Hole confinement and dynamics in δ-doped Ge quantum dots

We report picosecond pump–probe studies of the dynamics of inter-level transitions in p-type Ge quantum dot structures using a free electron laser as a source of intense mid-infrared pulses. The wavelength-independent lifetime of around 210 ps is much longer than have been recently reported in SiGe/Si multiple quantum-well structures. The observed pump–probe signal is ascribed to photo-excitation within the confined hole state. The presence of the dot potential reduces the phonon emission and the lifetime is strongly temperature dependent falling to 50 ps at 100 K. The origin of the relaxation process is unknown but is probably by acoustic phonon emission.