We employ plane-wave with ultrasoft pseudopotential method to calculate and compare the total density of states and partial density of states of bulk-phase GaN, Gao.9375N, and GaNo.9375 systems based on the first-principle density-functional theory （DFT）. For Ga and N vacancies, the electronic structures of their neighbor and next-neighbor atoms change partially. The Gao.9375N system has n-type semiconductor conductive properties, whereas the GaNo.9375 system has p-type semiconductor conductive properties. By studying the optical properties, the influence of Ga and N vacancy defects on the optical properties of GaN has been shown as mainly in the low-energy area and very weak in high-energy area. The dielectric peak influenced by vacancy defects expands to the visible light area, which greatly increases the electronic transition in visible light area.
The electronic structure and optical properties of A1 and Mg co-doped GaN are calculated from first principles using density function theory with the plane-wave ultrasoft pseudopotentiai method. The results show that the optimal form of p-type GaN is obtained with an appropriate AI：Mg co-doping ratio rather than with only Mg doping. A1 doping weakens the interaction between Ga and N, resulting in the Ga 4s states moving to a high energy region and the system band gap widening. The optical properties of the co-doped system are calculated and compared with those of undoped GaN. The dielectric function of the co-doped system is anisotropic in the low energy region. The static refractive index and reflectivity increase, and absorption coefficient decreases. This provides the theoretical foundation for the design and application of A1-Mg co-doped GaN photoelectric materials.
The adsorption characteristics of Cs on GaN （0001） and GaN （0001） surfaces with a coverage from 1/4 to 1 monolayer have been investigated using the density functional theory with a plane-wave uttrasoft pseudopotential method based on first-principles calculations. The results show that the most stable position of the Cs adatom on the GaN （0001） surface is at the N-bridge site for 1/4 monolayer coverage. As the coverage of Cs atoms at the N-bridge site is increased, the adsorption energy reduces. As the Cs atoms achieve saturation, the adsorption is no longer stable when the coverage is 3/4 monolayer. The work function achieves its minimum value when the Cs adatom coverage is 2/4 monolayer, and then rises with Cs atomic coverage. The most stable position of Cs adatoms on the GaN （000i） surface is at H3 site for 1/4 monolayer coverage. As the Cs atomic coverage at H3 site is increased, the adsorption energy reduces, and the adsorption is still stable when the Cs adatom coverage is 1 monolayer. The work function reduces persistently, and does not rise with the increase of Cs coverage.
