dc.description.abstract | In order to manifest ZnO material into application and develop novel oxide-based materials, I have systematically investigated on three research areas over the past few years, including the development of n-type ZnO-based conductive layers, the influences of point defects on electrical properties in ZnO, and demonstration of ZnSeO alloys by plasma-assisted molecular beam epitaxy (MBE).
Highly conductive n-type ZnO has potential to employ into transparent conductive layers (TCL) and Al, Ga, and In are major dopant sources for achieving n-type characteristics. Al and Ga doped ZnO films have been extensively studied, while few efforts on the growth and characterization of In-doped ZnO films have been reported. Therefore, a serious of In-doped ZnO films was grown by MBE and the characteristics in respect of crystal quality, electrical, and optical properties were investigated and estimated for TCL applications. For electrical characteristics, In-doped ZnO films can easily increase their electron concentration over 1020 cm-3. However, the large size mismatch between Zn and In leads to a degradation on crystal quality and photoluminescence. Even though a degenerated concentration is achieved, no metallic transport behavior was seen and attributed a weak-localization effect caused by atomic disorder arrangement. Thus, when annealing process is introduced to improve crystal quality, a transition from semiconductor to metallic behavior can be observed. The resistivity can be reduced to 1×10-3 Ω∙cm, and it is still not good enough compared to Al and Ga doped ZnO films.
The characteristics of Ga-doped ZnO films were also investigated, following an introduction of in-situ post thermal annealing under Zn flux. It’s found that this method effectively suppress the thermal desorption of Zn and the formation of acceptor-like defects. The resistivity of Ga-doped ZnO can reach as low as 9×10-5 Ω∙cm in this study. Compared to reported findings, Ga-doped ZnO shows lower compensation ratio (NA/ND) at any electron concentration. Ga-doped ZnO with low compensation ratio and low resistivity are achieved simultaneously. In-situ annealing in Zn flux might become an important technology to manifest Ga-doped ZnO into transparent electrodes.
On the other hand, ZnO with large band gap is also suitable for transparent thin film transistor (TTFT). However, the influences of intrinsic defects on electrical properties might lead to unstable output characteristics in devices. Therefore, undoped and Ga-doped ZnO films grown by MBE were prepared for systematic investigations on the influences of native defects on their electrical properties. Distinct electrical properties of the undoped and Ga-doped ZnO films are observed after thermal treatments in nitrogen and oxygen ambient. It is found that the undoped ZnO films show improved characteristics when annealed in oxygen ambient, and Ga-doped ZnO films exhibit stable characteristics when annealed in nitrogen ambient. As revealed by defect emission in photoluminescence, the variation of electrical properties could be attributed to the generation and annihilation of native defects, depending on the ambient of treatments. It is concluded that the annealed undoped ZnO films are affected mainly by oxygen vacancies, antisite oxygen and oxygen interstitials, while the annealed Ga-doped ZnO films are dominated by oxygen interstitials. For specific applications, undoped and Ga-doped ZnO are annealed in different environments.
For development of novel oxide materials, characteristics of ZnSeO alloy were investigation in this thesis. A method for growing ZnSeO alloys on GaAs substrates by carefully controlling the oxygen flow rates was exploited. ZnSeO with varying oxygen content and even near 10 % oxygen was first demonstrated. The optical band gap can be reduced from 2.8 eV (ZnSe) to 2.2 eV. The characteristics including structural properties, optical properties, carrier dynamic and etc. are discussed in this chapter. For the first time, the n-type ZnSeO doped by Cl is obtained. Combined with band gap tuning, it can cover a wide range of wavelength region used in optical devices.
The dissertation shows that the ZnO-based materials with high Ga doping exhibit low resistivity, high transparency, and high stability. Meanwhile, the clarification of ZnO native defects shows very useful information in usage of electronic devices. A development of ZnSeO alloys makes a new opportunity for optoelectronic applications. According to the characteristics above-mentioned, ZnO-based materials show great potential to realize in transparent electrode, electronic devices, and PVs. | en_US |