CHEMICAL-STRUCTURE OF AS-STABILIZED SURFACE DURING GAAS METALORGANIC VAPOR-PHASE EPITAXY STUDIED BY SURFACE PHOTOABSORPTION

As-stabilized surface structures during GaAs metalorganic vapor phase epitaxy (MOVPE) are investigated by taking a difference in surface photo-absorption spectrum between two principal axes, which produces the surface anisotropic spectra mainly due to As dimers on the surface. We established the surface phase diagram as a function of substrate temperature and AsH3 partial pressure. Below 630 degrees C, independently of AsH3 partial pressure, the surface is c(4 x 4)-like whose As dimers have a bond axis parallel to [110]. As the substrate temperature increases and the AsH3 partial pressure decreases, the surface evolves to (2 x 4)gamma-like which contains As dimers having a bond axis parallel to [$($) over bar$$ 110]. At a substrate temperature of around 600 degrees C, a phase diagram comparison between MOVPE and molecular beam epitaxy (MBE) shows that the c(4 x 4)-like surface is stable in MOVPE, while the (2 x 4) surface is stable in MBE. The characterization of step structure by atomic force microscopy shows that the difference of As structures has a significant effect on the step straightening mechanism.