MICROSTRUCTURE OF ANNEALED LOW-TEMPERATURE-GROWN GAAS-LAYERS

The crystal structure and orientation of As precipitates in annealed low-temperature GaAs (LT-GaAs) layers have been investigated by transmission electron microscopy. Three types of As precipitates were identified in layers grown by molecular beam epitaxy at substrate temperatures from 180-degrees to 210-degrees-C. In the monocrystalline LT-GaAs layers small "pseudocubic" As precipitates (2-3 nm diameter) coherent with the GaAs lattice were observed. These precipitates lose their coherency when a certain critical size is exceeded. Precipitates of similar sizes are occasionally found for which a TEM lattice image cannot be obtained. These precipitates are believed to be amorphous. Larger As precipitates with a hexagonal structure (> 4 nm diameter) were also found in the layers. These hexagonal As precipitates were observed to be largest near structural defects. The effect of these precipitates on the structure and on the electronic properties of the host GaAs is discussed.