Normal incident infrared absorption is observed in intervalencehyphen;subband transition of Si1minus;xGex/Si multiple quantum wells for the first time. The observed absorption peak wavelength, strength, and broadness are shown to be strongly dependent on the Ge content in the well. The intervalencehyphen;subband absorption peak shows up at a shorter wavelength with the beam polarized parallel to the growth plane (normal incidence), compared with the previously reported intersubband transition between two heavy hole subbands, which occurs when the optical field is polarized along the growth direction. The dependence of the absorption peak height with the Ge composition and thus the direct gap energy in the quantum well indicates that the origin of this transition is due to the intervalencehyphen;subband transition from the heavy hole ground state to another valence subband. The normal incident detection suggests the convenient fabrication of focal plane infrared detector arrays using Si1minus;xGex/Si multiple quantum wells.
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