Thermally Driven Intrinsic and Extrinsic Doping Mechanisms in Amorphous Oxide Semiconductors : 열에너지에 의한 비정질 산화물 반도체의 내인성 및 외인성 도핑 메커니즘 연구

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dc.description학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2016. 2. 주영창.-
dc.description.abstractAmorphous oxide semiconductors (AOSs) have been considered as one of the most promising materials for implementation of next-generation electronic devices that are flexible, transparent, and large-area applicable due to their novel properties. AOSs have long-range film uniformity induced by long-range structural disorder and show excellent electron mobility comparable to the corresponding crystalline oxide semiconductors. However, the use of AOSs in electronic devices has been hindered by the lack of controllability of electrical properties as well as stability, which is induced by various electronic states of dopant in subgap region. Although electron mobility is relatively insensitive to structural disorder, electronic states of both intrinsic and extrinsic dopants are affected by the local atomic structure. As a result, dopants form various electronic states and irregular doping efficiency is observed. Moreover, degree of structural disorder tends to decrease because amorphous structure is thermodynamically metastable, which is referred as structural relaxation (SR). Thus, doping efficiency of dopants as well as distribution of subgap states could change by thermal stress. Therefore, understanding the continuous change of doping efficiency of intrinsic and extrinsic dopants driven by thermal stress is necessary for controlling the electrical properties and improving the of stability of AOSs.
The objective of this thesis is to unravel the intrinsic and extrinsic doping mechanisms in AOSs with respect to thermal history and to provide guidelines for not only delicate control of electrical properties, but also creation of new functionality in AOSs. Before investigating thermally driven doping mechanisms in AOSs, it is necessary to regulate the additional reactions in AOSs such as redox reactions. In this study, novel metal/AOSs/metal structured devices are designed to prevent unwanted reactions of AOSs with the ambient. Based on the devices, changes in electrical properties of AOSs induced by intrinsic atomic rearrangement as well as extrinsic dopant migration were investigated.
First, concentration of oxygen vacancy (VO) as an intrinsic donor in amorphous In-Ga-Zn-O (a-IGZO) was modulated by solely SR. As annealing temperature increases from 300 °C to 450 °C, concentration of VO in the shallow donor state 1000 time increases. The SR-driven intrinsic doping effect depends strongly on the annealing temperature but not on the annealing time. The Arrhenius activation energy of the SR-driven doping effect is 1.76 eV, which is similar to the bonding energies in a-IGZO. Free volume in a-IGZO decreases during SR and VO in either deep-donor or electron-trap states consequently transforms into shallow-donor state.
The second focus is to identify the electronic states of extrinsic Cu dopant in AOSs. Amorphization of Cu-based metal oxides have induced peculiar electrical characteristics with loss of p-type characteristics of Cu dopant in the corresponding crystalline oxides. Therefore, unravelling the doping mechanism of Cu in AOSs is essential to determine the exact electronic states of Cu in AOSs. In the early stage of annealing, Cu dominantly diffuses into a-IGZO through the free volume and acts as an electron donor and increases electrical conductivity of a-IGZO. Moreover, resistive switching (RS) characteristics are generated in Cu-doped a-IGZO due to the electrochemical migration of Cu at the free volume. With further annealing, substitutional Cu becomes predominant which prefers In to Ga or Zn. After annealing, inter-diffused Cu and In form crystalline Cu-ln-O clusters in a-IGZO. Cu-In-O clusters not only form bulk-heterogeneous pn junction, but also give rise to negative differential resistance behavior in a-IGZO. RS performance can be modulated by Cu doping concentration at the free volume as well as the formation of Cu-In-O clusters.
This study reported thermally-driven intrinsic and extrinsic doping mechanism in AOSs without any reactions of AOSs with the ambient using the novel metal/AOSs/metal structured devices. A systematic study on electrical conduction mechanism analysis of the devices, microstructural and chemical analysis provided useful information for understanding the changes in electronic state of intrinsic and extrinsic dopants according to the structural location and suggested that extrinsic doping control gives rises to new-functionality in AOSs such as resistive switching in addition to the modulation of electrical conductivity.
dc.description.tableofcontentsChapter 1. Introduction 1
1.1. Amorphous oxide semiconductor-based electronic devices 1
1.2. Doping issues in amorphous oxide semiconductors 5
1.2.1. Effect of structural disorder on doping efficiency 5
1.2.2. Intrinsic doping: oxygen vacancy with hydrogen 15
1.2.3. Extrinsic doping: copper 20
1.3. Objective of the thesis 26
1.4. Organization of the thesis 29

Chapter 2. Theoretical Background 30
2.1. Subgap states in amorphous oxide semiconductors 30
2.1.1. Structural disorder 31
2.1.2. Chemical disorder 33
2.2. Structural relaxation in amorphous solid 44
2.3. Electrical conduction mechanisms in metal/semiconductor/metal structures 46
2.3.1. Schottky emission 46
2.3.2. Poole-Frenkel conduction 49
2.3.3. Space-charge-limited conduction 50
2.3.4. Tunneling-based conduction 51
2.4 Introduction to resistive switching devices 52

Chapter 3. Experimental Procedures 63
3.1. Sample preparation 63
3.1.1. Device fabrication 63
3.1.2. Multi-layer thin-films 66
3.1.3. Van der Pauw samples 67
3.2. Structural and optical analysis 68
3.3. Post-fabrication annealing 69
3.4. Electrical analysis 70
3.5. Microstructural and chemical analysis 71
3.6. Computation 72

Chapter 4. Development of the Devices Able to Regulate Extrinsic Reactions of Amorphous Oxide Semiconductors 73
4.1. Introduction 73
4.2. Experiments 74
4.3. Characteristics of as-deposited oxide films 75
4.4. Microstructural changes 79
4.5. Chemical state changes 84
4.6. I-V characteristics changes 87
4.7. Summary 95

Chapter 5. Structural Relaxation-Driven Intrinsic Doping 97
5.1. Introduction 97
5.2. Experiments 98
5.3. I-V characteristics changes 98
5.4. Conduction mechanism analysis 105
5.4.1. Schottky-thermionic emission 105
5.4.2. Ohmic and Poole-Frenkel conduction 110
5.4.3. Extraction of free electron/doping concentration 114
5.5. C-V characteristics of Schottky conducting devices 127
5.6. Effect of H dopant 142
5.7. Summary 145

Chapter 6. Dynamical Changes in Cu Doping Effect 146
6.1. Introduction 146
6.2. Experiments 147
6.3. I-V characteristics changes 148
6.4. Conduction mechanism analysis 155
6.5. Electrical breakdown behavior 159
6.5.1. The origin of resistive switching 159
6.5.2. Resistive switching characteristics 165
6.6. Microstructural and chemical analysis 171
6.7. Cu diffusion modeling 178
6.8. Summary 182

Chapter 7. Conclusion 183
7.1. Summary of results 183
7.2. Future works and suggested research 185
7.2.1. Effect of O non-stoichiometry on SR-driven doping 185
7.2.2. Flexible Cu-doped AOSs memristor 187

Reference 189

Abstract (In Korean) 206
dc.format.extent19508135 bytes-
dc.publisher서울대학교 대학원-
dc.subjectamorphous oxide semiconductors-
dc.subjectoxygen vacancy-
dc.subjectelectronic states-
dc.subjectstructural relaxation-
dc.subjectresistive switching-
dc.titleThermally Driven Intrinsic and Extrinsic Doping Mechanisms in Amorphous Oxide Semiconductors-
dc.title.alternative열에너지에 의한 비정질 산화물 반도체의 내인성 및 외인성 도핑 메커니즘 연구-
dc.contributor.affiliation공과대학 재료공학부-
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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)
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