It is very important to understand the electronic structure of GaAs and GaSb for fabrication of electronic and optoelectronic devices. Recently, tenary compounds, GaAs1-xSbx began to attract much attention from researchers because they were synthesized and expected to be new materials for optoeletronic devices. For x = 1 and x = 0, there have been a lot of computational studies to investigate their structural, electronic and optical properties, but few studies for general cases. Moreover, it has been known that “regular” density functional calculations does not describe the correct electronic structures including underestimated band gap and incorrect band splitting. To resolve such issues, we consider spin-orbit coupling effects and GW corrections on GaAs1-xSbx with the composition of x = 0, 0.25, 0.5, 0.75, and 1. To calculate their band structures, we make use of maximally localized Wannier functions to interpolate GW quasiparticle band structures. Based on our GW calculations, we discuss the trend of band gaps at various high symmetric points in the Brillouin zone with the composition x.
We report the existence of latent order during core relaxation in the high-angle grain boundaries (GBs) of GaN films using atomic-resolution scanning transmission electron microscopy and ab initio density functional theory calculations. Core structures in the high-angle GBs are characterized by two pairs of Ga-N bonds located next to each other. The core type correlates strongly with the bond angle differences. We identify an order of core relaxation hidden in the high-angle GBs by further classifying the 5/7 atom cores into a stable 5/7 core (5/7(S)) and a metastable 5/7 core (5/7(M)). This core-type classification indicates that metastable cores can exist at real high-angle GBs under certain circumstances. Interestingly, 5/7(M) exhibits distinct defect states compared to 5/7(S), despite their similar atomic configurations. We investigate the reconstruction of defect states observed in 5/7(M) by analyzing the real-space wave functions. An inversion occurred between two localized states during the transition from 5/7(S) to 5/7(M). We suggest an inversion mechanism to explain the formation of new defect states in 5/7(M).
We have studied the growth and characterization of patterned and uniformly distributed GaN microcrystals with well-defined facets and epitaxy. The microcrystals were grown on a mask pattertned by lithography, and formed by selective-area epitaxy using metal-organic chemical vapour deposition. The GaN microcrystals have similar sizes and shapes. Each microcrystal consists of an upper and lower part, which are rotated by 30 deg. Transmission electron microscopy shows that there is a rather clear interface between the two parts of the crystal, suggesting a sudden change in the growth direction. We performed ab initio calculations for the surface energies of hexagonal GaN, and the growth morphology is explained based on surface energy consideration.