The Pennsylvania State UniversityMOLYBDENUM DISULFIDE MOSFETS
A Thesis in
for the Degree of
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The thesis of Yosuke Endo was reviewed and approved by the following:
Suzanne E. Mohney Professor of Materials Science and Engineering Professor of Electrical Engineering Thesis Advisor
Thomas N. Jackson Robert E. Kriby Chair Professor of Electrical Engineering and Computer Science
Saptarshi Das Assistant Professor of Engineering Science and Mechanics
John Mauro Professor of Materials Science and Engineering Chair of the Intercollege Graduate Degree Program in Materials Science and Engineering
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ABSTRACT
Since the use of Si for densification for integrated circuit in metal-oxide-semiconductor field effect
transistors (MOSFETs) is approaching its limits, two-dimensional (2D) materials with interesting properties
are drawing attention; however, the contact resistance of the 2D FETs is still higher than the value for
practical use. In this thesis, we proposed and tested two ideas to reduce the contact resistance of MoS2
MOSFET.
The first study in this research was to utilize an electron beam (EB) to change the properties of
MoS2 by forming defects. We irradiated the EB on the contact region of Au contact MoS2 FETs with various
dose conditions (300 ~ 10000 μC/cm2), and investigated the electrical properties. Some variation was
included in the contact resistance measured by the transfer length method (TLM) due to the exfoliation of
MoS2 flakes. Therefore, we made another configuration to compare the various EB irradiated devices
fabricated on the same MoS2 flake to mitigate the variation between exfoliated flakes. As a result, we found
EB irradiation of 1000 μC/cm2 on the contact region slightly increased the electrical conduction compared to
300 μC/cm2 of EB irradiation or had little negative effect on the contact region at least. On the other hand,
too much EB irradiation on the contact region just degraded the electrical conduction, possibly due to lattice
scattering, and affected the threshold voltage. Also, intentional EB irradiation on the channel region degraded
the electrical properties even in small doses such as 500 μC/cm2. We found the appropriate amount of EB
irradiation may improve the contact but degrade the channel.
The second study in this thesis was to propose and verify the new idea of the hybrid contact,
initially proposed by Walter et al. The concept of the hybrid contact is to utilize the edge contact by increasing
the contact area to reduce the total contact resistance. The first attempt of the hybrid contact was to find the
appropriate metal mask for Walter’s hybrid contact idea since the Au etch mask they used deteriorates during
the Cl2 etching step. We tried Pt as an etch mask and verified the controllability of the mask shape and
durability during the etching. Furthermore, we tried a Ti/Au hybrid contact, designed to the reaction between
Ti and MoS2. We used the Au film with interconnected islands as a mask on MoS2 and deposited Ti on it.
We expected to form TixSy pillars in the MoS2, resulting in an increase of the contact area from TixSy and the
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MoS2 interface. We made the FETs to compare the electrical conductance and found one of the Ti/Au hybrid
contact FETs before annealing had slightly better conductivity than Au or Ti contact FETs. However, TEM
observation showed that Ti did not exist between the Au islands and reacted with MoS2 after the annealing,
and the hybrid contact we designed was not obtained through our fabrication process. Therefore, we could
not judge if our idea was practical. Thus, we need to consider the new fabrication process for further study.
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1.1 Background .............................................................................................................. 1 1.2 Molybdenum disulfide (MoS2) ................................................................................. 4 1.3 Metal-Semiconductor contacts ................................................................................. 6
1.3.1 Ohmic contact and Schottky contact .............................................................. 6 1.3.2 Fermi level pinning ........................................................................................ 8 1.3.3 Conduction mechanisms and current-voltage relationship for Schottky
cotacts ............................................................................................................. 9 1.3.4 Electrical measurement of contact resistance ............................................... 13 1.3.5 Previous studies of MoS2-based MOSFET for reduction of contact
resistance ...................................................................................................... 16 1.3.6 Defects in TMDs and impact on the electrical properties ............................ 21
1.4 Motivation and Goal............................................................................................... 26
Chapter 2 Defect engineering of MoS2 MOSFET by electron beam .................................... 28
2.1 Defect engineering concept for the reduction of contact resistance of MoS2 MOSFET ................................................................................................................ 28
2.2 Fabrication process of EB-irradiated MoS2 MOSFET ........................................... 29 2.3 Formation of defects on monolayer MoS2 by EB irradiation and
characterization ...................................................................................................... 32 2.4 Electrical characterization of EB-irradiated MoS2 MOSFET by TLM .................. 36 2.5 Evaluation of EB-irradiated FETs with a mitigating variation of exfoliated
flakes ...................................................................................................................... 47 2.6 EB irradiation effect on the channel region of MoS2 MOSFET............................. 50 2.7 Summary ................................................................................................................ 53
Chapter 3 Ideas for improvements of hybrid contact MoS2 MOSFET ................................. 54
3.1 Utilizing of Pt as an etch mask on MoS2 for Cl2 based plasma etching ................. 54 3.2 Film structure control of Au film for Ti /Au hybrid contacts................................. 59 3.3 Fabrication process of Ti/Au MoS2 MOSFET ....................................................... 66 3.4 Electrical characterization of Ti/Auhybrid contact MoS2 MOSFET ...................... 67 3.5 Summary ................................................................................................................ 73
Chapter 4 Summary and future work .................................................................................... 75
Appendix A Fabrication process of Mn contact MoS2 MOSFET and findings .................... 78
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LIST OF FIGURES
Figure 1-1: Moore's law on the exponential increase of transistors per chip (for IntelTM processor chips). Dashed line corresponds to doubling in 20 months. Image from Grundmann et al. [1] ................................................................................... 3
Figure 1-2: Complementary Metal Oxide Semiconductor (CMOS) structures composed of p-type MOSFET and n-type MOSFET for Moore’s Law continuation. Adapted from [86]. .............................................................................................. 4
Figure 1-3: Three polymorphous of MoS2, 1T (Tetragonal, metallic), 2H (Hexagonal, semiconductor), and 3R (Rhombohedral, semiconductor). Images from Toh et al. [8] ............................................................................................................... 5
Figure 1-4: Metal – n-type semiconductor contacts according to the Schottky Mott theory before and after contact. describes an electron. ............................................... 7
Figure 1-5: Metal – p-type semiconductor contacts according to the Schottky Mott theory before and after contact. describes a hole. ....................................................... 8
Figure 1-6: The schematic image of Thermionic Emission, Thermionic/Field Emission, and Field emission for the metal – n-type semiconductor contact. Images from Schroder. [17] .................................................................................................... 10
Figure 1-7: Schematic of current flow through the metal-2D material interface and its band diagram. Adapted from [88]. ............................................................................. 13
Figure 1-8: The global back gate structure of the MoS2 MOSFET and its resistances. ........ 14
Figure 1-9: The test structure of TLM and the plot of total resistance (RT) as a function of the gap (d), showing contact resistance (Rc) and sheet resistance (Rsh). ............ 15
Figure 1-10: Schematic model of current flow from semiconductor to metal and an equivalent circuit model represented by Rsh and ρc. Adapted from Schroder [17]. ................................................................................................................. 15
Figure 1-11: MoS2-based MOSFET structure proposed in the previous study. (a) ~ (d) belong to the top contact structure, and (e) ~ (g) belong to the edge contact structure. ......................................................................................................... 17
Figure 1-12: Fabrication process and device structure of the "hybrid contact" MoS2
MOSFET. Cited from Walter et al. [41] ......................................................... 20
Figure 1-13: STM images of the surface of MoS2 and depth profiles of the lines in STM image, displaying the existence of the sulfur vacancy. Images from Vancsó et al. [45]. ........................................................................................................ 22
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Figure 1-14: Electron-induced damage on MoS2 due to “knock on” at the accelerating voltage are 120 kV and 200 kV. Images from Gracia et al. [49] ...................... 22
Figure 1-15: Local Density of States at several types of defects with or without Au atom in monolayer MoS2, showing defect states in the bandgap. Images from Hus et al. [46]. .......................................................................................................... 24
Figure 1-16: 3D STM image of MoS2 surface showing higher conductivity at defect region. Image from McDonnell et al. [47]. ................................................................... 24
Figure 1-17: (a) Cross-sectional schematic of the WSe2 flake with Se defects on the H2 plasma exposed region and KPFM image showing the doped and undoped region. (b) Total resistance vs gate voltage of the WSe2 MOSFET showing significant Rc decreasing by H2 plasma. Images from Tosun et al. [68] ........... 25
Figure 1-18: Contact resistance for different values of channel sheet resistivity for top- contacted TMDs devices. Image from Allain et al. [69]. .................................. 27
Figure 2-1: Expected device structure of EB-irradiated MoS2 MOSFET and the schematic band diagram showing thermionic field emission and field emission of the electrons. ............................................................................................................. 29
Figure 2-2: The fabrication process of EB irradiated MoS2 MOSFET with global back gate structure. The process for the adhesion metal was omitted from this figure. ...... 31
Figure 2-3: The image of a written pattern of the adhesive layer, radiation region, and contact metal in L-edit software. ......................................................................... 31
Figure 2-4: The microscopic images of irradiated area (10 x 10 μm) monolayer MoS2. The darker square area corresponds to the irradiated area. ......................................... 32
Figure 2-5: PL spectrum of monolayer MoS2. From the top, ref. 500 μm/cm2, 1,000 μm/cm2, and 10,000 μm/cm2. .............................................................................. 34
Figure 2-6: The intensities of the PL peaks of each dose condition. ...................................... 34
Figure 2-7: Raman spectrum of monolayer MoS2. From the top, reference, 500 μm/cm2, 1,000 μm/cm2, and 10,000 μm/cm2. .................................................................... 35
Figure 2-8: The intensity and width of the peaks of E1 2g and A1g of Raman spectrum. .......... 35
Figure 2-9: Electrical measurement conditions for EB-irradiated FETs. ............................... 37
Figure 2-10: Microscopic images of the example FETs with 10000 μC/cm2 total EB doses for each step, after exfoliation, after EB irradiation (doses = 9700 μC/cm2), and after metal deposition (Total doses = 10000 μC/cm2, including metal contact patterning). The red arrows in the center image are guides for the irradiation areas. S and D in the right image stands for source and drain in TLM method...................................................................................................... ..37
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Figure 2-11: IDS-VDS characteristics at various VGS (-20 V to +20 V, steps of 5 V) of the typical MoS2 FETs with the various EB dose irradiation (300, 1000, or 10000 μC/cm2) at the channel length of 300 nm. ............................................................................... 39
Figure 2-12: IDS-VDS characteristics at various VGS (-20 V to +20 V, steps of 10 V) of the typical MoS2 FETs with the various EB dose irradiation (300, 1000, or 10000 μC/cm2) at the channel length of 300 nm. ............................................................................... 40
Figure 2-13: IDS-VGS characteristics (log plot) of the typical MoS2 FETs with the various EB dose irradiation (300, 1000, or 10000 μC/cm2) at different channel lengths. ..................... 41
Figure 2-14: Images of the trap states at the negative or positive gate voltage. ............................... 41
Figure 2-15: IDS-VGS characteristics of the typical MoS2 FETs with the various EB dose irradiation (300, 1000, or 10000 μC/cm2) at different channel lengths. We can see the threshold voltage difference between the channel length (300, 500, 1000 nm) as ΔVth. ............... 42
Figure 2-16: Contact resistance of all the FETs we measured as a function of 2D carrier density of the channel. The number in the guide means the EB dose, and A ~ H shows the different device sets for TLM. i.e., 300 μC-A~H (blue) means the total EB dose of the device was 300 μC/cm2, and 8 different FETs (A ~ H) were measured. ............................................................................................... 44
Figure 2-17: Rt·W vs. channel length plot at the over drive voltage of +9 V. ......................... 45
Figure 2-18: The relation between the number of layers of MoS2 flake and contact resistance at a 2D carrier density is 3.1 x 1012 [1/cm2]. .................................... 46
Figure 2-19: The number of layers dependence of contact resistivity. Image from Li et al. [84] ...................................................................................................................... 46
Figure 2-20: Microscopic images of fabricated FETs with 300 nm channel length. (a) 300 μC/cm2 irradiated FET and 1000 μC/cm2 irradiated FET on flake 1. (b) 300 μC/cm2 irradiated FET and 10000 μC/cm2 irradiated FET on flake 2. ............. 47
Figure 2-21: (a) IDS-VDS curves of 300 μC/cm2 irradiated FET and 1000 μC/cm2 irradiated FET at the VOD are +17.5 V and +17 V, respectively. (b) 300 μC/cm2 irradiated FET and 10000 μC/cm2 irradiated FET at the VOD of +13 V and +14 V, respectively. .......................................................................................... 49
Figure 2-22: (a) IDS-VGS curves of 300 μC/cm2 irradiated FET and 1000 μC/cm2 irradiated FET at the VDS = +0.1 V. (b) 300 μC/cm2 irradiated FET and 10000 μC/cm2 irradiated FET at the VDS = +0.1 V. .................................................................. 49
Figure 2-23: The back gate MoS2 MOSFET structure for EB irradiation on the channel. ..... 50
Figure 2-24: (a) IDS-VGS curves of the EB irradiated FET at the cumulative doses are 0, 500, 1000, 5000, and 10000 μC/cm2. (b) Vth of the EB irradiated FET as a function of cumulative doses. ........................................................................... 53
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Figure 2-25: (a) IDS-VDS curves of the EB irradiated FET at the cumulative doses are 0, 500, 1000, 5000, and 10000 μC/cm2. (b) Effective mobility of the EB irradiated FET as a function of cumulative doses. ............................................ 52
Figure 3-1: Au film on MoS2 before (a) and after (b) Cl2 plasma (Au thickness: 6nm, 100% Cl2 at 25 for 10s at 40 mTorr and 45 SCCM with the plasma power of 50W and chuck bias power of 20W., all the scale bars are 200 nm). Images from Walter et al. [41]. ................................................................................................. 55
Figure 3-2: SEM images of deposited Pt film on MoS2 with various deposition conditions. Sample 1~3 show the dependence on the thickness, and Sample 4~6 show the dependence on the base pressure. The squares (35 nm x 200 nm) in SEM indicates the minimum transfer length (35 nm) of MoS2 device from [24]. ....... 57
Figure 3-3: Pressure changes during heating source metal process at various base pressure. The pressure at 0 mA shows the base pressure. Around 180 to 200 mA are appropriate current for EB evaporation of Pt. ..................................................... 58
Figure 3-4: SEM images of Sample 2 before and after Cl2 based plasma etching (100% Cl2 at 25 for 10s at 40 mTorr and 45 SCCM with the plasma power of 50W and chuck bias power of 20W). .................................................................................. 59
Figure 3-5: The diffusion of Ti atoms into MoS2. (a) device structure of Ti/Au contact MoS2 MOSFET. (b) and (c) TEM images of the interface between Ti and MoS2. (d) STEM image of the interface and EELS results showing the Ti fraction of each layer. (e) Reaction between Ti and MoS2 showing the Ti atom penetration into the MoS2. All images from [76]. ............................................... 61
Figure 3-6: The device structure of Ti/Au hybrid contact MoS2 MOSFET and expected electron path circuit. The contact resistance value of the top contact of Au or TixSy (Ti contact) is the value from [30]. ............................................................ 62
Figure 3-7: SEM images of Ti films with different film thickness on MoS2. ......................... 63
Figure 3-8: The device structure of Ti/Au hybrid contact MoS2 MOSFET and electron path circuit. .......................................................................................................... 64
Figure 3-9: SEM images of Au films on MoS2 at different annealing temperature. The image of 110 annealing films contains 10 nm scale bar at the lower left corner for the comparison. ................................................................................... 65
Figure 3-10: Cross-sectional SEM images of the 110 annealed Au film after 30 nm of Ti deposition. .................................................................................................... 65
Figure 3-11: Fabrication process of Ti/Au hybrid contact MoS2 MOSFET. .......................... 67
Figure 3-12: (a) The appearance of the set of FETs (Ti/Au hybrid contact, Au contact, Ti contact) with 110 annealed Au mask for Ti/Au hybrid contact. (b) The appearance of the set of FETs (Ti/Au hybrid contact, Au contact, Ti contact)
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with as-depo Au mask for Ti/Au hybrid contact. The SEM images at the lower left of the optical microscopic image shows the Au mask structure deposited on dummy MoS2 flakes. The coverage of the Au are 74% for (a) and 70% for (b). ................................................................................................ 68
Figure 3-13: (a) shows the comparison of the IDS-VDS curves of the series of FETs including Ti/Au FET with 110 annealing Au mask, and (b) shows the IDS-VDS curves after 300 annealing. (c) shows the comparison of the IDS-VDS curves of the series of FETs including Ti/Au FET with as-depo Au mask, and (d) shows the IDS-VDS curves after 300 annealing. ................................ 71
Figure 3-14: TEM images of Ti/Au hybrid contact FETs with Au mask annealed at 110 after 300 annealing. The sample preparation and TEM observation (Titan G2) were operated by Trevor Clark and Ke Wang .. ........................................... 72
Figure 3-15: HAADF image and EDS mapping of Ti/Au hybrid contact FETs Au mask annealed at 110 after 300 annealing. The sample preparation and TEM observation (Titan G2) were operated by Trevor Clark and Ke Wang.. ............. 73
Figure A-1: The device structure of Mn/Ti/Ag/SiO2 contact MoS2 MOSFET device. .......... 79
Figure A-2: Unintentional crystal growth between electrodes after lift-off at the EB patterning doses of 300 μC/cm2........................................................................... 79
Figure A-3: Optical microscopic image and SEM image of Mn/Ti/Ag/SiO2 electrodes. ....... 81
Figure A-4: EDS mapping of Mn/Ti/Ag/SiO2 electrodes. ...................................................... 81
Figure A-5: The device structure of Mn/Al contact MoS2 MOSFET device. ......................... 82
Figure A-6: Optical microscopic image of Mn/Al contact MoS2 MOSFET. .......................... 83
Figure A-7: SEM images of the particles on MoS2 flakes at Mn/Al contact devices. ............ 83
Figure A-8: EDS mapping of the particles on MoS2............................................................... 83
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LIST OF TABLES
Table 2-1: EB patterning condition for monolayer MoS2 for defect creation. ....................... 33
Table 3-1: Energetic and Structural Parameters for the Adsorption of the Metals on MoS2(001)a Eads. δE1 and δE2 are the adatom diffusion barriers between two η4 sites along the path through η1 and η6 sites respectively. The table is adapted from Saidi [73].. ................................................................................................... 55
Table 3-2: EB evaporation condition of Pt film deposition. ................................................. 56
Table A-1: EB evaporation current for Mn contact and other metals in Edwards EB evaporation system. The deposition rate is 1.0 /s. ............................................. 80
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ACKNOWLEDGEMENTS
I would like to thank:
My thesis advisor, Dr. Suzanne Mohney for all her support not just for academics but for my life at Penn State during this challenging year.
My thesis committee, Dr. Thomas Jackson and Dr. Saptarshi Das, for taking their time to read my thesis and giving me advice and precious knowledge.
Mohney research group helping with my experiments and sharing their knowledge. I would
like to especially thank Timothy Walter for his guidance with the fabrication process and electrical characterization for 2D MOSFET, Nailah Oliver for sharing her data and valuable discussion, Ama Agyapong for assisting me with Raman and PL measurement, Ian Campbell for assisting me with ALD, and Alex Molina for assisting me with EB evaporation.
Redwing research group for offering 2D materials.
All of my friends in MATSE, Nanofab staff, and MCL staff for their friendship and technical
support.
Mr. Nakamura and JX Nippon Mining and Metals Corporation for allowing me to study in this valuable environment and for their generous support.
My parents, Seishi and Sadako Endo, and my parents-in-law, Toshihiro and Yasuko Kudo, for
their warm support and understanding during my single stay in the United States.
Lastly, my precious wife, Kanako Endo, and my wonderful daughters, Akiho and Michika Endo for all of their love and patience throughout my study in the United States.
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large-scale integrated circuits mostly using silicon-based semiconductor technology. The
densification of the integrated circuits with metal-oxide-semiconductor field effect transistors
(MOSFETs) has progressed for faster and cheaper computing along with Moore's law (Figure 1-1),
which is an empirical observation about the densification of the circuits, with improved
nanofabrication technology. Moreover, the Dennard rule shows the improvement in operating
speed and the reduction in power consumption due to the miniaturization of transistors [2]. The
rule shows the principle of constant-field scaling, which means the device dimensions and the
device voltages should be scaled down such that the electric fields remain essentially constant. As
the miniaturization of MOSFETs progressed, the channel length and gate oxide layer thickness
were getting smaller and smaller. Due to the miniaturization, phenomena different from the
characteristics predicted for long-channel MOSFETs have been observed, such as short-channel
effects [16], gate oxide tunneling [2], and leakage current between the source and drain [3]. To
suppress these phenomena, halo doping [85], high dielectric constant gate oxides (e.g., HfO2 [2]),
and new MOSFET structures such as silicon on insulator (SOI) and FinFETs have been introduced,
and the silicon-based MOSFET technology continues to evolve even now [3]. Figure 1-2 shows
the complementary metal oxide semiconductor (CMOS) structures composed of p-type MOSFETs
and n-type MOSFETs for Moore’s Law continuation [86]. The FinFET is a current standard
MOSFET structure, and the channel is covered by a gate from three directions, leading to more
gate controllability and less current leakage than planar MOSFETs. Then, the gate-all-around FET
(GAAFET) will be introduced into mass production using Si nanowires or Si nanosheets. The
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channel in the GAAFETs is covered by the gate from all directions, resulting in better gate control.
In the GAAFETs, Si is still used as a semiconductor material in the form of wire or sheet. The
miniaturization of the Si-based device will be limited by the quantum confinement effect, which
increases the bandgap of the semiconductor depending on size, especially as size reaches a few
nanometers. For example, the bandgap of the Si nanocrystal increases from 1.1 eV (bulk Si) to 3.02
eV when the diameter was decreased to 2.6 nm [87], which can no longer be used as a
semiconductor. Therefore, new materials with the appropriate bandgap for sizes on the order of a
few nanometers are needed to continue the development of a high-performance transistor according
to Moore’s Law. One family of promising materials is transition metal dichalcogenides (TMDs)
with the chemical formula of MX2 (where M represents the transition metal: Mo, W, Hf, Zr, Sn Re,
etc. and X represents the chalcogen: S, Se, Te). TMDs have a two-dimensional (2D) structure and
can show metallic or semiconducting properties depending on their phases. Since they have a
suitable bandgap for a semiconductor, even with a thickness of a few nanometers, they can be used
as materials for the next generation of circuits integrated with silicon MOSFETs. The expected 2D
material stacked MOSFET structure is drawn in Figure 1-2. This structure allows further
densification of the transistor vertically. In addition, since the extremely thin film can be made from
the TMDs, they are also expected to be implemented on flexible devices [4], where it is challenging
to use Si substrates. Also, due to its large surface area per volume, it is expected to be applied to
catalysts for hydrogen evolution [5], batteries [6], and gas sensors [7]. In this study, we focus on
MoS2 since it is one of the most popular semiconducting TMDs.
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Figure 1-1: Moore's law on the exponential increase of transistors per chip (for IntelTM processor chips). Dashed line corresponds to doubling in 20 months. Image from Grundmann et al. [1]
Figure 1-2: Complementary metal oxide semiconductor (CMOS) structures composed of p-type MOSFET and n-type MOSFET for Moore’s Law continuation. Adapted from [86].
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1.2 Molybdenum disulfide (MoS2)
MoS2 consists of molybdenum and sulfur, and there are three polymorphous: 1T with
metallic properties, and 2H and 3R showing semiconducting characteristics (Figure 1-3 [8]). In
addition, MoS2 has a layered structure and is bound by covalent bonds within the layers and van
der Waals bonds between the layers. Therefore, it is easy to peel between the layers, and it has been
generally used as a lubricant for a long time due to its weak van der Waals bonding. In 2004,
Novoselov et al. [9] succeeded in delaminating graphene using Scotch tape. Similar to graphene,
MoS2 also has been studied as an electronic material due to the successful exfoliation of a single or
a small number of layers. The bandgap of 2H-type MoS2 is 1.88 eV (direct transition bandgap) for
monolayer and 1.35 eV (indirect transition bandgap) in bulk [88], and it is also possible to adjust
the band gap by alloying [10]. Due to the crystal symmetry, the 2H type has remarkably different
conductivity in the a, b-axis direction, and c-axis direction, with 0.16 to 5.12 1/Ωcm and 1.02 to
5.89 x 10-4 1/Ωcm, respectively [11]. Several layers of MoS2 can be transparent in the visible light
range, and it is expected to utilize the transparent and flexible characteristics for the transparent
and flexible devices such as transparent displays [12].
In 2011, Radisavljevic et al. fabricated a MOSFET using a monolayer MoS2 as a
semiconductor and hafnium oxide as a gate dielectric and demonstrated room-temperature mobility
of above 200 cm2/Vs with the current on/off ratio to be 1 x 108 [13], comparable to the thin silicon
films. Moreover, electron mobility of 517 cm2/Vs and on/off ratio of 108 was achieved using a
back-gated SiO2/MoS2 MOSFET with Al2O3 top layer for thicker MoS2 film [14]. Even though
these mobility values were obtained in the relatively long channel length devices (1.5 μm and 9 μm,
respectively), which typically show good value [89][91], the values are still promising. These
studies regarding the use of MoS2 as a semiconductor material shed light on the practical use of
MoS2 for the next generation MOSFET.
5
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Metal-semiconductor contacts play an important role in MOSFETs, especially for short
channel devices used in the modern industry, because considerable contact resistance could
dominate the total resistance or mobility, resulting in poor performance of MOSFETs. There are
two types of metal-semiconductor contacts, which are ohmic contacts and Schottky contacts.
An ohmic contact is an ideal contact for source/drain contacts and has linear current-
voltage characteristics. In modern Si MOSFETs, the ohmic contacts are realized by forming a
heavily doped region under the contact metal by ion implantation, increasing tunneling current.
Therefore, the voltage drop across the contact is relatively tiny compared to the voltage drops across
the channel regions.
Schottky contacts are often explained by the Schottky-Mott theory. In the Schottky-Mott's
theory, we assume that contact between metal and semiconductor is an ideal contact and has no
interfacial layer. Figure 1-4 shows the energy band diagrams of metal – n-type semiconductor
contacts with different work functions of the metals (Φm) before and after contact, for the conditions
of Φm < Φs, Φm = Φs, Φm > Φs. The work function of a solid (Φs) is defined as the energy gap
between the vacuum level (EVAC) and the Fermi level (EF), and the small letters' m' and 's' mean
'metal' and 'semiconductor', respectively. χs is the electron affinity of the semiconductor given by
the energy difference between EVAC and the bottom of the conduction band (EC). When metal and
semiconductor contact electrically, the Fermi level of both metal and semiconductor corresponds
to each other by moving of carriers (electrons and holes), and the left-behind fixed ion generates
an electric field, forming a depletion region at thermal equilibrium. Therefore, in the energy band
diagram, the bands near the interface are bent by the electric field. There are three types of energy
barriers, as shown in Figure 1-4, which are named "accumulation," "neutral," and "depletion."
Ideally, accumulation type contacts correspond to the ohmic contact because the Schottky barrier
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height is the smallest among the three, resulting in easier electron transport from metal to
semiconductor. In the case of the metal – p-type semiconductor contact (Figure 1-5), accumulation
type contacts show the ohmic contact as well because the holes in the metal experience a small
energy barrier.
Figure 1-4: Metal – n-type semiconductor contacts according to the Schottky Mott theory before and after contact. describes an electron.
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1.3.2 Fermi level pinning
The ideal Schottky barrier height (Φb) mentioned in section 1.3.1 is described as the
following equation by the Schottky Mott theory.
= − (Eq. 1-1)
According to the equation, the Schottky barrier height only depends on the work function
of the contact metal and the electron affinity of the semiconductor; however, it is challenging to
alter the Schottky barrier height by using contact metals with different work function. In reality,
the Schottky barrier height is only weakly dependent on the work function of the metal, and this
phenomenon is observed for the common semiconductor materials with a strong tendency for
covalent bonds [1] (i.e., n-Si, n-GaSb [18], n-Ge [19], n-InGaAs [19]) and also TMDs (i.e., MoS2
[20]). The reason for the deviation from the theory is known as Fermi level pinning (FLP), where
Figure 1-5: Metal – p-type semiconductor contacts according to the Schottky Mott theory before and after contact. describes a hole.
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the Fermi level in the semiconductor is pinned at some energy level in the bandgap. The degree of
the FLP named pinning factor (S) is defined by the following equation.
=
(Eq. 1-2)
If there is no FLP, the pinning factor equals 1. On the other hand, if the degree of FLP is
high, the pinning factor approaches close to 0. FLP is induced by interface states between metal
and semiconductor because of the crystal imperfection due to dangling bonds, defects, an oxidized
layer, chemical bonds, diffusion, absorbates, impurities, strain, etc. [16][17][20]. In the case of
TMDs, there are fewer dangling bonds on the ideal surface because of their 2D crystal structure,
while there are many dangling bonds on the surface of other common semiconductors such as Si.
However, the defects in TMDs such as chalcogen vacancies could be caused by atom bombardment,
back-scattered electrons, X-ray during the fabrication process, or even exposure to the air
[20][21][22]. Therefore, TMDs typically show low pinning factor (i.e., S = 0.09 for metal-MoS2
contacts fabricated by electron beam evaporation [20]).
Metal induced gap states (MIGS) are another factor in the Fermi level pinning, which
occurs from the extended wavefunction from the metal. The wave function perturbs the original
wave function of the semiconductor, resulting in the metal states in the bandgap in the
semiconductor. This also affects the position of the Fermi level.
1.3.3 Conduction mechanisms and current-voltage relationship for Schottky contacts
In micro-scale 3D semiconductors, there are three types of conduction mechanisms for
Schottky contact, which are Thermionic Emission, Thermionic/Field Emission, and Field Emission
depending on the doping concentration (ND or NA) of the semiconductor. For the lightly doped
semiconductors, the current flows by electrons thermally going over the barrier height (Thermionic
Emission (TE), Figure 1-6 (a)). For the intermediate doped semiconductors, the current flows by
10
tunneling mechanism assisted by the thermally excited electrons as shown in Figure 1-6 (b),
Thermionic/Field Emission (TFE). In the case of the high doping concentration, the depletion width
is narrow enough for the electrons to penetrate by the tunneling mechanism, so the Field Emission
(FE) mechanism dominates the conduction (Figure 1-6 (c)). In the case of the Si-based MOSFET,
ion plantation or diffusion can be used to form a local high doping concentration region; however,
it is still challenging for TMD thin films to form the high doping concentration region that can
work as equivalent as the n+ (or p+) region in the Si-based MOSFET because limited doping
techniques are available for TMDs.
As discussed in 1.3.2, we often have a Schottky barrier at the metal-semiconductor
junction due to the Fermi level pinning. For lightly doped semiconductors, thermionic emission
dominates the current. The net current density J is described as the following equation, assuming
the barrier height is much larger than kT for using Maxwell-Boltzmann approximation.
= ∗2exp − b exp
− 1 (Eq. 1-3)
Figure 1-6: The schematic image of Thermionic Emission, Thermionic/Field Emission, and Field emission for the metal – n-type semiconductor contact. Images from Schroder. [17]
11
A* is called the effective Richardson constant for thermionic emission (= 120 (m*/m) [A/cm2K2]),
and n is the ideality factor. m* n is the effective mass of the electron and is approximately 10m for
the direction along the c-axis, and 0.01m for the direction perpendicular to the c-axis in the case of
MoS2 [25]. In the case of 2D materials, the net current density J2D is described as the following
equation [93].
− 1 (Eq. 1-5)
2 (Eq. 1-6)
We should also consider image force lowering, which modifies the Schottky barrier height as a
function of source-drain voltage (E). Then, we can write JST as below.
= ∗2exp − b exp
(Eq. 1-7)
(Eq. 1-8)
Next, we consider the tunneling current through the Schottky barrier. The depletion width
(W) of the Schottky barrier to an n-type semiconductor is the function of the donor concentration
(Nd) and is written as below.
= 2sbi+applied d
(Eq. 1-9)
Thus, as the Nd increases, the depletion width becomes narrower, and the probability of tunneling
through the barrier increases. The tunneling current density (Jt) increases exponentially with Nd and
is written as the below equation.
∝ exp − b 00

∗ (Eq. 1-11)
In the case of 2D FET, the tunneling current can be calculated by the following equation
[88].
(Eq. 1-13)
S (Eq. 1-14)
where h is Planck’s constant, m* is the electron tunneling effective mass, f (E) is the Fermi-Dirac
distribution of the contact metal, M2D (E) is the number of 2D conducting modes in the
semiconductor channel, ψS is the surface potential, TWKB (E) is the Schottky barrier transmission
probability, and λSB is the Schottky barrier tunneling width. In the case of ultra-thin body devices,
λSB can be calculated by the below equation.
SB = body OX
body (Eq. 1-15)
where εbody and εox are the dielectric constants of the semiconducting channel and gate oxide
respectively and, the tbody and tox are the thickness.
We also should mention the model of the conduction mechanism proposed for the global
back gate structure of 2D FET [88][98]. In the 2D FET, the thickness of the semiconductor is often
less than the depletion width, and it is expected that the conduction mechanism is different from
the conventional model. Figure 1-7 shows a schematic of the top and edge contact of the metal-2D
material junction, and the band diagram of the global back gate structure FET, showing three types
of current flow paths. Φb is a Schottky barrier height, λSB is the tunneling barrier width, tb is the
thickness of the body. Path-1 is the current flow from top contact metal assuming TFE, and it
dominates the current when the tb is larger than λSB. When the thickness of the body is thinner than
the depletion width, path-2 dominates the current. The carrier is injected from the contact metal,
13
and the electrons are electrostatically induced by the gate bias, resulting in dominating current.
Path-3 is an edge contact. The carrier is injected into each layer directly without passing through
the van der Waals barrier between layers, which is expected to be efficient in-plane carrier injection.
1.3.4 Electrical measurement of contact resistance
Figure 1-8 shows the schematic diagram of a global back-gate structure of MoS2
MOSFET used in this research. The circuit can be separated into two sections in terms of the type
of resistance, which are the contact resistance (Rc) [Ω] and the semiconductor sheet resistance (Rsh)
[Ω/square]. We can ignore the resistance of the contact metal since the value is usually quite small
compared to the other resistances. The total resistance (RT) of this device in Figure 1-8 is described
as the following equation.
T = 2c +
(Eq. 1-16)
where d is the gap between the contacts and W is the width of the semiconductor.
Figure 1-7: Schematic of current flow through the metal-2D material interface and its band diagram. Adapted from [88].
14
In general, a contact is characterized by two quantities, which are contact resistance (Rc)
[Ω] and specific contact resistance (ρc) [Ωcm2], which includes specific interfacial resistance (ρi)
and the resistances right above and right below the interface (a part of the metal right above the
interface, a part of the semiconductor right below the interface, and any interfacial layer between
the metal and semiconductor contact. It is difficult to separate the specific interfacial resistance (ρi)
from the measured specific contact resistance (ρc). Therefore, we use and discuss the specific
contact resistance (ρc). For the measurement of the contact resistance (Rc), specific contact
resistance (ρc) and sheet resistance (Rsh = bulk resistivity (ρ) / film thickness (t)) [Ω/square], the
transfer length method (TLM) is generally used for 3D materials. The simple test structure of TLM
is shown in Figure 1-9. Typically, more than three different gaps (d) between contacts are used.
One can calculate contact resistance (Rc) and sheet resistance (Rsh) by plotting total resistance (RT)
as a function of d as shown in Figure 1-9.
When current flows from the semiconductor to the metal, the current flows through the
path with the lowest resistance. This model is described schematically in Figure 1-10, using Rsh, ρc,
transfer length (LT), and the distance from the edge of the contact, x. The potential distribution as a
function of the distance from the edge of the contact pad, x, is calculated by the following equation.
() = shc
cosh[(−) ⁄ ] sinh( ⁄ ) (Eq. 1-17)
Figure 1-8: The global back gate structure of the MoS2 MOSFET and its resistances.
15
Then, we can calculate the LT by the following equation.
T = c sh⁄ (Eq. 1-18)
We should note that we assume that the Rsh is uniform in the simple transmission line
model as described in Figure 1-9; however, the assumption cannot be applied to the back gate
structure since the Rsh under the contact (Rsh, contact) is different from the Rsh of the channel (Rsh, ch)
as described in Figure 1-10 due to the electrostatically induced electrons from the gate bias.
Therefore, we can not determine the LT accurately for this test structure. Even though we cannot
calculate the exact value of the LT, the concept of the LT is still important, especially for the ultra
Figure 1-9: The test structure of TLM and the plot of total resistance (RT) as a function of the gap (d), showing contact resistance (Rc) and sheet resistance (Rsh). This simple model requires modification to apply to 2D gated MOSFETs.
Figure 1-10: Schematic model of current flow from semiconductor to metal and an equivalent circuit model represented by Rsh and ρc. Adapted from Schroder [17].
16
small scale MOSFET. We can think of the LT as the distance that most of the current transfers from
the semiconductor into metal and vice versa. The length of contact beyond x ≈ 1.5 LT is not effective
for conduction. If the device is very small scale and contact length (L) is reduced below 1.5 LT,
contact resistance (Rc) increases. In the case of MoS2 MOSFET, the value of LT is reported as 630
to 1260 nm for single-layer MoS2 [23] and 35 nm for 4.5 nm thickness of MoS2 fabricated using
ultra-high vacuum metal deposition [24]. The LT in MoS2-based MOSFETs is not a constant
number as both Rsh and ρc at the contact regions are dependent on the gate bias. For the operation
of the MoS2-based MOSFET, we have an accumulation of electrons at the MoS2 channel by
controlling the gate bias, reducing the Rsh. The gate bias also leads to a narrower Schottky barrier,
also reducing the ρc. Therefore, LT in the MoS2-based MOSFET is also modified by the gate bias,
and the switching of MoS2 could be achieved by tuning the Schottky barrier width or the effective
Schottky barrier height rather than the accumulation of the channel of MoS2 [23].
1.3.5 Previous studies of MoS2-based MOSFET for reduction of contact resistance
In the previous study, several types of MoS2-based MOSFET structures shown in Figure
1-11 have been proposed to study the electrical properties of MoS2, such as electron or hole
mobility and contact resistance. Structure (a) ~ (d) belong to top contact and structure (e) ~ (g)
belong to edge contact.
17
Structure (a) is the most general top contact structure, and the contact metal is generally
deposited by electron beam (EB) evaporation. Various metals with different work function have
been studied as contact metals such as indium [26], tin [27], scandium [28], gold [20][29], titanium
[23][28], silver [20][30], platinum [20][28], copper [20], nickel [28], indicating the existence of the
Fermi level pinning. Typically, the electron mobility of 10 ~ 200 cm2/Vs is obtained in this simple
structure depending on the channel length. Furthermore, by applying advanced techniques, mobility
can be improved. For example, Liu et al. applied a transfer method for making silver metal contact
to avoid the damage due to atom bombardment and back-scattered electrons during EB evaporation,
and obtained the high electron mobility of 260 cm2/Vs for the channel length of 10 μm with the
help of SiO2/PMMA stacked film as the gate dielectric [20]. Furthermore, Radisavljevic et al. add
a hafnium oxide gate dielectric and top gate metal on their monolayer MoS2 MOSFET, which
corresponds to structure (d) and achieved 200 cm2/Vs [13], which could be due to the charge
transfer mechanism [14][61], suppression of Coulomb scattering, or modification of phonon
dispersion in MoS2 monolayers [43]. Also, Das et al. applied Al2O3 dielectric layer on MoS2 and
scandium with low work function as a contact metal, improving the electron mobility from 184 to
700 cm2/Vs for the channel length of 5 μm device [28]. In some cases, even oxygen molecules
Figure 1-11: MoS2-based MOSFET structure proposed in the previous study. (a) ~ (d) belong to the top contact structure, and (e) ~ (g) belong to the edge contact structure.
18
absorbed on MoS2 channel can capture the electrons in the channel and can change the device mode
from depletion-mode to enhance-mode [62]. Therefore, we can consider the substances on the
channel to modify the electrical properties of the device. As for the contact resistance, 0.74 kΩμm
is obtained for gold metal contact deposited under ultra-high vacuum [24] and 0.8 kΩμm is
obtained for indium metal contact [26].
Structure (b) is a top contact structure with an interlayer between contact metal and MoS2
for field emission, Fermi level depinning due to dipole at interlayer, and suppression of the damage
during EB deposition. In the previous research, ZnO [31], h-BN [32], Ta2O5 [33], TiO2 [92], and
even 2D semiconductor [97] are selected as the interlayer. For Ti/ZnO/MoS2 contact, 0.9 kΩμm
and 66.2 cm2/Vs of electron mobility were achieved. ZnO was chosen because the electron affinity
of ZnO (χ = 4.30 eV) is almost the same as MoS2 (χ = 4.28 eV), resulting in a slight conduction
band offset (-0.02 eV). Wang et al. use h-BN as an interlayer between MoS2 and Ni and improved
contact resistance from 5.1 kΩμm to 1.8 kΩμm with reducing of Schottky barrier height of
158meV to 31meV. As for Ti/Ta2O5/MoS2 contact, 95 meV of Schottky barrier height with Ti
contact is decreased to 29 meV with inserting Ta2O5 layer between Ti and MoS2.
Structure (c) is a type of top contact structure that locates diffusion layer or metallic 1T
phase under the contact metal. Even though this is a type of top contact, it should be noted that
diffusion layer or metallic phase connects to MoS2 channel with top and also edge contact. In the
case of monolayer MoS2 MOSFET, there is only edge contact in the device. Ag is used as a contact
metal and diffused into MoS2 at 300 , resulting in the reduction of contact resistance from 0.8-
3.5 kΩμm for the as-deposited Ag contact to 0.2-0.7 kΩμm for annealed samples [30]. Pelella et
al. irradiate electron beam (10kV) onto Ti contact on monolayer MoS2 MOSFET and find the
lowering of the Schottky barrier and increasing transistor current probably due to metal diffusion
into MoS2 [52]. Furthermore, alloying or hybridization of MoS2 and Au by thermal treatment is
also beneficial for reducing contact resistance [34][35]. However, it is noted that the diffusion of
19
the contact metal into MoS2 does not always improve the contact resistance (i.e., Ni diffusion can
degrade the contact resistance. [36]). 1T phase (metallic) is also a promising interlayer material and
can be transformed from 2H phase (semiconductor) by applying energy media such as electron
beam [58][59], plasma [56][57], laser [53] on MoS2 or organolithium chemical method [54][55].
By forming 1T phase between contact metal and MoS2 channel by laser, Cho et al. show decreasing
of the Schottky barrier and increasing of the mobility of MoTe2 MOSFET. As for MoS2, Kappera
et al. found that 1T phase between gold contact metal and monolayer MoS2, which means the
formation of edge contact between 1T MoS2 and 2H MoS2 channel, can decrease contact resistance
from 1.1 kΩμm to 0.24 kΩμm [54].
Structure (e) is a basic edge contact structure in which the metal connects the edge of
MoS2 with chemical bonds. One of the previous studies of edge contact using graphene as a contact
metal shows the ohmic behavior and relatively high contact resistance of 30 kΩμm due to small
contact area [37]. Structure (f) and (g) are edge contact structures with h-BN on and/or below MoS2.
One of the benefits of the encapsulated structure is to be able to keep the intrinsic quality of MoS2
through the fabrication process [38]. MoS2 is often sensitive to the exposed air, resulting in the
occurrence of electron accumulation surface due to sulfur vacancy [39]. The residue on MoS2 from
the process can affect the quality of contact as well. Another benefit of the encapsulation is to be
able to suppress FLP due to the smaller contact area by edge contact, leading to low Schottky barrier
height when contact metal with appropriate work function is selected [40]. However, even though
the contact resistivity of the edge contact is superior to top contact, the contact resistance still is
large due to the narrow contact area. Therefore, increasing the contact area of the effective edge
contact area is one of the promising clues to improve edge contact TMDs MOSFET. Walter et al.
proposed a new structure, which is called "hybrid contact," to increase the edge contact area and to
utilize both top contact and edge contact effectively [41][42]. They utilized gold islands on MoS2
as an etching mask for dry etching to form an extremely minute trench structure on MoS2 and
20
fulfilled it by Ti, making a hybrid contact (Figure 1-12). They successfully found a promising result
that the contact resistance of the device (1.5 kΩμm) is relatively lower than the conventional Ti
top contact devices ( > 7 kΩμm [24]). Oliver simulated the voltage drop and current density of the
hybrid contact using a circuit model with COMSOL, suggesting that the contact area of the edge
contact with efficient specific contact resistance indeed improves total contact resistance [43].
Figure 1-12: Fabrication process and device structure of the "hybrid contact" MoS2 MOSFET. Cited from Walter et al. [41]
21
1.3.6 Defects in TMDs and impact on the electrical properties
Naturally, TMDs can contain a certain number of defects, such as chalcogen vacancies. In
the case of MoS2, a high native point defect concentration of 1013 cm-2 can be observed (Figure 1-
13), and the dominant defects are identified as sulfur vacancies [45]. The defects in TMDs can also
be created artificially or intentionally. Irradiation damage by the electron beam on MoS2 during
TEM observation has been investigated, showing sulfur vacancies formation and other structural
damages. For example, Gracia et al. and Algara-Siller et al. observed electron-induced damage on
MoS2 due to a "knock-on mechanism" (Figure 1-14) [49][50] by accelerating electrons that have
enough energy to break the bonds between each atom, depending on the accelerated voltage. They
showed the threshold accelerating voltage to break the bonds both theoretically and experimentally.
The calculated theoretical displacement energy of the sulfur atoms in MoS2 corresponds to 93 kV
of accelerating voltage, and the generation of sulfur vacancies at the accelerating voltage between
120 kV to 200 kV was observed, while they did not see damage at the accelerating voltage of 80
kV. In addition, Komsa et al. showed that it requires 560 kV accelerating electrons to knock on Mo
atoms from MoS2, which means sulfur vacancy is much easier to be generated in MoS2 than Mo
vacancy during electron beam irradiation [51]. There is another mechanism that creates defects in
materials, which is called the "radiolysis mechanism." This mechanism can occur even relatively
lower accelerated voltage than 80 kV, especially in semiconducting, insulating, or organic materials.
Defects in TMDs can also be created by plasma [65], laser [53], and ion beam [63][66] and can be
decreased by chemical reaction using thiol [64].
22
Sulfur vacancies in MoS2 or chalcogen vacancies in TMDs create defect states in the
bandgap and forming free electrons as described in Eq. 1-19. Some research shows the local density
of states at the single sulfur vacancy, suggesting the position of the defect states in the mid-gap in
monolayer MoS2, depending on the type of the defects (Figure 1-15) [45][46][60]. These defects
often affect the electrical properties of TMDs. For instance, McDonnell et al. found that a sulfur-
poor region in MoS2, which indicates the existence of a sulfur vacancy, showed n-type behavior
Figure 1-13: STM images of the surface of MoS2 and depth profiles of the lines in STM image, displaying the existence of the sulfur vacancy. Images from Vancsó et al. [45].
Figure 1-14: Electron-induced damage on MoS2 due to “knock on” mechanism at the accelerating voltage are 120 kV and 200 kV. Images from Gracia et al. [49]
23
and higher conductivity than a non-defective region. On the other hand, the sulfur-rich region in
MoS2, which can contain interstitial sulfur or a molybdenum vacancy, shows p-type behavior,
indicating that the type of defect can dominate even the type of conductivity at the surface (Figure
1-16) [47]. Besides, their simulation shows the increasing of current density at the contact as the
defect concentration increases at the same Schottky barrier height. Furthermore, Siao et al. show
that the Fermi level of the MoS2 surface is overlapped on the conduction band due to the plenty of
defects formed by MoS2 exposed to the air, which leads to strong n-type semiconductor behavior
[48]. In fact, according to most of the previous research, MoS2 devices typically show the n-type
behavior independent of the work function of the contact metal. In the previous research, defects
typically have a bad effect on contact resistance or conductivity of the channel. This is because that
defects can degrade the periodicity of the crystal lattice, resulting in poor carrier mobility, even
though they could create electrons. For example, Wu et al. suggested that an electron beam free
WSe2 device fabricated by transfer method for contact metal can achieve more than 100 times
higher mobility than the device fabricated by electron beam lithography [67]. However,
interestingly, Tosun et al. reported a significant reduction of contact resistance of Ni/WSe2
MOSFET from 1 MΩμm to 8 kΩμm using mild H2 plasma. They explained that the n+ region
under the contact due to Se vacancy contributes to increase tunneling probability of electron
injection into the semiconductor (Figure 1-17) [68]. According to these two articles that showed
opposite results of the electrical properties, it might be a good try to consider the effect of utilization
of defects in TMDs on MOSFET devices in this research, since all the papers mentioned above
shows that the defects in TMDs are one of the critical and promising factors to control the electrical
properties.
24
Figure 1-15: Local Density of States at several types of defects with or without Au atom in monolayer MoS2, showing defect states in the bandgap. Images from Hus et al. [46].
Figure 1-16: 3D STM image of MoS2 surface showing higher conductivity at defect region. Image from McDonnell et al. [47].
25
Figure 1-17: (a) Cross-sectional schematic of the WSe2 flake with Se defects on the H2 plasma exposed region and KPFM image showing the doped and undoped region. (b) Total resistance vs gate voltage of the WSe2 MOSFET showing significant Rc decreasing by H2 plasma. Images from Tosun et al. [68]
26
1.4 Motivation and Goal
For now, much research to reduce contact resistance has been done as shown in the
previous section; however, the industry needs a further reduction of contact resistance in TMD
MOSFETs for practical use, as shown in Figure 1-18 [69]. Therefore, we need further study to
reduce contact resistance. The goal of this research is to find clues to decrease the contact resistance
of the MoS2 based MOSFET. We propose two ideas that could be utilized to improve the electrical
properties of 2D MOSFET devices.
The first idea is based on defect engineering to reduce contact resistance by increasing
doping concentration under the contact by using an electron beam (EB). We used EB patterning
system to irradiate the contact region of MoS2 with an EB using several dose conditions and
considered the effect of the defects on electrical properties using TLM and other techniques.
The second idea is based on the "hybrid contact" proposed by Walter et al. [41], which
can increase the contact area utilizing Au as an etch mask. At first, we considered a Pt film on MoS2
to see if it could be used as an etch mask. Next, we considered a different type of hybrid contact to
try to utilize the reaction between Ti and MoS2 and conducted electrical measurements.
27
Figure 1-18: Contact resistance for different values of channel sheet resistivity for top-contacted TMD devices. Image from Allain et al. [69].
28
Defect engineering of MoS2 MOSFET by electron beam
2.1 Defect engineering concept for the reduction of contact resistance of MoS2 MOSFET
As shown in chapter 1, defects can be one of the important factors to control the electrical
properties of the MOSFET since they could increase the doping concentration, which can aid field
emission or thermionic field emission through the Schottky barrier. However, defects also could
degrade the electrical path created by orbital overlapping of the lattice, resulting in the reduction of
mobility. Therefore, finding the appropriate concentration of defects to "balance" the effects will
be needed. In this research, EB patterning technique is applied to control the defect region under
the contact to fabricate the TLM electrodes set to measure the contact resistance and the amounts
of the defect, which is a different way from the technique used in previous research. Figure 2-1 is
the concept device structure with several layers of MoS2 as a semiconductor, and a schematic image
of the band diagram showing conduction mechanism by field emission (FE) and thermionic field
emission (TFE). The defects induced by the EB are expected to contribute to the electron
conduction through FE or TFE mechanism since the tunneling probability is a function of the
doping concentration (Eq. 1-13).
2.2 Fabrication process of EB-irradiated MoS2 MOSFET
The fabrication process of a global back gate MoS2 MOSFET with EB irradiation after
fabrication of the substrate is described in Figure 2-2. The substrate was made by the following
process. Highly doped (> 1019 cm-3) p-type Silicon wafer with a 63 nm dry thermal oxide (SiO2) on
one side was used as a substrate, gate oxide, and gate electrodes. The substrate wafer was patterned
with Ti (20nm)/ Au (50nm) alignment markers by EB lithography and EB deposition to make it
easy to locate the flakes and for EB lithography. The wafers were diced into 1 x 1 cm pieces and
degreased in acetone, isopropyl alcohol (IPA), and deionized water (DIW) followed by blowing
dry with N2. Finally, all the pieces were exposed to UV-ozone treatment with dry 80% N2/ 20% O2
gas for 10 min for cleaning. Then MoS2 flakes were exfoliated from bulk MoS2 and moved onto
the substrate by Scotch tape, and then the substrate was soaked with acetone for more than 12 h to
remove adhesive material due to the tape, followed by IPA and DIW cleaning followed by blowing
dry with N2. After the cleaning, the locations of flakes with several layers, which are identified as
dark blue, black, or brown color depending on the thickness (~10 nm), were identified along with
the alignment marker by microscope with x20 and x100 magnification. The x20 magnified image
Figure 2-1: Expected device structure of EB-irradiated MoS2 MOSFET and the schematic band diagram showing thermionic field emission and field emission of the electrons.
30
was modified by Photoshop to bicolor, and TLM test electrode patterns and EB irradiation patterns
were written on the file by L-edit software for EB patterning (Figure 2-3). Each flake has at least 3
devices with a different channel length (100, 200, 300, 500, 1000 nm, etc.) depending on the length
and the shape of the flake. For EB irradiation on the contact region, PMMA495A3 (100 nm
thickness) was coated on the substrate, and the contact region of MoS2 was irradiated in several
dose conditions below to create defect regions. By coating PMMA495A3, we can confirm the
irradiated area by optical microscopy. We used EBPG 5200 (Raith GmbH.) for both EB irradiation
and pattering. The irradiation doses on the contact area were 0 μC/cm2 as a reference, 700 μC/cm2,
or 9700 μC/cm2. Since we irradiated 300 μC/cm2 for the electrode patterning later, the total doses
were 300 μC/cm2 (ref.), 1000 μC/cm2, or 10000 μC/cm2. In this experiment, there is a risk of
misalignment of EB patterning detailed later since we use EB irradiation twice. After the irradiation,
the PMMA was removed by acetone for more than 12 h followed by IPA, DIW cleaning, and then
the substrate was coated by the double-layer resist for EB patterning for contact metal deposition.
We used a double-layer resist consisting of PMMA495A6 as the bottom layer (200 nm thickness)
and PMMA950A3 as the top layer (100 nm thickness) to make the lift-off process easy. The coated
substrate was baked at 180 for 3 min after each coat. Electron dose for lithography was 300
μC/cm2 for several hundred nm line width and 450 μC/cm2 for electrode pad (100 x 100 μm), and
the current was 5 nA at 100 kV. After the EB patterning, the resist was developed by 1 DIW: 5 IPA
developer for 2 min. To prevent metal electrodes from peeling off from SiO2/Si substrate, at first,
we deposited Ti (10 nm) /Au (15 nm) by EB evaporation as adhesion metal for the pad and circuit
near the flakes. Next, we deposited 60 nm of Au as a contact metal since Au is often used as a
contact metal in previous studies. Temescal (FerroTec Co., Ltd.) was used as EB evaporator at a
deposition rate of 1 /s, and the base pressure was below 5.0 x 10-6 Torr. Only acetone was used
for the lift-off process, and the substrate was soaked in it for more than 12h followed by IPA, DIW
cleaning with N2 dry blowing. The device after deposition was lifted off by acetone for more than
31
12 h followed by IPA and DIW cleaning with N2 dry blowing. Then the electrical characterization
described in 2.4 was conducted. The film thickness was measured by AFM and the channel length
was measured by SEM (GeminiSEM 500, Carl Zeiss Microscopy GmbH) after all the electrical
measurements were finished since the electron beam influences the electrical properties of the
device.
Figure 2-2: The fabrication process of EB irradiated MoS2 MOSFET with global back gate structure. The process for the adhesion metal was omitted from this figure.
Figure 2-3: The image of a written pattern of the adhesive layer, radiation region, and contact metal in L-edit software.
32
2.3 Formation of defects on monolayer MoS2 by EB irradiation and characterization
As a preliminary work on EB irradiation test, monolayer MoS2 was used to determine the
appropriate electron dose to create defects. Monolayer MoS2 has a direct bandgap and emits light
when the monolayer absorbs energy above the bandgap. Therefore, Photoluminescence (PL)
spectroscopy is a powerful tool to detect defects since defect states in the bandgap reduce the direct
transition of electrons between the valence band and conduction band, resulting in a decrease in the
PL intensity. We also used Raman spectroscopy to detect the degradation of the crystal lattice.
For this experiment, monolayer MoS2 (grown by CVD, 2DCC) was transferred from a
sapphire substrate to SiO2/Si substrate. Before the transfer, monolayer MoS2 was covered by
PMMA495A6 (200 nm thickness) and soaked in 80 NaOH solution for 1 h, and then the
substrate was soaked in 80 hot water. The monolayer was peeled off from the sapphire substrate
and transferred onto the Si substrate, followed by cleaning in acetone to remove PMMA, and IPA,
and DIW. And then, the substrate was heated up in the air to make the adhesion between the MoS2
and substrate strong [70]. After the transfer, each 10 x 10 μm area was irradiated by EB with
different doses, and the amount of defect and the degree of deterioration of the crystal structure
was determined by photoluminescence (PL) and Raman spectroscopy (XploRa, HORIBA Ltd.).
According to Gracia et al. and Algara-Siller et al., we need above 80 kV accelerated voltage
to create sulfur vacancy by knock-on mechanism [49][50]. We used 100 kV accelerated voltage
and several electron dose conditions that are shown in Table 2-1. For 10,000 μC/cm2, the irradiation
was separated into 5000 μC/cm2 x 2 times to suppress heating. Figure 2-4 shows the appearance of
PMMA coated MoS2 after EB irradiation. We can see the irradiated area (10 x 10 μm) by color.
After the irradiation, PL and Raman spectrum was measured (Figure 2-5, Figure 2-6). The power
of the laser for PL and Raman was 0.1 mW to prevent the influence of the laser during measurement.
The intensity and peak positions are shown in Table 2.2. The intensity of the peak (E = 1.89 eV)
was decreased as the dose increased, and the intensity is almost zero in 10,000 μC/cm2 dose
33
irradiation. This implies there will be plenty of defect states in the bandgap, which may act as a
donor or change the surface potential, which can affect the position of the Fermi level pinning.
From the Raman spectrum shown in Table 2.3, we cannot see the difference of the frequencies and
peak intensities of the main peaks (E1 2g ≈ 388.5 cm-1 (in-plane vibration mode) and A1g ≈ 408.5
cm-1 (out-plane vibration mode)) between reference and EB irradiated samples. This indicates that
there will not be significant structural degradation under 10,000 μC/cm2 dose irradiation. On the
other hand, the width of both peaks was slightly increased as doses increased, indicating the
existence of the defects [66].
Table 2-1: EB patterning condition for monolayer MoS2 for defect creation.
Figure 2-4: The microscopic images of irradiated area (10 x 10 μm) monolayer MoS2. The darker square area corresponds to the irradiated area.
No. Dose [μC/cm2] Accelerating voltage [kV] Current [nA]
Ref. 0 - -
34
Figure 2-5: PL spectrum of monolayer MoS2. From the top, ref. 500 μm/cm2, 1,000 μm/cm2, and 10,000 μm/cm2.
Figure 2-6: The intensities of the PL peaks of each dose condition.
35
Figure 2-7: Raman spectrum of monolayer MoS2. From the top, reference, 500 μm/cm2, 1,000 μm/cm2, and 10,000 μm/cm2.
Figure 2-8: The intensity and width of the peaks of E1
2g and A1g of Raman spectrum.
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2.4 Electrical characterization of EB-irradiated MoS2 MOSFET by TLM
We conducted three sets of electrical characterization described in Figure 2-9 to evaluate
the electrical properties, such as MOSFET behavior, the judgment of ohmic contact or Schottky
contact, contact resistance, threshold voltage (Vth), and effective carrier mobility (μeff). The
electrical characterization was performed in a Lakeshore CRX-VG probe station at room
temperature and below 1.0 ×10-4 Torr. Drain current (IDS) - gate voltage (VGS) and drain current
(IDS) - drain voltage (VDS) relation were measured on all MOSFETs using a Keithley SCS 4200
semiconductor parameter analyzer. Since the width of each flake was different, the current was
normalized by dividing the current by the width, resulting in Rt·W = V/(I/W). To plot the contact
resistance as a function of 2D carrier density (n) for normalization, the carrier density was
calculated by n = VOD (COX/q), where COX is the capacitance of 63 nm thickness SiO2 (5.5 x 10-8
F/cm2). VOD is an overdrive voltage defined by VGS – VTH. VTH is a threshold voltage extrapolated
by IDS - VGS relation curve at VDS = + 0.1V (VGS was swept from –20 to +20 V with the step of 0.5
V). Due to the small size of the flakes obtained by the exfoliation method and the device often not
working well, 3 or 4 device sets on the same flake were typically used for TLM, resulting in having
a certain variation in the contact resistance. As the gate control is limited in the global back-gated
structure in terms of the film thickness, we used relatively thin exfoliated MoS2 with 5 to 15 layers.
37
Figure 2-10 shows the appearance of the flakes before and after EB irradiation. The
microscopic image of the flakes after EB irradiation shows the location of the irradiated areas with
the darker color of PMMA, and we can confirm the areas were overlapped by Au contacts in the
range of microscopic observation.
Figure 2-9: Electrical measurement conditions for EB-irradiated FETs.
Figure 2-10: Microscopic images of the example FETs with 10000 μC/cm2 total EB doses for each step, after exfoliation, after EB irradiation (doses = 9700 μC/cm2), and after metal deposition (Total doses = 10000 μC/cm2, including metal contact patterning). The red arrows in the center image are guides for the irradiation areas. S and D in the right image stands for source and drain in TLM method.
38
We show two types of IDS-VDS characteristics (Figure 2-11, Figure 2-12), IDS-VGS
characteristics (Figure 2-13), and Rt·W vs. channel length plot (Figure 2-14) of the typical FETs
with the three different EB irradiation doses (300 (Ref.), 1000, and 10000 μC/cm2). All the FETs
showed overall typical n-type MOSFET behavior and the ON currents of the magnitude of 10-4
A/μm (Figure 2-11) with the ON/OFF ratios of 106 to 109 (Figure 2-13). Some of the IDS-VDS curves
also showed the local current decrease in the saturation regime region. This will be due to the Joule
heating during electrical measurement by the resistance in the FET. They also showed a slight
increase of the current at the higher VDS region because of hot electrons, which are created by
accelerated electrons at near drain contact. Some IDS-VDS curves showed instability or noise, maybe
due to the absorbed molecules or residue of the resist. IDS-VDS curves of all the FETs in Figure 2-
12 look linear, but there is a slight curvature, indicating the existence of a Schottky barrier. Even
though there is a Schottky barrier, the electrons under the source contact electrostatically induced
by the gate bias will assist the FE or TFE, decreasing the depletion width and increasing the
tunneling current. Also, the IDS increases as the value of VDS increases by the image force lowering
[95], as shown in Eq. 1-12 and Eq. 1-13. As for the IDS-VGS characteristics in Figure 2-13, large
hysteresis was found in all the FETs maybe due to the trap states at the interface between MoS2
and gate oxide. The band diagram of the trap states in MoS2 created by the EB can be described as
shown in Figure 2-14. The trap states are charged positive or neutral depending on the Fermi level
controlled by the gate bias, resulting in hysteresis; however, the width of the hysteresis was not
dependent on the EB irradiation doses. This means that the change of the trap state density from
EB irradiation was not enough to influence the large hysteresis up to a 10000 μC/cm2 dose, or the
influence of the EB irradiation was small enough to be hidden by other factors, such as absorbed
molecules or residue of the resists. Of particular note in the extrapolation of threshold voltage in
the IDS-VDS curves in Fig. 2-15 is that the variation of the threshold voltage (ΔVth) between the
channel length of 300, 500, and 1000 nm increases from 2 V for 300 μC/cm2 irradiated FET to 9 V
39
for 10000 μC/cm2 irradiated FET as the EB dose increases. The threshold voltage is a function of
the oxide charge, doping concentration typically in the channel region, and oxide thickness.
Therefore, we could say that EB irradiation affected the doping concentration of MoS2 in the
channel region or the fixed oxide charge in SiO2 gate oxide. There could be tens of nm
misalignment between the patterning step and EB irradiation step. This can be the cause of the
change of the doping concentration in the channel and affect on the threshold voltage.
Figure 2-11: IDS-VDS characteristics at various VGS (-20 V to +20 V, steps of 5 V) of the typical MoS2 FETs with the various EB dose irradiation (300, 1000, or 10000 μC/cm2) at the channel length of 300 nm.
40
Figure 2-12: IDS-VDS characteristics at various VGS (-20 V to +20 V, steps of 10 V) of the typical MoS2 FETs with the various EB dose irradiation (300, 1000, or 10000 μC/cm2) at the channel length of 300 nm.
41
Figure 2-13: IDS-VGS characteristics (log plot) of the typical MoS2 FETs with the various EB dose irradiation (300, 1000, or 10000 μC/cm2) at different channel lengths.
Figure 2-14: Images of the trap states at the negative or positive gate voltage.
42
Figure 2-16 shows the contact resistance for all the FETs we made in this experiment as a
function of 2D carrier density. The values were calculated from the Rt·W vs. channel length plot by
TLM shown in Figure 2-17 as examples. From this result, we found the lowest value of the contact
resistance, which is 0.8 kΩμm at a carrier density is 5.1 x 1012 1/cm2, was achieved in the 300
μC/cm2 irradiated FETs. However, 1000 μC/cm2 irradiated FETs also show relatively low value,
which is 1.4 kΩμm at the carrier density of 4.1 x 1012 1/cm2. We also considered the influence of
the thickness of the flakes on contact resistance since previous researchers have shown the contact
resistance dependence on the number of layers [42][83][84]. Li et al. showed that contact resistivity
Figure 2-15: IDS-VGS characteristics of the typical MoS2 FETs with the various EB dose irradiation (300, 1000, or 10000 μC/cm2) at different channel lengths. We can see the threshold voltage difference between the channel length (300, 500, 1000 nm) as ΔVth.
43
sharply decreases as the number of the layer increases in the range of 1 to 4 layers since the bandgap
increases due to the quantum confinement effect. Also, the contact resistivity slightly increases as
the number of the layer increase in 5 layers or more [84]. Figure 2-18 shows the contact resistance
of all our FETs as a function of number of layers of MoS2 at the 2D carrier density is 3.1 x 1012
1/cm2, and Figure 2-19 shows the number of layers dependence of the contact resistivity from [84].
Contact resistivity does not change significantly between 6 and 7 layers from Figure 2-19. Thus,
we can compare the contact resistance for 1000 μC/cm2 irradiated FETs and 300 μC/cm2 FETs at
6 to 7 layers FETs. It will be difficult to conclude which is better since the Rc are 2.1 ~ 7.3 kΩμm
for 300 μC/cm2 FETs and 3.3 ~ 6.3 kΩμm for 1000 μC/cm2 FETs. The result of 10000 μC/cm2
irradiated FETs was not terrible, and it may imply that even such a high dose EB irradiation does
not degrade the metal-MoS2 contact intensely. However, we should still consider the experimental
variation in this series of TLM using exfoliated flakes since the flakes have an ununiform shape
and may have a different surface condition or unintentional dopant concentration in each flake.
Therefore, we conducted another experiment to compare the conductivity of the FETs more
accurately in the next section.
44
Figure 2-16: Contact resistance of all the FETs we measured as a function of 2D carrier density of the channel. The number in the guide means the EB dose, and A ~ H shows the different device sets for TLM. i.e., 300 μC-A~H (blue) means the total EB dose of the device was 300 μC/cm2, and 8 different FETs (A ~ H) were measured.
45
Figure 2-17: Rt·W vs. channel length plot at the over drive voltage of +9 V.
46
Figure 2-18: The relation between the number of layers of MoS2 flake and contact resistance at a 2D carrier density is 3.1 x 1012 [1/cm2].
Figure 2-19: The number of layers dependence of contact resistivity. Image from Li et al. [84]
47
2.5 Evaluation of EB-irradiated FETs with a mitigating variation of exfoliated flakes
It was difficult to conclude the influence of EB irradiation on contact resistance by the
series of TLM results. Therefore, we fabricated sets of FET on the same exfoliated flake to mitigate
the variation between the flakes and compared the IDS. Figure 2-20 shows the configuration of the
FETs with 300 nm channel length. We could only put two FETs on the same flake by the limitation
of the flake size. One is the 300 μC/cm2 irradiated FET as a reference, and the other is the 1000 or
10000 μC/cm2 irradiated FET. The electrical measurement was performed in the same condition as
section 2.4.
Figure 2-21 (a) shows the IDS-VDS curves of 300 μC/cm2 irradiated FET and the 1000
μC/cm2 irradiated FET at the VOD is +17 V and +17.5 V, respectively. From these IDS-VDS curves,
we can see that the current of 1000 μC/cm2 irradiated FET (IDS = 1.7 x 10-4 A/μm at VDS = +4 V) is
slightly higher than the 300 μC/cm2 irradiated FET (IDS = 1.6 x 10-4 A/μm at VDS = +4 V), even
though the overdrive voltage of the 1000 μC/cm2 irradiated FET (VOD = +17 V) is lower than that
of the 300 μC/cm2 irradiated FET (VOD = +17.5 V). This difference might not be large enough to
Figure 2-20: Microscopic images of fabricated FETs with 300 nm channel length. (a) 300 μC/cm2 irradiated FET and 1000 μC/cm2 irradiated FET on flake 1. (b) 300 μC/cm2 irradiated FET and 10000 μC/cm2 irradiated FET on flake 2.
48
state the positive effect of EB irradiation on the contact region, but we can say below 1000 μC/cm2
of EB irradiation had little negative effect on the contact region at least. We can consider three
reasons for the increase of the drain current. First, this might be due to the increase of doping
concentration in MoS2 in the contact region, resulting in an increasing FE or TFE mechanism.
Second, the induced interface defect states in MoS2 might change the pinning position, decreasing
Schottky barrier height. Third, the positive gate oxided charge induced by EB irradiation [96]
helped to increase the electrostatically induced carrier concentration under the contact, resulting in
increasing FE or TFE. As for the 10000 μC/cm2 irradiated FET shown in Figure 2-21 (b), the current
(IDS = 1.9 x 10-4 A/μm at VDS = +4 V) was significantly lower than the current of 300 μC/cm2
irradiated FET (IDS = 3.4 x 10-4 A/μm at VDS = + 4 V). This indicates too much EB irradiation could
degrade the contact region in MoS2, although we could not see the evidence in the Raman spectrum.
Figure 2-22 (a) shows the comparison of the IDS-VGS curves of 300 μC/cm2 irradiated FET and 1000
μC/cm2 irradiated FET. The Vth of the 1000 μC/cm2 irradiated FET was almost same as the Vth of
the 300 μC/cm2 irradiated FET. The Vth of the 10000 μC/cm2 irradiated FET was 1V lower than the
Vth of the 300 μC/cm2 irradiated FET, but the 1V of the difference might be within the margin of
error in this series of experiment. We also calculated the mobility from the IDS-VGS curves, and the
mobility was decreased significantly by the 10000 μC/cm2 EB irradiation from 46.2 to 24.9 cm2/Vs.
This will be due to surface scattering due to the induced oxide charge near the contact or
degradation of the electron path in MoS2, leading to lattice scattering.
49
Figure 2-21: (a) IDS-VDS curves of 300 μC/cm2 irradiated FET and 1000 μC/cm2 irradiated FET at the VOD are +17.5 V and +17 V, respectively. (b) 300 μC/cm2 irradiated FET and 10000 μC/cm2 irradiated FET at the VOD of +13 V and +14 V, respectively.
Figure 2-22: (a) IDS-VGS curves of 300 μC/cm2 irradiated FET and 1000 μC/cm2 irradiated FET at the VDS = +0.1 V. (b) 300 μC/cm2 irradiated FET and 10000 μC/cm2 irradiated FET at the VDS = +0.1 V.
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2.6 EB irradiation effect on the channel region of MoS2 MOSFET
While we focused on the EB irradiation effect on the contact region in the previous section,
we also had an interest in the EB irradiation effect on the channel region. Thus, we fabricated a
MoS2 FET with the channel length is 300 nm and the irradiated area at the center of the FET
described in Figure 2-23. The fabrication process was the same as section 2.2 except for the EB
irradiation. Also, as the heat treatment to remove absorbed molecules [24], we annealed the sample
at 300 in the Ar atmosphere for 5 min. We used Au as a contact metal as well. 75 nm clearance
was made to the EB irradiation area from the metal contact edge, considering the misalignment of
the EB system. We irradiated EB on the channel region of the same FET repeatedly at various doses
(irradiation doses in each step: 0 → 500 → 500 → 4000 → 5000 μC/cm2, cumulative doses: 0 →
500 → 1000 → 5000 → 10000 μC/cm2). The electrical characterization was performed right after
each EB irradiation step in the same condition as section 2.4.
Figure 2-24 (a) shows the IDS-VGS curves after each EB irradiation step and (b) shows the
Vth as a function of the cumulative doses. The Vth was decreased as the cumulative doses increased.
This will be due to the induced positive oxide charge or increase of doping concentration in MoS2,
but we could not distinguish them from this data. The rate of the Vth decline per EB dose was
gradually slowed down as the cumulative doses increased, and it was saturated at 5000 and 10000
Figure 2-23: The back gate MoS2 MOSFET structure for EB irradiation on the channel.
51
μC/cm2 cumulative doses. Figure 2-25 (a) shows IDS-VDS curves of the EB irradiated FET at the
cumulative doses are 0, 500, 1000, 5000, and 10000 μC/cm2 and (b) shows the effective mobility
of the EB irradiated FET as a function of cumulative doses. The current was clearly decreased as
the cumulative EB doses increases. The effective mobility was also decreased as the cumulative
EB doses increases, but the mobility was saturated at 5000 to 10000 μC/cm2 as same as Vth. The
induced positive oxide charge should increase the current at the same applied VGS, and the induced
carrier by the defect also should increase the current. Thus, the decrease of the current and the
mobility will be due to the increase of the lattice scattering by degradation of the MoS2 lattice by
the EB irradiation. Therefore, we can summarize that the EB irradiation on the channel region of
MoS2 decreases the Vth by EB irradiation-induced oxide charge or doping, and decreases the
electrical conduction by the degradation of the MoS2.
Figure 2-24: (a) IDS-VGS curves of the EB irradiated FET at the cumulative doses are 0, 500, 1000, 5000, and 10000 μC/cm2. (b) Vth of t