A Simple Method for Elimination of Gallium-Source Related Oval Defects in Molecular-Beam Epitaxy of GaAs.

By evaporating gallium from an aluminum coated pBN crucible in a molecular beam epitaxy (MBE) system, `Ga-spitting' and formation of Ga-cell related oval defects have been eliminated. MBE-GaAs layers as thick as 20microns were totally free from the Ga-cell related oval defects. Remaining oval defects were those related to particulates and substrate contamination and their densities were 100 and 500 cm sup (-2) for 5 and 20microns thick layers of GaAs. These defects also were hardly seen when GaAs was grown on Si substrates. Aluminum wets and reacts with the pBN crucible. Condensed gallium near the orifice wets the aluminum treated surface. As a result, Ga droplets and related oval defects do not form. The method also avoids any gallium oxide in the Ga-melt and provides a better outgassed pBN crucible. Contamination of the initial GaAs layers with Al was negligibly small with Al content x = 0.0016 which reduced in subsequent layers. The results are presented for GaAs layers but the method should be equally applicable for evaporation of indium to prevent In-related oval defects in In-based compounds.