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 layers in GaAs grown at low substrate temperature (250 degrees C) by molecular beam epitaxy (LTMBE). It is now established that LTMBE-GaAs contains an excess of arsenic which causes an increase in the lattice parameter. After annealing, this arsenic is redistributed and forms precipitates resulting in the relaxation of the lattice. Previous positron beam studies have shown that the as-grown material has a large concentration of gallium vacancies, and after annealing the S-parameter increases above the as-grown value indicating that vacancy clusters have formed. A region depleted of arsenic precipitates has been shown to form near to aluminium delta layers, and this work is the first to study the vacancy distribution associated with this depletion region. We observe that the as-grown material has a peak value of the normalized S-parameter that is similar to 3.5\% higher than the substrate, which 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 findings with SIMS and TEM data and propose a mechanism involving compositional disordering due to the aluminium layers. (C) 1999 Elsevier Science B.V. All rights reserved