Short-period Si/Si1-xGex multiple quantum wells: real-time spectro-ellipsometric characterization during growth by synchrotron-radiation-excited chemical-beam epitaxy

A short-period Si/Si0.53Ge0.47 multiple quantum wells (MQW) was grown at 250 deg. C by synchrotron-radiation-excited chemical-beam epitaxy. Two-dimensional strained-layers with atomically abrupt interfaces were obtained. Atomic-scale roughness at the interfaces between the Si and Si0.53Ge0.47 layers is the main relaxation channel for misfit strain. The ellipsometric angles ψ and Δ exhibited short-period oscillations over time, which are the result of alternation of the top layer material. At transparent energy levels, long-period modulation caused by optical interference is superimposed on the shorter oscillations. Two different procedures for the real-time characterization of the MQW structure by fitting of simulation to the experimental data were investigated; one provided a wide-range fit to the long-period modulation where the whole MQW is approximated as a pseudo-dielectric film and the other was a local-fit, under virtual-interface approximation, to the short-period oscillation for the absorbent range of photon energy