Low-temperature GaAs (LT-GaAs) of a high resistivity and a short carrier lifetime (0.2-20 ps) is promising for device applications. High temperature annealing of LT-GaAs often leads to formation of twinned As precipitates in the GaAs matrix. Here high resolution electron microscopy has been applied to study the structure of As precipitates formed in MBE grown LT-GaAs layers during rapid thermal annealing at 850 and 950 °C. Twinning in the rhombohedral As was a special focus of this study.
Fig. 1 shows a HREM image of a typical As precipitate formed in a LT-GaAs layer after annealing at 850 °C. The precipitate has a rhombohedral structure with a=0.376 nm and c= 1.055 nm and a polyhedral shape with distinct facets. The long facets bounded by the {111}A planes of the GaAs are planar while the others have a high density of steps.