Optical Pump — THz Probe Study of SI-GaAs:Cr with Long Charge Carrier Lifetime

Abstract

SI-GaAs:Cr is a material for ionizing radiation detectors [1], in which, on the contrary to materials for THz devices [2], a long charge carrier lifetime, at the level of tens and hundreds of nanoseconds, is required. Despite the fact that SI-GaAs:Cr has been successfully used to create ionizing radiation detectors highly demanded by nuclear physics research centres [3] the mechanisms responsible for the increase in charge carrier lifetime with the introduction of chromium are not completely revealed. THz pump-probe methods are contactless and relevant to study ultrafast phenomena in semiconductors. Nevertheless, there are considerable differences in the values of charge carrier lifetime in GaAs obtained from terahertz pump-probe measurements and from measurements of photoconductivity decay or charge collection efficiency [4]. This can be attributed to influence of surface states and high levels of injection at pump-probe measurements. To address these issues in the present study semiconductors with chemically and mechanically treated surfaces were studied. To distinguish the contributions of different recombination mechanisms, an approximation based on the equation 1/τ eff =A+B-N+C-N2 =−1/NdN/dt was applied, where τ eff is the effective nonequilibrium charge carrier lifetime, the A coefficient is responsible for the influence of surface and volume Shockley-Read-Hall (SRH) recombination, the B coefficient is associated with the effect of interband radiative transitions and Auger recombination through trap levels and the C coefficient is responsible for the effect of interband Auger recombination.