We have investigated the structural and defect characteristics of GaAs and AlxGa1−xAs grown at low substrate temperature (250 °C) by molecular beam epitaxy. Using x-ray diffraction we have observed an increase in lattice parameter for all as-grown layers, with the AlxGa1−xAs layers showing a smaller expansion than the GaAs layer. However, infrared absorbtion measurements revealed that the concentration of neutral arsenic antisite defect, [AsGa]0, was not significantly affected by aluminum content(x), with only a small reduction forx=0.36. Positron beam studies showed that the low temperature layers had a higher concentration of vacancy-related defects (∼1017cm−3) than the semi-insulating substrate, with the AlxGa1−xAs layers having the highest values. After annealing (600 °C, 15 min) the lattice constants relaxed to those of conventionally grown material and [AsGa]0was reduced in all cases, with the smallest reduction occurring for thex=0.36 layer, indicating that the Al atoms strengthen the lattice against excess arsenic incorporation and hold the arsenic antisite atoms more strongly in position. X-ray photoelectron spectroscopy showed that arsenic diffused out of the surface region and was replaced by oxygen, possibly due to an insufficient overpressure of forming gas during the anneal. This oxygen penetration was greater for the GaAs layer than for the AlxGa1−xAs layers. Extra Raman peaks at 200 and 257 cm−1confirmed that the surface was very disordered. There was, nevertheless, a large increase (4&percent;) in the positronSparameter in the bulk of the annealed layers, suggesting the formation of vacancy clusters, whereas in the surface region we find evidence that AsGadiffusion proceeded at a faster rate in thex=0.36 than thex=0.2, in agreement with the vacancy-enhanced AsGadiffusion model. ©1997 American Institute of Physics.