Fast relaxation of free carriers in compensated n- and p-type germanium

The relaxation of free holes and electrons in highly compensated germanium doped by gallium (p-Ge:Ga:Sb) and antimony (n-Ge:Sb:Ga) has been studied by a pump-probe experiment with the free-electron laser FELBE at the Helmholtz-Zentrum Dresden-Rossendorf. The relaxation times vary between 20 ps and 300 ps and depend on the incident THz intensity and compensation level. The relaxation times are about five times shorter than previously obtained for uncompensated n-Ge:Sb and p-Ge:Ga. The results support the development of fast photoconductive detectors in the THz frequency range

Extremely fast electron capture in moderately doped germanium

We report the observation of ultrafast capture of free electrons (n-Ge:Sb:Ga) and holes (p-Ge:Ga:Sb) in moderately doped (net concentration > 2×10^15/cm3) Ge with high compensation, up to 50%. The typical range of the relaxation times for photoinduced (far-infrared photo- ionization) transmittance is in the range of 20-300 ps dependent on the sample characteristics and the far-infrared light intensity. Measurements of the photoinduced decay have been carried out by a contactless pump-probe technique at the infrared free electron laser facility at HZDR, Dresden. The observed relaxation times of free charge carriers are up to two orders of magnitude shorter than the ~ 2 ns response time observed in the photoconductive response of neutron transmutation doped isotope-fixed compensated p-Ge:Ga:Se:As. This indicates the potential for very fast broad band detection of infrared pulses by extrinsic photoconductive germanium detectors with optimized doping, compensation, geometry and an appropriate electrical readout circuit

Towards a life-time-limited 8-octave-infrared photoconductive germanium detector

Ultrafast, ultra-broad-band photoconductive detector based on heavily doped and highly compensated germanium has been demonstrated. Such a material demonstrates optical sensitivity in the more than 8 octaves, in the infrared, from about 2 mm to about 8 μm. The spectral sensitivity peaks up between 2 THz and 2.5 THz and is slowly reduced towards lower and higher frequencies. The life times of free electrons/holes measured by a pump-probe technique approach a few tenths of picoseconds and remain almost independent on the optical input intensity and on the temperature of a detector in the operation range. During operation, a detector is cooled down to liquid helium temperature but has been approved to detect, with a reduced sensitivity, up to liquid nitrogen temperature. The response time is shorter than 200 ps that is significantly faster than previously reported times

Carrier dynamics in doped Ge measured at the free electron laser facility FELBE

Cooled germanium (Ge) photoconductive detectors are one of the most sensitive detectors at terahertz (THz) frequencies. They are widely used in laboratory spectroscopy and imaging experiments. The speed of a Ge photoconductive detector is set by technical limitations such as the bias circuit, the geometry of the detector crystal and the electric field applied to the .detector. The recovery speed of the detector material is, however, fundamentally limited only by the lifetimes of the intraband relaxation of the free charge carriers within the valence or conduction band and by band-to-impurity relaxation (capture) down to the impurity ground state. Therefore, capture and intraband relaxation processes have been measured for different dopants in uncompensated and compensated n- and p-type Ge by a pump-probe technique at the free electron laser facility FELBE

Towards a life-time-limited 8-octave-infrared photoconductive germanium detector

Ultrafast, ultra-broad-band photoconductive detector based on heavily doped and highly compensated germanium has been demonstrated. Such a material demonstrates optical sensitivity in the more than 8 octaves, in the infrared, from about 2 mm to about 8 μm. The spectral sensitivity peaks up between 2 THz and 2.5 THz and is slowly reduced towards lower and higher frequencies. The life times of free electrons/holes measured by a pump-probe technique approach a few tenths of picoseconds and remain almost independent on the optical input intensity and on the temperature of a detector in the operation range. During operation, a detector is cooled down to liquid helium temperature but has been approved to detect, with a reduced sensitivity, up to liquid nitrogen temperature. The response time is shorter than 200 ps that is significantly faster than previously reported times

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

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