臺灣博碩士論文加值系統

氧化鋅(ZnO)為一多功能性之II-VI族化合物半導體材料，在室溫下具有寬的直接能隙(3.37 eV)、大的激子束縛能(60 meV)以及好的化學穩定性與生物相容性等特點，使其吸引廣大的研究興趣並已使用在相關的產品上。由於氧化鋅為六方晶系中之纖鋅礦結構(wurtzite structure)，此結構具有六方對稱，且沒有對稱中心，此種特殊結構使氧化鋅晶體擁有極性面(polar plane)與非極性面(nonpolar plane)，此兩種平面具有不同的表面原子排列結構與相異的物理與化學性質，使之研究與應用變得更為多樣化。氧化鋅之極性面為(0002)面向，此面向為氧化鋅之最低表面能方向，因此大部分的氧化鋅奈米結構皆沿著此方向做成長。相較於極性面之氧化鋅，(101 ̅0)及(112 ̅0)為氧化鋅之非極性面向，此結晶方向之氧化鋅製備則較為困難，使得相關的研究相對地稀少，但其特殊之晶體結構與獨特的物理性質，卻具有相當大的應用潛力。因此本實驗將利用電漿輔助式化學氣相沉積技術(plasma enhanced chemical vapor deposition, PECVD)合成出極性與非極性之氧化鋅薄膜，藉由操控不同的合成參數，包括合成壓力、溫度、以及前驅物氣體流量比例等等，能夠精確地控制氧化鋅薄膜之成長方向。研究中將PECVD製程之合成溫度、合成壓力、前驅物二氧化碳(CO2)與二乙基鋅(DEZn)之比例對於氧化鋅薄膜之特性與影響進行一系列系統性的探討，結果顯示出合成溫度是影響氧化鋅結晶方向的關鍵參數，並且結晶品質可以藉由改變合成壓力與前驅物比例獲得改善。另外，透過光發射光譜(optical emission spectroscope)臨場監控薄膜合成時之電漿組成並結合薄膜成長理論驗證提出一可能之成長機制來解釋氧化鋅結晶方向改變之原因。
於功能性應用上，將分別對於極性與非極性氧化鋅薄膜用於紫外光感測器上做一完整性之探討，並提出相關之可能響應機制。本研究透過光微影製程(photolithography)與射頻磁控濺鍍技術(RF sputtering)將指插狀白金薄膜沉積於氧化鋅薄膜上作為接觸電極以形成一感測元件。接著經由光暗電流量測與電流-時間響應曲線來判定感測器之可靠度、響應性、穩定性與靈敏度。實驗結果指出極性與非極性氧化鋅薄膜對於紫外光顯現出不同的響應特性。非極性氧化鋅薄膜對於紫外光有較佳之感測能力，響應性為4708.88 μA/W，反應及回復時間分別為0.141及0.125秒。而極性氧化鋅薄膜則無法達到較佳的響應速度，其響應性為3367.73 μA/W也較非極性氧化鋅薄膜差。本研究進一步透過後退火處理的方式，將極性氧化鋅分別在大氣、真空與氮氣氣氛下進行退火，以進一步改善極性氧化鋅薄膜感測器之效能。結果顯示在大氣退火下能夠大幅提昇感測器之響應性，而在氮氣環境下退火則可以大大改善感測器之穩定性與可靠度。
非極性氧化鋅為近幾年才興起之研究方向，且已被證實應用於光電元件上比極性氧化鋅更具有優勢。本實驗成功地使用電漿輔助式化學氣相沉積法合成出非極性之氧化鋅薄膜，並將其應用於紫外光感測器上證實其本質就具有比極性氧化鋅薄膜更佳之紫外光響應能力與反應速度。而極性氧化鋅則可藉由後退火處理的方式進一步改善其響應性能。

ZnO is a promising semiconductor material for many kinds of functional optoelectronics applications due to its wide direct band gap of 3.37 eV and high exciton binding energy of 60 meV at room temperature. ZnO is a hexagonal wurtzite structure and exhibits a non-centrosymmetric structure, which causes that the ZnO possesses polar and nonpolar plane. The polar and nonpolar planes show the different surface atomic configurations and physic and chemistry properties anisotropic, making that the ZnO attracts extensive research interests and diversity applications. The polar plane for ZnO is (0002) plane, which is a lowest surface energy plane. The most of studies are focus on growth of polar c-orientation ZnO nanostructures and explore its material properties and relative applications. Compared with polar ZnO, the nonpolar planes of (101 ̅0) and (112 ̅0) ZnO are seldom reported due to the difficulty of preparation. However, nonpolar ZnO is considered as a candidate material for next generation high efficiency optoelectronic device due to absence of spontaneous polarization effect in the crystal. Therefore, this present study used plasma enhanced chemical vapor deposition (PECVD) system to synthesize polar and nonpolar ZnO thin films. By adjusting the synthesis temperature, synthesis pressure, and precursor gas flow rate ratio to obtain the high quality ZnO thin films are the first step in this study. The experimental results indicate that the synthesis temperature is a dominated process parameter for controlling the crystallographic orientation of ZnO, and the crystal quality can be improved by altering the synthesis pressure and precursor gas flow rate ratio. Moreover, the possible growth mechanism of ZnO with different crystallographic orientations have been proposed based on the OES analyses and SEM observations.
Both the polar and nonpolar ZnO thin films are used as a sensing layer for UV photodetectors applications. The interdigitated Pt thin films were deposited onto the polar and nonpolar ZnO thin films as a contact electrodes via conventional lithography process and RF sputtering. The performance including responsivity, reliability, and sensitivity of both detector were determined by typical current-voltage (I-V) characterization under dark and UV light illumination and time-dependent photoresponse measurement. The photoresponse results indicated that the nonpolar detector possesses better responsivity (4708.88 μA/W) and faster response (0.141 s) and recovery times (0.125 s) than the polar one (3367.73 μA/W, the response time cannot be determined). The performance of the polar ZnO-based UV photodetector can be improved by using RTA system annealed in different ambients. The photodetector annealed in air revealed the largest responsivity at operating temperature of 25 oC while the detector annealed in nitrogen showed a stable responsivity.
The nonpolar ZnO-based UV photodetector with Pt as a contact electrode natively possesses good sensitivity and acceptable responsivity and reliability, but the polar ZnO-based UV photodetector have to be improved through a post-annealing assistance to exhibit a good performance.

ABSTRACT IN CHINESE i
ABSTRACT IN ENGLISH iii
ACKNOWLEDGEMENTS v
CONTENTS vii
LIST OF TABLES xi
LIST OF FIGURES xii
Chapter 1: INTRODUCTION 1
1.1 Background 1
1.2 Motivation 4
1.3 Aims of this study 5
Chapter 2: LITERATURE REVIEW 7
2.1 Properties of ZnO 7
2.1.1 Crystal structure of ZnO 7
2.1.2 Optical and electrical properties of ZnO 10
2.1.3 Defects in ZnO 16
2.2 Growth of ZnO Crystal 19
2.2.1 Background 19
2.2.2 Growth of polar and nonpolar ZnO 22
2.2.3 Plasma enhanced chemical vapor deposition (PECVD) 31
2.3 Thin films growth mechanism 33
2.4 Metal-semiconductor contacts 35
2.4.1 Ohmic contact 36
2.4.2 Schottky contact 37
2.5 UV photodetector 38
Chapter 3: EXPERIMENTAL DETAILS 41
3.1 Experimental flowchart 41
3.2 Homemade PECVD system 42
3.3 Experimental steps 43
3.3.1 Substrate preparation and cleaning 43
3.3.2 DEZn preparation and preservation 44
3.3.3 PECVD chamber preparation and synthesis of ZnO thin films 44
3.3.4 Preparation of interdigitated-like pattern onto as-synthesized ZnO thin film 46
3.3.5 Deposition of Pt top electrode and chemical lift-off 47
3.3.6 RTA process 48
3.4 Material characterization instruments and principles 53
3.4.1 X-ray diffraction (XRD) 53
3.4.2 X-ray photoelectron spectroscopy 55
3.4.3 Photoluminescence spectroscopy 56
3.3.4 Field emission scanning electron microscopy 57
3.3.5 Transmission electron microscopy and focus ion beam milling 57
3.3.6 Atomic force microscope 58
3.3.7 Optical emission spectroscopy 59
3.3.8 Water contact angle goniometer 60
3.3.9 UV photoresponsivity analysis 61
Chapter 4: RESULTS AND DISCUSSION 62
4.1 Investigation of process parameters on ZnO crystallinity 62
4.1.1 Introduction 62
4.1.2 Synthesis temperature effect 63
4.1.3 Synthesis pressure effect 71
4.1.4 Gas flow rate ratio effect 77
4.1.5 Summary 83
4.2 Investigations on crystallographic orientation evolution of ZnO thin films and their crystal growth mechanism 85
4.2.1 Introduction 85
4.2.2 Synthesis of different crystallographic orientation ZnO thin films 86
4.2.3 Crystal structure, morphology, and optical properties 87
4.2.4 Crystal growth mechanism 93
4.2.5 Electrical property 97
4.2.6 Surface wettability 101
4.2.7 Summary 106
4.3 Investigation on effect of synthesis pressure on crystallinity of nonpolar ZnO thin films 107
4.3.1 Introduction 107
4.3.2 Synthesis of high quality nonpolar ZnO under different pressures 108
4.3.3 Crystal structure and surface morphology 108
4.3.4 Optical properties 114
4.3.5 Summary 115
4.4 Fabrication of polar and nonpolar ZnO-based UV photodetector and comparison between their performances 116
4.4.1 Introduction 116
4.4.2 Fabrication of interdigitated electrode onto polar and nonpolar ZnO thin films 118
4.4.3 Polar ZnO-based UV photodetector 119
4.4.4 Nonpolar ZnO-based UV photodetector 123
4.4.5 Discussion of performance between the polar and nonpolar ZnO-based UV photodetector 127
4.4.6 Responsivity and reliability tests of nonpolar ZnO-based UV photodetector 132
4.4.7 Summary 135
4.5 Responsivity improvement of polar ZnO-based UV photodetector by post-annealing 137
4.5.1 Introduction 137
4.5.2 Effects of annealing ambients on polar ZnO-based UV photodetector 138
4.5.3 Investigations on the response mechanism of polar ZnO-based UV photodetector annealed in different ambients 144
4.5.4 Summary 147
Chapter 5: CONCLUSIONS 151
Chapter 6: FUTURE WORK 154
REFERENCES 155
LIST OF SYMBOLS 179
CURRICULUM VITAE 181
PUBLICATION 182

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