Recombination processes in SiGe/Si quantum wells measured by photoinduced absorption spectroscopy

We used photoinduced absorption spectroscopy of optical transitions between valence subbands of nominally undoped SiGe/Si quantum wells to study the recombination processes of photogenerated carriers in these heterostructures. We measured the photoinduced absorption as a function of the ambient temperature, photoexcitation density, and modulation frequency. The energy band structure of the SiGe/Si quantum well and the optical transitions between its valence subbands are calculated using an eight-band $\mathbf{k}\mathbf{\cdot }\mathbf{p}$ model. The model, which includes sets of bulk Si and Ge parameters, agrees very well with the observed intersubband transition and its magnitude. Thus, the photoinduced absorption intensity is a direct measure of the photogenerated excess carrier density, for which we measured a characteristic time scale for a decay of $\simeq$2.5 μsec at 80 K, and longer times at lower temperatures. Our measurements show that the recombination kinetics is governed by both an extrinsic monomolecular and intrinsic bimolecular terms. We incorporate this density dependence into a simple rate model that takes into account thermal activation of the photogenerated holes out of the SiGe quantum wells. The model describes very well the measured temperature dependence of the photoinduced absorption and it provides quite an accurate determination of the intrinsic recombination rate of electrons and holes within the well regions in these heterostructures. The rate that we obtain is faster than the measured recombination rates in bulk Si and Ge. We believe that this recombination rate enhancement is due to carriers confinement within the SiGe quantum-well regions.