Relaxation of THz impurity photoconductivity in GaAs/InGaAsP and Ge/GeSi quantum well heterostructures

Abstract

Abstract. In n-GaAs/InGaAsP and p-Ge/GeSi quantum well heterostructures excited by both the picosecond broad band and the nanosecond narrow band pulses of THz radiation the relaxation times of the impurity photoconductivity were measured. In one of the sample Ge/GeSi this more narrow quantum well we observed the first time increases maximum relaxation time from 9 to 14 ns at the narrow band THZ radiation frequency decrease from 60 cm -1 down to 25 cm -1 that can be explained by a dispersion of shallow acceptor binding energies. The frequency of 60 cm -1 nearly corresponds to the binding energy of acceptors situated nearby Ge QW center while that of 25 cm -1 corresponds to ionization energy for the hole (in Ge QW) bound with a remote acceptor ion in the center of GeSi barrier. We explain the observed larger relaxation time for the acceptor with lower binding energy by an enhanced probability for the hole captured by an excited impurity state to be thermally activated again into the valence band. IntroductionNowadays there is a growing interest to the nonequilibrium phenomena and possibilities to produce the intraband population inversion in semiconductor nanostructures doped with shallow impurities. In order to design THz laser operating at the optical transitions involving shallow impurity states direct information on lifetimes of carriers both excited into conduction (valence) band and captured by excited impurity states is required. In this paper we study the relaxation of impurity photoconductivity at T = 4.2K in n-GaAs/In 0.1 Ga 0.9 As 0.8 P 0.2 and p-Ge/Ge 0.9 Si 0.1 quantum well (QW) heterostructures excited by broadband THz radiation (0.5 -3 THz) generated by optical rectification of femtosecond laser pulses and by narrow band (0.2 cm -1 ) tunable THz pulses of nanosecond duration.