Dependence of the degree of selectivity on the Al content during the selective area growth of AlGaAs on GaAs(001) by PSE/MBE

Selective area growth (SAG) of AlGaAs and, for the first time, of AlAs on GaAs(0 0 1) substrates was studied by applying the periodic supply epitaxy (PSE) technique to molecular beam epitaxy (MBE). Under growth conditions and PSE parameters suitable to obtain a smooth epitaxial layer, the degree of selectivity was investigated for AlGaAs at different Al/Ga flux ratios and for AlAs at different growth rates. Zones free of AlGaAs or AlAs polycrystalline islands ("depleted zones") have been observed on the mask, after the growth under selective conditions, next to each GaAs open window. These depleted zones were formed due to the migration, that is enhanced by the PSE technique, of Al and Ga adatoms from the mask toward the window region. The depleted zone width next to a large GaAs open window was found to be a good measure of the degree of selectivity. During SAG of both AlGaAs and AlAs, we observed that when Al flux is increased, the degree of selectivity is quickly reduced. This reduction in the selectivity can be explained in terms of the higher amount of thermal energy required to break Al-As bonds and of the shorter migration length of Al adatoms. Despite the difficulties, it is shown that AlGaAs can be grown under selective conditions at any Al content by PSE/MBE. (C) 1998 Elsevier Science B.V. All rights reserved.