Fabrication and Characterization of ZnO Nanorods Based ...454302/FULLTEXT01.pdf · Hussain, S....
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Linköping Studies in Science and Technology
Dissertation No. 1401
Fabrication and Characterization of ZnO Nanorods Based Intrinsic
White Light Emitting Diodes (LEDs)
Nargis Bano
Physical Electronics and Nanotechnology Group
Department of Science and Technology (ITN)
Linköpings Universitet, Campus Norrköping
SE-601 74 Norrköping
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Copyright © 2011 by Nargis Bano
ISBN: 978-91-7393-054-3
ISSN 0345-7524
Printed by Liu-Tryck, Linköping University,
Linköping, Sweden
Linköping Studies in Science and Technology
Dissertation No. 1401
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Fabrication and Characterization of ZnO Nanorods Based Intrinsic White Light Emitting Diodes
Nargis Bano
Department of Science and Technology, Linköping University Norrköping Sweden
Abstract:
ZnO material based hetero-junctions are a potential candidate for the design and realization of intrinsic white light emitting devices (WLEDs) due to several advantages over the nitride based material system. During the last few years the lack of a reliable and reproducible p-type doping in ZnO material with sufficiently high conductivity and carrier concentration has initiated an alternative approach to grow n-ZnO nanorods (NRs) on other p-type inorganic and organic substrates. This thesis deals with ZnO NRs-hetero-junctions based intrinsic WLEDs grown on p-SiC, n-SiC and p-type polymers. The NRs were grown by the low temperature aqueous chemical growth (ACG) and the high temperature vapor liquid solid (VLS) method. The structural, electrical and optical properties of these WLEDs were investigated and analyzed by means of scanning electron microscope (SEM), current voltage (I-V), photoluminescence (PL), cathodoluminescence (CL), electroluminescence (EL) and deep level transient spectroscopy (DLTS). Room temperature (RT) PL spectra of ZnO typically exhibit one sharp UV peak and possibly one or two broad deep level emissions (DLE) due to deep level defects in the bandgap. For obtaining detailed information about the physical origin, growth dependence of optically active defects and their spatial distribution, especially to study the re-absorption of the UV in hetero-junction WLEDs structure depth resolved CL spectroscopy, is performed. At room temperature the CL intensity of the DLE band is increased with the increase of the electron beam penetration depth due to the increase of the defect concentration at the ZnO NRs/substrate interface. The intensity ratio of the DLE to the UV emission, which is very useful in exploring the origin of the deep level emission and the distribution of the recombination centers, is monitored. It was found that the deep centers are distributed exponentially along the ZnO NRs and that there are more deep defects at the root of ZnO NRs compared to the upper part. The RT-EL spectra of WLEDs illustrate emission band covering the whole visible range from 420 nm and up to 800 nm. The white-light components are distinguished using a Gaussian function and the components were found to be violet, blue, green, orange and red emission lines. The origin of these emission lines was further identified. Color coordinates measurement of the WLEDs reveals that the emitted light has a white impression. The color rendering index (CRI) and the correlated color temperature (CCT) of the fabricated WLEDs were calculated to be 80-92 and 3300-4200 K, respectively.
Keywords: Zinc Oxide nanorods, White light emitting diode, Photoluminescence, Cathodoluminescence, Electroluminescence, Deep level transient spectroscopy (DLTS).
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List of publications included in the thesis
This thesis consists of an introductory text and the following papers:
I. Study of radiative defects using current-voltage characteristics in ZnO rods
catalytically grown on 4H-p-SiC
N. Bano, I. Hussain, O. Nur, M. Willander, and P. Klason, Journal of Nanomaterials,
Article ID 817201(2010).
II. Study of luminescent centers in ZnO nanorods catalytically grown on 4H-p-SiC
N. Bano, I. Hussain, O. Nur, M. Willander, P. Klason and A. Henry, Semicond. Sci.
Technol. 24, 125015 (2009).
III. Depth-resolved cathodoluminescence study of zinc oxide nanorods catalytically
grown on p-type 4H-SiC
N. Bano, I. Hussain, O. Nur, M. Willander, Q. Wahab, A. Henry, H. S. Kwack and D. Le
Si, Dang, Journal of Luminescence 130, 963–968 (2010).
IV. Study of Au/ZnO nanorods Schottky light-emitting diodes grown by the low-
temperature aqueous chemical method
N. Bano, I. Hussain, O. Nur, M. Willander, H. S. Kwack and D. Le. Si Dang, Appl. Phys
A. 100, 467–472 (2010).
V. ZnO-organic hybrid white light emitting diodes grown on flexible plastic using low
temperature aqueous chemical method
N. Bano, S. Zaman, A. Zainelabdin, S. Hussain, I. Hussain, O. Nur, and M. Willander,
J. Appl. Phy. 108, 043103 (2010).
VI. Study of intrinsic white light emission and its components from ZnO-nanorods/p-
polymer hybrid junctions grown on glass substrates
I. Hussain, N. Bano, S. Hussain, O. Nur and M. Willander, J. Mater Sci.46, 7437 (2011).
VII. Study of the distribution of radiative defects and reabsorption of the UV in ZnO
nanorods-organic hybrid white LEDs
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I. Hussain, N. Bano, S. Hussain, Y. Soomro, O. Nur, and M. Willander, Materials 4,
1260-1270 (2011).
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Related Papers not included in the thesis
1. ZnO as an energy efficient material for white LEDs and UV- LEDs
M. Willander, O. Nur, A. Zainelabdin, N. Bano. I. Hussain, S. Zaman, and M. Q. Israr,
submitted (2011).
2. Intrinsic white light emission from zinc oxide nanorods heterojunctions on large
area substrates
Magnus Willander, O. Nur, S. Zaman, A. Zainelabdin, G. Amin, J. R. Sadaf, M. Q. Israr,
N. Bano, I. Hussain, and N. H. Alvi, Proceedings of SPIE 7940, 79400A (2011).
3. Zinc Oxide nanorods/polymer hybrid heterojunctions for white light emitting
diodes
M. Willander, O. Nur, S. Zaman, A. Zainelabdin, N. Bano, and I. Hussain, J. Phys. D:
Appl. Phys. 44, 224017 (2011).
4. Luminescence study of ZnO hybrid white LEDs grown on cheap/disposable
substrates by low temperature chemical growth
I. Hussain, N. Bano, M. Y. Soomro, O. Nur, and M. Willander Nanoscinece and
Nanotechnology submitted (2011).
5. Inorganic-organic ZnO based heterostructures for lighting
M. Willander, N. Bano, and O. Nur, ECS Transactions, 19 (12) 1-12 (2009).
6. Luminescence from zinc oxide nanostructures and polymers and their hybrid
devices
M. Willander , O. Nur, J. R. Sadaf, M.Q. Israr, S. Zaman, A. Zainelabdin, N. Bano and I.
Hussain, Materials 3, 2643-2667 (2010).
7. Zinc oxide nanorod-based heterostructures on solid and soft substrates for white-
light-emitting diode applications
M. Willander, O. Nur, N. Bano and K. Sultana, New Journal of Physics 11, 125020
(2009).
8. Different interfaces to crystalline ZnO nanorods and their applications
M. Willander, M. H. Asif, S. Zaman, A. Zainelabdin, N. Bano, S. M. Al-Hilli, and O.
Nur , Phys. Status Solidi C 6, No. 12, 2683–2694 (2009).
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9. Current-transport studies and traps extraction of hydrothermally grown ZnO
nanotubes using gold Schottky diode
G. Amin, I. Hussain, S. Zaman, N. Bano, O. Nur, and M. Willander, Phys. Status Solidi
A 207, No. 3, 748–752 (2010).
10. Photonic nano-devices and coherent phenomena in some low dimensional systems
M. Willander ,Yu. E. Lozovik, S. P. Merkulova, O. Nur, A. Wadeasa, P. Klason, B.
Nargis, N. H. Alvi, and S. Kishwar, 214th ECS Meeting, Abstract #2034, © The
Electrochemical Society.
11. Enhancement of zinc interstitials in ZnO nanotubes grown on glass substrate by the
hydrothermal method
M. Y. Soomro, I. Hussain, N. Bano, S. Hussain, O. Nur and M. Willander, Appl. Phys A
submitted (2011).
12. Growth and characterization of ZnO nanotubes on disposable-flexible paper
substrates by low temperature chemical method
M. Y. Soomro, I. Hussain, N. Bano, Jun Lu, O. Nur and M. Willander (manuscript).
13. Nanoscale elastic modulus of single horizontal ZnO nanorod using nano-indentation
experiment
M. Y. Soomro, I. Hussain, N. Bano, E. Broitman, O. Nur and M. Willander, Nanoscale
Research Letters, submitted (2011).
14. ZnO nanorods based nanogenerator on flexible paper substrate
M. Y. Soomro, I. Hussain, N. Bano, O. Nur and M. Willander, (manuscript).
15. Study of deep level defects in ZnO nanorods grown on p-GaN by low temperature
chemical method
I. Hussain, N. Bano, M. Y. Soomro, M. Asghar, O. Nur and M. Willander, (manuscript).
16. Comparative study of ZnO nanorods based WLEDs grown on p-SiC by high and
low temperature growth methods
N. Bano, I. Hussain, M. Y. Soomro, O. Nur and M. Willander, (manuscript).
17. The Study of surface states in ZnO nanorods/p-GaN heterojunctions
I. Hussain, N. Bano, M. Y. Soomro, S. M. Faraz, O. Nur and M. Willander, (manuscript).
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Acknowledgement
All praise goes to ALLAH, who created the universe and appointed the man as His
vicegerent. I offer my humble thanks to ALLAH who blessed and enabled me to complete
this dissertation in stipulated time frame. All the blessings to His Prophet Muhammad
(peace be upon him), who is the source of guidance and knowledge for humanity.
Special appreciation goes to my supervisor Associate Prof. Omer Nour for his supervision
and constant support. His invaluable help of constructive comments and suggestions
throughout this work which have contributed to the success of this research.
I would like to warmly acknowledge my co-supervisor Prof. Magnus Willander for his
guidance and input throughout the process of this research.
Not forgotten, my appreciation to the ex-research administrator Lise-Lotte Lönndahl
Ragnar and our present group research administrator Ann-Christin Norén for their
administrative help during my studies and research work.
I would like to express my deep and obedient appreciation gratitude to Dr. Anne. Henry, Dr.
D. Le. Si Dang, Dr. H. S. Kwack, Dr. Peter Klason, Dr. Asghar Hashmi and Dr. Qamar ul
Wahab and Sajjad Hussain for their endless support and co-operation in my research work.
I am really thankful to Higher Education Commission (HEC), government of Pakistan for
partial financial help in during my research work. I am also very thankful to Dr. Atta ur
Rehman ( Ex- Chairman HEC), Dr. Javeed Laghari (Chairman HEC), Muhammad
Ashfaq, project manager (HEC), Dr. Sohail Naqvi, and Dr. Yasir Jameel for their
cooperation and good wishes.
I am also thankful to all of my group members. Many thanks for your cooperation and nice
company.
I offer my sincerest wishes and warmest thanks to Muhammad Yousuf Soomro and his
family. They help me and give me a nice company. I am also thankful to my sincere friends
Sobia Shakir, Aisha Rana and Mona Ijaz.
It is those who are near to us that must bear the full force of our own inadequacies, so I must
appreciate my brothers and sisters Haji Abdul Quyam, Khazar Hayat, Abdul Qadeer,
Abdul Zaheer, Muhammad Aqil, Haji Imtiaz Hussain, Hafiz Mumtaz Hussain, Faraz
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Hussain, Shehzad Hussain and my sister Zatoon Fatima, Humeera Hussain and Dr.
Naghmana Batool they always have given me support and love. I really appreciate them
from the bottom of my heart.
I would like to express my profound admiration and salute to my father Haji Muhammad
Iqbal my mother Sikandra Bibi, my father in law Muhammad Hussain and my mother in
law Sarwri Begum. I would essentially have not been able to achieve this noble goal without
their help and kindness. I wish to express my appreciation for their love and affection on me
in every aspect of life.
At last I am very thankful to my loving husband Dr. Ijaz Hussain Asghar and my sweet son
Muhammad Mughees without their unconditional support and encouragements nothing was
possible.
Nargis Bano
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Table of Contents Chapter 1 ................................................................................................................................ 1
Introduction ................................................................................................................... 1
Chapter 2 ............................................................................................................................... 7
Zinc oxide material .............................................................................................................. 7
2.1 Basic properties of ZnO ............................................................................................... 7
2.2 Crystal structure of ZnO................................................................................................ 7
2.3 Physical properties of ZnO at room temperature ........................................................... 8
2.4 Electrical properties of ZnO as compared to GaN.......................................................... 9
2.5 Properties and device applications ................................................................................. 9
2.5.1 Direct and wide band gap .......................................................................................... 9
2.5.2 Large exciton binding energy .................................................................................... 9
2.5.3 Strong luminescence ............................................................................................... 10
2.6 Optical properties of ZnO ........................................................................................... 10
2.7 ZnO nanostructures ..................................................................................................... 11
2.8 Native point defects in ZnO ....................................................................................... 11
2.9 Classification of Defects ............................................................................................. 11
2.10 Excitons .................................................................................................................... 12
2.10.1 Frenkel excitons ..................................................................................................... 13
2.10.2 Wannier excitons ................................................................................................... 13
2.11 Recombination………………………………………………………………………..13
2.10.1 Radiative recombination (Band to band recombination)…………………………...13
2.10.2 Auger recombination.................................................................................................13
2.10.3 Shockley-Read-Hall recombination or (Trap-assisted recombination)…………….13
Chapter 3 ............................................................................................................................. 19
Growth and characterization of ZnO nanorods based LEDs ................................................ 19
3.1 Growth methods of ZnO nanorods ............................................................................. 19
3.1.1 Low temperature chemical growth ........................................................................... 19
3.1.2 Growth of ZnO nanorods on PEDOT:PSS coated flexible plastic substrate .............. 19
3.1.3 Growth of ZnO nanorods on paper substrate ............................................................ 20
3.1.4 Growth of ZnO nanorods on glass substrate ............................................................. 20
3.2 Growth of ZnO nanorods by high temperature or vapor-liquid-soild (VLS) mechanism ...................................................................................................................... 20
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3.2.1 Cleaning of the substrate .......................................................................................... 20
3.2.2 Evaporation and cutting of the substrate ................................................................... 21
3.2.3 Vapor-liquid-soild (VLS) growth method ................................................................. 21
3.2.4 Growth of ZnO nanorods on 4H-P-SiC substrate by the VLS methods ..................... 21
3.3 Bottom contact ........................................................................................................... 23
3.4 Spin coating of photo resist, plasma etching and top contact deposition ...................... 23
3.5 Scanning electron microscope (SEM) ......................................................................... 23
3.6 Photoluminescence (PL) ............................................................................................. 24
3.6.1 Uses of Photoluminescense ...................................................................................... 25
3.7 Cathodoluminesence (CL)........................................................................................... 27
3.7.1 Type of optical-CL ................................................................................................. 28
3.8 Current-voltage characteristics ................................................................................... 29
3.9 Electroluminescence (EL) ........................................................................................... 29
3.10 Deep level transient spectroscopy (DLTS) ................................................................ 29
3.11 Color rendering index (CRI) ..................................................................................... 30
Chapter 4 ............................................................................................................................. 33
Device application of ZnO nanostructures ........................................................................... 33
4.1 ZnO nanorods (NRs) based intrinsic white light emitting diode (WLEDs).................... 33
Chapter 5 ............................................................................................................................. 41
Results and discussion………………………………………………………………………..41
5.1 Identification of the violet-blue luminescent centers in ZnO NRs/p-SiC hetrojunctions 41
5.2 Spatial distribution of the luminescence centers in ZnO NRs/p-SiC hetrojunctions....... 45
5.3 Schottky LEDs grown by the low temperature aqueous chemical growth method......... 48
5.4 Hybrid white LEDs grown on flexible plastic substrates using low temperature aqueous chemical growth method ............................................................................... 52
5.5 ZnO-nanorods/p-polymer hybrid white LEDs grown on glass substrates using the low temperature aqueous chemical growth method .............................................. 57
5.6 Distribution of radiative defects and reabsorption of the UV in ZnO nanorods-organic hybrid white LEDs ......................................................................................... 63
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List of Figures Figure 2.1: The hexagonal wurtzite structure of ZnO. The O-atoms are the large red spheres
and the Zn-atoms are the small grey spheres [5, 6] ........................................... 8
Figure 2.2: PL spectrum for ZnO NRs measured using an Ar-laser operating at 150 W at
room temperature [30]. ................................................................................. 10
Figure 2.3: SEM image of ZnO nanorods grown by the ACG method …………………..11
Figure 2.4: Carrier recombination mechanisms in semiconductors [37] ........................... 14
Figure 3.1: The VLS growth method [5]. ......................................................................... 22
Figure 3.2: Tube furnace setup [5]. .................................................................................. 22
Figure 3.3: A schematic diagram of the SEM [8]. ........................................................... 24
Figure 3.4: Schematic PL experimental setup [3]. ........................................................... 25
Figure 3.5: Energy vs. crystal momentum for a semiconductor with a direct band gap [9]26
Figure 3.6: Luminoscope optical-CL attached to an optical microscope .......................... 27
Figure 3.7: Top and cross-sectional views of a luminoscope optical-CL system attached to an optical microscope. Modified after Marshall (1988) [10]. .......................... 28
Figure 4.1: Schematic illustration of ZnO/ p-SiC nanorods hetrostructure device. ............ 34
Figure 4.2: Room temperature EL spectrum of ZnO NRs/p-SiC heterojunction LED. ...... 35
Figure 4.3: Schematic illustration of ZnO NRs/ n-SiC Schottky LED............................... 35
Figure 4.4: (a) Room temperature EL spectrum and Gaussian fitting of the ZnO NRs/n-SiC Schottky LED ............................................................................................... 36
Figure 4.5: Schematic illustration of ZnO NRs/PFO hybrid device on PEDOT:PSS coated flexible plastic. .............................................................................................. 36
Figure 4.6: Room temperature EL spectrum and Gaussian fitting of the PFO/ZnO hybrid NRs LED. ...................................................................................................... 37
Figure 4.7: Schematic illustration of ZnO NRs/PFO/PEDOT:PSS heterostructure device 35
Figure 4.8: Room temperature EL spectrum and Gaussian fitting of the ZnO NRs/PFO/PEDOT:PSS/glass heterojunction LED ......................................... 38
Figure 5.1: Typical current-voltage characteristics for ZnO nanorods and the inset is the diode resistance (dV/dI) as a function of the voltage. ..................................... 41
Figure 5.2: Log-log plot for the I-V data of ZnO nanorods. .............................................. 43
Figure 5.3: (a) Representative DLTS spectra of ZnO NRs. Filled circles (experimental) and solid line (line shape fit) which shows two traps E1 (electron trap) and P1 (hole trap). ..................................................................................................... 44
Figure 5.3: (b) Schematic band diagram of the three observed luminescence process. ...... 44
Figure 5.4: Depth dependent CL spectra of ZnO NRs taken at a spot size of 50 nm at
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Room temperature. ........................................................................................ 46
Figure 5.5: (a) The energy loss characteristic dE/dx versus the depth curves for two energies in ZnO nanorods. ............................................................................. 46
Figure 5.5: (b) The emission intensity (right axis) of the NBE, green band and red as well as the ratio of the NBE and the green bands (left axis) as a function of the penetration depth .......................................................................................... 47
Figure 5.6: Comparison of the CL emission spectra from ZnO NRs and a nearby area
without ZnO NRs (4H-SiC area). ................................................................... 47
Figure 5.7: Typical room temperature current-voltage characteristics for the Au/ZnO NRs Schottky diode ............................................................................................... 49
Figure 5.8: Room temperature EL spectra of Au/ZnO NRs Schottky diode. ..................... 49
Figure 5.9: (a) Depth dependent CL spectra of ZnO NRs at room temperature ................ 51
Figure 5.9: (b) The emission intensity (right axis) of the NBE, and DLE as well as the ratio of NBE and DLE (left axis) as a function of penetration depth ............ .51
Figure 5.10: Comparison of the CL emission spectra from ZnO NRs and 4H-SiC area.. .... 52
Figure 5.11: Typical room temperature I-V characteristics of the ZnO NRs/PFO hybrid WLED and the inset show the semilog plot of I-V characteristics. ................. 53
Figure 5.12: (a) Room temperature PL spectrum of the ZnO NRs grown by the same parameters on plastic substrate with no PFO film.. ......................................... 54
Figure 5.12: (b) Room temperature PL spectrum of the ZnO NRs/PFO hybrid structure and
the inset shows the room temperature PL spectrum of the PFO on PEDOT:PSS plastic ........................................................................................................... 54
Figure 5.13:Room temperature EL spectrum and the Gaussian fitting of the PFO/ZnO hybrid WLED. ............................................................................................... 56
Figure 5.14: Typical color coordinates characteristics of the ZnO-PFO hybrid WLEDs.. ... 56
Figure 5.15: Room temperature PL spectrum of the ZnO NRs/PFO hybrid structure and the
PL spectrum of the PFO on glass substrate. ................................................... 58
Figure 5.16: Typical room temperature I-V characteristics of the ZnO
NRs/PFO/PEDOT:PSS heterojunction. .......................................................... 58
Figure 5.17: (a) Room temperature EL spectrum after a Gaussian fitting............................ 59
Figure 5.17: (b) The EL spectrum showing the whole visible range which is distinguished by a Gaussian fitting.. ................................................................................... 59
Figure 5.17: (c) The chromaticity diagram of the HWLED... ............................................. 60
Figure 5.18: Room temperature EL and PL spectra of HWLEDs with PL spectra of PFO
(without ZnO). .............................................................................................. 61
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Figure 5.19: (a) Energy band diagram of ZnO NRs/PFO hybrid heterojunction showing the EL emissions from ZnO NRs.. ..................................................................... 61
Figure 5.19: (b) Energy band diagram of ZnO NRs/PFO hybrid heterojunction under forward bias showing the band banding at the interface and the charge accumulation.................................................................................................. 62
Figure 5.20: Depth dependent CL spectra of the hybrid LED at room temperature... .......... 64
Figure 5.21: Room temperature CL spectrum of the hybrid LED after the Gaussian fittin .. 64
Figure 5.22: Comparison of the CL emission spectra from the hybrid LED and an area with PFO only and the inset shows a magnified CL emission spectra of PFO.. ...... 65
Figure 5.23: The emission intensity (right axis) of the UV, violet and DBE as well as the ratio of the UV and the DBE (left axis) as a function of the penetration depth…………………………………………………………………………..66
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List of Tables
Table 2.1: Basic physical properties of ZnO at RT [4, 5, 7-9]…………………………………8
Table 2.2: Electrical properties of ZnO and GaN at RT [4, 5, 10-13]………………………….9
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Chapter 1
Introduction
The history of light emitting diodes (LEDs) goes back to 1907 [1] when it was
reported for the first time by Henry Joseph Round but gained practical importance only in
1962 [2] when Nick Holonyak invented the first red light emitting diode. Since then, the field
of LEDs has attracted a huge amount of attention both from industry and academia. However,
not until recently, 50 years later, the fruit of the first report has been harvested through
commercialization of LEDs. These days LEDs of all colors with reasonable efficiency are
available which opens the opportunity to use LEDs in areas beyond conventional signage and
indicator applications. Now LEDs can be used in high-power applications thereby enabling
the replacement conventional light sources. LEDs light sources, which are more compact,
more durable and capable to change color in real time, are finding more applications in
domestic, commercial and industrial environments. However, at the moment LEDs have high
prices, and nonlinear optical and electrical characteristics, which are major barriers for
widespread lighting applications.
Since the last decade zinc oxide (ZnO) is being considered a potential candidate
for LEDs due to several potential advantages over GaN-based material system. ZnO is an II-
VI semiconductor material with direct band gap ~ 3.37 eV and the room temperature exciton
binding energy ~ 60 meV, the large exciton energy makes it possible to employ excitonic
recombination process as a lasing mechanism [3]. The sudden enormous increase of the
global interest in ZnO material is due to the fact that it has diverse range of nanostructures
including nanorods (NRs), nanobelts and nanotubes which can be grown on any substrate
without the need of lattice matching [4]. The growth of ZnO NRs can be achieved using
different high and low temperature growth methods. The most usual high temperature
techniques used are metal organic chemical vapor deposition (MOCVD) and vapor liquid
solid (VLS) and the aqueous chemical growth (ACG) method is low temperature techniques
[4]. ZnO nanostructures possess novel integrated nano-systems with excellent optoelectronic
properties for LEDs because of their extremely small volume and modified light-matter
interaction [5]. ZnO NRs are the most important nanostructures which offer additional
advantages for light emission due to the increased junction area, enhanced polarization
dependence of reflectivity, and improved carrier confinement in one dimensional
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nanostructure [4]. In addition ZnO NRs have extremely smooth ends which make them
perfect mirror planes and vertical ZnO NRs are like natural waveguide cavities for making the
emitted light to travel to the top of the device. At the same time, ZnO NRs can reduce the
threshold of simulated emission which appeared due to the possible radial quantum
confinement effects in two dimensions in which the finite size of nano-scale materials that
confines the spatial distribution of the electrons gives rise to the quantized energy level, due
to that ZnO NRs possess high density of states at the band edge [6-8]. The quantum
confinement effect plays an important role and enhancs the oscillation strength of the
excitons, which is favorable for radiative recombination of excitons at room temperature
these properties open the way to fabricate ZnO NRs based LEDs [9].
ZnO NRs have large number of intrinsic and extrinsic deep-level defects that emit
different colors of light including violet, blue, green, yellow, orange and red, i.e. all
constituents of the white light [10,11]. This implies that ZnO NRs have potential of being
used as intrinsic white light emitting diodes (WLEDs). In order to develop ZnO NRs based
WLEDs, the most important issue is the fabrication of low-resistivity p-type ZnO. Because
unintentionally undoped ZnO shows typically n-type properties, acceptors may be
compensated by native defects such as zinc interstitial (Zni), oxygen vacancy (VO), or
background impurities such as hydrogen [6]. Thus the lack of a reliable and reproducible p-
type doping in ZnO with sufficiently high carrier concentration has instigated an approach to
grow ZnO nanostructures on other p-type substrates to realize ZnO-based p–n hetero-
junctions. Moreover, in order to achieve high internal quantum efficiencies, free carriers need
to be spatially confined. This requirement also led to the development of p-n hetero-junction
LEDs consisting of different semiconductors.
This thesis presents some effectual ways to produce ZnO NRs based hetero-junctions
based intrinsic WLEDs on p-SiC, n-SiC and p-type polymers by ACG and VLS methods and
provide some beneficial results in aspects of their structural, electrical and optical properties,
which builds experimental and theoretical foundation for much better understanding the deep
level emission physics.
Typically ZnO NRs exhibit one sharp ultraviolet (UV) peak and possibly one or two
deep level emissions (DLE) bands due to deep level defects within the band gap. The
dominant emitted colors depend on the growth conditions and methods. This means that the
emitted colors can be controlled and that the luminescence efficiency is directly or indirectly
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related to deep level defects [4]. Therefore the defects are investigated using deep level
transient spectroscopy (DLTS) and current-voltage characteristics in paper I and II.
The performance of ZnO NRs based WLEDs depend upon the defect chemistry, the
distribution of defects and the origin of specific emissions from ZnO NRs and the substrates,
which are the subjects of recent theoretical and experimental studies. Therefore in paper III
the detailed information about the origin of specific emission, growth dependence of the
optically active defects and their spatial distribution the depth resolved cathodoluminescence
spectroscopy are performed. The cathodoluminescence is a powerful technique for
characterizing the optical properties of ZnO NRs because it has a resolution below the
diffraction limit of light [12].
In paper IV we reported for the first time ZnO NRs based Schottky LEDs grown on n-
SiC substrates by the ACG method. These days ZnO-organic hybrid LEDs becomes one of
the most exciting research areas because hybrid materials promise good properties that may
not easily be achieved from conventional materials such as combining the high flexibility of
polymers with the structural, chemical and high functional stability of inorganic materials.
Such a route may pave the way for the apprehension of large-area photonic devices that could
serve as building blocks for the next generation of flexible display panels. In paper V we
reported white light luminescence from ZnO-organic hybrid LEDs grown at 90 oC on flexible
plastic substrates. This will provide lighting designers the ability to put flexible white light
panels in places previously impossible or prohibitively expensive.
In paper VI we reported white-light luminescence from ZnO-organic hybrid LEDs
grown on glass substrate. The configuration used for the LEDs consists of two-layers of
polymers on glass with top ZnO nanorods (NRs). Electroluminescence spectra show the
combination of emission bands arising from radiative recombination in polymer and ZnO
NRs. The transitions causing these emissions are identified and discussed in terms of the
energy band diagram of the hybrid junction and the influence of the transparent ITO ohmic
contact on the emitted intensity is discussed.
The UV emission can be internally reabsorbed by the ZnO crystal itself within
1µm range this reabsorbed UV emission can excite defect states in the ZnO material resulting
in an increase of the intensity of the DBE. Thus, it is possible that part of the UV emission
may contribute to the enhancement of the DBE. Therefore in paper VII depth dependent CL
spectra was examined to investigate the UV re-absorption and the detailed information of the
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luminescence and especially the spatial distribution of the defects that cause the UV and the
wide visible emissions from ZnO nanorods-organic hybrid white LEDs.
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References:
[1] H. J. Round, Electrical World 19, 309 (1907).
[2] N. Holonyak, and S. F. Bevacqua, Appl. Phys. Lett. 1, 82 (1962).
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6
7
Chapter 2
Zinc oxide material
In the past few years zinc oxide (ZnO) has received much attention because it
has a wide range of properties that depend on doping, including a range of conductivity from
metallic to insulating (including n-type and p-type conductivity), high transparency,
piezoelectricity and wide-bandgap semiconductivity. Without much effort, it can be grown in
many different nanoscale forms on any substrate, thus allowing various novel devices to be
achieved. In this chapter we discuss the different properties of ZnO nanostructures.
2.1 Basic properties of ZnO
ZnO is a wide band gap semiconductor with a large number of attractive
properties for electronics and optoelectronics devices. At room temperature the bandgap
energy is 3.37 eV. ZnO belongs to II-VI semiconductor group and the native doping of the
ZnO is n-type. It has large excitons binding energy 60 meV due to that the excitons in ZnO
are thermally stable at the room temperature. This semiconductor has many favorable
properties like high electron mobility, good transparency, strong room temperature
luminescence, ect. Those properties are used in electronic applications of ZnO as thin-film
transistors and light-emitting diodes [1-3].
2.2 Crystal structure of ZnO
At room temperature (RT) ZnO has a hexagonal wurtzite crystal structure with
lattice parameters a=3.25 Å and c=5.12 Å. The ZnO structure consists of alternating Zn and O
layers as shown in Fig. 2.1. Each oxygen anion is surrounded by four zinc cations at the
corner of the tetrahedron, and vice versa. Although the entire unit cell of ZnO is neutral. The
polarity observed in the ZnO crystal is due to the tetrahedral structure. ZnO has four common
surfaces, the polar Zn (0001) and O (0001) terminated faces and the nano-polar (1120) and
(1010) faces. The (1120) and (1010) surfaces contain equal numbers of Zn and O atoms. At
relatively modest external hydrostatic pressures wurtzite ZnO can be transformed to the
rocksalt (NaCl) like other II-VI semiconductors. As compared to the wurtzite structure
zincblende structure has lower ionicity, this lead to the lower carrier scattering and higher
doping efficiencies [5, 6].
8
Figure 2.1: The hexagonal wurtzite structure of ZnO. The O-atoms are the large red spheres
and the Zn-atoms are the small grey spheres [5, 6]
2.3 Physical properties of ZnO at RT Some basic physical properties of ZnO at 300K are listed in the Table 2.1 [4, 5,
7, 8]. The thermal conductivity variation is due to crystal defects so some of the values in the
table have uncertainty [9] and because of the problems of producing robust and reproducible
p-type doping there is also uncertainty in the values of the hole mobility and effective mass.
Table 2.1: Basic physical properties of ZnO at RT [4, 5, 7-9]
Parameters
Values
Lattice constant a = 0.32495 nm, c =0.52069 nm
Density (g cm-3) 5.606
Stable phase at RT Wurtzite
Melting point (oC) 1975
Thermal conductivity 0.13, 1-1.2
Static dielectric constant 8.656
Refractive index 2.008, 2.029
Bulk Young´s modulus, E (GPa) 111.2±4.7
Electron effective mass 0.23m0
Hole effective mass 0.54m0
9
2.4 Electrical properties of ZnO as compared to GaN ZnO has several advantages as compared to its chief competitor GaN. ZnO has a
direct bandgap of 3.37 eV at RT and higher exciton binding energy 60 meV at RT as
compared to GaN 25 meV. However, GaN has a higher electron Hall mobility at RT as
compared to ZnO but ZnO has higher effective mass and larger optical phonon parameters.
ZnO has a higher saturation velocity which is very important for high speed devices [4].
Table 2.2: Electrical properties of ZnO and GaN at RT [4, 5, 10-13]
Parameters ZnO GaN
Band gap energy (eV) 3.37 3.39
Electron mobility (cm2/Vs) ∼210 1000-1350
Hole mobility (cm2/Vs) ∼10 100-400
Exciton binding energy (meV) 60 21-25
2.5 Properties and device applications ZnO displayed a wide range of useful properties as it has been recognized for a
long time [14]. Due to the fact that ZnO is a semiconductor with a direct band gap of 3.44 eV
(at low temperature) it captured most of the attention in recent years [15–17]. ZnO in
principle enables optoelectronic applications in the blue and UV regions of the spectrum.
A list of the properties of ZnO that distinguish it from other semiconductors or render it useful
for applications includes:
2.5.1 Direct and wide band gap
The band gap of ZnO is 3.37 eV at room temperature and 3.44 eV at low
temperatures but the respective values for wurtzite GaN are 3.50 eV and 3.44 eV [15, 18].
Due to these properties ZnO is an important material for light emitting diodes, laser diodes,
photodetectors and optoelectronics in the blue/UV region [4, 5, 7, 19, 20]. Reports on p–n
homojunctions have recently appeared in the literature [21-24], but stability and
reproducibility have not been established.
2.5.2 Large exciton binding energy
Due to the large free-exciton binding energy (60 meV) [25, 26] an efficient
excitonic emission in ZnO can persist at room temperature and higher [25, 26]. Due to this
large free-exciton binding energy ZnO is most promising material for optical devices that are
based on excitonic effects.
10
2.5.3 Strong luminescence
ZnO is a strong luminescence material because it possesses a number of intrinsic
and extrinsic radiative defect levels which emit light in a wide range within the visible region
[27] due to this property ZnO is also a suitable material for phosphor applications. Due to n-
type conductivity, ZnO is one of the best material for applications in vacuum fluorescent
displays and field emission displays. The origin of the luminescence mechanism and the
luminescence center are still under discussion, without any clear evidence, being frequently
attributed to oxygen vacancies or zinc interstitials [28].
2.6 Optical properties of ZnO Due to a number of intrinsic and extrinsic radiative defect levels ZnO emit light
in a wide range within the visible region [27]. There are a variety of experimental techniques
available for the study of optical transitions in ZnO such as reflection, photoreflection,
transmission, optical absorption, photoluminescence (PL), cathodoluminescence (CL),
spectroscopic ellipsometry, and calorimetric spectroscopy [29].
The room temperature photoluminescence spectrum of ZnO is characterized by
the two main peaks, a sharp UV peak centered on 380 nm and another broad deep band
emission that lies literally between 400 nm and up to close to 800 nm. Figure 2.3 shows a
typical PL spectrum of ZnO nanorods at room-temperature [30].
Due to the radiative defects different wavelength emissions from ZnO have been
observed. The deep broad band emission from the ZnO exhibited violet, blue, green, yellow
and orange-red color emissions [29], i.e. it covers the whole visible region.
Figure 2.2: PL spectrum for ZnO NRs measured using an Ar-laser operating at 150 W at room
temperature [30].
11
2.7 ZnO nanostructures
ZnO has a rich family of nanostructures such as nanowires, nanorods, nanocages
nanocombs, nanotubes, nanosprings and nanorings. The different kinds of nanostructures can
be obtained by changing growth condition. By the fabrication of white light emitting diode we
use ZnO nanorods as shown in figure 2.3.
Due to their potential use in nanotechnology ZnO nanostructures has received
broad attention. Light emitting diodes (LEDs), field effect transistors (FET), different sensors
and other devices using ZnO nanostructures have been demonstrated. Due to the intrinsic and
extrinsic defects ZnO has the possibility of white light emission. Nanostructured LEDs are of
great interest due to white light emission from ZnO [38, 39].
Figure 2.3: SEM image of ZnO nanorods grown by the ACG method.
2.8 Native point defects in ZnO Native or intrinsic defects are imperfections in the crystal lattice that involve
only the constituent elements [31]. The enormous interest in semiconductors is that both
electrical and optical properties of semiconductors can be modified by incorporation of small
amounts of impurities or other defects. We can introduce defects during the growth process
and also through annealing or ion implantation.
2.9 Classification of defects Defects can be classified into
1. Line defects
12
2. Point defects
3. Complex defects
1. Line defects
Line defects involve rows of atoms, e.g. dislocations.
2. Point defects
Point defects involve isolated atoms in localized regions of the host crystal.
3. Complex defects
Complex defects are a composition of more than one point defect.
Intrinsic and Extrinsic defects
If the defects involve foreign atoms, i.e. impurities, they are referred to as extrinsic defects but
the defects that only consist of host atoms are called intrinsic defects [5].
Classification of point defects
Point defects are further classified as follows:
Vacancy
Missing atoms at regular lattice positions called vacancy. The notation is VA. This defect is
always intrinsic. In ZnO both zinc (VZn) and oxygen (Vo) vacancies can be formed.
Substitutional
In case of substitutional a host atom A is replaced with an atom C, and it is denoted by CA.
Interstitial
Interstitials (extra atoms occupying interstices in the lattice) and antisites (a Zn atom
occupying an O lattice site or vice versa). An intrinsic interstitial defect is often called self-
interstitial, whereas the extrinsic interstitial defect is referred to as an interstitial impurity.
Antisite
Antisite is a special kind of substitutional defect in this a host atom A is replaced by another
host atom B. The anisite is denoted by the same notation as for substitutional defects. These
defects are intrinsic and in case of ZnO both Zn and O exist.
2.10 Exciton A bound state of an electron and hole which are attracted to each other by the
electrostatic Coulomb force is called an exciton. It is a quasiparticle that exists in insulators,
semiconductors and some liquids. An exciton can move through the crystal and transport
energy but cannot change the charge since it is electrically neutral. An exciton forms when a
photon is absorbed by a semiconductor. This excites an electron from the valence band into
the conduction band. Exciton has been studied in two cases [32, 33].
13
2.10.1 Frenkel exiton
In materials with a small dielectric constant, the Coulomb interaction between
an electron and a hole may be strong and the excitons thus tend to be small, of the same order
as the size of the unit cell. The Frenkel excitons, named after Yakov Frenkel has strong
electron-hole interaction, i.e. the excitons are tightly bound. They have a typical binding
energy on the order of 0.1 to 1 eV. Frenkel excitons are realized in alkalihalide crystals and in
organic molecular crystals composed of aromatic molecules [32, 33].
2.10.2 Wannier excitons
The dielectric constant is generally large in semiconductors. Consequently, the
Coulomb interaction between electrons and holes tends to reduced by electric field screening.
So in most of the semiconductors the excitons are weakly bond. These excitons are called
Wannier exciton, and the binding energy is usually much less than the hydrogen atom,
typically on the order of 0.01 eV [32, 33, 34].
2.11 Recombination
Recombination is a process in which the electrons combine with holes in one or
multiple steps and eventually disappear. The energy difference between the initial and final
state of the electron is released and can leads to one or more possible classification of the
recombination processes, e.g. radiative recombination, Auger recombination or Shockley-
Read-Hall recombination (SRH) [35, 36].
2.11.1 Radiative recombination (Band to band recombination)
In radiative recombination the electron in the conduction band directly combines
with the hole in the valance band and releases a photon therefore it is also called band to band
recombination. The light produced from a light emitting diode (LED) is the most obvious
example of radiative recombination in a semiconductor device [35, 36].
2.11.2 Auger recombination
In Auger recombination the electron and a hole recombine, but no
electromagnetic radiation is emitted, and the excess energy and momentum of the
recombining electron is given up to another electron. Auger recombination is mostly
important at high temperatures and in heavily doped materials.
2.11.3 Shockley-Read-Hall recombination or (Trap-assisted recombination)
14
Recombination through defects or Shockley-Read-Hall (SRH) recombination
occurs when an electron trapped by defect level within the bandgap. One can envision this
process either as a two-step transition of an electron from the conduction band to the valence
band [35, 36].
Figure 2.4: Carrier recombination mechanisms in semiconductors [37]
15
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C. Czekalla, G. Zimmermann, and M. Grundmann, A. Bakin, A. Behrends, M. Al-
Suleiman, A. El-Shaer, A. Che Mofor, B. Postels and A. Waag, N. Boukos and A.
Travlos, H. S. Kwack, J. Guinard and D. Le Si Dang, Nanotechnology 20, 332001
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17
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18
19
Chapter 3
Growth and characterization of ZnO nanorods based LEDs
ZnO nanostructures can be grown by using different high as well as low
temperature (< 100 oC) growth approaches. In our work, we used the vapor-liquid-solid
(VLS) and the aqueous chemical growth (ACG) methods. Due to the lack of a reliable and
reproducible p-type doping in ZnO material we grow n-ZnO on other p-type substrates. In this
chapter we discuss the growth methods of the ZnO nanorods on different substrate, for the
fabrication of light emitting diode. In order to investigate the structural, morphological,
optical and electrical properties of ZnO nanostructures and light emitting diode (LEDs) we
used different experimental and characterization techniques.
3.1 Growth methods of ZnO nanorods
3.1.1 Low temperature chemical growth
To grow ZnO nanostructures there are several different chemical growth
methods but in recent years wet chemical methods have received more attention and are now
commonly used method for the growth of ZnO nanostructures. One of its major advantages is
that the growth temperature is < 90 oC and with such low temperature we can grow ZnO
nanorods on very cheap substrates e.g. plastic, paper and glass. On these substrates we can use
p-type polymers when we want to produce a pn-hybrid junctions with ZnO nanorods, as the
ZnO nanorods are n-type.
3.1.2 Growth of ZnO nanorods on PEDOT:PSS coated flexible plastic substrate
To grow ZnO nanorods on Poly (3, 4-ethylenedioxythiophene) poly
(styrenesulfonate) PEDOT:PSS coated flexible plastic substrate, the substrate was cleaned in
ultrasonic cleaner in acetone, isopropyl alcohol and de-ionised water sequentially. We cover
small portion of the PEDOT:PSS which later work as an electrode then the substrate was spin
coated with Polyfluorene (PFO) and backed at 100 oC for 5 minutes. The PFO was mixed at a
1:1 molar ratio in toluene solvent. This gave a PFO film with 20 nm thickness. To improve
the ZnO nanorods growth quality, distribution and density the substrate was coated with a
seed layer and baked at 100 oC for 10 min [1]. The sample was placed in the solution
containing a mixture of 0.01M zinc nitrate hexa hydrate (Zn (NO3)2.6H2O) and 0.01M
Hexamethyl tetra-mine (HMT) (C6H12N4) and was heated for 5 hours at 90 C°.
20
3.1.3 Growth of ZnO nanorods on paper substrate
For the growth of ZnO nanorods on paper we have to overcome the rough
surface of paper because the paper is synthesized from cotton/pulp contains cellulose due to
that it has very rough surface. In order to overcome the surface roughness the surface is
modified by evaporating Cr/Ag on paper. This Cr/Ag used as a electrode and also reduces the
surface roughness to enhance the alignment and uniformity of the ZnO NRs on the paper
substrate [2]. A hole transporting layer of Poly (3,4-ethylenedioxythiophene)
poly(styrenesulfonate) PEDOT: PSS was spin-coated onto the paper substrates and baked at
80 oC for 5 minutes to form a uniform film. A layer of Polyfluorene (PFO) was spin-coated on
the PEDOT: PSS film and was backed for 5 mints at 80 oC. To grow ZnO nanorods we used
the hydrothermal growth technique. In this method 0.01M zinc nitrate hexa hydrate (Zn
(NO3).6H2O) was mixed with 0.01M Hexamethyl tetra-mine (HMT) (C6H12N4). The sample
was then placed in the solution and was heated at 80 oC for 6 hours.
3.1.4 Growth of ZnO nanorods on glass substrate
In order to clean the glass substrate it was washed in ultrasonic cleaner using
acetone, isopropyle and de-ionised water for 10 min. A hole transporting layer of Poly (3, 4-
ethylenedioxythiophene) poly (styrenesulfonate) PEDOT: PSS was spin-coated onto the glass
substrate for 5 minutes and baked at 80 oC to form a uniform film. On top of the PEDOT: PSS
film a layer of Polyfluorene (PFO) was spin-coated and backed at 80 oC. To grow ZnO
nanorods we used the hydrothermal growth method. In this method we mixed 0.01M zinc
nitrate hexa hydrate (Zn (NO3).6H2O) with 0.01M Hexamethyl tetra-mine (HMT)
(C6H12N4).The sample was then placed in the solution and was heated at 80 oC for 6 hours.
During the process the ZnO nanorods were formed on the substrate.
3.2 Growth of ZnO nanorods by high temperature method or the vapor-
liquid-soild (VLS) mechanism
3.2.1 Cleaning of the substrate
For the growth of ZnO nanorods by the chemical vapour deposition we used 4H-
p-SiC substrate. In this growth method before any growth can take place, all samples need to
prepared. For the growth of ZnO nanorods by the vapor-liquid-soild (VLS) mechanism
cleaning of the samples is very important because if the substrate is clean then one can obtain
21
good, reliable and reproduceible results. In order to clean the substrate we use a standard
cleaning process RCA -1. During the RCA-1 cleaning insoluble organic residues are removed
from the wafer surface [3].This was followed by RCA-2 to remove any metallic species.
3.2.2 Evaporation and cutting of the substrate
The vapour-liquid-solid (VLS) technique is a catalytic growth so the substrates
need to have a catalyst metal on the surface. In this study we used a very thin film (1-5 nm) of
Au on p-SiC. The Au was deposit by a standard evaporation technique. After the evaporation
of Au we used a diamond saw to cut the substrate into small pieces Since for the boat used
during the VLS growth a substrate of 6x8 mm2 is suitable [3].
3.2.3 VLS growth method
We adopted the vapour-liquid-solid technique for the growth of ZnO NRs [3].
The VLS mechanism was first time mentioned by R.S. Wagner and W.C Ellis in 1964 [4]. In
recent years the growth of various nanostructures was achieved by the VLS processes. In this
method a thin layer of Au nano-particles were deposited on the substrate which is use as a
catalyst and heated to form a liquid droplet of the catalyst and the nanostructures on the
surface. The main principle is still the same as in Wagner and Ellis paper [4].
3.2.4 Growth of ZnO nanorods on 4H-P-SiC substrate by the VLS methods
We adopted the vapour-liquid-solid (VLS) technique for the growth of ZnO NRs
on 4H-P-SiC [3]. The substrate coated with Au which is used as a catalyst is placed in a
horizontal tube furnace. The substrate is placed within a certain distance from the boat
containing the reactant, i.e. the ZnO. A mixture of 1:1 ZnO:C powder with respect to weight,
was placed into the boat and the substrate was placed above the mixture, with the catalyst
pointing towards to ZnO:C powder. The boat was placed in the middle of the quartz tube.
22
Figure 3.1: The VLS growth method [5].
A constant Ar gas flow of 80 sccm was applied for 5-10 min to maintain a stable environment
inside the tube. Then we increased the temperature from room temperature to around 890 oC.
The temperature was varied between 875-910 oC and the growth time varied from 5 min to 1
hour. At high temperature the ZnO is reduced to Zn vapor. The reaction is given by:
ZnO (s) + Cgraphite (s) → Zn (g) + CO (g)
ZnO (s) + CO (g) → Zn (g) + CO2 (g)
Figure 3.2: The tube furnace setup [5].
We commonly used argon as an inert gas; the reaction products are transported to the
substrate, where the Zn vapour forms a liquid alloy with the catalyst. The formation of ZnO is
assumed to be a result of a reaction between Zn and CO/CO2.
23
3.3 Bottom contact
ZnO-based semiconductors are of great technological importance for the
fabrication of optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes.
Further improvement of the LED performance can be achieved through the enhancement of
the external quantum efficiency. In this regard, high quality ohmic electrodes having low
contact resistance and high transmittance (or reflectivity), along with thermal stability, must
be developed because ohmic contacts play a very important role in the performance of LEDs.
An ohmic contact on a semiconductor device has been prepared so that the current-voltage (I-
V) curve of the device is linear and symmetric [6].
In this work, different metals were used to form the bottom contacts to the 4H-p-
SiC and p-polymer. A Ni/Al alloys with a thickness of Ni and Al layer of 50 nm and 400 nm,
respectively were used to form ohmic contacts on 4H-SiC. The samples were annealed in
nitrogen and argon (N2/Ar) gass for 2 minutes at 500 oC. This contact gives specific contact
resistance of 7.9x 10-6 Ω-cm-2 [7].
3.4 Spin coating of photo resist, plasma etching and top contact deposition
An insulating photo-resist layer was spin coated on the ZnO nanorods to fill the
gap between them and to isolate electrical contacts from reaching the p-type substrate. After
the spin coating of the photo-resist, using oxygen plasma ion etching top of the ZnO nanorods
were exposed. Al metal was used to form the ohmic contact to the ZnO.
3.5 Scanning electron microscope
The nanostructures grown by both the ACG and the VLS methods were
characterized by a scanning electron microscope (SEM). The result show that the ZnO NRs
were grown vertically aligned. Using the SEM it is possible to determine if any growth has
been taken place because the SEM gives information on the morphology of the surface of the
sample. However from the SEM images we cannot say that the obtain nanostructures actually
consists of ZnO. From the SEM images we can find the diameter, length shape and growth
rate of the nanostructures. Figure 3.3 shows the schematic diagram of the SEM apparatus. In
our experiment a JEOL JSM-6301F scanning electron microscope was used.
24
Figure 3.3: A schematic diagram of a SEM [8] .
The pressure inside the chamber is about 10-6 mbar and the gun voltage used is 12 KV. We
can achieve maximum resolution of upto 10nm. In order to take the images through the SEM
we have no need to do a lot of preparation on the sample.
3.6 Photoluminescence (PL)
In order to probe the electronic structure of materials photoluminescence
spectroscopy is a common nondestructive method. In this method when light falls onto the
sample some portion of the light is absorbed by the sample and imparts rest of the energy into
the material. This process is called photo-excitation. During the process of photo-excitation
excess energy can be dissipated by the sample through the emission of light, or luminescence
and this luminescence is called photoluminescence (PL). This technique has been used to
determine the bandgap and impurity levels, we can also use it to investigate the intrinsic and
the extrinsic properties of semiconductors.
In PL light is used to create electron hole pairs in the semiconductor and the
electrons will accumulate in the conduction band minimum and the hole in the valence band
maximum.
25
When these electrons return to their equilibrium states, they will recombine and the excess
energy is released from the sample, if the light is emitted from the sample then it is called a
radiative process but if there is no emission of light then it is a nonradiative process. The
emitted light is detected with a detector. The amount of energy emitted during the
photoluminescence process is equal to the difference in energy levels between the two
electron states involved in the transition between the excited state and the equilibrium state.
Figure 3.4: A schematic PL experimental setup [3].
A schematic diagram of our PL setup is given in the figure 3.3; in this a laser line from an
Ar+ laser was used as the excitation source. In order to disperse and detect the emission from
ZnO a double grating monochromator and photomultiplier detector were used [3].
3.6.1 Uses of photoluminescense
(a) Band gap determination
PL is use to determine the bandgap of semiconductors. In semiconductors the
radiative transition between states in the conduction and the valence bands, with the energy
difference being known as the band gap.
(b) Impurity levels and defect detection
Radiative transitions in semiconductors also involve localized defect levels and
the energy associated with these levels during the photoluminescence measurement can
26
identify specific defects, and also with the relative intensity of photoluminescence we can
determine their concentration.
(c) Recombination mechanisms
When an electron returns to its equilibrium position, a process known as
"recombination," it can involve both radiative and nonradiative processes. In semiconductors
there are several possible recombinations mechanisms such as band-to-band transition and
donor-acceptor pair recombination. Direct bandgap semiconductor materials have high
probability for radiative recombination of electron-hole pair. Such semiconductors have
strong bandgap radiation and can be used for LEDs.
(d) Band-to-band transition
Energy vs. crystal momentum for a semiconductor with a direct band gap, show
(figure 3.4) that an electron can shift from the lowest-energy state in the conduction band
(green) to the highest-energy state in the valence band (red) without a change in crystal
momentum. Band to band recombination dominates at higher temperatures where all shallow
defects are ionized. The carriers can be frozen on defects at lower temperature. In case of free
to bound transition a free carrier recombine with a bound carrier [9].
Figure 3.5: Energy vs. crystal momentum for a semiconductor with a direct band gap [9].
27
(e) Excitonic emission in ZnO
In ZnO exciton recombination is more efficient than band-to-band transitions
and ZnO has high exciton binding energy. At room temperature the excitons are thermally
stable and accordingly ZnO has a strong near band edge emission. At room temperature the
free excitons (FE) dominate the excitonic emission and FE emission peak is centered around
380 nm (3.37 eV). At low temperature the bound excitons dominates the excitonic emission.
(f) Material Quality
We can determine the materials quality and device performance in the
nonradiative process because in the nonradiative process there are localized defect levels.
With the amount of radiative recombination we can identify the material quality.
3.7 Cathodoluminesence (CL)
High energy electron bombardment of materials can cause emission of light, the
process is known as cathodoluminescence (CL). In order to get the detailed emission
information and to explain the origin of specific emission from specific small area, even from
individual nanostructures, cathodoluminescence spectroscopy is performed, which records
luminescence after creating electron–hole pairs by high energy electron bombardment. During
the CL measurement the sample was on an evacuated CL-stage to an optical microscope. The
CL-stage attachment uses a cathode gun to bombard a sample with a beam of high-energy
electrons. The resulting luminescence in minerals allows us to see textures and compositional
variations that are not otherwise evident using light microscopy.
.
Figure 3.6: Luminoscope optical-CL system which is attachmed to an optical microscope.
28
3.7.1 Type of optical-CL
Optical-CL attachements can be categorized in two types:
1. Cold-cathode CL
2. Hot-cathode CL
Cold-cathode CL
The commonly used optical-CL system is called a cold-cathode CL. This system
is attached to a microscope so one can examine the sample optically with the microscope
and with CL in the same area. In a cold-cathode CL system the electron beam is generated
by a discharge that takes place between the cathode at a negative high voltage and an
anode at ground potential in an ionized gas at a moderate vacuum of ~10-2 Torr. The result
is relatively low-intensity CL in most CL-active silicate minerals. Because some of the
electrical charges at the discharge may reach the sample and neutralize the static charge
built up, it is not necessary to coat the sample with a conductive coating. The CL response
in the sample can be viewed through the objective lens of the microscope or the image can
be recorded with high-speed film or with a digital camera (Figure 2) [10].
Figure 3.7: Top and cross-sectional views of a luminoscope optical-CL system .Modified after
Marshall (1988) [10].
29
Hot-cathode CL
A hot-CL instrument generates the electrons by a heated filament and the
electrons are directed at lower voltage toward the anode. The vacuum is generally lower (<10-
5 Torr) and the accelerating potential is about 14 keV. This stage has a more stable beam and
has greater CL intensity. There are several non-commercial versions of the hot-CL
instrument. These instruments have higher sensitivity and the ability to detect short-lived
luminescence in minerals.
3.8 Current-voltage characteristics
The current versus voltage (I vs V) measurements were performed for the
fabricated LEDs by using semiconductor parameter analyzer (SPA). The I-V curves provide
different information about the information about the LEDs such as its turn-on voltage, break-
down voltage forward and reverse currents, ideality factor, and parallel and series resistance
of the fabricated LEDs.
3.9 Electroluminescence (EL)
Electroluminescence (EL) is an optical and electrical phenomenon in which a
material emits light in response to the passage of a strong electric field. In this phenomenon as
a result of radiative recombination of electrons and holes in a material, usually in a
semiconductor we get an emission spectrum. In semiconductors electroluminescent devices
such as LEDs, Prior to recombination, electrons and holes may be separated either by doping
the material to form a p-n junction or through excitation by impact of high-energy electrons
accelerated by a strong electric field.
In this thesis the electroluminescence (EL) from n-ZnO nanostructures/p-4H-
SiC, p-polymers LEDs is investigated. The electroluminescence (EL) measurements were
performed at room temperature. The fabricated LEDs were illuminated by applying a forward
biasing across the electrodes by using kiethely 2400 apparatus with the help of metal probes.
3.10 Deep level transient spectroscopy (DLTS)
The crystal quality of a semiconductor material is affected by defects such as
dislocations, interstitial, vacancies and antisites. The impurity atoms might get into the crystal
during the growth or by diffusion of different kinds of materials, metal deposition, chemical
30
treatments, and annealing and ion bombardment. Due to the presence of all these defects and
impurities, some localized energy levels called deep levels appear inside the band gap. All
defects are not equally important. Their properties like the position in the band gap or the
optical or electrical capture cross section that determine which one the critical defects.
Several techniques were proposed to get information about these deep levels but
the ingenuous idea of using the change of capacitance under bias conditions due to the
refilling of deep levels was proposed in the sixties [11]. In 1974 D. V. Lang proposed the
deep level transient spectroscopy (DLTS) technique [12] which is a high frequency (MHz
range) junction capacitance versatile technique for the determination of the parameters
associated with traps including density, thermal cross section, energy level and spatial profile.
By monitoring the capacitance transients produced by pulsing the semiconductor junction at
different temperatures, a spectrum is generated which exhibits a peak for each deep level. The
height of the peak is proportional to the trap density, its sign allows one to distinguish
between the majority and the minority traps and the position on the temperature axis leads to
the determination of the fundamental parameters governing the thermal emission and capture
(activation energy and cross section). The DLTS is a quantitative, electrical-characterization
method, which is used to survey deep levels in the band gap of semiconductor materials. Each
deep level in the band gap results in a peak in the DLTS spectrum and a spectrum usually
contains several peaks corresponding to different defects. From the peak position, the
apparent energy position and apparent carrier capture cross section can be extracted.
3.11 Color rendering index (CRI)
The color rendering index (CRI) is a quantitative measure of the ability of a
light source to reproduce the true colors of the objects in comparison with a natural light
source. The color rendering index (CRI) is one of the most important parameter when we use
LEDs for indoor and outdoor lighting applications and in that case the CRI of LEDs should be
high.
The fabricated LEDs were illuminated by applying a forward biasing injection
current across the electrodes by using Kiethely 2400 apparatus with the help of metal probes.
The EL spectra of the emitted light from the fabricated LEDs were measured by a
photomultiplier detector. The value of the color rendering index (CRI) and the correlated
color temperature (CCT) were extracted from the EL spectra of fabricated LEDs by using
computer software.
31
References:
[1] M. Vafaee and H. Youzbashizade, Mat. Sci. Forum 553, 252 (2007).
[2] A. Manekkathodi, M. Y. Lu, C. W. Wang and L. J. Chen, Adv. Mater. 22, 4059 (2010).
[3] P. Klason P 2008 Ph.D. Thesis, Department of Physics, University of Gothenburg,
ISBN 978-91-628-7492-6
[4] R. S. Wagner and W. C. Ellis, Appl. Phys. Lett. 4, 89 (1964).
[5] http://www.mtixtl.com/xtlflyers/vlspapers/JustinHwaReport.pdf
[6] http://en.wikipedia.org/wiki/Ohmic_contact
[7] J. Vang, M. Lazar, P. Brosselard, C. Raynaud, P. Cremillieu, J. L. Leclercg, J. M. Bluet,
S. Scharnholz, and D. Planson, Superlattices and Microstructures 40, 626 (2006).
[8] http://www.britannica.com/EBchecked/media/110970/Scanning-electron-microscope
[9] http://en.wikipedia.org/wiki/Direct_and_indirect_band_gaps
[10] http://serc.carleton.edu/research_education/geochemsheets/opticalcl
[11] R. Williams, J. Appl.Phys. 37, 3411 (1966).
[12] D. V. Lang, J. Appl. Phys. 45, 3023 (1974).
32
33
Chapter 4
Device application of ZnO nanostructures
Light emitting diodes (LEDs) were reported for first time in 1907 but they
gained practical importance only in 1962 when the first red light emitting diode was invented.
Today, LEDs made with different semiconductor materials are available in different colors
with reasonable efficiency and this opens the possibility to replace conventional light sources
with these LEDs.
4.1 ZnO nanorods (NRs) based intrinsic white light emitting diode
(WLEDs)
ZnO is one of the prime materials for intrinsic white light emitting diodes (WLEDs)
with potential advantages over the III-nitride system due to many reasons, among them is the
existence of many deep radiative levels [1]. These radiative defects emit different colors such
as blue, green, yellow and red colors which covers the whole visible region [1, 2]. ZnO
typically exhibits one sharp UV peak and possibly one or two broad deep band emissions
(DBE) due to radiative defects within the band gap [1-3]. The dominant emitted colors depend
on the growth method and conditions; this implies that the emitted color lines can be
controlled [1, 2].
ZnO is considered to be an alternate to GaN for WLEDs owing to its relatively low
production cost and superior optical properties. However, during the last few years
reproducible p-type doping in ZnO material with sufficiently high conductivity and carrier
concentration remains to be the main obstacle for the realization of ZnO-based
homojunctions. The lack of a reliable p-type doping has initiated an alternative approach to
grow ZnO nanostructures on other inorganic/organic p-type substrates to realize ZnO-based
p–n heterojunctions. Therefore ZnO-based WLEDs are potentially useful in efficient solid
state lighting where intrinsic white light can be achieved by using an appropriate external p-
type electrode [4]. In this thesis we demonstrate ZnO nanorods (NRs) based WLEDs on
inorganic/organic substrates such as p-SiC, n-SiC and Polyfluorene (PFO). ZnO NRs may
offer additional advantages for light emission due to the increased junction area, enhanced
polarization dependence of reflectivity, and improved carrier confinement in one dimensional
nanostructure [4, 5]. At the same time, both ends of ZnO NRs are extremely smooth, making
34
them perfect mirror planes and vertical NRs are like natural waveguide cavities for making
the emitted light to travel to the top of the device, also minimizing partial leakage and thus
enhancing the light extraction efficiency from the surface [6].
ZnO NRs based heterojuctions LEDs which are used in this thesis are given as:
1. ZnO NRs/p-SiC
2. ZnO NRs/n-SiC
3. ZnO NRs/PFO/PEDOT:PSS/Plastic
4. ZnO NRs/PFO/PEDOT:PSS/Glass
A device diagram of ZnO NRs based heterojunction LEDs grown on p-SiC is illustrated
schematically in Fig.4.1. The corresponding EL spectrum demonstrates three peaks at 425
nm (violet), 525 nm (green) and 685 nm (red) as shown in Fig. 4.2.
Figure 4.1: Schematic illustration of ZnO NRs/ p-SiC hetrostructure device.
p-SiC
n-SiC
Ni/Al
PMMA layer
ZnO NRs
Al
35
400 500 600 700 800
EL In
tens
ity (a
.u.)
Wavelength (nm)
Figure 4.2: Room temperature EL spectrum of ZnO NRs/p-SiC heterojunction LED.
The schematic device diagram of ZnO NRs/n-SiC Schottky LEDs is shown in Fig .4.3.
Figure 4.4 shows that the electroluminescence (EL) spectrum which reveals a broad emission
band extending from 400 nm to 800 nm. In order to distinguish the white-light EL spectrum
components we used a Gaussian function to simulate the experimental data. The simulated EL
spectrum shows three different emissions at 476 nm, 538 nm and 690 nm which are
associated with blue-green, green and red, respectively.
Figure 4.3: Schematic illustration of ZnO NRs/n-SiC Schottky LED.
Ni
PMMA layer
ZnO NRs
Au
n-SiC
36
400 500 600 700 800
690 nm
538 nm
EL In
tens
ity (
a. u
.)
Wavelength (nm)
476 nm
Figure 4.4: (a) Room temperature EL spectrum and the Gaussian fitting of the ZnO NRs/n-
SiC Schottky LED.
A schematic diagram of the ZnO NRs/PFO heterojunction LEDs on PEDOT:PSS
coated flexible plastic substrate is shown in Fig. 4.5. Figure 4.6 shows the EL spectrum
covering the whole visible region. In order to distinguish the white-light EL spectra
components and identify the contribution of the PFO we used a Gaussian function to simulate
the experimental data. The simulated EL spectrum shows five different emissions at 448 nm,
469 nm, 503 nm, 541 nm and 620 nm which are associated with violet, blue, green and red,
respectively. The emissions at 448 nm and 469 nm are due to exciton emission of the PFO.
Figure 4.5: Schematic illustration of ZnO NRs/PFO hybrid device on PEDOT:PSS coated
flexible plastic.
PEDOT coated Plastic
PFO layer
PMMA
ZnO
Al electrode
PEDOT:PSS
37
400 500 600 700 800
EL In
tens
ity (a
. u.)
Wavelength (nm)
448 nm469 nm
503 nm
541 nm
620 nm
Figure 4.6: Room temperature EL spectrum and the Gaussian fitting of the PFO/ZnO hybrid
LED.
Figure 4.7 shows the schematic diagram of ZnO NRs/PFO/PEDOT:PSS
heterojunction LEDs on glass substrates. The RT-EL spectra of hybrid white light emitting
diodes (HWLEDs) reveal a broad emission band from 420 nm to 800 nm covering the whole
visible region as shown in Fig. 4.8. In order to distinguish the white-light EL spectra
components we used a Gaussian function to simulate the experimental data. The simulated
EL spectrum shows four emissions centered at 454 nm, 540 nm, 617 nm and 680 nm which
are associated with blue, green, orange and red, respectively
Figure 4.7: Schematic illustration of the ZnO NRs/PFO/PEDOT:PSS heterostructure device.
Glass Substrate
PFO
PEDOT
PMMA
ZnO
ITO
A
38
400 500 600 700 800
EL In
tensit
y (a
. u.)
Wavelength (nm)
454 nm
540 nm
617 nm
680 nm
Figure 4.8: Room temperature EL spectrum and the Gaussian fitting of the ZnO
NRs/PFO/PEDOT:PSS/glass heterojunction LED.
39
References:
[1] M. Willander, O. Nur, Q. X. Zhao, L. L. Yang, M. Lorenz, B. Q. Cao, J. Zúñiga Pérez,
C Czekalla, G Zimmermann, and M. Grundmann, A. Bakin, A. Behrends, M. Al-
Suleiman, A. El-Shaer, A. Che Mofor, B. Postels and A. Waag, N. Boukos and A.
Travlos H. S. Kwack, J. Guinard and D. Le Si Dang, Nanotechnology 20, 332001
(2009).
[2] N. Bano, S. Zaman, A. Zainelabdin, S. Hussain, I. Hussain, O. Nur, and M. Willander, J.
Appl. Phys. 108, 043103 (2010).
[3] A. B. Djurisic, Y. H. Leung, Optical properties of ZnO nanostructures. Small 2, 944–
961 (2006).
[4] C. Y. Chang, F. C. Tsao, C. J. Pan, G. C. Chi, H. T. Wang, J. J. Chen, F. Ren, D. P.
Norton, S. J. Pearton, K. H. Chen, and L. C. Chen, Appl. Phys. Lett. 88, 173503 (2006).
[5] R. Könenkamp, R. C. Word and C. Schlegel, Appl. Phys. Lett. 85, 6004 (2009).
[6] E. Lai, W. Kim and P. Yang, J. Nano Research 2 (1), 123 (2008).
40
41
Chapter 5
Results and discussion
5.1 Identification of violet-blue luminescent centers in ZnO NRs/p-SiC
hetrojunctions
In paper I and II the violet-blue luminescent centers in ZnO NRs grown by the VLS
method was identified using current transport mechanisms and deep level transient
spectroscopy (DLTS) technique complemented by photolumincence. We have different
results from previously published paper on n-ZnO thin films on p-SiC. We found that the
dominant emission is originating from the ZnO NRs.
The ZnO NRs/p-SiC heterojunction exhibited good rectifying I-V characteristics as
shown in Fig. 5.1 and the inset shows the dynamic diode resistance R=dV/ dI. The diode
dynamic resistance R decreases with increasing the forward bias and it maintains a constant
resistance above the turn-on voltage. This means that the diode resistance would first decrease
upon the increase of the forward bias voltage (i.e the lowering of the barrier). When the
forward bias is large enough and the barrier is negligible; the diode resistance becomes
constant, this is due to bulk material series resistance [1].
-6 -4 -2 0 2 4 6
0.0
2.0x10-6
4.0x10-6
0 2 4 6
0.0
2.0x105
4.0x105
6.0x105
dV/d
I (oh
m)
Voltage (V)
Curre
nt (A
)
Voltage (V)
Figure 5.1: Typical current-voltage characteristics of ZnO nanorods and the inset is the
diode resistance (dV/dI) as a function of voltage
42
The slow rise in the forward current under low bias could be explained by the division
of the voltage of the non-ideal contact between the Al electrode and ZnO nanorods. Non-ideal
contact shows higher resistance under forward bias.The series resistance (Rs) is calculated as
90 kΩ by the Cheung method [2]. The shape of the I–V characteristics depend on the Rs of
the device if the Rs is high, the I–V curve will show wide curvature, and if the effect of Rs is
relatively small, then the non-linear region of forward bias I–V curve will be small [3].
The ideality factor and barrier height were found to be 3-4 and 0.9 eV, respectively.
The higher value of the ideality factor indicates that the transport mechanism is no longer
dominated by the thermionic emission. To understand which mechanisms that control this
junction behavior, the I-V characteristics of the device is studied in the log-log scale. The log-
log plot of the I-V data is shown in Fig. 5.2 which demonstrates that the current transport
mechanism exhibit three different regions. The current in region I follow a linear dependence,
i.e. I~V. This indicates the current transport is dominated by tunneling at low voltages. The
boundary for this region was determined to be below 0.03 V. In region II (0.04-1 V) the
current increases exponentially as a relation of I~ exp (cV). In region III (above 1 V) the
current follows a power law (I~V2), indicating a space-charge limited current (SCLC)
transport mechanism. Lampert and Mark have developed a single carrier SCLC model with
the presence of a trap above the Fermi level [4]. At an applied voltage of V>VTFL (TFL stands
for trap filled limit) all the traps are filled and the conduction would become space charge
limited and the current follows the Mott and Gurney SCLC [5]. The deep level parameters
were also calculated from the experimental VTFL at room temperature. In these calculations
the depletion region thickness at zero bias capacitance was used as the active layer thickness.
The values of the trap concentration Nt and the location of deep level states (traps) below the
conduction band were obtained to be 3.4 × 1017 cm-3 and ∼0.24 ±0.02 eV, respectively. The
deep level states observed are in approximate agreement with the reported data of zinc
interstitial (Zni) level which is theoretically located at 0.22 eV below the conduction band [6].
43
10-2 10-1 100 101
10-10
10-9
10-8
10-7
10-6
10-5
VTFL
I∼V 2
I∼exp(cV)I∼V
Curre
nt (A
)
Voltage (V)
Von
I
II
III
Figure 5.2: Log-log plot for the I-V data of ZnO nanorods/p-SiC
To gain greater insight into the ZnO bandgap the DLTS measurements are carried out.
The DLTS spectrum of ZnO under identical conditions: (VP/VR = 0/-5, tp = 20 µs, rate
window 4550 s-1) is shown in Fig. 5.3 (a). This DLTS spectrum shows two levels E1 and P1
and yielded activation energies (capture cross sections) as 0.52 eV (2.7 × 10-15 cm2) and 0.635
eV (2 × 10-14 cm2), respectively. The trap concentration (Nt) for the E1 and P1 were found to
be 1×1015 cm-3 and 7×1014 cm-3, respectively. The traps observed in our ZnO NRs are of
great interest because of the fact that the defect-induced electronic states in the band gap can
significantly alter the optical performance of the device under investigation.
Several groups have reported that different defect centers in ZnO are responsible for
blue, green, yellow, and red emissions. We have correlated E1 and P1 traps emit blue, violet
and green-yellow in the probable emission spectra.
44
200 300 400 500
-8
-4
0
4 P1
E1
DLTS
Sig
nal (
a.u)
Temperature (K)
Figure 5.3: (a) Representative DLTS spectra of ZnO NRs. Filled circles (experimental) and
solid line (line shape fit) which shows two traps E1 (electron trap) and P1 (hole trap).
We have used energy–wavelength relation: E = hc/λ, to calculate the wavelength and
identified the components in the emission spectrum.
Violet
Blue
Green-yellow
Zni (E1)
UV
VZn (P1)
C.B
V.B
0.52 eV
0.635 eV
446 nm
465 nm
575 nm 380 nm
Figure 5.3: (b) Schematic band diagram showing the three observed luminescence
processes.
45
As a result, blue (465 nm) and violet (446 nm) emissions are found to be emitted by
direct transition/recombination of carriers from the conduction band to the radiative centre
VZn (P1) and from radiative center Zni (E1) to valance band. Moreover green-yellow (575 nm)
emission is found to be emitted by transition of carriers from traps Zni to VZn. The detail of
this emission model is illustrated in the Fig. 5.3 (b).
5.2 Spatial distribution of the luminescence centers in ZnO NRs/p-SiC
hetrojunctions
In paper III depth resolved cathodoluminescence spectroscopy technique was used to
gain greater insight into the defect chemistry of ZnO NRs and the spatial distribution of the
luminescence centers along the vertical direction in ZnO NRs. For this study the ZnO NRs
were grown by theVLS method on 4H-p-SiC substrates.
Figure 5.4 shows the RT-CL spectra at different accelerating voltages (2-30 kV) and
with beam current is 56 pA. The CL spectra exhibit NBE emission at 380 nm, a green band
centered at 521 nm and a red emission at 750 nm. To investigate the electron penetration
depth (U0) in the ZnO the graph for the energy loss per unit depth, dE/dx, as a function of the
depth into the ZnO is shown in Fig. 5.5 (a). These curves are the polynomial fit to the
Everhart-Hoff depth dose relation [7]. The penetration depth (U0) varies with the incident
electron accelerating voltages such that U0 = 0.04, 0.1, 0.4, 1.14 and 2.16 µm for Vb = 2, 5,
10, 20 and 30 kV, respectively. The electron acceleration voltage of 20 kV corresponds to
1µm of penetration depth [8]. The RT-CL spectra exhibit orders of magnitude lower NBE and
high DBE which shows that defect concentrations is varying in orders of magnitude with the
depth along the ZnO nanorod. The CL emission intensities of the NBE, DBE and red
emissions as a function of the penetration depth are shown in Fig .5.5 (b). The band-edge and
the green emission show distinct differences in depth dependence but the red emission
illustrates weak depth dependence. For the U0 from 1 to 2.21 µm the green emission intensity
is higher than the NBE intensity. It was found in bulk and thin films of ZnO that the UV
emission from ZnO can be internally reabsorbed by the crystal itself within a 1µm range [9].
However, this reabsorbed UV emission can excite defect states in the material resulting in
DBE. Thus, part of the UV emission may contribute to the enhancement of the NBE [10]. The
ratio of the band edge to the DBE intensity, which is used as a criterion for the nanorods
quality, decreases with penetration depth within the whole length of our NRs. As a result from
Fig. 5.5 (b), it is concluded that there are more deep defects at the root of ZnO NRs.
46
Willander et al. demonstrated that ZnO NRs have more structural defects at the interface ZnO
NRs and the substrate than at the top, by using HR-TEM study [11]. Chien-Lin et al. reported
that the NBE and DBE are emitted separately from two opposite halves of the nanorods [12].
By using CL spectroscopy it is concluded that ZnO NRs have more radiative defects at the
interface.
400 500 600 700 800
CL In
tens
ity (a
rb.u
.)
Wavelength (nm)
02 kV 05 kV 10 kV 15 kV 20 kV 30 kV
Figure 5.4: Depth dependent CL spectra of ZnO NRs taken at a spot size of 50 nm at room
temperature.
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.20
100
200
300
400
30 kV
Ener
gy lo
ss d
E/dx
(eV/µm
)
depth (µm)
20 kV
47
Figure 5.5: (a) The energy loss characteristic dE/dx versus depth curves for two energies in
ZnO nanorods.
0 1 20
1
2
3
4
5Intensity ratio
Red
Green
NBE
NBE/Green
Penetration Depth (µm)
CL in
tens
ity (a
rb. u
.)
Fig. 5.5 (b) The emission intensity (right axis) of the NBE, green and red band as well as the
ratio of NBE and the green bands (left axis) as a function of the penetration depth.
The Figure 5.6 shows a CL spectrum from an area containing no ZnO, i.e. SiC.
400 500 600 700 800
CL In
tensit
y (ar
b.u.)
Wavelength (nm)
SiC
ZnO NRs
Figure 5.6: Comparison of the CL emission spectra from ZnO NRs and nearby area without
ZnO NRs (4H-SiC area).
48
The CL spectra of SiC have very weak emission at 390 nm and 540 nm but the CL
spectrum of ZnO NRs show stronger emissions at 380 nm and at the DBE (420-720 nm)
centered at 521 nm. Therefore this confirms that most of the emission is originating from the
ZnO NRs, and the SiC has less contribution to the emission spectrum.
5.3 Schottky LEDs grown by low temperature aqueous chemical growth
method
In Paper VI we report ZnO NRs based Schottky LEDs. ZnO NRs were grown by the
low temperature aqueous chemical growth (ACG) technique on 4H-n-SiC substrates. The
electrical and optical properties of the Schottky diodes were investigated by the current-
voltage (I-V), EL and CL measurements.
Schottky contacts to ZnO have been studied for over 40 years, since Mead’s
pioneering work in the 1960s on vacuum cleaved ZnO surfaces [13]. Figure 5.7 shows the
current-voltage I-V characteristics for Au Schottky diode. The ideality factor and the barrier
height were found to be 3 and 1.0 eV, respectively. The corresponding EL spectrum
demonstrate two peaks at 533 nm (green) and 700 nm (red) as shown in Fig. 5.8. Several
groups have reported that different defect centers in ZnO are responsible for the blue, the
green, and the red emissions [14]. In the literature, there is still controversy about the origin of
the luminescence centers observed in ZnO materials [15]. The variation in the green emission
peak position is attributed to different relative contributions from, e.g. VO, VZn, Zni, Oi and Cu
related defects caused by the fluctuations in the growth conditions for different growth
methods [14, 16]. Recently, the green emission has been explained to be originating from
more than one deep level defect, VO and VZn with different optical characteristics were found
to contribute to the broad green luminescence band [16, 17]. The red emission at 700 nm (1.8
eV) is attributed to the zinc vacancies or excess oxygen [18].
49
-10 -5 0 5 10
0.0
4.0x10-7
8.0x10-7
1.2x10-6
Curre
nt (A
)
Voltage (V)
Figure 5.7: Typical room temperature current-voltage characteristics of the Au/ZnO NRs
Schottky diode.
400 500 600 700 800
EL In
tens
ity (
a. u
.)
Wavelength (nm)
533 nm
700 nm
Figure 5.8: Room temperature EL spectra of Au/ZnO NRs Schottky diode.
50
For the detailed emission information and to explain the origin of specific emission
from specific small area depth resolved cathodoluminescence (CL) spectroscopy is
performed. Depth resolved cathodoluminescence spectroscopy technique involves an incident
electron beam of energy that produces free electron-hole pair recombination across the
bandgap or between deep levels and the band edges. The CL penetration depth increases with
increasing the beam energy, as determined by the energy-range relationships for energy loss
within a solid [8, 19]. CL signals from different depth positions within the bandgap of the
material can be excited and thus an average depth distribution of the luminescence can be
determined. The acceleration voltage for this study was varied from 2 to 30 kV, which
corresponds to a maximum penetration depth of 0.04 – 2.16 µm as calculated from the
Kanaya-Okayama model [19].
Figure 5.9 (a) exhibits RT-CL spectra as a function of the accelerating voltage (5-30
kV). The number of excited carriers is considered to be in proportion to accelerating voltage.
The beam current is kept at 180 pA. The CL spectra exhibit NBE emission at 380 nm and
DLE centered at 690 nm. The depth of U0 peak which is related to the electron-hole pair
creation rate, varies with the incident electron accelerating voltages such that U0=0.04, 0.1,
0.4, 1.14 and 2.16 nm for Vb = 2, 5, 10, 20 and 30 kV, respectively. The CL emission
intensities of the NBE emission and the DLE as a function of the penetration depth are shown
in Fig. 5.9 (b). The band-edge and the DLE show distinct differences in depth dependence.
For the penetration depth in the range of 0-0.75 µm, the DLE intensity is increasing linearly
and tends to reach saturation afterwards but the NBE emission increases slowly and decreases
afterwards. Remarkably, the ratio of the band edge to the DLE intensity, which is used as
criteria for the nanorods quality, decreases with the penetration depth within the whole length
of the NRs as shown in Fig. 5.9 (b). The CL spectrum of SiC has a very weak emission as
shown in Fig 5. 10 which confirms that most of the emission is originating from the ZnO
NRs, and that the SiC contributes a little in the observed emission. To fully understand the
correlation between the luminescence and the transport characteristics, more systematic
investigation of samples grown by different techniques are necessary. The inhomogeneities
observed among ZnO nanostructures can be attributed to fluctuations during growth.
Investigation of inhomogeneities among ZnO nanostructures grown by different methods can
shed some light on the fundamental growth mechanisms and to identify ways for possible
improvements.
51
400 500 600 700 800 900
5kV 10kV 20kV 30kV
CL In
tens
ity (a
. u.)
Wavelength (nm)
Figure 5.9: (a) Depth dependent CL spectra of ZnO NRs at room temperature.
0 1 2
0 1 2
Penetration Depth (µm)
Intensity Ratio
DLE
NBE
NBE/DLE
CL In
tens
ity (a
. u.)
Figure 5.9: (b) The emission intensity (right axis) of the NBE, and DL emission as well as the
ratio of NBE and DL emission (left axis) as a function of the penetration depth.
52
400 600 800
CL In
tens
ity (a
. u.)
Wavelenght (nm)
ZnO NRs n-SiC
Figure 5.10: Comparison of the CL emission spectra from ZnO NRs and 4H-SiC area.
5.4 Hybrid white LEDs grown on flexible plastic substrates using low
temperature aqueous chemical growth method
I paper V we present an approach which can provide the possibility of achieving low
cost large area flexible lighting sources. We demonstrate white-light luminescence from ZnO-
organic hybrid LEDs grown on PEDOT:PSS coated flexible plastic substrates by the low
temperature ACG method. The PEDOT:PSS coating layer is used as an electrode for the ZnO-
organic hybrid white light emitting diodes (WLEDs) and structural, electrical and optical
properties of these WLEDs were measured and analyzed using different techniques.
The current-voltage (I-V) characteristics of the hybrid WLED is shows in Fig. 5.11
and the inset show the semi-log plot of the I-V characteristics. The value of the ideality factor
of the hybrid WLED was found to be in the range 3-3.8 for all the diodes investigated which
is comparable to the recently reported values of the ideality factor for polymer based hetero-
junctions [20].
To be able to separate the emission sources we grew ZnO nanorods on the same
plastic substrate without the use of an underlying polymer films. Figure 5.12(a) shows the
RT-PL spectrum of the ZnO nanorods which illustrate a sharp UV peak with additional two
broad deep level emissions centered at the green and red wavelengths. The RT-PL spectrum
53
of ZnO-organic hybrid structure consisted of ultraviolet (UV) emission centered at around
380 nm, PFO violet and blue emissions at 435 nm, 464 nm respectively, and two deep level
emissions DLE bands green at 513 nm and the red at 666 nm from ZnO NRs as shown in Fig.
5.12(b). It is extensively reported that the DLE is a superposition of many different deep
levels emitting at the same time [21]. The peak position of the deep band emission is defined
according to the relative density of these radiative defects which controls directly or indirectly
the luminescence efficiency in ZnO. The RT-PL spectra of PFO (without ZnO NRs) reveals
two violet emissions at 424 nm and 446 nm, and one broad blue emission centered at 490 nm
as shown in the inset of Fig 5.12(b). However it was difficult for the PFO to obtain pure and
stable blue emission due to the presence of undesirable green emission from
electroluminescence devices which might originate from an oxidation of the 9-monoalkylated
fluorenes during device processing [22]. Recently PFO peaks at 424 nm, 446 nm and 520 nm
were reported [22].
-4 -2 0 2 4
0.0
1.0x10-5
2.0x10-5
3.0x10-5
-4 -2 0 2 4
-20
-16
-12
-8
Cur
rent
(A)
Voltage (V)
Voltage (V)
ln I
(A)
Figure 5.11: Typical room temperature I-V characteristics of the ZnO NRs/PFO hybrid
WLED and the inset shows the semilog plot of I-V characteristics.
54
400 500 600 700
PL In
tens
ity (a
rb. u
.)
Wavelength (nm)
UV
Green
Red
Figure 5.12: (a) Room temperature PL spectrum of the ZnO NRs grown by the same
parameters on plastic substrate without the PFO film.
400 500 600 700
400 500 600
490 nm446 nm
PL
Inte
nsity
(ar
b. u
.)
Wavelength (nm)
424 nmPFO
PL
Inte
nsity
(arb
. u.)
Wavelength (nm)
UV
ZnO
Violet
PFO
Blue
PFO
Green
ZnO + PFO
Red
ZnO
Figure 5.12: (b) Room temperature PL spectrum of the ZnO NRs/PFO hybrid structure and
the inset shows the room temperature PL spectrum of the PFO on PEDOT:PSS plastic.
55
The electroluminescence (EL) spectrum reveals a broad emission band ranging from
420 nm to 750 nm as shown in Fig. 5.13. In order to distinguish the white-light EL spectrum
components and the contribution of the PFO emission we used a Gaussian function to
simulate the experimental data. The simulated EL spectrum shows five different emissions at
448 nm, 469 nm, 503 nm, 541 nm and 620 nm which are associated with violet, blue, green
and red, respectively. The emissions at 448 nm and 469 nm are due to exciton emission of
PFO. The minute contribution in green emission at 503 nm from the PFO is due to the
formation of excimer as stated above. In our PL spectrum of the PFO the three peaks are
appearing at 424 nm, 446 nm and 490 nm while in the EL spectra these peaks emerge at 448
nm, 469 nm and 503 nm which is consistence with the previous reports. The emissions at
541 nm (green) and 620 nm (orange-red) are associated with intrinsic deep level defects in
ZnO. The white-light EL might be attributed to the excitonic emissions resulting from the
recombination of the electrons and the holes at the ZnO NRs/PFO interface. The lifetime of
the electrons existing in the ZnO/PFO interface is much larger than that in the ZnO NRs [23].
The electrons existing at the interface between the conduction band edge of the ZnO NRs and
the lowest unoccupied molecular orbit (LUMO) level of the PFO layer slowly drop to the
lower states due to their longer life time. Therefore the emission with various wavelengths is
emitted due to the continuous recombinations between the electrons and the holes during
electron transitions from the upper states to the lower states [23]. We have many states in the
ZnO band gap due to native defects in ZnO. The energy states of various native defects have
been calculated or experimentally measured in the band gap of ZnO. The EL emissions at 448
nm (violet) and 469 nm (blue) are due to the exciton emission in the PFO [22]. These PFO
related violet and blue emissions combined with ZnO defect related emissions results in the
observed white-light emission. The combination of ZnO NRs with low temperature growth
process and the low cost transparent flexible substrates makes the organic-inorganic hybrid
junction LEDs attractive for the development of large-area electronic devices such as flat
panel screens and flexible electronics that can be folded up.
56
400 500 600 700
EL In
tens
ity (a
rb. u
.)
Wavelength (nm)
448 nm469 nm
503 nm
541 nm
620 nm
Figure 5.13: Room temperature EL spectrum and the Gaussian fitting of the PFO/ZnO hybrid
WLED.
The color quality was investigated for this ZnO-organic hybrid WLEDs and figure
5.14 the shows color coordinates measurement of the present ZnO-organic hybrid WLEDs.
The color rendering index (CRI) and the correlated color temperature (CCT) of the white light
emitting diodes were calculated to be 68 and 5800 K, respectively.
Figure 5.14: Typical color coordinates characteristics of the ZnO-PFO HWLEDs.
57
5.5 ZnO-nanorods/p-polymer hybrid white LEDs grown on glass substrates
using the low temperature aqueous chemical growth method
In paper VI we demonstrate an intrinsic white-light luminescence from ZnO-
nanorods/p-polymer hybrid white LEDs grown on glass substrates by the low temperature
ACG method. The configuration used for the hybrid white light emitting diodes (HWLEDs)
consists of two-layers of polymers PEDOT:PSS and PFO on glass with top ZnO NRs. The
structural, electrical and optical properties of the HWLEDs were measured and analyzed
using different techniques. The transitions causing radiative emissions are discussed in terms
of the energy band diagram of the ZnO-nanorods/p-polymer hybrid junction.
Figure. 5.15 shows the RT-PL spectra of the PFO and the HWLEDs. The RT-PL
spectrum of the PFO (without ZnO NRs) reveals two violet emissions at 429 nm and 451 nm,
and one broad blue-green emission centered at 482 nm. The RT-PL spectrum of the
HWLEDs consisted of a UV emission centered at around 380 nm, a PFO violet-blue emission
at 432-450 nm and two DLE bands from ZnO NRs, a green centered at 512 nm and red at 652
nm. Figure 5.16 shows the current-voltage (I-V) characteristics of the heterojunction. The
ideality factor was found to be in the range of 3-3.5. The value of the ideality factor is
comparable to the recently reported value of ideality factor for polymer heterojunction [2].
The RT-EL spectrum of the HWLEDs illustrates a broad emission band from 420 to
800 nm covering the whole visible region as shown in Fig. 5.17(a). The simulated EL
spectrum shows four emissions at 454 nm (blue), 540 nm (green), 617 nm (orange) and 680
nm (red). The emission at 454 nm (blue) is due to radiative recombination in the PFO and the
green-red (540-680) emissions are associated with radiative defects in the ZnO. Figure 5.17
(b) demonstrates the RT-EL spectrum with all emissions which are distinguished by a
Gaussian function. To investigate the color quality of the LEDs color chromaticity
coordinates are plotted in Fig 5.17 (c) which shows the emitted light has a white impression.
The color rendering index (CRI) and correlated color temperature (CCT) of the LEDs were
calculated to be 70 and 5500 K, respectively.
58
400 500 600 700
ZnOZnO
Red
482nm
451 nm
PL In
tens
ity (a
rb. u
.)
Wavelength (nm)
HWLED PFO
429 nm
UV
ZnO
Green
Figure 5.15: Room temperature PL spectrum of the ZnO NRs/PFO hybrid structure and the
PL spectrum of the PFO on glass substrate.
-4 -2 0 2 4 6 8 100.0
2.0x10-4
4.0x10-4
6.0x10-4
Curre
nt (A
)
Voltage (V)
Figure 5.16: Typical room temperature I-V characteristics of ZnO NRs/PFO/PEDOT:PSS
heterojunction.
59
400 500 600 700 800
EL In
tensit
y (a
rb. u
.)
Wavelength (nm)
454 nm
540 nm
617 nm
680 nm
MeasurementSimulationSimulated comp.
Figure 5.17: (a) Room temperature EL spectrum after a Gaussian fitting.
Figure 5.17: (b) The EL spectrum showing the whole visible range which is distinguished by
a Gaussian fitting.
60
Figure 5.17: (c) The chromaticity diagram of the HWLED.
The room temperature EL and PL spectra of the HWLEDs with the PL spectra of PFO
(without ZnO) are shown in Fig. 5.18. Figure 5.18 obviously demonstrates that the EL bands
of the HWLEDs are consistent with the sum of the individual PL bands of the PFO and the
ZnO components. It is essential to mention that the PFO blue peaks are completely intermixed
with the DLE bands of the ZnO NRs resulting in a single broad band. But the UV emission of
the ZnO NRs is not detected in the EL spectra probably due to the self-absorption of the UV
light by the ZnO direct bandgap and/or absorption by the PFO at the ZnO/PFO interface [24].
Do-Hoon Hwang et al. reported that polyfluorenes (PFs) exhibit absorption maxima close to
380 nm. The concentration of the PFO polymer has vital a role on the optical efficiency and
light quality of the HWLEDs [25].
We discuss the EL emission of the WLEDs by using the energy band diagram. The
energy band diagrams of the WLEDs without and with bias voltage are shown in Fig. 5.19(a)
and (b) respectively. It can be seen in Fig. 5.19(a) that there is a 0.9 eV barrier for hole
injection from the Ag to the PEDOT:PSS highest occupied molecular orbit (HOMO) and a
0.6 eV barrier from the PEDOT:PSS to the PFO HOMO. Also there is a 2 eV barrier for hole
injection from the PFO HOMO to the ZnO valence band. The electron injection barrier is 0.5
eV between the ITO Fermi level and the ZnO conduction band, and 1.7 eV barriers for the
electron injection from the ZnO conduction band to the PFO lowest unoccupied molecular
orbit (LUMO). When the device is biased the barriers normally cause hole and electron
accumulation and energy band bending at the organic/ZnO interface [26]. The electrons and
the holes under forward bias accumulate at the ZnO NRs/PFO interface, as shown in Fig.
5.19(b).
61
400 500 600 700
Inte
nsity
(arb
. u.)
Wavelength (nm)
PL HWLED PL PFO EL HWLED
Figure 5.18: Room temperature EL and PL spectra of the HWLEDs with the PL spectrum of
the PFO (without ZnO).
Figure 5.19: (a) Energy band diagram of the ZnO NRs/PFO hybrid heterojunction showing
the EL emissions from the ZnO NRs.
PED
OT
:PSS
PFO
VO
Oi
4.4 eV
6.22 eV 6.5 eV
Zni
540 nm
617 nm
680 nm
4.3 eV
3.5 eV
5.2 eV
2.5 eV
5.6 eV
Ec= 4.2 eV
Ev= 7.6 eV
4.7eV Ag ITO
LUMO
HOMO
ZnO
62
Figure 5.19: (b) Energy band diagram of the ZnO NRs/PFO hybrid heterojunction under
forward bias showing band bending and charge accumulation at the interface.
The white-light EL might be attributed to the excitonic emissions resulting from the
recombination of the electrons and the holes at ZnO NRs/PFO interface. The lifetime of the
electrons existing in the ZnO/PFO interface is much larger than that in the ZnO NRs [23]. The
electrons existing at the interface between the conduction band edge of the ZnO NRs and the
LUMO level of the PFO layer slowly drops to the lower states due to their longer life time.
Therefore the emission with various wavelengths is emitted due to continuous recombination
between the electrons and the holes during electron transitions from the upper states to the
lower states [23]. We have many radiative defects in the ZnO NRs and in the bulk of the PFO
polymer. The energy states of various native radiative defects had been calculated or
experimentally measured in the band gap of ZnO. Lee et al. reported that the EL emission at
454 nm (blue) is due to the exciton emission in the PFO [27]. This PFO related blue emission
combined with ZnO defect related emissions results in the observed intrinsic white-light
emission. But the brightness and efficiency of the white light is dependent on the
concentration/thickness of the polymers and the transparent contact. The combination of
transparency in the visible range, thin polymer layers and low temperature procedure makes
the organic-inorganic hybrid junction WLEDs attractive for the development of large area
lighting source compatible with existing armature technologies.
PED
OT:
PSS
PFO
Zni
Oi
VO
Ec
Ev
ZnO
63
5.6 Distribution of radiative defects and reabsorption of the UV in ZnO
nanorods-organic hybrid white LEDs
In paper VII we used depth resolved cathodoluminescence to gain detailed information
on the luminescence centers and verify the origin of specific emissions from the ZnO
nanorods-organic hybrid white LEDs. The information derived from the depth resolved
cathodoluminescence provides a tool for the growth and processing of the state-of-the-art
ZnO NRs based hybrid white LEDs. The distribution of the radiative defects and reabsorption
of the UV are responsible for the wide modification of the luminescence properties of the
hybrid white LEDs based on ZnO NRs. Therefore it is important to obtained a detailed
information of the luminescence and verify the origin of these specific emissions from ZnO
nanorods-organic hybrid white LEDs.
Figure 5.20 shows the RT-CL spectra at 5-20 kV accelerating voltages which exhibit
UV emission at 381 nm and to two DBE bands at 417 nm and 625 nm. We used a Gaussian
function in order to distinguish the emission components of the PFO and the ZnO as shown in
Fig 5.21. The simulated CL spectrum shows five emissions at 381 nm, 417 nm, 551 nm, 617
nm and 676 nm which are associated with the UV, violet, green, orange and red lines
respectively. To attain greater insight into the deep defect and contribution of emissions in the
hybrid white LED structure we investigate the CL spectrum of the PFO. The CL spectrum of
PFO have weak emission band centered at 400 nm and at 560 nm but the CL spectrum of the
ZnO NRs shows stronger emissions at 381 nm, violet emission at 417 nm and DBE band
centered at 625 nm as shown in Figure 5.22. The inset of Figure 5.22 illustrates that the CL
spectrum of PFO have a violet-blue emission band from 350-490 nm this implies that the PFO
a have contribution to the emission in the violet-blue range and most of the emission observed
in the ZnO NRs/PFO hybrid LED is originating from the ZnO NRs.
64
400 500 600 700 800
CL In
tens
ity (a
. u.)
5K 10K 15K 20K
Wavelength (nm)
Figure 5.20: Depth dependent CL spectra of the fabricated hybrid LED at room temperature.
400 500 600 700 800
675
nm
617
nm
551
nm
CL In
tens
ity (a
. u.)
Wavelength (nm)
381
nm
417
nm
Figure 5.21: Room temperature CL spectrum of the hybrid LED after the Gaussian fitting.
65
300 400 500 600 700 800
400 500 600
CL
Inte
nsity
(a. u
.)
Wavelength (nm)
PFO CL spectra ZnO NRs PFO
CL
inte
nsity
(a. u
.)
Wavelength (nm)
Figure 5.22: Comparison of the CL emission spectra from the hybrid LED and an area with
PFO only and the inset shows a magnified CL emission spectra of PFO.
To attain greater insight into the defect chemistry in the hybrid white LEDs structure
the depth dependent CL spectra were examined to investigate the UV re-absorption and
especially the spatial distribution of the defects that cause the UV and the wide visible
emissions from the ZnO nanorods-organic hybrid white LEDs. The CL emission intensities of
the UV and the DBE as a function of the penetration depth are shown in Fig.5.23. The UV
and the DBE show distinct differences in depth dependence. The experimental DBE intensity
data is compared with the simulation (red line) the results obtained by exponential increase
fitting procedure confirms that the ZnO NRs have exponential distribution of defects along
the ZnO nanorods as shown in Fig. 5.23. The intensity ratio of the DBE to the UV emission is
very useful in exploring the origin of the deep level emission and the distribution of the
recombination centers. The intensity ratio of the DBE to UV emission decreases with the
penetration depth within the whole length of our ZnO NRs. As a result from Fig. 5.23, it is
concluded that there are more deep defects at the root of ZnO NRs compared to the upper
part. N. Bano et al. demonstrated that ZnO NRs have more structural defects at the interface
of ZnO NRs and the substrates [28]. By increasing the accelerating voltage, the electron beam
is expected to penetrate deeper into the ZnO and excites more electron-hole pairs hence more
emission centers will be excited by the electron bombardment. ZnO crystal can reabsorb the
UV within 1µm range this reabsorbed UV emission can excite more defect states in the
66
material resulting in an increase of the intensity of the DBE [29]. Thus, it is possible that part
of the UV emission may contribute to the enhancement of the DBE [28].
The present CL spectroscopy technique implemented here gives in depth information
about the radiative defects. Using this method it is found that ZnO NRs have more radiative
defects at the PFO interface. It is also proposed that some part of the UV might be reabsorbed
within the ZnO NRs. The confirmation of this issue needs more investigations.
0.0 0.8 1.6 2.4
Penetration Depth (µm)
CL In
tens
ity (a
. u.) Intensity Ratio
0.0 0.8 1.6 2.4
UVViolet
DBE
UV/DBE
Figure 5.23: The emission intensity (right axis) of the UV, violet and DBE as well as the ratio
of the UV and the DBE (left axis) as a function of the penetration depth.
67
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