Abstract—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. I. INTRODUCTION AND BACKGROUND OOLED germanium (Ge) photoconductive detectors are characterized by a very low noise equivalent power and high quantum efficiency. They have been serving for decades as one of the most sensitive THz detectors in spectroscopy and imaging for laboratory research as well as for astronomy and planetary research [1]. The sensitivity of extrinsic photoconductive Ge detectors doped by shallow hydrogen-like impurity centers, such as gallium (p-Ge:Ga) and antimony (n-Ge:Sb), peaks at wavelengths around 100 µm (3 THz) and extends towards the infrared. Achieving high sensitivity requires low doped (~10 14 /cm 3 ) and weakly compensated Ge [2]. These broadband low-noise and high sensitivity operation features dominated the research on Ge photodetectors. As a rule of thumb an increased doping concentration and high compensation level increase the speed of response of these detectors while the sensitivity decreases and the dark current increases. Despite this fact the availability of intense short pulsed THz sources still demands fast, broad- band detectors. So far the shortest response times are a few ns obtained with a moderately compensated (32%) Ge in direct detector operation [3]. A more fundamental approach for determining the speed of response of extrinsic Ge photoconductors can be made with the application of the pump-probe technique providing a few ps temporal resolution which is available for example at free electron lasers [4]. In this work we present results of lifetime measurements obtained with a dedicated single-color pump- probe setup at the free electron laser FELBE. In particular we investigated the influence of the compensation level on the relaxation times in p-Ge:Ga:Sb as well as n-Ge:Sb:Ga detector material. II. EXPERIMENTAL The speed of response of an extrinsic Ge-photodetectors on a technical level is determined by parameters such as the electric bias field, the speed of the detector bias circuit or the geometry of the detector crystal, whereas the fundamental temporal limitation is determined by the lifetimes of the ionized charge carriers in the conduction/valence band. This work is a continuation of the investigation of these relaxation processes in extrinsic Ge with a dedicated single-color pump- probe setup at the infrared/THz free electron laser facility FELBE. The focus was set to the influence of the degree of compensation in Ge on the free carrier capture rate. Therefore, different samples with varying levels of compensation, i.e. relative amounts of Sb donor atoms and Ga acceptor atoms in germanium crystals, have been produced by the Institute of Crystal Growth. The samples were grown by the Czochalski method and the doping material was introduced directly into the crucible. Due to the depletion during the growth and the different segregation coefficients of Sb and Ga the concentration of the impurities changes along the growth axis of the ingot. The crystal was then sawed to samples of 10x10x0.5 mm³ size. The approximate concentration of impurity atoms obtained in the samples is shown in Tab. 1. TABLE I. DOPING CONCENTRATIONS OF INVESTIGATED GERMANIUM SAMPLES Sample NGa (cm -3 ) NSb (cm -3 ) Nnet (cm -3 ) Comp (%) Ge-539-1 2.8×10 16 2.6×10 16 (p) 2×10 15 92.9 Ge-539-3 3.5×10 16 3.3×10 16 (p) 2×10 15 94.3 Ge-538-0 2.3×10 16 2.7×10 16 (n) 3×10 15 85.2 Ge-538-1 2.9×10 16 3.3×10 16 (n) 4×10 15 87.9 Ge-538-3 3.4×10 16 4.0×10 16 (n) 6×10 16 85. Ge-538-4 4×10 16 4.7×10 16 (n) 7×10 15 85.1 Additionally, the absorbance spectra of these samples have been measured by Fourier transform spectrometry (FTS) at liquid helium temperatures. They show broad a absorption band at the position of impurity absorption lines (range 5-10 meV, Fig. 1) with a large absorbance (up to 100 cm -1 ) that indicates on higher net levels of impurity concentration than those calculated from the original dopants in the crucible. In accordance to our previous pump-probe measurements we chose a wavelength of the FEL radiation of 105 µm (photon energy of 11.8 meV or 2.85 THz). N. Deßmann a , S. G. Pavlov b , M. Mittendorff c , S. Winnerl c , R. Kh. Zhukavin d , V. V. Tsyplenkov d , D. V. Shengurov d , V. N. Shastin d , N. V. Abrosimov e , H. Riemann e and H.-W. Hübers a,b a Institut für Optik und Atomare Physik, Technische Universität Berlin, Berlin, Germany b Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt, Berlin, Germany, c Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany d Institute for Physics of Microstructures, Russian Academy of Sciences, Nizhny Novgorod, Russia e Leibniz-Institute for Crystal Growth, Berlin, Germany Fast relaxation of free carriers in compensated n- and p-type germanium C 978-1-4673-4717-4/13/$31.00 ©2013 IEEE