Substrate Rotation Induced Composition Modulation In Epitaxial AlGaAs Grown By Molecular Beam Epitaxy

We observe a periodic variation of AlAs mole fraction along the growth direction in AlGaAs grown by molecular beam epitaxy (MBE) 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 semi-empirically to be abut 6% in Al sub (0.3) Ga sup (0.7) As. Composition modulation was not observed Al sub (0.7) Ga sub (0.3) As, indicating an improvement in the uniformity of the Al beam flux profile across the wafer at a high source temperature.