Space-charge limited free carrier transport in semi-insulating III-V compounds

Abstract

We demonstrate a novel way to analyse carrier recombination and transport processes in photorefractive semiconductors via kinetics and exposure characteristics of light induced diffraction. The results of a picosecond degenerate four-wave mixing on free carrier gratings in semi-insulating GaAs and InP bulk crystals are discussed. The role of a deep trap recharging in carrier diffusion and recombination is sensitively revealed through a feedback effect of a space-charge field to nonequilibrium carrier transport or directly through linear electrooptic effect. IntroductionDegenerate four-wave mixing (DFWM) has been demonstrated to be a sensitive tool for studying carrier generation, transport, and recombination processes in bulk crystals, epilayers, and heterostructures [1,2]. In particular, the technique provides good means for nondestructive characterization of deep-impurity-related effects in semi-insulating (SI) materials by monitoring the formation of light-induced space-charge (SC) field E sc either through a linear electrooptic effect [3] or indirectly through its feedback to nonequilibrium carrier transport [4]. For this goal, the kinetics of free carrier grating at small grating periods Λ (because of relationship E sc ~ 1/Λ ) and various excitations are usually measured.In the given work, we present a novel way to study deep-impurity-related photoelectrical processes in SI GaAs and InP bulk crystals by exploring the features of exposure characteristics (EC) of diffraction, i.e. the dependence of diffraction efficiency on optical excitation energy. Aside from the well established fact that EC is very informative if carrier generation mechanisms have to be characterized [5], we demonstrate as well that this type of dependence, under certain experimental conditions, contains valuable information on carrier recombination and transport. Experimental Results and DiscussionA mode-locked Nd:YAG laser at λ = 1064 nm wavelength (hν = 1.17 eV) has been used for recording and probing of transient gratings [6]. The pulse width was 27 ps at FWHM. The incident laser beam was divided into two s-polarized parts of approximately equal energy which recombined at an angle Θ to form a grating with period Λ = λ / [2 sin(Θ / 2)] in the bulk of the crystal. The decay of a transient grating as well as its dependence on the excitation energy I 0 was studied by Bragg diffraction of the probe beam I prb [6]. The setup enabled us to record the diffracted signal I 1 on free carrier (FC) grating (p-diffracted component of p-polarized probe beam) or photorefractive (PR) grating (p-diffracted component of s-polarized probe beam). In both cases, the diffraction efficiency η has been defined as a ratio of the diffracted signal intensity to the transmitted signal intensity. Further details on