Comparative study of adsorption characteristics of Cs on the GaN(0001) and GaN(000) surfaces

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.

First principle study of the influence of vacancy defects on optical properties of GaN

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.

Transmission-mode GaN photocathode based on graded Al_xGa_(1-x)N buffer layer

We create a GaN photocathode based on graded AlxGa1-xN buffer layers to overcome the influence of buffer-emission layer interface on the photoemission of transmission-mode GaN photocathodes. A gate- shaped spectral response with a 260-nm starting wavelength and a 375-nm cut-off wavelength is obtained. Average quantum efficiency is 15% and short wavelength responses are almost equivalent to long wavelength ones. The fitted interface recombination velocity is 5×10^4 cm/s, with negligible magnitude, proving that the design of the graded buffer layers is efficient in obtaining good interface quality between the buffer and the emission laver.

Photoemission properties of GaAs （100） β2（2×4） and GaAs （100） （4×2） reconstruction phases

Using the first-principles plane-wave pseudopotential method, based on the density function theory, the electron structure and optical properties of GaAs （100） β2（2×4） and GaAs （100） （4×2） reconstructions are calculated. The formation energy of As-richβ2 （2 ×4） reconstruction is minus and the formation energy of Ga-rich （4×2） reconstruction is positive; As-rich β2（2×4） reconstruction is stable and Ga-rich （4×2） reconstruction is unstable. Ga-rich （4×2） reconstruction owns lower work function. The electrons at two reconstructions both move into the bulk and form a band-binding region. Both the absorption and the refleetivity of As-rich β2（2×4） reconstruction are smaller than the Ga-rich （4× 2） reconstruction. As- rich β2（2×4） reconstruction is more benefit for the movement of photos through the surface to emit photoelectrons.