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 ${\mathit{T}}_{\mathit{g}}$=450 \ifmmode^\circ\else\textdegree\fi{}C on a 〈100〉-oriented silicon substrate on top of which a thin intermediate ${\mathrm{Si}}_{0.4}$${\mathrm{Ge}}_{0.6}$ 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\ifmmode\times\else\texttimes\fi{}0.15${\mathit{V}}^{1/2}$, where n is an integer.