Substrate rotation-induced composition modulation in epitaxial AlGaAs grown by molecular-beam epitaxy

Abstract

We observe a periodic variation of Al mole fraction along the growth direction in AlGaAs grown by molecular-beam epitaxy on GaAs substrate by cross-sectional transmission electron microscopy (TEM). The wavelength of such composition modulation in AlGaAs is inversely proportional to the substrate rotation speed during growth and is independent of the growth temperature used. Uniform composition along the growth direction is achieved by increasing the rotation speed such that the modulation period merges into a continuum composition. The minimum change in the Al mole fraction required to obtain a visible periodic contrast in the [200] dark-field TEM images is estimated semiempirically to be about 6% in Al0.3Ga0.7As. Composition modulation was not observed in Al0.7Ga0.3As, indicating an improvement in the uniformity of the Al-beam flux profile across the wafer at a high source temperature. Although Hall measurements and high-resolution x-ray diffractometry did not show any difference in layers grown with and without composition modulation, both the photoluminescence intensity and its uniformity across the wafer improved considerably in Al0.3Ga0.7As with increasing rotation speed from 5 to 16 rpm. A deep donor-acceptor recombination with an activation energy around 157 meV was observed in Si-doped Al0.36Ga0.64As.