Relaxation dynamics of photoionized carriers in n-type and p-type germanium

Abstract

Intense and short pulsed THz sources require fast, broad-band detectors with large dynamic range. Extrinsic germanium photodetectors made from moderately compensated (32%) crystals have response times of a few ns. The low temperature capture of free charge carriers into ionized, hydrogen-like antimony and gallium centers in germanium has been studied by a dedicated, degenerate pump-probe experiment (λPump= λProbe) using the free-electron laser (FEL) FELBE at the Helmholtz-Zentrum Dresden-Rossendorf. The analysis of the pump-probe signal reveals typical capture times (τc) in the order of a few nanoseconds, which decrease with increasing (pump) photon flux density (ΦPump) on the probed surface spot on the sample. The results support an acoustic-phonon assisted cascade capture model. Additionally, we have determined the capture time in high doped and up to almost 100% compensated n-Ge:Sb:Ga and p-Ge:Ga:Sb. The capture time varies between 20 and 350 ps. With such material a sub-ns broadband THz-detector could be realized.