A study of the vacancy-defect distribution in a GaAs/Al_xGa_1-xAs multi-layer structure grown at low temperature

We have grown a multilayer structure of GaAs and Al_xGa_1-xAs (x = 0.25) by molecular beam epitaxy at low substrate temperature (250°C) in order to investigate the effects of annealing on arsenic precipitation and vacancy-defect formation. The as-grown heterostructure contained ∼5 × 10¹⁷ cm^-3 gallium vacancies, but information about the individual layers was lost because the layer width ( ∼ 45 nm) was smaller than the average positron diffusion length ( ∼ 70 nm). Annealing at 500 and 600°C showed increases in the S-parameter (above the bulk value) of ∼2.5% and ∼1.5%, respectively, smaller than for a single LT-GaAs layer (∼3-4%). We explain this phenomenon by suggesting that the excess arsenic which migrates from the material with the higher precipitate/matrix energy (LT-Al_xGa_1-xAs) reduces the gallium vacancy concentration in the LT-GaAs and hence the S-parameter. This hypothesis is supported by SIMS data which shows a build-up of As in the LT-GaAs layers, and TEM images which indicate that arsenic precipitation occurs to a greater extent in the LT-GaAs than in the LT-Al_xGa_1-xAs.