Positron beam study of low-temperature-grown GaAs with aluminum delta layers

We report on the use of a slow-positron beam to examine aluminium delta lay ers in GaAs grown at low substrate temperature (250 degrees C) by molecular beam epitaxy (LTMBE). It is now established that LTMBE-GaAs contains an ex cess of arsenic which causes an increase in the lattice parameter. After an nealing, this arsenic is redistributed and forms precipitates resulting in the relaxation of the lattice. Previous positron beam studies have shown th at the as-grown material has a large concentration of gallium vacancies, an d after annealing the S-parameter increases above the as-grown value indica ting that vacancy clusters have formed. A region depleted of arsenic precip itates has been shown to form near to aluminium delta layers, and this work is the first to study the vacancy distribution associated with this deplet ion region. We observe that the as-grown material has a peak value of the n ormalized S-parameter that is similar to 3.5% higher than the substrate, wh ich is much larger than that for a single layer LT-GaAs structure (0.8-1.5% ). After annealing in the range 600-800 degrees C the S-parameter collapses down to the substrate, value, which again is opposite to the LT-GaAs case and indicates that few precipitates have formed. We correlate these finding s with SIMS and TEM data and propose a mechanism involving compositional di sordering due to the aluminium layers. (C) 1999 Elsevier Science B.V. All r ights reserved.