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Gallium Oxide Solar-blind Photodetectorsfor Harsh Environment Applications: From
Thin Film Growth to Device Packaging
BHERA RAM TAK
INDIAN INSTITUTE OF TECHNOLOGY DELHIOctober 2020
©Indian Institute of Technology Delhi (IITD), New Delhi, 2020
Gallium Oxide Solar-blind Photodetectorsfor Harsh Environment Applications: From
Thin Film Growth to Device Packaging
by
BHERA RAM TAKDepartment of Physics
Submitted
In fulfillment of the requirements of the degree of
Doctor of Philosophyto the
INDIAN INSTITUTE OF TECHNOLOGY DELHIOctober 2020
This thesis is dedicated to my parents for their unconditional love and support
Certificate
This is to certify that the thesis entitled “Gallium Oxide Solar-blind Photode-
tectors for Harsh Environment Applications: From Thin Film Growth to
Device Packaging” being submitted by Mr. Bhera Ram Tak to Indian Institute
of Technology Delhi for the award of the degree of Doctor of Philosophy is a
record of bonafide research work carried out by him. He has worked under my guidance
and supervision and has fulfilled the requirements, which to our knowledge have reached
the requisite standard for the submission of the thesis. The results contained in this
thesis have not been submitted in part or full to any other University or Institute for
the award of any degree or diploma.
Prof. Rajendra Singh
Department of Physics
Indian Institute of Technology Delhi
New Delhi-110016, India
Date:
i
Acknowledgments
My journey of Ph.D. has been tough and challenging as well as joyful. During this
time, hard luck always motivated me to be patient and work hard. The outcome of this
thesis also involves support, criticism and encouragement of lots of people. I also wish
to thank all the institutions that supported me for the accomplishment of my thesis
work.
First of all, I would like to express my sincere gratitude to my Ph.D. supervisor
Professor Rajendra Singh. It was an honor for me to work in this guidance. I am grateful
to him for his suggestions, motivation for research and giving me the freedom to work
on research ideas. It has been a great time to learn and improve myself professionally as
well as personally. I appreciate him for his valuable time and devotion for my research
work, research fundings and collaborations which made this thesis work productive and
stimulating. Finally, a special thanks to him for pushing me hard to execute device
packaging work.
Besides my advisor, I wish to thank my Ph.D. research committee members Professor
Sujeet Chaudhary, Professor J. P. Singh and Professor Samaresh Das for their time to
time evaluations, feedback, comments and appreciation during this research work. I
would like to acknowledge the Department of Physics, Indian Institute of Technology
Delhi for giving me the opportunity to work here.
I would also like to thank the funding agencies department of science and technology
(DST), India and British Council, United Kingdom for awarding me as a Newton-
Bhabha Ph.D. fellow to execute some part of work at the University of Warwick, United
Kingdom. I would like to express my gratitude to Professor Marine Alexe for hosting
me during this program. It was a learning and productive experience to work under his
guidance. I wish to thank him for providing research facilities and training me during
this program. His passion for research and his way towards dealing research problem
always impressed me. I would like to thank Dr. Mingmin Yang, Mr. Afan, Ms. Daniela-
Emilia Dogaru, Mr. Hangbo Zhang and other group members for helping and training
me on various experimental tools.
I acknowledge the department of science and technology (DST), India for awarding
me INSPIRE fellowship and contingency grant to execute my Ph.D. degree. I am also
iii
thankful to the Indian Institute of Technology Delhi for providing funds to attend an
international conference outside India. I would like to acknowledge Nano Research
Facility (NRF), IIT Delhi for providing funding support for my research visit at the
University of Kolkata and Indian Institute of Science (IISc), Bangalore.
I acknowledge the department of Physics, NRF, central research facility (CRF) IIT
Delhi for providing various experimental tools to perform my research work. I would
also like to express my gratitude to Professor Vinay Gupta and Dr. Monika Tomar,
Department of Physics and Astrophysics, University of Delhi for providing me Pulsed
Laser Deposition (PLD) system to deposite gallium oxide thin films. I wish to thank
his group members especially Sheetal Dewan and Surbhi Gupta for helping me during
my time in his lab.
I am obliged to Prof. Xiaohang Li, Advanced Semiconductor Laboratory, King Ab-
dullah University of Science and Technology (KAUST) and Professor Ying-Hao Chu
from National Chiao Tung University, Taiwan for the cross-sectional TEM measure-
ments. Professor Chu is also acknowledged for providing muscovite substrates. I am
thankful to Dr. Samaresh Das and Veerendra Dhiyani from Centre of Applied Elec-
tronics, IIT Delhi, Professor B. R. Mehta, Professor B. D. Gupta, Mr. Mujeeb Ahmad
and Mr. Vivek Semwal from Department of physics, IIT Delhi for helping in metal
deposition.
I would like to thank following collaborators for their help during my Ph.D. work:
• Dr. Ashok Kapoor from Solid state physics laboratory (SSPL), New Delhi
• Dr. Raman Kapoor from Solid state physics laboratory (SSPL), New Delhi
• Dr. Anshu Goyal from Solid state physics laboratory (SSPL), New Delhi
• Dr. K. Asokan, Inter University Accelerator Centre, India
• Dr. S. Nagarajan from Aalto University, Finland
• Dr. Ashish Kumar, Inter University Accelerator Centre, India
Working at Advanced Semiconductor Materials and Devices Laboratory, IIT Delhi,
has been an enriching experience. I would like to thank my seniors Dr. Uday Dadwal,
Dr. Ashish Kumar, Dr. Ashutosh Kumar, Dr. Sudheer Kumar, Dr. Mukesh Kumar, Dr.
iv
Chandra Shekhar Pathak, Dr. Manjari Garg, Dr. Monika Moun for their valuable sug-
gestions and help. I also express my gratitude to my colleagues Ravi Pathak, Chandan
Sharma, Aditya Singh, Kapil Narang, Shuchi Kaushik, Sahin Sorifi, Madan Pancholi,
Hardhyan, Pallavi Agarwal, T. Arundeepth, Prithu Bhatnagar, Kalyani Thakur, Anjali
Chauhan, Aarti, Sukhdeep Gill, Mohd. Danish Ali, Swapnil, Arun, Shabbin Rahiman
K., Suresh Bhambhu and Rahul Agarwal for the scientific and personal discussions.
Once again, I thank all these members for making my stay at IIT Delhi unforgettable.
This PhD would have been more challenging without the good and bad times that
I shared with my friends at IIT Delhi. All the tea and coffee sessions and parties
with Balwant Singh, Vikas Sharma, Mujeeb Ahmad, Sonu, Minakshi and Vivek Semwal
will be cherished forever. Also excursions and dinners with Manjari Garg, Monika,
Ravi Pathak, and Aditya Singh will be treasured. All the moments spent with friends
specially Nakul Jain, Rajesh Jangir, Balwant Singh, Vikas Sharma, Minakshi, Barkha,
Manisha and Dora during trekking and camping will always be cherished. My days at
IIT Delhi will be incomplete without all my friends with whom I used to play cricket
and badminton.
Finally, words are inadequate to express my deep and heart filled gratitude towards
my family for their blessings, love, care, unconditional support, encouragement, patience
and sacrifices during all the stages of this Ph.D. and in both good times and bad times.
Thank you.
Bhera Ram Tak
v
Abstract
Gallium oxide (Ga2O3) is a most promising and emerging wide bandgap semiconductor
material for optoelectronics, high-power devices and radio-frequency power electronics
applications. Owing to its wide bandgap, β-Ga2O3 is a potential contender for solar-
blind photodetector applications. The Ga2O3 technology is at the early stage of research
which provides a great opportunity to identify different challenges. A good quality sin-
gle crystalline material is inevitably required for high performance photodetectors. The
effect of oxygen growth conditions on the quality of Ga2O3 is still not investigated
that set the first challenge to begin this work. The investigation of the photodetector
performance for harsh environmental conditions such as temperature and radiation is
necessary for defense, security, environmental and space applications. The photocur-
rent transport both at high temperature and gamma irradiation environments is also not
studied for Ga2O3 material. Flexible and self-powered deep ultraviolet (UV) photode-
tectors are pivotal for future technologies. The fabrication of epitaxial β-Ga2O3 thin
films is challenging on flexible substrates due to high-temperature growth requirements.
In order to address the aforementioned problems, this thesis work is accomplished in the
direction of rigid as well as flexible photodetectors for harsh environmental conditions.
In the beginning of this work, the growth of β-Ga2O3 thin films was optimized by
varying oxygen content. The thin films deposited at 0.5 mT oxygen growth pressure
at 800 ◦C temperature possessed minimum point defects. It was also found that the
Fermi level was pinned at the mid-gap energy in both oxygen-deficient conditions and
oxygen-rich conditions which are attributed to oxygen and gallium vacancy related de-
fects. Further, the metal-semiconductor-metal (MSM) photodetectors were fabricated
on the best quality thin film sample. The photodetector exhibited an ultra-low dark
current of 8.6 ± 3.4 fA at zero bias. An ultra-low noise current of 9.1 ×10−16 A/Hz1/2
at 1 Hz was also obtained in the self-powered condition. Such ultra-low noise current
suggests the potential of this device in detecting very weak optical signals. The linear
dynamic range (LDR) of 88.5 dB was achieved which is very useful for high-resolution
imaging. The dark current, noise floor, and LDR of the photodetector are the bench-
mark for β-Ga2O3 self-powered deep UV photodetectors. We also demonstrated a 3×4
two-dimensional photodetector array with uniform dark and photocurrent across all the
vii
pixels. The outcomes of the present work are encouraging for imaging applications of
β-Ga2O3 based energy-efficient deep UV photodetectors. In subsequent work, High-
temperature operation of MSM UV photodetectors fabricated on pulsed laser deposited
β-Ga2O3 thin films has been investigated. These photodetectors were operated up to
250 ◦C temperature under 255 nm illumination. The photo to dark current (PDCR)
ratio of about 7100 was observed at room temperature (RT) and 2.3 at high temper-
ature of 250 ◦C with 10 V applied bias. A decline in photocurrent was observed until
a temperature of 150 ◦C beyond which it increased with temperature up to 250 ◦C.
The suppression of the UV and blue band was also observed in the normalized spec-
tral response curve above 150 ◦C temperature. Temperature-dependent rise and decay
times of temporal response were analyzed to understand the associated photocurrent
mechanism at high temperatures. Electron-phonon interaction and self-trapped holes
were found to influence the photoresponse in the devices. Further, thermally stimulated
current (TSC) measurements were also performed to identify deep level traps in thin
films. The deep level trap of 1.03 eV energy was found dominant trap responsible for
persistent photocurrent.
Further, the radiation hardness of Ga2O3 MSM solar-blind photodetectors has also
been investigated under the exposure of 60Co γ-source. It was observed that the metal
contacts were not degraded and the dark current of photodetector was slightly improved
from 3.27× 10−7 A to 1.88 × 10−7 A. The photo to dark current ratio (PDCR) was
observed to increase from 5.1 to 14.1 with increasing γ-radiation exposure. The apparent
Schottky barrier height (SBH) evaluated from current-voltage characteristics were found
to increase with irradiation. The increased SBH was explained using image force induced
barrier lowering. The obtained results reveal that the Ga2O3 solar-blind photodetectors
are relatively less susceptible to the radiation environment.
For flexible photodetectors, wearable solar-blind photodetector based on amorphous
gallium oxide grown at muscovite mica is reported for room temperature as well as high
temperature operations. The ultra-high photoresponsivity of 9.7 A/W is obtained for
5 V applied bias at room temperature under 75 µW/cm2 weak illumination of 270 nm
wavelength. The detector enables very low noise equivalent power (NEP) of 9×10−13
W/Hz1/2 and ultra-high detectivity of 2×1012 jones which shows the magnificent detec-
tion sensitivity. Further, bending tests are performed for robust utilization of flexible
viii
detectors up to 500 bending cycles with each bending radius of 5 mm. After 500 bending
cycles, device shows a slight photocurrent decrease. The bending performances exhibit
excellent potential for wearable applications. Moreover, photocurrent and dark current
characteristics above room temperature demonstrate the outstanding functionalities till
523K temperature which is remarkable for flexible photodetectors. Further, to improve
the performance of flexible photodetectors, β-Ga2O3 (20 1) films are hetero-epitaxially
grown on ultra-thin and environment-friendly muscovite mica which is the first time β-
Ga2O3 epitaxy growth on any flexible substrate. The integration of Gallium oxide with
muscovite enables high-temperature processing as well as excellent flexibility compared
to polymer substrates. Additionally, the metal-semiconductor-metal (MSM) photode-
tector on β-Ga2O3 layer shows an ultra-low dark current of 800 fA at zero bias. The
photovoltaic peak responsivity of 11.6 µA/W is obtained corresponding to very weak
illumination of 75 µW/cm2 of 265 nm wavelength. Thermally stimulated current (TSC)
measurements are employed to investigate the optically active trap states. Among these
traps, trap with an activation energy of 166 meV dominates the persistence photocur-
rent in the devices. Finally, photovoltaic detectors have shown excellent photocurrent
stability under bending induced stress up to 0.32%. Hence, this novel heteroepitaxy
opens the new way for flexible deep UV photodetectors. In the last, the solar-blind
photodetectors fabricated on β-Ga2O3 /sapphire were packaged in the transistor out-
line header. The packaged devices showed the same photoresponse before and after wire
bonding.
ix
xiii
सार
गलियम ऑकसाइड (Ga2O3) ऑपटोइिकरॉनिक, उचच-शककि उपकरणो और रडडयो-आवतति पावर इिकरॉनिकस क उपयोग क लिए एक सबस आशाजिक और उभरिा हआ ततवसिि बडगप समीकडकटर ह। इसक ततवसिि बडगप क कारण, β- Ga2O3 सोिरबिाइड फोटोडडटकटर क लिए एक सभाततवि दावदार ह। Ga2O3 िकिीक अिसधाि क परारलभक चरण म ह जो ततवलभनि चिौनियो की पहचाि करि का एक शािदार अवसर परदाि करिा ह। उचच परदशशि फोटोडडटकटसश क लिए एक अचछी गणविा वािी एकि करिसटिीय मटररयि अनिवायश रप स आवशयक ह। Ga2O3 की गणविा पर ऑकसीजि क परभाव की अभी भी जाच िही की गई ह जो इस काम को शर करि क लिए पहिी चिौिी निधाशररि करिी ह। रकषा, सरकषा, पयाशवरण और अिररकष अिपरयोगो क लिए कठोर पयाशवरणीय पररकसिनियो जस िापमाि और ततवकरकरण क लिए इि फोटोडडटकटसश क परदशशिो की जाच आवशयक ह। उचच िापमाि और गामा ततवकरकरण वािावरणो म फोटो करट पररवहि को Ga2O3
मटररयि क लिए समझा िही गया ह। िचीि और सव-सचालिि गहर पराबगिी (यवी) फोटोडडटकटर भततवषय की परौदयोगगकरकयो क लिए महतवपणश ह। उचच िापमाि की आवशयकिाओ क कारण िचीि सबसरटस पर एततपटककसयि β- Ga2O3 पििी करफलमो का निमाशण चिौिीपणश ह। उपयशकि समसयाओ को सबोगधि करि क लिए, कठोर पयाशवरणीय पररकसिनियो क लिए यह िीलसस कायश, अिमय और साि ही िचीि फोटोडडटकटसश की ददशा म परा करकया जािा ह।
इस काम की शरआि म, ऑकसीजि की मातरा को बदि करक β- Ga2O3 पििी करफलमो क गरोि को अिकलिि करकया गया । 800 °C िापमाि ििा 0.5 mT ऑकसीजि गरोि परशर म बिाई पििी करफलमो म नयििम बबद दोष ि। ऑकसीजि की कमी वािी कसिनियो और ऑकसीजि अगधकिा वािी कसिनियो म फमी सिर मधय-अिराि ऊजाश पर ततपि हआ पाया गया िा, कजसक लिए ऑकसीजि और गलियम ररककि सबधी दोष कजममदार ह। इसक आग, धाि-अधशचािक-धाि (MSM) फोटोडडटकटसश को सबस अचछी गणविा वािी पििी करफलम पर गढा गया िा। फोटोडडटकटर ि शनय वोलटज पर 8.6 ± 3.4 fA क अलरा-िो डाकश करट का परदशशि करकया। 1Hz पर 9.1 ×10-16 A/Hz1/2 का अलरा-िो िोइज करट भी सव-सचालिि कसिनि म परापि करकया गया िा। इस िरह क अलरा-िो िॉइज करट बहि कमजोर ऑकपटकि लसगिि का पिा िगाि म इस डडवाइस की कषमिा बिािा ह। 88.5 dB की िीनियर डायिलमक रज (LDR) हालसि की गई जो उचच-ररजॉलयशि इमकजग क लिए बहि उपयोगी ह। फोटोडडटकटर का डाकश करट, िोइज िवि, और LDR β-Ga2O3 सव-सचालिि गहर यवी फोटोडडटकटसश क लिए बचमाकश ह। हमि सभी ततपकसिो म एक समाि डाकश और फोटो करट वािी एक 3 × 4 दततव-आयामी फोटोडडटकटर सरणी को परदलशशि करकया ह। विशमाि कायश क पररणाम β-
Ga2O3 आधाररि ऊजाश कशि गहर यवी फोटोडडटकटर क इमकजग अिपरयोगो क लिए परोतसादहि करि वाि ह। बाद क काम म, पलसड िजर स बिाई β- Ga2O3 की पििी करफलमो पर गढ गए MSM यवी फोटोडडटकटर क उचच िापमाि पर सचािि की जाच की गई ह। य फोटोडडटकटर 255 nm परकाश म 250 °C िापमाि िक
xiv
सचालिि करकए गए ि। 10 V पर फोटो स डाकश करट (PDCR) का अिपाि कमर क िापमाि (RT) पर िगभग 7100 और 250 °C क उचच िापमाि पर 2.3 दखा गया। फोटो करट म 150 °C क िापमाि िक गगरावट दखी गई िी और उसक ऊपर 250 °C िापमाि िक इसम वदगध हई । यवी और बि बड का दमि भी 150 °C िापमाि स ऊपर सामानयीकि वणशिमीय परनिकरिया वि म दखा गया िा। उचच िापमाि स सबगधि फोटो करट मकनिजम को समझि क लिए टमपोरि परनिकरिया क िापमाि-निभशर वदगध और कषय समय का ततवशिषण करकया गया िा। इिकराि-फोिोि इटरकशि और सलफ-रपड होि को उपकरणो म फोटोरसपोनस को परभाततवि करि क लिए कजममदार पाया गया। इसक अिावा, पििी करफलमो म गहर सिर क रपस की पहचाि करि क लिए िमशि रप स उिकजि करट (TSC) माप भी करकए गए ि। 1.03 eV ऊजाश क गहर सिर क रप को परलससटट फोटो करट क लिए कजममदार परमख रप पाया गया।
इसक अिावा, Ga2O3 MSM सोिरबिाइड फोटोडडटकटर की ततवकरकरण कठोरिा की जाच 60Co γ- सरोि क ससगश म भी की गई ह। यह दखा गया करक धाि कोनटकट खराब िही हई िी और फोटोडडटकटर क डाकश करट म 3.27 × 10-7 A स 1.88 × 10-7 A िक िोडा सधार हआ िा। फोटो स डाकश करट (PDCR) का अिपाि γ-
रडडएशि ससगश क साि 5.1 स बढकर 14.1 हआ पाया गया। करट-वोलटज वि मलयाकि स निकािी गई सपषट शोटकी बररयर ऊचाई (SBH) को ततवकरकरण क साि बढा हआ पाया गया। वदगध हई SBH को छततव बि स परररि अवरोध कमी स समझाया गया । परापि पररणामो स पिा चििा ह करक Ga2O3 सोिरबिाइड
फोटोडडटकटर ततवकरकरण पयाशवरण क लिए अपकषाकि कम सवदिशीि ह।
िचीि फोटोडडटकटसश क लिए, मसकोवाइट माइका पर उगाए गए अमोरफस गलियम ऑकसाइड पर आधाररि ततवयरबि सोिरबिाइड फोटोडडटकटर को कमर क िापमाि क साि-साि उचच िापमाि सचािि क लिए सपाददि करकया गया ह। 9.7 A/W की अलरा-उचच फोटोररसपोलसततवटी 5V क लिए कमर क िापमाि पर 75
W/cm2 एव 270 nm िरग दधयश की कमजोर रोशिी क िहि परापि की जािी ह। डडटकटर 9 × 10-13 W/Hz1/2 क बहि कम िोइज समककष पॉवर (NEP) और 2 × 1012 Jones की अनि-उचच डडटककटततवटी क सकषम ह जो शािदार पहचाि सवदिशीििा को दशाशिा ह। इसक अिावा, 5 mm की परतयक झकाव बतरजया क साि 500 झकाव चिो िक िचीि डडटकटरो क मजबि उपयोग क लिए झकाव परीकषण करकए जाि ह। 500 झकाव चिो क बाद, डडवाइस फोटोकरट म एक मामिी कमी ददखािा ह। झकाव परदशशि क पररणाम ततवयरबि अिपरयोगो क लिए उतकषट कषमिा ददखाि ह। इसक अिावा, कमर क िापमाि क ऊपर 523K
िापमाि िक फोटोकरट और डाकश करट उतकषट कायशकषमिा परदलशशि करिी ह जो िचीि फोटोडडटकटर क लिए उलिखिीय ह। इसक अिावा, िचीि फोटोडडटकटर क परदशशि को बहिर बिाि क लिए, β- Ga2O3 (-
2 0 1) करफलम अलरा-पििी और पयाशवरण क अिकि मसकोवाइट माइका पर दहटरो-एततपटककसयिी उगाई जािी ह, जो करकसी भी िचीि सबसरट पर पहिी बार β- Ga2O3 एततपटकसी गरोि ह। मसकोवाइट क साि गलियम ऑकसाइड का एकीकरण बहिक सबसरट की िििा म उचच िापमाि परससकरण क साि-साि
xv
उतकषट िचीिापि दिा ह। इसक अनिररकि, β- Ga2O3 परि पर MSM फोटोडडटकटर शनय वोलट पर 800 fA
की एक अलरा-िो डाकश करट ददखािा ह। 11.6 μA/W की फोटोवोकलटक लशखर रसपोनस 265 nm िरग दधयश एव 75 μW /cm2 की बहि कमजोर रोशिी क ससगश म परापि की जािी ह। परकालशक सकरिय रप सिरो की जाच क लिए TSC माप काम म लिया गया ह। इि रपो म स, 166 meV की सकरियण ऊजाश का रप उपकरणो म परलससटट फोटोकरट क लिए हावी ह। अि म, फोटोवोकलटक डडटकटरो ि 0.32% िक झकाव स परररि ििाव क िहि उतकषट फोटोकरट कसिरिा ददखाई ह। इसलिए, यह िया दहटरो-एततपटकसी िचीिी गहरी यवी फोटोडडटकटसश क लिए िया रासिा खोििा ह। आखखरी म, राकजसटर आउटिाइि हडर म β- Ga2O3/सफायर पर निलमशि सोिरबिाइड फोटोडटकटर पक करकए गए ि। पक करकए गए उपकरणो ि वायर बकनडग स पहि और बाद म एक ही फोटोरसपोनस ददखाया।
Contents
Certificate i
Acknowledgments iii
Abstract vii
List of Figures xix
List of Tables xxv
Appendix xxvii
1 Introduction 1
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Fundamentals of photodetectors: Figures of merit . . . . . . . . . . . . . 2
1.2.1 External quantum efficiency and responsivity . . . . . . . . . . . 2
1.2.2 Photo to dark current ratio (PDCR) and UV to visible rejection
ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.3 Transient response or speed . . . . . . . . . . . . . . . . . . . . . 4
1.2.4 Noise equivalent power (NEP) . . . . . . . . . . . . . . . . . . . . 5
1.2.5 Specific detectivity (D∗) . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.6 Linear dynamic range (LDR) . . . . . . . . . . . . . . . . . . . . 6
1.3 Technology roadmap for next-generation UV detectors . . . . . . . . . . 6
1.4 Schottky MSM architecture . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.5 Material selection for Solar-blind photodetector . . . . . . . . . . . . . . 9
1.6 Organization of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2 Experimental and characterization methods 17
2.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1.1 Material target fabrication . . . . . . . . . . . . . . . . . . . . . 17
xv
2.1.2 Pulsed laser deposition . . . . . . . . . . . . . . . . . . . . . . . . 18
2.1.3 Device fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2 Material characterizations . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.1 Structural characterizations . . . . . . . . . . . . . . . . . . . . . 21
2.2.2 Scanning probe microscopy . . . . . . . . . . . . . . . . . . . . . 22
2.2.3 Optical characterization . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.4 Elemental characterization . . . . . . . . . . . . . . . . . . . . . . 25
2.3 Gamma irradiation facility . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.4 Device characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.4.1 Photoelectrical measurements . . . . . . . . . . . . . . . . . . . . 26
2.4.2 Noise measurements . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.4.3 Flexibility measurements of devices . . . . . . . . . . . . . . . . . 29
2.5 Thermally stimulated current spectroscopy . . . . . . . . . . . . . . . . . 29
2.6 Device packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.6.1 Dicing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.6.2 Wire bonding and packaging . . . . . . . . . . . . . . . . . . . . 31
3 Growth of epitaxial β-Ga2O3 thin films on sapphire 33
3.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1.1 β-Ga2O3 target fabrication . . . . . . . . . . . . . . . . . . . . . 35
3.1.2 Thin film growth . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1.3 Material Characterizations . . . . . . . . . . . . . . . . . . . . . . 36
3.1.4 Kelvin Probe Force Microscopy Measurement . . . . . . . . . . . 36
3.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.2.1 Structural and elemental characterization of Ga2O3 target . . . . 36
3.2.2 Material characterizations of Ga2O3 thin films . . . . . . . . . . 37
3.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4 Solar-blind photodetectors fabricated on β-Ga2O3/sapphire 49
4.1 Room temperature performance of photodetectors arrays . . . . . . . . . 49
4.1.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.1.2 Material characterizations . . . . . . . . . . . . . . . . . . . . . . 52
4.1.3 Photoresponse measurements . . . . . . . . . . . . . . . . . . . . 53
xvi
4.1.4 Noise measurements . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.1.5 NEP and LDR measurements . . . . . . . . . . . . . . . . . . . . 56
4.1.6 Performance of 2D-array . . . . . . . . . . . . . . . . . . . . . . . 57
4.1.7 Photocurrent transport mechanism of MSM detector . . . . . . . 59
4.1.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.2 High-temperature performance of photodetectors . . . . . . . . . . . . . 62
4.2.1 Experimental: thin film growth and MSM device fabrication . . . 63
4.2.2 Photo to dark current ratio . . . . . . . . . . . . . . . . . . . . . 64
4.2.3 Photoresponse . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.2.4 Spectral response . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.2.5 Temporal response . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4.2.6 Physical mechanisms of photocurrent transport . . . . . . . . . . 69
4.2.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5 Analysis of trap states using thermally stimulated current spectroscopy 73
5.1 Experimental: TSC spectroscopy . . . . . . . . . . . . . . . . . . . . . . 75
5.2 Thermally stimulated current measurements . . . . . . . . . . . . . . . . 75
5.3 Fractional emptying of traps . . . . . . . . . . . . . . . . . . . . . . . . . 76
5.4 TSC mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
6 Gamma irradiation effect on β-Ga2O3 photodetectors 83
6.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
6.2 Material characterizations . . . . . . . . . . . . . . . . . . . . . . . . . . 85
6.3 Photoresponse of γ-irradiated device . . . . . . . . . . . . . . . . . . . . 86
6.4 Schottky barrier height calculations . . . . . . . . . . . . . . . . . . . . . 89
6.5 Photoluminescence of irradiated device . . . . . . . . . . . . . . . . . . . 91
6.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
7 Gallium oxide flexible solar-blind photodetectors on muscovite mica 93
7.1 Growth of amorphous gallium oxide and photodetector performance thereon
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
7.1.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
7.1.2 Material characterizations . . . . . . . . . . . . . . . . . . . . . . 96
xvii
7.1.3 Photoresponsivity . . . . . . . . . . . . . . . . . . . . . . . . . . 97
7.1.4 Temporal response . . . . . . . . . . . . . . . . . . . . . . . . . . 99
7.1.5 Optical power dependent photoresponse . . . . . . . . . . . . . . 100
7.1.6 Photocurrent gain, detectivity and NEP . . . . . . . . . . . . . . 101
7.1.7 Effect of bending induced strain on photoresponse . . . . . . . . . 102
7.1.8 High temperature performance of flexible photodetector . . . . . 105
7.1.9 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.2 Growth of epitaxial gallium oxide and photodetector performance thereon 108
7.2.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
7.2.2 Structural characterizations . . . . . . . . . . . . . . . . . . . . . 110
7.2.3 Photoelectrical measurements . . . . . . . . . . . . . . . . . . . . 112
7.2.4 Investigation of traps using TSC spectroscopy . . . . . . . . . . . 115
7.2.5 Effect of bending induced strain on photoresponse . . . . . . . . . 118
7.2.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
8 Wire bonding and packaging of photodetectors 121
8.1 Experimental: device fabrication, dicing and packaging . . . . . . . . . . 123
8.2 Photoelectrical measurements before and after wire bonding . . . . . . . 125
8.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
9 Summary and future perspective of the work 129
9.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
9.2 Future perspectives and challenges . . . . . . . . . . . . . . . . . . . . . 130
References 133
Publications in international journals 147
International/national conference presentations 149
Bio-data 151
xviii
List of Figures
1.1 UV exposure limit for the humans . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Applications of solar-blind photodetectors . . . . . . . . . . . . . . . . . 7
1.3 Technology roadmap for next generation UV photodetectors [25] . . . . . 8
1.4 Energy band diagram of MSM structure under (a) thermal equilibrium
(b) applied bias V in dark condition and (c) applied bias V upon optical
illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.5 Unit cell of monoclinic β-Ga2O3 . . . . . . . . . . . . . . . . . . . . . . . 11
1.6 Band structure of monoclinic β-Ga2O3 . . . . . . . . . . . . . . . . . . . 11
2.1 Process flow for pellet fabrication . . . . . . . . . . . . . . . . . . . . . . 18
2.2 Pulsed laser deposition systems at (a) University of Warwick, United
Kingdom and (b) University of Delhi, India . . . . . . . . . . . . . . . . 19
2.3 Clean room process flow for device fabrication . . . . . . . . . . . . . . . 20
2.4 Bragg’s diffraction condition on a lattice of interplanar distance d and
(b) Schematic of Bragg-Brentano geometry of XRD system . . . . . . . . 21
2.5 Gamma chamber facility at IUAC, New Delhi . . . . . . . . . . . . . . . 26
2.6 Schematic of photodetector system . . . . . . . . . . . . . . . . . . . . . 27
2.7 Camera image of complete photodetector system in the laboratory . . . . 28
2.8 Schematic of noise measurement system . . . . . . . . . . . . . . . . . . 28
2.9 Module for flexible detector measurements . . . . . . . . . . . . . . . . . 29
2.10 Process flow involved in the TSC technique . . . . . . . . . . . . . . . . 30
2.11 Camera image of the dicing system . . . . . . . . . . . . . . . . . . . . . 31
3.1 Camera image of sintered β-Ga2O3 target . . . . . . . . . . . . . . . . . 35
3.2 (a) XRD 2θ-scan and (b) EDAX of Ga2O3 target . . . . . . . . . . . . . 37
3.3 (a) XRD 2θ scans (b) XRD Φ scan of the film deposited at 10 mT
pressure (c) Bandgap variation of Ga2O3 thin films deposited at with
various growth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
xix
3.4 ((a) Transmittance spectra of thin films (b) bandgap of thin films cacu-
lated from tauc plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.5 XPS of C 1s spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.6 XPS spectra of O1s peak of β-Ga2O3 films deposited at (a) 0.2 mTorr
(b) 0.5 mTorr (c) 10 mTorr and Ga 3d peak deposited at (d) 0.2 mTorr
(e) 0.5 mTorr (f) 10 mTorr . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.7 (a) Raman phonon modes for oxygen and gallium vacancies (b) valence
band spectra for thin films grown at different oxygen pressures . . . . . . 44
3.8 (a) Surface potential mapping (b) Gaussian distribution of VCPD (c)
calculated the work function of thin films deposited at various oxygen
pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.9 Band diagram of samples deposited at (a) 0.2 mT (b) 0.5 mT and (c) 10
mT oxygen pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.1 Optical microscope image of three devices among 3×4 two-dimensional
array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.2 X-ray diffraction (a) 2θ scan which shows (2 0 1) plane orientation of
β-Ga2O3 thin film (b) rocking curve of (2 0 1) plane (c) phi-scan of (4 0
1) plane and (d) surface topography of the thin film . . . . . . . . . . . 52
4.3 (a) Cross-sectional TEM (CS-TEM) of Ga2O3/Al2O3 interface (b) Mag-
nified TEM image of thin film from the cross-sectional area and (c) in-
terplanar distance of (2 0 1) planes of β-Ga2O3 thin film in the HRTEM
image . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.4 (a) Dark current and photocurrent of device exhibiting zero bias response.
(b) spectral response of the detector at zero bias. Inset is the logarithmic
plot of responsivity versus wavelength . . . . . . . . . . . . . . . . . . . . 54
4.5 (a) Noise current as a function of frequency (b) total noise current at 1Hz
as a function of power density (c) Dynamic photocurrent response at zero
bias. Greenline represents the total noise current per Hz bandwidth at
zero bias (d) State-of-the-arts linear dynamic range of self-powered solar-
blind photodetectors as a function of dark current . . . . . . . . . . . . . 56
xx
4.6 (a) Dark and photocurrent of a 2D array, histograms of (b) dark and (c)
photocurrent of pixels and (d) current image of two pixels illuminated
with 250 nm light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.7 Energy band diagram at zero bias of MSM photodetector for (a) ideal
and (b) real case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.8 Current-voltage fitting of thermionic emission model for both the contacts 60
4.9 Contact potential difference distribution of Ga2O3 thin film at (a) posi-
tion 1 and (b) position 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.10 SEM image of fabricated photodetector at 100 µm scale . . . . . . . . . 64
4.11 Variable temperature (a) dark current-voltage and (b) photocurrent-
voltage measurement of Ga2O3 photodetector . . . . . . . . . . . . . . . 65
4.12 (a) Temperature-dependent PDCR (b) photocurrent and peak respon-
sivity of β-Ga2O3 photodetector at 10V bias and 255 nm illumination . . 67
4.13 Spectral response of fabricated photodetector with temperature variation
ranging from 23 ◦C to 250 ◦C . . . . . . . . . . . . . . . . . . . . . . . . 67
4.14 Real-time current change of fabricated photodetector at different tem-
peratures under 255 nm UV illumination . . . . . . . . . . . . . . . . . . 68
4.15 (a) Rise times (τr1 and τr2) and decay times (τd1 and τd2) at 5V bias under
255 nm illumination with detector temperature (b) Arrhenius plot of slow
components of rise times shows activation energies of 73 and 205 meV
and (c) decay times depicts activation energies of 58 and 168 meV (d)
Model for photocurrent mechanism below 150 ◦C with electron capture
process having activation energies of 58 and 73 meV and (e) Model for
photocurrent mechanism above 150 ◦C with the transition of STH to a
mobile hole having activation energies of 205 and 168 meV . . . . . . . . 70
5.1 (a) heating and cooling curve of TSC at 7 K/min heating rate (b) net
TSC spectrum of the β-Ga2O3 thin film . . . . . . . . . . . . . . . . . . 76
5.2 Process flow of fractional emptying method for TSC peak resolution . . . 77
5.3 Resolved peaks of TSC spectrum for all the traps . . . . . . . . . . . . . 77
5.4 Net TSC peaks with characteristic peak teamperatue of all the traps . . 78
5.5 Arrhenius plot of dark current . . . . . . . . . . . . . . . . . . . . . . . . 79
xxi
5.6 Band diagram of (a) trap filling via optical injection of charge carriers
(b) after complete trapping and (c) under thermal release of carriers . . . 81
6.1 Schematic of the fabricated β-Ga2O3 based Ni/Au/Ni metal-semiconductor-
metal photodetector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
6.2 (a) X-ray diffraction pattern showing (201) orientation. Inset shows the
rocking curve of (201) plane and (b) AFM image of β-Ga2O3 thin film
grown on c-plane sapphire . . . . . . . . . . . . . . . . . . . . . . . . . . 85
6.3 Current-voltage (I-V) measurements under (a) dark conditions and (b)
illumination using 245 nm wavelength, for the MSM photodetector after
gamma exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
6.4 (a) Photo to dark current ratio (PDCR) and (b) peak responsivity at
10 V of the fabricated photodetector with radiation dose under 245 nm
illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.5 Spectral responsivity of photodetector with γ-ray irradiation at 10 V bias 88
6.6 SEM images of MSM photodetectors (a) pristine (b) after 100 kGy irra-
diation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
6.7 Logarithmic plots of exp(eV/kT) vs V. Inset shows the magnified scale
of linear fit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.8 Variation in dark current and Schottky barrier height (SBH) with γ-ray
irradiation dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
6.9 Photoluminescence spectra of Ga2O3 thin film with increasing cumulative
radiation dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
7.1 (a) XRD 2θ scan of β-Ga2O3 thin film (b) Highly smooth surface mor-
phology of thin film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
7.2 (a) photoluminescence spectra of Ga2O3 having various defect bands of
UV, blue, green and red (b) XPS survey scan of thin film and (c) X-ray
photoelectron spectroscopy of Ga 3d and (d) O 1s core levels . . . . . . . 97
xxii
7.3 (a) Schematic of photodetection measurements on interdigitated elec-
trode structures fabricated on Ga2O3/Mica (b) variation of dark current
and photocurrent at a peak wavelength of 270 nm with an applied bias to
the detector (c) demonstration of spectral responsivity at 5V bias rang-
ing from 240 nm to 600 nm wavelength. Inset shows the camera image
of fabricated devices on highly transparent and flexible Ga2O3/Mica and
(d) time response curve of the detector at 5V bias with biexponential
fitting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
7.4 Optical power dependent (a) photocurrent and (b) responsivity at peak
wavelength of 270 nm of a photodetector which was kept at 5V external
bias during measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 100
7.5 (a) Responsivity (blue curve) and gain (red curve) of the fabricated device
with varying voltages (b) variation of detectivity (blue curve) and NEP
(red curve) with voltages exhibiting sensitivity of the devices . . . . . . . 101
7.6 CCD camera image corresponding to (a) flat condition and (b) 5 mm
bending radius (c) dark and photocurrent at the various bending radius
(d) Photocurrent pulses with bending cycles of the device having 5 mm
bending radius (e) Plot of photocurrent and dark current with bending
cycles showing a small decrease in current after 500 cycles . . . . . . . . 104
7.7 (a) Dark current (blue curve) and photocurrent (red curve) at 2V bias
with temperature up to 523K (b) PDCR of the detector at elevated
temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
7.8 Comparisons of reported maximum working temperature of solar-blind
photodetectors based on flexible substrates . . . . . . . . . . . . . . . . . 107
7.9 (a) XRD 2θ-scan of β-Ga2O3 /muscovite (b) rocking curve of (201)-plane
and (c) surface morphology of β-Ga2O3 thin film . . . . . . . . . . . . . 111
7.10 Cross-sectional TEM image of β-Ga2O3 /muscovite mica interface (b)
SAED pattern of β-Ga2O3 thin film and (c) SAED pattern of muscovite
mica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
7.11 (a) Current-voltage measurements under dark and 265 nm wavelength
illumination of device (b) peak photoresponsivity (c) time-response mea-
surement at zero bias and (d) normalized spectral responsivity . . . . . 113
xxiii
7.12 TSC curve of the device with 5 ◦C/min heating and cooling rates at zero
bias (b) net TSC plot with two broad current peaks (c) and (d) are the
Gaussian fits of both peaks . . . . . . . . . . . . . . . . . . . . . . . . . 115
7.13 (a) Measurement setup for bending tests of the device (b) Tensile strain
versus bending radius of the device (c) CCD camera images of a flexi-
ble device under flat (d) 16 mm bending radius condition (e) Dark and
photocurrent of the device at zero bias with bending radius of muscovite. 118
8.1 SEM image of (a) ball bond and (b) wedge bond [247] . . . . . . . . . . 122
8.2 Wire bonding process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
8.3 Optical image of (a) single diced device (b) wedge bond on the device
pad (c) camera image of a mounted device with a wire loop and ball
bond and (d) packaged photodetector on TO-header with quartz optical
window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
8.4 (a) Dark current and (b) photocurrent as a function of voltage before
and after wire bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
8.5 (a) Responsivity as a function of voltage before and after wire bonding
and (b)time response before and after bonding . . . . . . . . . . . . . . 127
xxiv
List of Tables
1.1 Material comparisions for solar-blind photodetectors application . . . . . 10
1.2 Physical properties of β-Ga2O3 . . . . . . . . . . . . . . . . . . . . . . . 12
3.1 Lorentzian width and FWHM of thin films grown at different oxygen
pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.2 Ga/O ratio of samples calculated from XPS spectra . . . . . . . . . . . . 42
3.3 Raman active phonon modes of all the thin films . . . . . . . . . . . . . 43
3.4 Average values of bandgap (from Tauc plot), intrinsic Fermi-level (from
theoretical calculations), work function (from KPFM) and energy gap
between VBM and Ef (from VB spectra) . . . . . . . . . . . . . . . . . . 47
4.1 Comparisons of Ga2O3 self-powered photodetector arrays . . . . . . . . . 59
5.1 Known origins of the traps in β-Ga2O3 . . . . . . . . . . . . . . . . . . . 74
5.2 List of traps and their activation energies in β-Ga2O3 thin film using
TSC method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.3 List of traps and their activation energies in β-Ga2O3 thin films by Ar-
rhenius plot of dark current . . . . . . . . . . . . . . . . . . . . . . . . . 80
7.1 State-of-the-arts room temperature performance parameters of flexible
solar-blind photodetectors . . . . . . . . . . . . . . . . . . . . . . . . . . 103
7.2 Comparisons of wearable photodetectors in the perspective of flexibility 105
7.3 Comparisons of photovoltaic UV-C detectors with different device struc-
tures and channel layer growth methods (Notations: TG - growth tem-
perature, Id - dark current, Ip- photocurrent and Popt - optical power
density) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
7.4 Distribution of bandgap states calculated from thermally stimulated cur-
rent spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
xxv
7.5 Comparisons of self-powered and flexible UV-C photodetectors based on
Ga2O3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
8.1 Comparisons of different wire bonding methods . . . . . . . . . . . . . . 121
8.2 Comparisons of different wire bonding methods . . . . . . . . . . . . . . 124
8.3 List of wire bonding parameters for gold wire . . . . . . . . . . . . . . . 124
xxvi
Appendix
Symbols
η Ideality factor
Ip Photocurrent
q Charge
Popt Optical power density
h Plank constant
ν Frequency of photon
c Speed of light
λ Wavelength
Rλ Spectral response at wavelength λ
Aeff Effective device area where charge carrier generates
φ Photon flux
r Reflectance of material
α Absorption coefficient of material
ξ Factor due to electron-hole recombination in material
κ Extinction coefficient of material
Id Dark current
I Current
I0 Steady state current
τ Relaxation time constant
A1 & A2 Constants
i2n Mean square of total noise current
is Shot noise
iT Thermal noise
R resistance of material
k Boltzmann constant
T Temperature
Pmax Maximum illumination power
ϕ Schottky barrier height
xxvii
Vb Built-in potential
β Beta phase
γ gamma
Angstrom
m0 Rest mass of electron
Be Beryllium
Li Lithium
Sn Tin
Si Silicon
Ge Germanium
Zr Zirconium
Ni Nickel
Au Gold
Co Cobalt
Cu Copper
Pt Platinum
Ir Iridium
Al Aluminum
Ti titanium
Fe Iron◦C Degree Celsius
K Kelvin
J Joule
θ Incident angle of x-ray with crystal plane
Φ Rotation angle with normal to the crystal plane
VCPD Contact potential difference
ϕtip Work function of the tip
ϕsample Work function of the sample
Eg bandgap
B Proportionality constant
Efi Intrinsic Fermi level
m∗p Hole effective mass
xxviii
m∗n Electron effective mass
SI Spectral density of current fluctuations
Sv Spectral density of voltage fluctuations
A Device area
ϕap Apparent Schottky barrier height
A∗ Richardson constant
Pλ Optical power density at λ wavelength
ET Activation energy of trap
Tm Characteristic peak temperature
Γ Heating rate
vth Thermal velocity
Nc Density of states in conduction band
σn Capture cross section
Efn Quasi Fermi level of electron
Efp Quasi Fermi level of hole
ϕb Barrier height
ϕif Image force induced barrier lowering
Nd Donor concentration
G Gain
D∗ Detectivity
ε Strain
tf Thickness of film
ts Thickness of substrate
Rc radius of curvature
Yf Young’s modulus of film
Ys Young’s modulus of substrate
ζtfts
kGy Kilo Gray
xxix
Abbreviations
UV Ultraviolet
EQE External quantum efficiency
PDCR Photo to dark current ratio
NEP Noise equivalent power
LDR Linear dynamic range
FET Field effect transistor
MSM Metal-semiconductor-metal
MOS Metal oxide semiconductor
SB Solar-blind
IoT Internet of things
DUV Deep ultraviolet
Ga2O3 Gallium oxide
AlGaN Aluminum gallium nitride
SiC Silicon carbide
VBM Valence band maximum
TSC Thermally stimulated current
TO Transistor outline
IDE Interdigitated electrodes
PVA Polyvinyl alcohol
PLD Pulsed laser deposition
TEM Transmission electron microscope
XRD X-ray diffraction
SPM Scanning probe microscopy
KPFM Kelvin probe force microscopy
LH Lift height
AC Alternative current
DC Direct current
PL Photoluminescence
XPS X-ray photoelectron microscopy
DLTS Deep level transient spectroscopy
TCO Transparent conducting oxide
xxx
UID Unintentionally doped
EDAX Energy dispersive X-ray spectroscopy
FWHM Full width at half maximum
DOS Density of states
VBM Valence band maxima
HOPG highly oriented pyrolytic graphite
FLP Fermi level pinning
SNR Signal to noise ratio
MBE Molecular beam epitaxy
MOCVD Metal oxide chemical vapor deposition
PECVD Plasma enhanced chemical vapor deposition
SBH Schottky barrier height
DUT Device under test
RT Room temperature
STH Self trapped holes
DLOS Deep level optical spectroscopy
TSDC Thermally stimulated depolarization current
EFG Edge-defined film-fed grown
MOSFET Metal oxide semiconductor field effect transistor
SEM Scanning electron microscope
ICNIRPInternational Commission on Non-Ionizing Radiation
Protection
PEN Polyethylene naphthalate
BN boron nitride
ZnO Zinc oxide
PET Polyethylene terephthalate
CCD Charge coupled device
HRXRD High resolution X-ray diffraction
FFT Fast Fourier transform
xxxi
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