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Ab-initio study on optical and electrical properties of oxide semiconductors
산화물 반도체의 광학 및 전기적 성질에 관한 제일원리 계산 연구

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Authors
강영호
Advisor
한승우
Major
공과대학 재료공학부
Issue Date
2015-08
Publisher
서울대학교 대학원
Keywords
oxide semiconductorsdensity functional theory (DFT)electron transportpoint defectlight absorption
Description
학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2015. 8. 한승우.
Abstract
Oxide semiconductor (OS) is a promising candidate for the application to
large-area and flexible opto-electrical devices, because OS which is deposited at
low temperature exhibits the outstanding electrical properties showing the high
electron mobility (~10 cm2/Vs) in comparison with the widely used semiconductors
such as amorphous Si (a-Si). In addition, since OS is transparent in the most
visible and near-infrared regions due to their large band gap, it is often called
transparent oxide semiconductor and can be expected to offer a solution for
fabrication of the transparent electronic device on flexible substrates. In spite of
the advantages of OS, previous experimental studies reported various obstacles to
be overcome for realization of the device utilizing OSs. For example, the OS
based thin film transistor (TFT) suffers from device instabilities under illumination
and bias stress. In addition, the underlying mechanism for the charge transport in
OS still remains unclear, which hinders improvement of the OS based device
performance.
In this research, we investigate the optical and electrical properties of OSs using
ab-initio calculations on the basis of density functional theory (DFT) to suggest the
strategy to improve the performance of OSs in terms of electron transport and
reliability. First, in order to investigate the electron transport in OSs, we introduce
the model Hamiltonian for the conduction band within tight-binding approximation
and carry out the calculation of the electron mobility considering several types of
scattering processes. It is found that the interaction between metal s and oxygen
p orbitals under tetrahedral and octahedral local atomic structure gives the
quasi-linear dispersion of the conduction band which plays an important role in
determining the electron mobility in OSs. In addition, we reveal that the electron
mobility in a multi-component OS like InGaZnO4 is dominated by the
cation-disorder scattering process.
Next, we investigate the influence of hydrogen impurity in OS. Previously, various
models to explain the threshold voltage shifts under illumination and bias stress
were suggested based on oxygen vacancy defect, but the clear mechanisms for
that are still controversial. In this study, the DFT calculation results turn out that
hydrogen in OSs can have +1/-1 charge state depending on Fermi level. This
bistability of hydrogen enables to cause the threshold voltage shift by alternating
its charge state under illumination and bias stress.
In a second part, the optical properties are mainly dealt with. For the more
realistic modelling, we study the ab-initio calculation methodology based on GW
approximation for obtaining the accurate band gap of OS. The DFT calculation
usually results in 30~40% underestimation of the band gap in typical insulator and
GW method improves the accuracy of the calculated band gap by correcting
exchange-correlation energy of the conventional DFT calculation. However,
previous GW calculation results for OSs still yield ~10% lower band gap than
experimental value. Thus, we investigate the theoretical reason for such wrong
description of GW method for OSs and suggest new method to further improve
the predictive power of GW method, which shows the mean absolute relative
error (MARE) of ~3%.
Finally, we study the visible light absorption of amorphous In-Ga-Zn-O since the
device degradation critically occurs when exposed to visible and UV light. It is
found that the relative downshift of the conduction band position comparing to
that of crystalline phase occurs as well as long tail states near the valence band
edge appear in amorphous phase. This is one of the main reasons for amorphous
OSs to absorb the visible light without any defects.
Language
Korean
URI
https://hdl.handle.net/10371/118014
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Materials Science and Engineering (재료공학부)Theses (Ph.D. / Sc.D._재료공학부)
  • mendeley

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