Photocurrent spectroscopy measurements of SiGe alloys and superlattices

We used photocurrent spectroscopy at 300 K, 77 K and 4.2 K to investigate the optical absorption processes of (001) SiGe strained layer superlattices and alloys with an average composition of 50%Si-50%Ge. We observed an evolution toward higher energies of the threshold in the photocurrent spectra as the period of the superlattices decreases, with the spectrum of the shortest period superlattices (2:2) approaching that of the alloy. The energy dependence of absorption in these SiGe heterostructures is quite distinct from that measured in elemental silicon.