Abstract :
We report on the use of a slow-positron beam to examine aluminium delta layers in GaAs grown at low substrate
temperature 2508C.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 ;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 600–8008C 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. q1999 Elsevier Science B.V. All rights reserved
