First-principles Study Of Defects And Dopability In ZnO

ZnO is a wide-band-gap semiconductor, with band gap of 3.37 eV at room temperature (RT). Particularly, its exciton binding energy is as large as 60 meV, which promises its applications at RT or even higher temperature in ultraviolet optoelectronic devices. However, its applications depend on high-quality n-type and p-type ZnO. Although high-quality n-type ZnO can be obtained easily, p-type ZnO is difficult to be produced. Recent years, many researches have been conducted to explore the dopability of p-type ZnO in both theoretical and experimental studies, which give many valuable results and simultaneously bring up many questions. In this dissertation, we investigate properties of native defects and acceptor-H complexes, as well as the conductivity of p-type ZnO. This dissertation consists of five chapters.In the first chapter, we review the researches on native defects and H as well as p-type doping in ZnO.In the second chapter, we introduce density functional theory which is widely used in theoretical studies, and then describe CI-NEB method which is usually used to compute the economical diffusion paths and the related energy barrier for the diffusion of an atom in bulk material.In the third chapter, we present our results on electronic structures of native defects and group-I impurities in ZnO, and optical and vibrational properties of some important native defects. By using B3LYP calculations, the energy levels arising from native defects of ZnO are accurately determined, from which we conclude that vacancies (V_P and V_Zn) and octahedral interstitials (O_i,o and Zn_i,o) may contribute to green photoluminescence (PL) observed in experimental investigations, which implies that it is improper to attribute the observed green PL to a single type of native defects as done in previous reports. Furthermore, we find O_i,o and Zn_i,o may contribute to yellow and red PL. For the group-I impurities, we find that the PL in Lior Na-doped ZnO may derive from defect complexes; while for Ag-doped ZnO, there are many defect energy levels induced by the doped Ag atom, most of which may contribute to the PL at 3.17 eV observed in experiments. We studied optical and vibrational properties of some important native defects (V_O,H_O and V_Zn,and find some typical optical absorption peaks and vibrational modes, which can be used to identify these defects. In the fourth chapter, we report the properties of N_O-H,Li_Zn-H and Cu_Zn-H complexes in ZnO. We firstly calculated the formation and dissociation of these complexes in ZnO. By computing the minimum energy paths of H diffusion, we predicted the energy barrier of the diffusions. We find that, these complexes can form below 200 K and be stable under RT. However, the dissociations of these complexes require activation energies of 1.25-1.48, 1.10-1.35 and 1.42-1.63 eV,respectively, which correspond to activation temperatures of 480-570, 420-520 and 540-620 K. Moreover, we investigated the electronic structures, defect energy levels, vibrational and optical properties of these complexes, which give further understandings of these defects in ZnO.In the fifth chapter, we investigate the instability and low efficiency of the conductivity in p-type ZnO. From analysis of possible defects in N-doped ZnO, we propose a new microscopic model to interpret the mechanism of the conductivity conversion induced by Blue/UV photo irradiation in N-doped ZnO. We find that, H_O (H is captured by V_O) plays an important role in this phenomenon. Accordingly, we predict that, if the concentration of V_O can be reduced so that the concentration of H_O will be sufficiently low, the stability of the conductivity of N-doped ZnO will beenhanced. On the other hand, we analyze some phenomena observed in N-doped ZnO from experimental studies, and speculate that the shallow acceptor levels are derived from incomplete ionization of the acceptors (N_O) in this material. From analysis of electronic structures and distributions of spin densities in N-doped ZnO, we find that the delocalized wavefunctions of defect states are the origin of incomplete ionization. Furthermore, we find that in P- and As-doped ZnO, the shallow acceptor levels are derived from incomplete ionization too. In addition, based on the proposed concept of incomplete ionization, we predicted that in Ag-doped ZnO, the acceptor levels are also shallow, being within 0.1-0.2 eV.