Electron Transitions in S i_1-x G e _x /G e Multiple Quantum Wells Grown on Si(001) Substrates

The intersubband absorption by an optical field corresponding to normally incident light has been observed in the conduction band of Si_{1- x} Ge _x /Ge quantum wells grown on Si(001) substrates using a Si molecular beam epitaxy (MBE) system. Normal incidence electron transitions are allowed due to the coupling of the optical field parallel to quantum well planes and the nonzero off-diagonal terms of the inverse effective mass tensor which are caused by the energy ellipsoids tilted toward the [001] growth direction. The quantum wells were formed by the conduction band offset of SiGe heterostructure and Sb δ-doping. An absorption coefficient of 17000 cm^-1 has been obtained as measured with a Fourier-transform infrared (FTIR) spectrometer. Absorption peaks in the range of 5∼10 µ m have been obtained for a variety of samples studied. The detection range can be easily controlled by changing either the doping concentration or the band offset or both. The results offer the opportunity for the application of focal plane arrays with Si integrated circuits built on Si(001) substrates.