Positron studies of arsenic precipitation in low-temperature GaAs grown by molecular beam epitaxy

Gallium arsenide grown at low substrate temperature by molecular beam epitaxy has been studied using a variable energy slow positron beam. As-grown LT-GaAs was found to have a higher concentration of vacancy-related defects (~10¹⁷ cm^-3) than the semi-insulating substrate. After annealing at 600°C, the positron S parameter results suggest the formation of clusters which we associate with arsenic precipitation. The lowering of the S parameter at the surface was thought to be due to oxygen and this was confirmed by XPS measurements. For the first time, we have examined aluminium delta-layers using a positron beam and found that the Al-layers can be resolved to depths of at least 1700 Å by this method. The lowering of the S parameter after annealing would suggest that the Al forms Al _xGa_1-xAs, and that the presence of the Al layers may inhibit the diffusion of arsenic, thereby reducing the formation of vacancy-defects