Intersubband lifetimes in p-Si/SiGe terahertz quantum cascade heterostructures

Time-resolved studies of the dynamics of intersubband transitions are reported in three different strain symmetrized p-Si/SiGe multiple-quantum-well and quantum cascade structures in the far-infrared wavelength range (where the photon energy is less than the optical phonon energy), utilizing the FELIX free-electron laser. The calculated rates for optical and acoustic phonon scattering, alloy disorder scattering, and carrier-carrier scattering have been included in a self-consistent energy balance model of the transient far-infrared intersubband absorption, and show good agreement with our degenerate pump-probe spectroscopy measurements in which, after an initial rise time determined by the resolution of our measurement, we determine decay times ranging from similar to 2 to similar to 25 ps depending on the design of the structure. In all three samples the lifetimes for the transition from the first light hole subband to the first heavy hole subband are found to be approximately constant in the temperature range 4-100 K.