Long intersubband relaxation times in n-type germanium quantum wells

We measured the non-radiative intersubband relaxation time in n-type modulation-doped Ge/SiGe multi-quantum wells of different thickness by means of degenerate pump-probe experiments. The photon energy was tuned to be resonant with the lowest conduction band intersubband transitionenergy (14-29 meV), as measured by terahertz absorption spectroscopy and in agreement with bandstructure calculations. Temperature-independent lifetimes in excess of 30 ps were observed.