| In the past decades, with the rapid developments of semiconductor technology, the so called third generation semiconductor materials, such as GaN, SiC, and ZnO, show significant application values. Especially, the GaN-based materials have the advantages of wide band-gap, fast electron drift velocity, high temperature resistance, high pressure tolerance and anti-radiation; have attracted much attention in the preparation of short wavelength luminescence device and high power microwave device. In fact, the as-grown GaN is an n-type semiconductor with many intrinsic defects such as N vacancy, so it is very difficult to dope GaN to a p-type semiconductor because of the compensation by the donor defects. Thus, the development and application of GaN-based optoelectronic devices are greatly limited due to the lack of GaN p-n junctions. So how to obtain p-type GaN is a focus task. Furthermore, GaN-based ultraviolet detector generally work in the solar-blind region, the working wavelength range is from 220 to 280nm. However, the intrinsic band-gap of GaN is 3.3eV (the corresponding wavelength is 376nm), so GaN films with wider band-gap are desired. It is shown that the band-gap of the GaN system can be enhanced by doping Al, and the wavelength range of the emission spectra covers the whole visible region and part of the ultraviolet region.Materials computation and materials design based on the advanced computer techniques play an important role in modern materials science. In this thesis, the doping problems of wurtzite GaN and blende GaN have been studied by the first-principles approach based on the density functional theory (DFT).The main contents of the thesis are as follows.(1) The structure, electricity properties, optical properties, the application of GaN-based materials, and the related doping researches are given. The calculation tool—ABINIT package and its theoretical foundation—DFT are briefly discussed.(2) The electron structure and optical properties of the pure blende GaN and Al doped GaN systems are calculated. The lattice constants, the energy band structures, the electronic density of states, the charge density difference, and the absorption spectra of Ga1-xAlxN systems are studied. The obtained results indicate that pure GaN is a direct band-gap semiconductor, the top of the valence band is mainly contributed by N2p electrons, the bottom of the conduction band is mainly contributed by Ga4s electrons, and the band gap is mainly determined by N2p electrons and Ga4s electrons. With Al doping concentration x increasing, the lattice constants change according to Vegard's law, the band gaps broaden, and the absorption spectra shift to the blue region. When x=0.5, the edge of the optical absorption is at about 270 nm. Thus, the solar-blind region ultraviolet detector's requirements can be met. Our results are in good agreement with experiments.(3) The electron structure of pure wurtzite GaN, Mg doped, and Mg-O co-doped GaN systems are calculated. The energy band structures, the electronic density of states, and the charge density difference are studied. The results show that the acceptor level is formed in the band-gap for the Mg doped GaN system, but the acceptor level is deep, and the carrier concentration is low. For the Mg-O co-doped GaN system, the acceptor level becomes shallow, the carrier concentration increases, and the system becomes more stable compared with the Mg doped case. Thus, the better p-type GaN-based material can be obtained. |
PDF Full Text Request