1.02 1.11 φBn 1.12 1.11 x 1.11 x 1.11 x 1.12 1.13 1.14

4 H

n

Tib η 1.03 1.03 x 1.05 x 1.05 x 1.06 1.14 1.06

φBp 1.60 x 1.61 x 1.66 x x x x x Nanoa

η 1.38 x 1.44 x 1.41 x x x x x φBp 1.70 x 1.73 x 1.79 x x x x x

p

Tib η 1.24 x 1.24 x 1.20 x x x x x

φBn 0.59 0.59 0.59 0.59 0.61 0.63 0.65 0.68 x x Nanoa

η 1.05 1.07 1.13 1.27 1.36 1.32 1.35 1.30 x x φBn 0.76 0.76 0.76 0.76 0.77 0.78 0.78 0.78 x x

6 H

n

Tib η 1.07 1.07 1.10 1.12 1.13 1.13 1.20 1.30 x x

x : not performed a Ti Schottky Contacts with embedded Au nano-particles b Particle-free control Ti Schottky contacts

(a) (b)

0.0 0.5 1.0 1.510-11

1x10-7

1x10-3

1x101

Temperature 250C, 1000C, 2000C, 3000C

Au-embedded Schottky Ti Schottky

4H-SiC (n-type)

Cur

rent

(A

)


0.0 0.5 1.0 1.510-11

10-7

10-3

101

Au-embedded Schottky Ti Schottky

6H-SiC (n-type)

Temperature (25,50,75,100,and 125oC)

C

urre

nt (

A)


Figure 5-26. The current (log I)-forward voltage (VF) characteristics of particle free control Ti Schottky contacts and Ti Schottky contacts with embedded Au nano-particles to (a) n-type 6H-SiC at different measurement temperature 25 oC, 50 oC, 75 oC, 100 oC, and 125 oC and (b) n-type 4H-SiC at different measurement temperature 25 oC, 100 oC, 200 oC, and 300 oC.


- 67 -

Figure 5-26 shows typical I-V characteristics of 350 oC annealed control Ti Schottky contact and Ti Schottky contacts with embedded Au nano-particles to n-type 6H- and 4H-SiC. The results are also summarized in Table 5-2. The results from the I-V measurements clearly indicated that the SBH for Ti Schottky contacts with embedded Au nano-particles on n-type SiC was 0.19 eV (4H-SiC) and 0.15 eV (6H-SiC) lower than those of the control Ti Schottky contacts. The SBHs for p-type Ti Schottky contacts with embedded Au nano-particles on SiC were also 0.02 ∼0.05 eV (4H-SiC) and 0.1∼0.13 eV (6H-SiC) lower than those for the control Ti Schottky contacts in the temperature range 25 ∼ 125 oC. In order to understand this reduction of the SBH for Ti Schottky contacts with embedded Au nano-particles to 4H- and 6H-SiC both from I-V and C-V measurements, it has been proposed that SBH lowering is caused by an enhanced electric field at the depletion region close to the surface of the semiconductor due to the small size of the Au nano-particles and the large SBH difference. According to Tung's dipole-layer approach of the potential and the electronic transport at metal-semiconductor (MS) interface, the potential distribution for circular patch geometry at MS interface is given by [99]

( )( )

+−∆−++

−= −

21

20

2AuTina

2

bi

Rz

z1VV

wz

1Vz,0,0V φ (5-10)

( )

+−

+∆−

−= − 32

02

2

20

2AuTi2bi

Rz

z

Rz

1w

z2w2

V)z,0,0(E φ (5-11)

where z is the distance from the surface of semiconductor, w is the depletion width, R0 is the radius of the circular patch, and ∆φTi-Au is the difference of the barrier height between Ti and Au metals. Figure 5-27 (a) and (b) shows the calculated conduction band potential and electrical field as a function of the distance from the surface and the radius of the circular patch using equation 5-10 and 5-11, respectively. Equation 5-10 also suggested that the potential at the metal-semiconductor interface be highly dependent on the radius of the circular patch (R0) and SBH difference (∆φTi-Au) between Ti and Au contacts. The magnitude of electric field for n- and p-type SiC was calculated to be 0.07 ×107 V/cm (n-4H- and 6H-SiC), 0.06 ×107 V/cm (p-4H-SiC), and 0.04 ×107 V/cm (p-6H-SiC), respectively. In order to calculate the value of Schottky barrier lowering, the calculated electrical field for each n- and p-type was plugged into following well-known equation 3-16. In general we could see that there is no image force lowering at the forward bias in Schottky diode due to low electric field at the forward bias. However, from our calculation results, there is enough electric field to reduce the barrier height due to the small size of metal particles and large difference of the barrier height between two metals. The ∆φ for our experimental results is in reasonable agreement with the theoretical calculation using a dipole layer approach with the circular patch [99] even though the ∆φ from the theoretical calculation is a factor of 2 lower than what we obtained from our measurements. The reason for this could be a much higher electric field than we expected and calculated at the metal-semiconductor interface. Detailed further studies are required for a more solid explanation. In order to evaluate the predominant current of lower barrier height Schottky contacts and confirm an enhanced high electric field, a 2-D simulation was performed with the device simulator ATLAS [20]. Figure 5-28 (a) and (b) shows ATLAS simulation set-up and


- 68 -

the simulation results of the forward I-V characteristics for Ti, Au and dual metal (Ti and Au) on SiC. The simple double barrier devices consisted of a silicon carbide with two metal electrodes (called M1 and M2, with 1nm spacing between them) of different work functions and barrier heights on the surface. The simulation results confirmed that the small barrier height Ti Schottky contact conducts current dominantly indicating the SBH values form simulation only depends on the lower SBH diodes as shown in Figure 5-28 (b). In addition, we did not observe the SBH difference between Ti Schottky and dual metal (Ti/Au) Schottky contacts from our simulation. However, ATLAS simulation showed the electric field increase at the triple interface (two metals and SiC

Figure 5-27. (a) calculated conduction band potential distributions and (b) electric field distribution for n-type 4H-SiC as a function of the radius of the circular patch and the depth from the surface (z) The insert shows a schematic diagram of a high barrier circular patch (Au) surrounded by low barrier metals (Ti) on SiC.

0 50 100 150 2000.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

n-type 4H-SiC

50nm

100nm

200nm

20nm

10nm (in this work)

R0=5nm (Au nano-particles)

Pot

entia

l (V

)

Depth from the surface Z (nm)-10 0 10 20 30 40 50 60 70 80 90 100

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

n-type 4H-SiC

20nm

50nm100nm200nm

10 nm (in this work)

R0=5nm (Au nano-particles)

Depth z (nm)E

lect

ric fi

eld

x 10

7 (V/c

m)

(a)

Z

φB (Ti)

φB-∆ (Au)R0

Metal

Semi

(b)

0.0 0.5 1.0 1.5 2.010-15

10-13

10-11

10-9

10-7

10-5

10-3

10-1

101

η=1.01, φB=1.13 eV

η=1.02, φB=1.11 eV

η=1.02, φB=1.64 eV

Ti/Au (Dual metals) Ti only Au only

Cur

rent

(A

)

Forward voltage (V)

Figure 5-28. (a) ATLAS simulation set-up and (b) the results of forward I-V characteristics for Ti, Au, and dual metals on silicon carbide at 300 K.

VDC

SiC (∼ 1018 cm-3) (0001)

4µm thick epi (4∼7×1015 cm-3)

M1(Au)M2 (Ti)

Backside ohmic

-

+

GND

1nmφBn=φm-χs

VDC

SiC (∼ 1018 cm-3) (0001)

4µm thick epi (4∼7×1015 cm-3)

M1(Au)M2 (Ti)

Backside ohmic

-

+

GND

1nmφBn=φm-χs


- 69 -

substrate) as compared to the control Ti/SiC interface as shown in Figure 5-29. The electric field at the triple interface was around 3 × 106 V/cm.

Figure 5-29. The lateral electric field distribution for dual metal on SiC.


- 70 -

5.3 Specific contact resistance measurements Linear transmission line method (LTLM) with mesa structures was used to characterize the Ohmic contacts to 4H- and 6H-SiC (mainly 4H-SiC).

Table 5-3. A review of the Ohmic contacts to SiC n or p

Metals Doping (cm-3)

ρc (Ωcm2)

Face Annealing Ref.

4.8×1017 1.0×10-4

∼1.6×10-5 4H (n)

Ni-Cr

1.3×1019 1.2×10-5

Si- 1100oC, 3 min

[101]

Si/Pt 4H(p) Al/Ti

1×1019 ∼10-3 ∼10-4

Si- 30oC ∼400oC 1100oC, 3 min

[102]

Ni Cr

4H(n)

W

1017∼1018 10-4∼10-6 Si- 1000oC∼ 1050oC, 5 min

[103]

4H(n) 1.3×1019 4×10-5 4H(p)

TiC >1020 6×10-5

Ti >1020 8×10-4

Si 950oC Paper II

6H(n) Ti >1020 2×10-5 Si 950oC [104] 4H(p)

TiC 2×1019 1×10-4 Si 850oC Paper III

4H(n)a 1 × 1019 6.0 × 10-6 4H(p)b 1 × 1021 1.5 × 10-4

1050oC 10 min

[105]

1 × 1019 1.5 × 10-5 1000oC, 5min [106] 4H(n) 1.1× 1019 7.5× 10-6 950oC, 30

min Paper VII

5 × 1019 1 × 10-6 1000oC, 5min [107] 7.8 × 1018 4∼9 × 10-6 950oC, 2min

3.2 × 1017 3 × 10-6

Si

1200oC, 1min [50]

6H(n)

Ni

2 ∼5 × 1018 8∼9 × 10-5 C 1000oC [108] 4H(n) 1.3 × 1019 2∼6 × 10-5 4H(p)

TiW (30:70)c 6 × 1018

∼ >1020 1.2 × 10-4

∼ 4 × 10-6

950oC, 30min Paper V, VI, VII

6H(n) 7 × 1018 1 × 10-4 750oC, 5min [109] 3C(n) 1.7 × 1020 7.8 × 10-5

6H(n)

TiW (10:90)c

6 × 1018 3.4 × 10-4

Si

900oC, 15min [110]

a Nitrogen was implanted. b Aluminum and carbon were co-implanted. c Weight ratio (Ti:W) In this thesis, various metals (Ti, TiC, TiW, and Ni) were used as a metal for Ohmic contact studies. According to previous works, the specific contact resistance to n- and p-type of 4H-SiC and 6H-SiC is in the range of 10-4 ∼ 10-6 Ωcm2 and 10-3 ∼ 10-5 Ωcm2,


- 71 -

respectively, which seems to be highly depending on the surface doping concentration, the choice of the metals, the post heat treatment, the sample preparation (deposition, cleaning and other processes), and also quality of the silicon carbide materials [100]. The review results of most promising Ohmic contacts to n- and p-type silicon carbide are summarized in Table 5-3 (also see ref. [100] in detail). 5.3.1 TiC and Ti on n- and p-SiC (Paper II, III, V)

TiC/n- and p-SiC (epi) A summary of Ohmic specific contact resistance as a function of measurement temperature and annealing temperature using Ti and TiC to n- and p-type 4H-SiC is shown in Table 5-4. The results of the specific contact resistance for p- type (a) and n-type (b) TiC is also shown in Figure 5-30 (a) and (b), respectively. The lowest specific contact resistance for as-deposited p-type TiC was 1.1 × 10-4 Ωcm2 at 25oC. After annealing at 700oC, the specific ρC did not improve. The specific ρC reached its lowest value of 1.9 × 10-5 Ωcm2 at 300oC after annealing at 950 oC. These results can be explained by a shorter transfer length and lower contact resistance caused by a smoother interface and no interface reaction. As shown in Figure 5-30 (b) n-type TiC have different behaviors. In order to explain the exact mechanism at the interface between the metals and epilayer, further investigation is needed. From the study of TiC contacts deposited by co-evaporation in a UHV system have promising advantages to make the lower contact formation on 4H-SiC. One reason for this is that no post-annealing is required. Ti/p-SiC (epi) As-deposited Ti contacts showed a good Ohmic characteristic with the lowest ρC of 2.5 × 10-4 Ωcm2 at 200oC and broadly uniform distribution of the specific contact resistance. After sequential annealing at 700oC and 950oC using RTA in 10%H2 /Ar the specific ρC increased by a factor of 2. A possible reason for this increase is due to the creation of

the new phase such as Ti5Si3 and TiC1-x which have a higher contact resistance (RC) than

0 50 100 150 200 250 300 35010-8

10-7

10-6

1x10-5

1x10-4

10-3

10-2

(b)

950oC,60S, RTA in 10% H2/Ar

As-deposited

Spe

cific

con

tact

res

ista

nce

(Ω c

m2 )

Temperature (oC)

0 50 100 150 200 250 300 35010-7

10-6

1x10-5

1x10-4

10-3

10-2

10-1

S

peci

fic c

onta

ct r

esis

tanc

e (Ω

cm

2 )

(a)

As-deposited 700 oC, RTA in 10% H

2/Ar

950 oC, RTA in 10% H2/Ar

Temperature (oC)

Figure 5-30. The results of the specific contact resistance of TiC Ohmic contacts to p-type (a) and n-type (b) to 4H-SiC as a function of the measurement and annealing temperature.


- 72 -

that of as-deposited contacts (see also section 5.1)[86]. The results from the LTLM showed and confirmed the above indication because the contact resistance (RC) for as-deposited contact at 25oC was about 40Ω, which is 40% lower than that of the 950oC annealed contacts measured at the same temperature.

Table 5-4. The summary of Ti and TiC Ohmic contacts to n- and p-type 4H-SiC for different measurement and annealing temperature (Paper II).

Average specific contact resistance (Ω cm2) Titanium Carbide (TiC) Titanium (Ti)

Annealing Temperature (oC)

Measurement Temperature (oC) n-type p-type p-type

As-deposited

25 100 200 300

9.28 × 10-6 3.07 × 10-6 7.38 × 10-7 5.26 × 10-6

1.08 × 10-4 1.31 × 10-4 1.12 × 10-4 2.32 × 10-4

3.44 × 10-4 2.91 × 10-4 2.50 × 10-4 2.93 × 10-4

700 oC, 180s RTA in 10%H2/Ar

25 100 200 300

X X X X

1.75 × 10-4 3.28 × 10-4 1.96 × 10-4 2.61 × 10-4

4.70 × 10-4 3.90 × 10-4 3.18 × 10-4 2.75 × 10-4

950 oC, 180s RTA in 10%H2/Ar

25 100 200 300

4.01 × 10-5 2.77 × 10-5 4.59 × 10-5 8.72 × 10-5

5.62 × 10-5 4.18 × 10-5 3.16 × 10-5 1.87 × 10-5

7.70 × 10-4 6.19 × 10-4 5.04 × 10-4 4.31 × 10-4

X : not performed TiC on implanted layers The results of the specific contact resistance as a function of measurement temperature and annealing temperature are shown in Figure 5-31 and Table 5-5 The best result was achieved after 500 oC annealing. For as-deposited contact (TLM1 structure, see Table 5-5) the sheet resistance (ρs) of the epilayer increased from 0.6 kΩ/£ to 1.4kΩ/£ when

0 50 100 150 200 250 300 35010-6

1x10-5

1x10-4

10-3

As-deposited 500 oC RTA 700 oC RTA 850 oC RTAC

onta

ct r

esis

tivity

ρC (Ω

cm

2 )

Temperature (oC)

Figure 5-31. The specific contact resistance of TiC Ohmic contact (TLM1) on implanted 4H-SiC versus measurement temperature and annealing temperature.


- 73 -

the temperature was increased from 25oC to 300oC. Considering the theoretical sheet resistance of 0.5kΩ/£, this would correspond to over 80% activation of Al acceptors. We also observed that the TLM structures had different values of ρs varying from 0.6 kΩ/£ to 6.3kΩ/£ for as-deposited contacts. This non-uniformity of the sheet resistance might be due to the different activation of Al in the epilayer or not completely recrystallized SiC after 1700oC annealing. This behavior affected the variation of the specific contact resistance as shown in Table 5-5. The specific contact resistance from TLM1 was as low as 2 × 10-5 Ωcm2 after 500oC. After sequential annealing at 700oC and 850oC, no further improvement of ρC was observed. As mentioned in the material characterization part (see also section 5.1), RBS and XPS depth profiles indicated that there were around 10 to 15% oxygen on the surface after 950oC RTA, but the oxygen content decreased to 1 at% close to the metal/substrate interface. The increase in the specific contact resistance after high temperature annealing (> 500oC RTA in 10%H2/Ar) was correlated with the oxygen detected from XPS and RBS analyses. The oxygen incorporation results in substantial degradation (increasing specific contact resistance) of the contacts, TiC/SiC.

Table 5-5. The summary of TiC Ohmic contacts on Al implanted 4H-SiC for 3 different sets (TLM1, TLM2, and TLM3) (Paper III).

TiC TLM 1 TiC TLM 2 TiC TLM 3

Annealing Temperature (°C)

Meas. Temp. (°C)

ρC ×10-4 Ωcm2

ρS Ω/£

ρC ×10-4 Ωcm2

ρS Ω/£

ρC ×10-4 Ωcm2

ρS Ω/£

25 0.87 601 1.66 2492 1.32 1810 100 1.07 629 1.65 2614 1.22 1865 200 1.12 743 2.08 3021 1.34 1993

As-deposited

300 0.48 1400 5.83 6273 2.87 3305 25 0.19 1947 8.63 8642 4.46 4757 100 0.32 1895 7.62 8168 4.27 4611 200 0.37 1936 7.70 8159 4.20 4656

500°C RTA

300 0.37 2096 8.34 8732 4.48 5118 25 0.48 1922 7.78 8265 4.17 4737 100 0.66 1780 7.05 7733 3.92 4506 200 0.74 1762 6.94 7692 3.78 4491

700°C RTA

300 0.37 2054 8.37 8758 4.16 4756 25 1.00 3350 X X X X 100 0.95 3033 X X X X 200 0.61 3088 X X X X

850°C RTA

300 0.99 3314 X X X X X denotes no measurement


- 74 -

5.3.2 Ni and TiW (30:70) contacts on n- and p-SiC (Paper V, VI, VII)

Ni/n-SiC Nickel contacts are widely used as Ohmic contacts to n-type both 4H- and 6H-SiC due to its lower specific contact resistance and the reproducibility after high-temperature annealing (see also Table 5-3). Normally high-temperature annealing (> 650 oC) creates a new polycrystalline δ Ni2Si phase at the interface as mentioned in section 5.1 (See also Figure 5-3, 5-12, and 5-14). The specific contact resistance for Ni is summarized in Table 5-3. Figure 5-32 (a) shows the SEM view of Ni contacts after 950oC annealing, indicating the new phase on the surface. Figure 5-32 (b) and 5-33 show the microscopic mapping of the specific contact resistances for annealed Ni Ohmic contacts on highly doped (1.1 × 1019 cm-3) n-type 4H-SiC (Paper VII).

Ni2Si 6,3E-6

6,3E-6

6,3E-64E-6

6,3E-6

1E-5

1 2 3 4 5 6 71

2

3

4

5

6

7

Y-

Pos

ition

X- Position

M7

1E-6

1,6E-6

2,5E-6

4E-6

6,3E-6

1E-5

1,6E-5

2,5E-5

4E-5

6,3E-5

1E-4

Figure 5-32. (a) SEM of Ni contacts after 950 oC annealing and (b) the contour mapping of specific contact resistance for Ni contacts. The solid line indicates the contour line of each measurement.

1 2 3 4 5 610-6

1x10-5

1x10-4

M7

A'

A

Top view

A'A

Ni on n+ 1.1x1019cm-3

Spe

cific

Con

tact

res

ista

nce

ρ C(Ω

cm2 )

TLM Position (A-A')

Figure 5-33. The bar-graph mapping of he specific contact resistance for Ni contacts. The right circle shows the exact position of the sample on the wafer.


- 75 -

TiW/n- and p-SiC As mentioned in the section on material characterization, polycrystalline (Ti,W)Si2 and (Ti,W)C1-x phases were created at the interface and then finally changed it to Ohmic behavior due to the high-temperature annealing (above 950oC). We found that it is reproducible, has low specific contact resistance, and compatible to the end process including bonding process. This makes TiW the best candidate for both n- and p-type Ohmic contacts to SiC [93]. As shown in Figure 5-34 (a) and (b), both p-and n-type TiW Ohmic contacts have a good uniformity with the specific contact resistance of 1.2 ×10-4 Ωcm2 and 3.3 × 10-5 Ωcm2, respectively. We will describe more detail about TiW in the next section with some important factors to make low resistivity Ohmic contacts.

0 5 10 15 20 2510-6

1x10-5

1x10-4

10-3

Average ρc

Con

tact

res

istiv

ity ρ

c(Ω

cm2 )

Data number0 50 100 150 200 250 300

10-6

1x10-5

1x10-4

10-3

After cooling down to 25oC

N-type (TiW) 1.3x1019cm-3

Temperature (oC)

ρ c(Ω

cm2 )

Figure 5-34. (a) The temperature dependence of the specific contact resistance for n-type TiW Ohmic contacts and (b) the distribution of the p-type TiW Ohmic contacts. The average specific contact resistance was 1.2 × 10-4 Ωcm2.


- 76 -

5.3.3 Microscopic mapping of specific contact resistance (Paper VI, VII)

For this study, the works on the microscopic mapping of the specific contact resistances using sputtered TiW contacts are described. Whole wafer mapping (n+-SiC)

The detailed sample preparation and differences among the samples are summarized in Table 5-6. The inset denotes the exact sample position on the 35 mm wafer. Figure 5-35 shows the contour mapping of the specific contact resistance for each sample (M1 to M6). In order to investigate the gradient behavior of the ρc for each adjacent sample the bar-graph mapping of the ρc with the standard deviation is also presented in Figure 5-36(a) to (d). Comparing sample M4 and M6, which was treated under exact same sample preparation, we found the specific contact resistance had gradient behaviors decreasing from the center to edge as shown in Figure 5-36(a). It indicates that the center region has a lower doping concentration than that of the edge region. Figure 5-36 (b) also shows similar behavior even though these samples are differently prepared.

Table 5-6. Summary of the process and specific contact resistances for all samples

Metals Pretreatment Specific contact resistance (Ωcm2)

Samplesj Mapping Long term

(Capping layersg)

Sub. Temp (oC)b

ICP etchinge (W/min)

S/Oh

Average STDEV

M1 TiW Xa 200 30W(3)→ 60W(3)

1250oC (1 hr)

2.34×10-4 4.15×10-4

M2a TiW Xa 200 30W(3)→ 60W(3)

None

1.27×10-3 1.40×10-3

M3 TiW Xa 200 30W(2) None

3.26×10-5 4.96×10-6

M4 TiW Pt/Ti/TiW 200 Xa 6.47×10-5 8.17×10-6

M5-1 Au/Ti/TiW 2.40×10-5 1.76×10-5 M5i M5-2

TiW

TiW

Nonec

Xa

3.95×10-5 5.79×10-6

M6-1 Au/Ti/TiW 1.84×10-5 1.47×10-6 M6i M6-2

TiW

TiW

200 Xa

3.77×10-5 6.31×10-6 M7d Ni Xa None

c Xa 7.47×10-6 2.47×10-6

a Not performed. b Substrate temperature for the TiW sputtering. c Without substrate heating. d M7 was prepared by e-beam evaporator for reference. e Platen power (30W or 60W) under the same coil power (600W). g Capping layer was performed by e-beam evaporator on annealed TiW layers and it is not

annealed by deposition. h S/O denotes sacrificial oxidation. i M5 and M6 samples were cut into two called M5-1, M5-2, M6-1, and M-6-2 M5-1 and M6-1 samples were capped with sequentially evaporated Ti and Au. j All samples are annealed at 950oC in a vacuum chamber for 30 min and measured at room

temperature.


- 77 -

4E-5

7

6

5

4

3

2

1

1 2 3 4 5 6 7

M6

XPosition

4E-5

5

4

3

2

1

1 2 3 4 5 6 7 8

M3

Y P

ositi

on

X Position

1E-5

1,6E-5

2,5E-5

4E-5

6,3E-5

1E-4

1,6E-4

2,5E-4

4E-4

6,3E-4

1E-3

1E-3

1 2 3 4 5 6 76

5

4

3

2

1

M2

X Position

2.5E-4

1.6E-5

4E-4

1 2 3 4 5 61

2

3

4

5 Y

pos

ition

M1

X Position

1E-5

1.6E-5

2.5E-5

4E-5

6.3E-5

1E-4

1.6E-4

2.5E-4

4E-4

6.3E-4

1E-3

6,3E-5

6,3E-5

6

5

4

3

2

1

1 2 3 4 5 6

M4

XPosition

4E-5

4E-5

6

5

4

3

2

1

1 2 3 4 5 6

M5

1E-5

1,6E-5

2,5E-5

4E-5

6,3E-5

1E-4

1,6E-4

2,5E-4

4E-4

6,3E-4

1E-3

Y P

ositi

on

X Position

Figure 5-35. The contour mapping of the specific contact resistance of sputtered TiW Ohmic contacts for sample M1, M2, M3, M4, M5, and M6. The measurement was performed at room temperature.


- 78 -

The influence of the surface pre-treatment (Paper VI)

We will discuss how the surface roughness induced by ICP etch affects the formation of the Ohmic contacts. For this study, we prepared three different samples (M1, M2, and M3) on the same wafer (see Table 5-6). The comparison of the I-V curves for three different samples (M1, M2, and M3) after annealing at 950oC shows that the samples etched with 60W (M1 and M2) have higher resistances compared to the 30W-etched sample M3. Figure 5-36 (c) and (d) also show big differences of the ρC and step behaviors compared to sample M3 and M5. The specific contact resistances for the un-etched sample (M6) and 30W etched, M3 (≈ 0.22 µm etched), was 3.8 × 10-5 Ωcm2 and 3.3×10-5 Ωcm2, respectively with good uniformity of the specific contact resistances. The specific contact resistances of these samples are almost identical even though sample M3 has slightly lower specific contact resistance, indicating sample M3 (30W etched sample) does not contain any critical damage, which is enough to degrade the specific contact resistances. However, the ρc for sample M2 increased by a factor of 40

0 5 10 15 2010-5

10-4

10-3

M5M3 M4

M5

A'A Top view

A'A

M4M3

Spe

cific

con

tact

res

ista

nce

ρ C(Ω

cm2 )

TLM Position (A-A')

(b)

0 1 2 3 4 5 6 7 8 9 10 11 12 1310-6

1x10-5

1x10-4

10-3

10-2

10-1

M5

M2

A'

A

Top view

A'A

M5M2

Spe

cific

con

tact

res

ista

nce

ρ C(Ω

cm2 )

TLM Position (A-A')

(c)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 1410-5

10-4

10-3

M6

M4

A'

A

Top view

A'AM6M4

Spe

cific

con

tact

res

ista

nce

ρ C(Ω

cm2 )

TLM Position (A-A')

(a)

0 1 2 3 4 5 6 7 8 9 10 11 1210-5

10-4

10-3

10-2

M3

M1

A'

A

Top view

TiW contact on n+ 4H-SiC

A'A

M3M1

S

peci

fic c

onta

ct r

esis

tanc

e ρ C

(Ωcm

2 )

TLM Position (A-A')

(d)

Figure 5-36. The bar-graph mapping of the specific contact resistances of different regions (a) M4→M5, (b) M3→M4→M5, (c) M1→M3, and (d) M2→M5. The insetindicates the position of each sample and scan directions on the wafer.


- 79 -

compared to the un-etched sample and 30W etched sample, having a much higher specific contact resistance of 1.3 × 10-3 ± 1.4×10-3 Ωcm2. We also observed that 5 out of 35 TLM structures did not work due to the severe damages induced by the ICP etch. To evaluate the passivation of a surface damaged sample, M1 was oxidized (1250oC, 1 hr, ≈ 58nm) after an ICP etch with 60W of platen power and the oxide was etched prior to metal deposition (sacrificial oxidation). From Figure 5-36 (c), we observed the specific contact resistance decreased by a factor of 6 with good uniformity on different TLM structures even though it did not reach the ρc for un-etched and 30W etched sample. This result indicates that the deep etching by ICP with medium power (60W) can cause severe surface damage, which is corresponding to our earlier study on Schottky contacts[64], and it directly affects the Ohmic contact formation. High temperature oxidation can remove some of the surface damage but not all. The oxide thickness was 58 nm, which means that we removed damage to a depth of about 30 nm. Figure 5-37 shows the comparison of the specific contact resistances depending on the roughness for sample M1, M2, M3 and M6. From Figure 5-37 we found that the Ohmic contacts formed on a much rougher surface had much higher specific contact resistance.

(Å)

0 10 20 30 40 50 60 70 80 9010-5

10-4

10-3

10-2

M2(60W etched)

M3 (30W etched)

M1 (60W, S/O)

M6 (un-etched)

Spe

cific

con

tact

res

ista

nce

ρ c (Ω

cm2 )

Roughness

Figure 5-37. Comparison of the specific contact resistance with roughness for sample M1, M2, M3, and M6.


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5.4 Long-term reliability tests at high temperature

5.4.1 In vacuum

Long-term operating Ohmic contacts are also required for stable performance of device at high temperature and high power operation. For the long-term study, we tested 3 different samples such as Au/Ti/TiW and Pt/Ti/TiW with reference contacts without capping layer. The results of long-term reliability tests at 500 and 600 oC in a vacuum furnace for 308 hours are shown in Figure 5-38. The initial ρc was 2×10-5, 6×10-5, and 4×10-5 Ωcm2 for Au/Ti/TiW, Pt/Ti/TiW and TiW, respectively. The difference of initial values between contacts was due to on the wafer (see also section 5.3.3). The reason to use capping layers (in our case, Pt and Au) is to protect degradation, e.g. oxidation and to be compatible to the wire bonding. As shown in Figure 5-38, all of the samples with a capping layer show excellent properties of the ρc without any Ohmic contact degradation. It also shows that the specific contact resistance of a sample without a capping layer increased slightly due to the oxidation at the interface or the surface. We also found that after 308 hours test the sample with Au capping layer had much surface degradation and was damaged by the long-term reliability tests (see Figure 5-39). However, the sample with a Pt capping layer did not show any surface damage or a specific contact resistance degradation.

0 50 100 150 200 250 300 35010-5

10-4

600oC500oC

#M5-1 (Au/Ti/TiW @ Alice) #M5-2 (TiW @ Alice) #M6-1 (Au/Ti/TiW @ ET sputter) #M6-2 (TiW @ ET sputter) #M4 (Pt/Ti/TiW @ ET sputter)

Annealing Time (hrs)

Spe

cific

con

tact

res

ista

nce

ρ C(Ω

cm2 )

Figure 5-38. The specific contact resistance for different capping layers as a function of annealing time at 500 oC or 600 oC in a vacuum furnace.


- 81 -

5.4.2 In oxidizing ambient

High-temperature operation chemical sensors with fast gas responses are of considerable interest, for instance, for the control of combustion processes in automotive industry. Time constants of less that about 20 ms enable the monitoring of individual cylinders in a normal automobile engine. In order to satisfy this automotive specification more stable sensors during the high-temperature operation are required [111]. In this section, we tested TiW with capping layer in oxidizing atmosphere as well as at high-temperature (500 oC or 600 oC)[112]. As shown in Figure 5-40, TLM structures were glued on heaters. A Pt-100 element is also glued on the heater for the temperature control. The heater is mounted on a 16-pin socket, which is put in an Al-block with a gas flow channel. The gas flow over the sensors was around 80 ml/min of 20% O2 in N2 and the annealing temperature was 500 or 600 oC.

Figure 5-39. (a) SEM view of Pt/Ti/TiW Ohmic contacts after a long-term reliability test for 308 hours.

Figure 5-39. (b) SEM view of Au/Ti/TiW Ohmic contacts after a long-term reliability test for 308 hours.

Figure 5-40. Measurement setup including gas-handling system. A gold plated 16-pin holder with a heater welded to the pins. On the heater, TLM structures, a MISiCFET, and Pt-100 element are mounted. The metal contacts of the sensor and TLM structures are gold bonded to the pins (after ref. [112]).


- 82 -

As-deposited samples had a specific contact resistance of ≈ 1×10-5 Ωcm2. The results of the measurement of two LTLM structures annealed in an oxidizing ambient at 500 oC and 600 oC are shown in Figure 5-41. The ρC increases over time, but the contacts were still working after more than 500 hours at 500oC with a 10-20 times higher values of ρC. At 600 oC, the ρC increases faster than for 500 oC and the measurements were stopped after 200 hours due to the failure of the contacts. Other work [113] with different metallization schemes such as Ni/TaSix/Pt and TaSix/Pt for high temperature gas sensor applications also confirms that the TiW/Ti/Pt shows the best performance while both Ni/TaSix/Pt and TaSix/Pt show a rather poor performance at high temperature (500-600 oC) in oxidizing ambient. Both metal stacks (Ni and TaSix based), which is commonly used for MISiCFET [111], did not work at all after less than 100 hours at 600oC in same oxidizing ambient [113].

0 200 400 60010-6

1x10-4

10-2

100

Spe

cific

con

tact

resi

stan

ce (Ω

cm2 )

Annealing time (hours)

20% O2in N

2 at 600 oC

20% O2in N

2 at 500 oC

Figure 5-41. Specific contact resistance for Pt/Ti/TiW/n-SiC measured at 500 oC and 600 oC in oxidizing ambient (20 %O2/N2) up to 500 hours.

- 83 -

B

6. Conclusions and future work

oth Schottky and Ohmic contacts to silicon carbide have been characterized by electrical and material measurements. Several promising metallization schemes

such as sputtered TiW, co-evaporated TiC, and evaporated Ti and Ni for both Ohmic and Schottky contacts to n- and p-type silicon carbide (4H- and 6H-SiC) were presented and characterized in this thesis. The main achievements, divided in two parts (Schottky and Ohmic parts), of each appended paper are summarized. Improvement of Schottky diode performance Sputtered TiW (weight ration 30:70) Schottky contacts have been fabricated and investigated on n- and p-type 4H-SiC. The thermally stable ideality factors for n- and p-type after annealing at 500 oC in a low-pressure vacuum furnace were 1.06 and 1.08, respectively. Our results from I-V and C-V measurements show that there is no Fermi-level pinning in sputtered TiW Schottky contacts on SiC after low-temperature annealing. This can reduce the barrier height inhomogeneities of Schottky diodes and improve the backside contacts. Observation of the relationship between SBH and metal work function (p-type contacts to silicon carbide) Schottky barrier diodes of several metals (Ti, Ni and Au), having different metal work function to p-type 4H-SiC (Si-face) were characterized using I-V and C-V measurements. From our measurements, the SBH and metal work function show a linear relationship of φBp=4.58-0.61φm and φBp=4.42-0.54φm for I-V and C-V characteristics at room temperature, respectively. It means that the SBH strongly depends on the metal work function even though the Fermi level is partially pinned. A new approach for Schottky contacts using Au nano-particles By the incorporation of size-selected gold nano-particles (normally ≈ 20 nm in diameter) in Ti Schottky contacts on silicon carbide, we observed considerably lower barrier height of the contacts. The reduction of the Schottky barrier height is explained and simulated using a model with enhanced electric field at the triple point (Ti-Au-SiC) due to the small size of the Au nano-particles and the large difference of the barrier height between Ti and Au on SiC. Low-resistivity titanium carbide (TiC) contacts Co-evaporated TiC on highly doped n- and p-type grown epilayers and aluminum-implanted layers was investigated. TiC was formed by UHV co-evaporation with Ti and C60 at low substrate temperature. The lowest specific contact resistances were 5×10-6, 2×10-5, and 2×10-5 Ωcm2 for TiC contacts on n+ epilayers, on p+ epilayers, and on Al implanted layers, respectively, even though there was a lack of homogeneity of the specific contact resistances.

Chapter 6 Conclusions and future work Sang-Kwon Lee

- 84 -

Ohmic contact formation on an ICP etched surface of SiC For this study, we used and tested sputtered TiW Ohmic contacts to n-type SiC. High temperature oxidation recovered some of the etch damage caused by ICP even though it did not fully recover etch damage for the sample etched medium power (60W). We also found that the specific contact resistance is highly related to the surface roughness and quality of the metals. A lower specific contact resistance was obtained due to the smoother interface. Microscopic mapping of specific contact resistance The mapping of specific contact resistance was performed on a whole wafer (35 mm wafer from CREE) using sputtered TiW in order to see the distribution on the wafer. The results show that the specific contact resistance had a decreasing distribution from the center to the edge region on the wafer. Long-term reliability tests Long-term reliability tests of contacts were performed at 500 – 600 oC in vacuum as well as in oxidizing ambient for high-temperature operation gas-sensor applications. Titanium tungsten (Pt/Ti/TiW) contacts with Pt capping layer shows the best results without surface degradation and have stable specific contact resistance at 500-600 oC in a vacuum chamber for 308 hours.. General conclusions Based on our studies, the best Ohmic contacts found so far to n-type SiC was annealed Ni, which had a lowest specific contact resistance of ≈ 8 × 10-6 Ωcm2 and a good reproducibility. However, for the device application point of view such as long-term reliability and complete metallization schemes (that is a contact layer, a diffusion barrier layer, and a wire-bond compatible conducting layer), Ni show a lack of reliability and manufacturability due to its rough surface after high-temperature annealing, while sputtered titanium tungsten (TiW) has much more flexibility instead. TiW can be applied for n-type as well as p-type contacts to SiC with a low specific contact resistance (≈ 10-4 – 10-5 Ωcm2), is compatible to the wire-bonding, and is very inert with respect to reaction with SiC under high-temperature due to its high melting point. TiC is also a promising material for low resistivity n- and p-type metallization on highly doped silicon carbide using a co-evaporation method even though it is still under development due to the lack of reproducibility, the limit of sample size, and slow deposition rate. Future work Finally, a more detailed study of Ohmic contact formation on ion implanted layers of SiC is needed since ion implantation can be used to make selective doping and it has many advantages even though its process is not fully optimized in SiC technology. The Ohmic contact formation correlated with Al and B implanted layers should be investigated more. In addition, as we considered three different methods of metal deposition, the sputter deposition should be highlighted because it offers many advantages as we discussed previously. One main reason is that sputter deposition can be used to deposit refractory materials, which is useful for long-term reliability, since such materials are often difficult to evaporate, and hence sputtering may be the best practical way.

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