First - Principles Study Of Zinc Oxide P - Type Doping System

ZnO has been regarded as a promising wide bandgap semiconductor because of its excellent optical and electrical properties. It can be used in the fields of solar cells, chemical and pressure sensitive sensors, surface acoustic wave devices, and light-emitting diodes, etc. In fact, ZnO is an n-type semiconductor due to the intrinsic donor defects such as O vacancy and interstitial Zn, so it is very difficult to dope ZnO to a stable p-type semiconductor compared with the n-type semiconductor. Thus, the practical application and development of ZnO-based optoelectronic devices are greatly limited, so the p-type doping of ZnO has caused widespread attention. Usually, the elements in Group I( Li, Na, K, etc.) and Group V( N, P, As, etc.) are chosen as the dopants for the p-type Zn O. Among the ZnO systems doped by Group I elements, Li-doped ZnO system has attracted much attention. Although there have been many experimental studies on the Li-doped ZnO system, the systematic theoretical research with different doping concentrations has not been reported. When the Group V elements N substitutes for O, the solubility of the dopant N is low and the system is instability. It has been shown that by Li- N codoping, the hole concentration can be improved and stable p-type ZnO can be achieved more easily compared with N or Li-doped ZnO, respectively. Up to now, most of the related studies are on experiments, little attention has been paid to the theoretical research, especially on the optical properties. In this thesis, the p-doping problems of wurtzite ZnO have been studied by the first-principles approach.The main contents of the thesis are as follows:(1) The crystal structure, electricity properties, optical properties of the pure ZnO, and the research progress of doping problem on ZnO are given. The density functional theory and the calculation tool based on it-- ADF package are briefly discussed.(2) The electronic structures and optical properties of the Li-doped ZnO systems has been studied. The obtained results indicate that the acceptor level can be introduced near the top of the valence band when Li is replaced with Zn, which makes the top of the valence band shift up. Meanwhile, the bottom of the conduction band moves up more. As a result, the band gap increases linearly with increasing doping concentration and the absorption edge gradually shifts to the short wavelength direction. Compared with pure ZnO, the optical properties of the doped systems are almost not influenced in the high-energy region, but they change significantly in the low-energy region. Due to the impurity level, new absorption peaks near the visible region emerge in the doped systems, it to say that the absorptivity in visible light region can be promoted by suitable doping. Thus, the photocatalytic properties of the system can be improved.(3) The electronic structure and optical properties of the Li- N codoped ZnO systems are studied. The obtained results show that for the N doped system, the impurity band is narrow and the degree of the localization of the hole carriers is high, which results in the low solubility of the dopant N and the instability of the system. For the Li- N codoped system, the impurity band is broadened, the localization degree of holes is weaken, and the solubility of N atoms is raised, thus a better and more stable p-type ZnO can be achieved by Li- N codoping. Due to the impurity level, new optical absorption peaks near the visible region are observed for the doped systems. The peak in Li- N codoped system are significantly higher than those of ZnO doped with Li or N alone, it means that the absorptivity of the Li- N codoped system in visible light region is much higher, thus the photocatalytic properties of the system can greatly be improved.