Photoluminescence and intersubband absorption spectroscopy of interdiffused Si/SiGe quantum wells

The interdiffusion and thermal stability of narrow Si/SiGe multi-quantum wells is investigated by photoluminescence and intersubband spectroscopy. The photoluminescence exhibits a blueshift as a function of the temperature of annealing. The activation energy of the intermixing process and the interdiffusion coefficient are deduced from the photoluminescence shift versus temperature of the anneal. The intersubband absorption is measured by photoinduced infrared spectroscopy on the interdiffused samples for light polarized perpendicular (z polarization) or parallel (x polarization) to the layer plane. In z polarization, the absorption of annealed samples exhibits a redshift respective to the as-grown sample which is enhanced as more levels are confined in the well. The magnitude of this shift is in good agreement with simulations based on the data obtained by photoluminescence experiments. The redshift of the intersubband absorption in x polarization is lower than in z polarization due to the lower dependence of the spin-orbit level on the quantum well profile. Bound-to-continuum absorption is broadened in interdiffused quantum wells. This enhancement is explained by the local variation of the effective mass along the growth axis of the interdiffused well. At high temperature of annealing and beyond the onset of relaxation, we still observe intersubband absorption with a ratio between z and x polarization measured by photoinduced spectroscopy which is slightly decreased. (C) 1996 American Institute of Physics.