A multi-layer structure of GaAs and Ga0.75Al0.25As was grown by means of molecular beam epitaxy at 250C. The as-grown heterostructure contained about 5 x 1017/cm3 of Ga vacancies, but information on individual layers was unavailable because the layer width (about 45nm) was smaller than the average positron diffusion length (about 70nm). Annealing at 500 and 600C led to increases in the S-parameter (above the bulk value) of about 2.5% and 1.5%, respectively. These values were smaller than those (3 to 4%) for a single low-temperature grown GaAs layer. This was explained by suggesting that excess As, which migrated from the material with the higher precipitate/matrix energy, reduced the Ga vacancy concentration and hence the S-parameter. This suggestion was supported by secondary ion mass spectrometry data, which revealed a build-up of As in the GaAs layers, and transmission electron microscopic images, which indicated that As precipitation occurred to a greater extent in GaAs than in GaAlAs.

A study of the vacancy-defect distribution in a GaAs/AlxGa1-xAs multi-layer structure grown at low temperatures S.Fleischer, C.Surya, Y.F.Hu, C.D.Beling, S.Fung, T.L.Smith, K.M.Moulding, M.Missous: Journal of Crystal Growth, 1999, 196[1], 53-61