OPTICAL STUDIES OF SHORT-PERIOD SI/GE SUPERLATTICES BY PHOTOCAPACITANCE

Photocapacitance measurements performed on short-period silicon-germanium superlattices are discussed in terms of Wannier-Stark localization previously only observed in III-V superlattices. The superlattices have been grown by molecular-beam epitaxy at a process temperature of T(g) = 450-degrees-C on a <100>-oriented silicon substrate on top of which a thin intermediate Si0.4Ge0.6 buffer layer has been grown for strain relief. It is believed that the spectrum of the photoionization cross section with a threshold energy of about 0.35 eV is defect related. The energy position of the defect is estimated to be 0.2 eV below the bottom of the conduction-band well. It is shown that the peaks observed in the spectral distribution shift in energy when the reverse bias V is changed following the relation E(eV) = 0.49 + n x 0.15V1/2, where n is an integer.