Wide band-gap and high conductivity ZnO-based transparent films grown by RF magnetron sputtering : effects of Mg, Be, and Ga doping and process parameters = RF 마그네트론 스퍼터링에 의해 성장 된 넓은 밴드 갭 및 높은 전도도를 가진 ZnO 기반 투명 필름 : Mg, Be 및 Ga 도핑 및 공정 파라미터의 영향

다국어 초록 (Multilingual Abstract)

ZnO and its alloy ZnMgO have gained much attention for applications in ultraviolet photodetectors, photo-catalyst, transparent conductive electrodes, and electronic devices. The increase of the band-gap in Zn1-xMgxO alloys with added Mg facilitates tunable control of the conduction band alignment and the Fermi-level position. However, the maximal conductivity achieved by doping decreases considerably at higher Mg concentrations, which limits practical application of this material as a wide-gap transparent conductive oxide.
This study suggests that the Mg content, band-gap energy, and structural phase in sputter-deposited ZnMgO films could be tailored over a wide range by controlling the growth temperature and sputtering ambient, which opens up an important research direction on deep UV materials. The Mg content in the films and the band-gap energy showed a strong dependence on the growth temperature and sputtering ambient, increasing with the growth temperature or decreasing by the oxygen addition into the plasma ambient. In this work, a wurtzite ZnMgO film with a wide band gap of 4.53 eV and a high Mg content of XMg = 0.53 was realized for the first time by sputtering the Zn0.7Mg0.3O alloy targets with Mg contents of 0.3 – 0.35 below 200 oC under a pure Ar ambient. The structural phase exhibits a dependence on the growth temperature, and the solubility limit of Mg in the single wurtzite ZnMgO phase without phase segregation was estimated at 0.53 – 0.56 and was confirmed to reduce with the growth temperature, possibly due to the lower formation Gibbs free energy of MgO than ZnO and the difference in the growth condition.
Next, the quaternary ZnMgBeO films with co-doped Mg and Be were prepared using an RF magnetron sputtering to avoid phase segregation, and the effects of oxygen partial pressure within the Ar process plasma on the optical, structural, and electrical properties ZnMgBeO films were investigated in detail. It was observed that the optical energy bandgap (Eg) values of the ZnMgBeO films substantially decrease with the oxygen addition, from 5.3 eV to 4.3 eV as the oxygen partial pressure increases from zero to one. The full-width-at-half-maximum (FWHM) values of the (0002) XRD peaks drastically decrease with the addition of a small amount of oxygen but then increase with further oxygen addition. All the films had very high sheet resistance, 1.3-1.4 G/. It was also observed that the concentration of Zn within the films significantly increased with the oxygen addition, which was proposed to be mainly responsible for the observed decrease in Eg. It was also proposed that the FWHM change due to the oxygen addition may be attributable to three factors, film composition, grain size, and point defect density, as confirmed by results of TEM and XPS investigations.
On the other hand, Ga has been chosen to improve the conductivity of the ZnO-based TCMs, as the element shows well-matched ionic size and covalent radii with the Zn ions, minimizing lattice deformations of the ZnO matrix even at high doping concentration. Ga heavily-doping of ZnO films has been studied to take advantage of the Brustein-Moss effect and to obtain a high conductivity and an extended Eg simultaneously. Sputter growth of highly conductive Ga-doped ZnO films with abnormally wide band-gap (Eg) is discussed in detail, as well as the accompanying defect behavior. It was observed that the optical Eg and the conductivity increased as the working pressure of the Ar plasma decreases. The ZnGaO films with Eg of 3.95 eV and resistivity of 2×10-4 Ω∙cm was obtained by simply lowering the working pressure to 0.3 Pa, which represent the highest Eg value so far reported for the ZnO-based transparent conducting materials with a resistivity lower than 10-3 Ω∙cm. It was proposed that this phenomenon may be attributed to the improved crystalline quality and the increased amount of the Ga incorporation into the Zn sites and the oxygen vacancy concentration. It was also suggested that the oxygen vacancy acts as the electron donor for the sputtered ZnO and ZnGaO films.
In a final study, transparent conductive ZnMgGaO films were sputter-grown on sapphire at lower working pressure in an effort to silmuneously improve the conductivity and the optical band-gap. However, the conductivity of the films decreases markedly at high Mg doping level, the Zn0.93Mg0.03Ga0.04O film have a Hall mobility (μ) of 18.8 cm2/VS and a carrier concentration of 1.1×1021 cm-3, yielding a resistivity of 3×10-4 Ω.cm, while the Zn0.66Mg0.3Ga0.04O film possesses significantly worse electrical properties (μ = 1.82 cm2/VS, n = 2×1017 cm-3, and ρ = 23.6 Ω.cm). It is demonstrated that the formation of acceptor-like compensating defects such as Zn vacancies and/or other complexes increases with the increasing Mg content, decreasing the free electron concentration and the mobility through the ionized impurity scattering. XPS investigation proves that the oxygen vacancies also decrease with increasing Mg content, resulting in the films with high resistivity at high Mg doping level. The ZnMgGaO films were post-annealed in Zn vapor to introduce point defects – Zn interstitial and lead to increase the carrier concentration of the films. It was suggested that oxygen ions emerge at the surface of the films and combine with zinc atoms from vapor to form ZnO on the surface, and the electrons given up at the surface by the incorporated zinc atoms convert the newly formed oxygen vacancies then migrate in to the annealed films. Zinc interstitials could also form since the surrounding zinc vapor is an obvious source of zinc atoms that could diffuse into the crystal. As a result, the annealed ZnMgGaO films with Eg = 3.97 eV and the resistivity of 1.6×10-4 Ω.cm could be obtained after post-treatment in Zn vapor ambient. The ZnMgGaO films at high Mg doping level could also be obtained after annealed in Zn vapor with an Eg = 4.26 eV and the resistivity of 4×10-3 Ω.cm, which presents a promising candidate for transparent electrode in the UV optoelectronic application.

목차 (Table of Contents)

• Contents
• Acknowledgements i
• Contents iii
• List of Publications vii
• List of Tables viii
• List of Figures ix
• Abstract xvi
• Chapter 1 Introduction and Background 1
• 1.1. Applications and the status of TCOs 1
• 1.2. Fundamentals of ZnO 5
• 1.2.1. Structural properties 5
• 1.2.2. Electrical properties 7
• 1.2.3. Band-gap engineering in ZnO 7
• 1.2.4. Native point defects in ZnO 11
• 1.2.5. Doping in ZnO 14
• 1.2.6. Optical properties 17
• 1.3. Motivation 19
• 1.3.1. Motivations to perform band-gap engineering in ZnO 19
• 1.3.2. Current challenge in band-gap engineering ZnO 22
• 1.4. Research Objectives 23
• 1.5. Thesis scope 23
• References 27
• Chapter 2 Deposition and Characterization techniques 44
• 2.1. Deposition method 44
• 2.1.1. Background of sputter deposition 44
• 2.1.2. Film growth and structures 51
• 2.2. Characterization techniques 53
• 2.2.1. Atomic Force Microscope (AFM) 53
• 2.2.2. X-ray diffraction (XRD) 55
• 2.2.3. Hall effect measurement 56
• 2.2.4. Four-point probe 58
• 2.2.5. UV-Vis spectroscopy 59
• 2.2.6. Scanning Electron Microscopy 60
• 2.2.7. Transmission Electron Microscopy 61
• 2.2.8. Inductively Coupled Plasma – Optical Emission Spectrometry 63
• 2.2.9. Photoluminescence (PL) 64
• 2.2.10. X-ray Photoelectron Spectroscopy 65
• References 67
• Chapter 3 Tailoring the Composition, Band-gap, and Structure Phase in ZnMgO Films by Simply Controlling Growth Temperature and Oxygen Partial Pressure during Sputter Deposition 69
• 3.1. Introduction 69
• 3.2. Experimental 71
• 3.3. Results and Discussion 72
• 3.4. Summary 88
• References 90
• Chapter 4 Effects of Oxygen Partial Pressure on the Characteristics of Magnetron-Sputtered ZnMgBeO Thin Films 95
• 4.1. Introduction 95
• 4.2. Experimental 96
• 4.3. Results and Discussions 96
• 4.4. Summary 105
• References 106
• Chapter 5 Realization of Highly Conductive Ga-doped ZnO Film with Abnormally Wide Band-gap Using Magnetron Sputtering by Simply Lowering Working Pressure 109
• 5.1. Introduction 109
• 5.2. Experimental 110
• 5.3. Results and Discussions 112
• 5.3.1. Characteristics of the sputtered ZnGaO films relying upon the working pressure. 112
• 5.3.2. Chemical composition of the ZnGaO films sputtered at different working pressures. 114
• 5.3.3. Characteristics of the undoped ZnO films sputtered at different working pressures. 118
• 5.3.4. XRD and SEM analysis of the ZnGaO films sputtered from various working pressures. 119
• 5.3.5. The dependence of the optical band-gap upon the carrier concentration. 123
• 5.4. Summary 125
• References 127
• Chapter 6 Wide Band-gap and Highly Conductive ZnMgGaO Transparent Films Grown by RF Magnetron Sputtering 132
• 6.1. Introduction 132
• 6.2. Experimental 133
• 6.3. Results and Discussions 135
• 6.4. Summary 146
• References 148
• Chapter 7 Summary and Future Directions 151
• 7.1. Summary 151
• 7.2. Future direction 154
• 초록 156