Silicon/silicon-germanium multiple quantum wells grown by gas-source molecular beam epitaxy: Hydrogen coverage and interfacial abruptness

We have grown silicon-germanium/silicon (Si1-xGex/Si, x < 0.30) multiple quantum wells (MQWs) by gas-source molecular beam epitaxy (GSMBE) using disilane (Si2H6) and germane (GeH4) as source gases, and have characterized their structural properties by secondary ion mass spectroscopy (SIMS), X-ray diffraction (XRD) rocking curve and transmission electron microscopy (TEM) techniques. A substrate temperature of 520 degrees C was maintained during growth resulting in a Si and SiGe growth rate-limited primarily by hydrogen desorption kinetics. Under these conditions, surface hydrogen is expected to function as a surfactant thereby enhancing interfacial abruptness at the Si/SiGe interface through suppression of Ge surface segregation. Independent of Ge composition in the Si1-xGex wells, we find abrupt interfaces, as determined from XRD measurements, and sharp SIMS decay lengths of the order of 2.5 nm/decade. For nominally identical Si barriers in all samples examined, we find thicker barriers for the structures with higher Ge content in the well. For the specimens with x = 0.30 in the wells, we find a noticeable well plus barrier period variation of approximately 5%-10% as determined from XRD rocking curves, as well as TEM evidence for the onset of strain relaxation via interface undulation formation in the first quantum well of the structure. A discussion of these results in terms of hydrogen desorption kinetics is presented.