Improvement in the crystalline quality of heteroepitaxial GaAs on Si films grown by modulated molecular beam epitaxy

GaAs films were grown on Si substrates by a two-step method in which a thin buffer layer was first deposited using a modulated molecular beam epitaxy (MBE) technique followed by a thick layer grown by conventional molecular beam epitaxy. The film quality was evaluated using 77 K photoluminescence (PL) and double-crystal x-ray rocking curves. It was found that GaAs films grown in this way have a superior crystalline quality compared to the films prepared by normal two-step MBE. To investigate the nucleation of this buffer layer, thin GaAs layers (150 Å) were grown on Si substrates and examined by plan-view and cross-sectional transmission electron microscope. A thin, two-dimensional nucleation pattern was found in this sample, in clear contrast to the three-dimensional nucleation islands found in samples grown by conventional MBE. This showed conclusively that modulated beam molecular beam epitaxy resulted in a more uniform and two-dimensional nucleation during the initial stage of the growth as speculated earlier and is believed to be the reason for the improvement of the crystalline quality of the overgrown layer.