OPTICAL STUDY OF DIFFUSION LIMITATION IN MBE GROWTH OF SIGE QUANTUM-WELLS

We report on detailed studies of the bandgap of Si/SixGe1-x quantum we ll structures grown on [001] Si by molecular beam epitaxy. Photocurren t 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 h igh growth temperature (720 degrees C). At a lower growth temperature (500 degrees C) diffusion is negligible. This is verified by p-i-n str uctures and p-type modulation-doped quantum wells. In the modulation-d oped samples the bandgap could be reduced to 1.5 mu m while still show ing intense bandgap related photoluminescence. As well as an alloy-rel ated onset the p-i-n diodes reveal a low-energy threshold, which is de fect related. Low growth temperatures lead to defects located in the S iGe layers. Raising the number of quantum wells and Ge content up to a lmost critical thickness we found a maximum external responsivity of 4 x 10(-4) A W-1 in normal incidence for mesa-type pin photodiodes.