The absorption characteristics of GexSi1−xquantum well infrared photodetector (QWIP) structures have been studied in samples over a range of germanium compositions and doping levels. In all these samples, quantum well intersubband transitions are either very weak or nonexistent for normally incident light. However, free carrier absorption in GexSi1−xquantum wells is a strong absorbing mechanism in the long wavelength infrared regime, and has been found to be stronger than in silicon for similar doping levels. Therefore, detectors relying upon free carrier absorption in GexSi1−xquantum wells may offer superior responsivity and quantum efficiency. ©1995 American Institute of Physics.