Optical study of diffusion limitation in MBE growth of SiGe quantum wells

We report on detailed studies of the bandgap of Si/Si_xGe _1-x quantum well structures grown on (001) Si by molecular beam epitaxy. Photocurrent and photoluminescence spectroscopy are used to determine the bandgap of the SiGe alloy up to x=0.67. We found that interdiffusion of the SiGe layers limited the maximum Ge content in the alloy layers at a high growth temperature (720 degrees C). At a lower growth temperature (500 degrees C) diffusion is negligible. This is verified by p-i-n structures and p-type modulation-doped quantum wells. In the modulation-doped samples the bandgap could be reduced to 1.5 mu m while still showing intense bandgap related photoluminescence. As well as an alloy-related onset the p-i-n diodes reveal a low-energy threshold, which is defect related. Low growth temperatures lead to defects located in the SiGe layers. Raising the number of quantum wells and Ge content up to almost critical thickness we found a maximum external responsivity of 4*10^-4 A W^-1 in normal incidence for mesa-type pin photodiodes.