Investigations were made of the structural and defect characteristics of samples which had been grown at 250C by means of molecular beam epitaxy. The use of X-ray diffraction techniques revealed an increase in the lattice parameter for all of the as-grown layers; with GaAlAs layers exhibiting a smaller expansion than that of GaAs layers. Infra-red absorption measurements revealed that the concentration of neutral As antisite defects, [AsGa]0, was not significantly affected by the Al content; with only a small reduction in the case of Ga0.64Al0.36As. Positron beam studies showed that the low-temperature layers contained a higher concentration (about 1017cm3) of vacancy-related defects than did the semi-insulating substrate. After annealing (600C, 0.25h), the lattice constants relaxed so as to equal those of conventionally grown material, and [AsGa]0 decreased in all cases; with the smallest reduction occurring in Ga0.64Al0.36As layers. This indicated that the Al atoms strengthened the lattice against excess As incorporation, and held the As antisite atoms more strongly in position. The X-ray photo-electron spectra showed that As diffused out of the surface region and was replaced by O; due possibly to an insufficient over-pressure of forming gas during annealing. This O penetration was greater for the GaAs layers than for the GaAlAs layers. The presence of extra Raman peaks at 200 and 257/cm confirmed that the surface was very disordered. However, there was a large (4%) increase in the positron S-parameter in the bulk of the annealed layers. This suggested the formation of vacancy clusters whereas, in the surface region, there were signs that AsGa diffusion occurred at a faster rate in Ga0.64Al0.36As than in Ga0.80Al0.20As. This was consistent with vacancy-enhanced AsGa diffusion.

S.Fleischer, C.D.Beling, S.Fung, W.R.Nieveen, J.E.Squire, J.Q.Zheng, M.Missous: Journal of Applied Physics, 1997, 81[7], 190-8