Epitaxial growth mode and silicon/silicon-germanium heterointerfaces

Abstract

Silicon-germanium/silicon (Si1 − xGe x /Si, x < 0.50) multiple quantum wells (MQWs) have been grown on (001) Si substrates by gas source molecular beam epitaxy (GSMBE) using disilane (Si2H6) and germane (GeH4) as source gases. Their structural properties have been evaluated by X-ray diffraction (XRD), rocking curve techniques and transmission electron microscopy (TEM). For the substrate temperatures used in this work (450 ‡C to 520 ‡C) the Si growth rate is limited by hydrogen desorption kinetics, whereas the growth of SiGe is limited primarily by the arrival rate of the source gases onto the Si substrates. XRD analysis of the structures indicates a significant well plus barrier period variation of approximately 5–10%, attributed to fluctuations in the substrate temperature during growth, since these cause significant variations in the growth rate of the Si barriers. For x < 0.30 we find nearly ideal Si/SiGe interfaces as determined from a comparison of the XRD data with dynamical simulations of the 004 X-ray reflectivity, although TEM micrographs indicate that the x = 0.30 samples exhibit undulations in the first SiGe/Si interface of the structures. For x = 0.50 such undulations occur throughout the MQW structure; the undulation amplitude decreases with decreasing growth temperature but the period remains unchanged. The observed improvement in the SiGe/Si interface planarity at lower growth temperatures is attributed to a reduction in the surface diffusion of Si and Ge with decreasing growth temperature.