Electrical Relaxation and Transport Properties of ZnGeP₂ and 4H-SiC Crystals Measured with Terahertz Spectroscopy

Terahertz photoconductivity and charge carrier recombination dynamics at two-photon (ZnGeP2) and three-photon (4H-SiC) excitation were studied. Thermally annealed, high-energy electron-irradiated and Sc-doped ZnGeP2 crystals were tested. The terahertz charge carrier mobilities were extracted from both the differential terahertz transmission at a specified photoexcitation condition and the Drude-Smith fitting of the photoconductivity spectra. The determined terahertz charge carrier mobility values are similar to 453 cm(2)/V center dot s for 4H-SiC and similar to 37 cm(2)/V center dot s for ZnGeP2 crystals. The charge carrier lifetimes and the contributions from various recombination mechanisms were determined at different injection levels using the model, which takes into account the influence of bulk and surface Shockley-Read-Hall (SRH) recombination, interband radiative transitions and interband and trap-assisted Auger recombination. It was found that ZnGeP2 possesses short charge carrier lifetimes (a similar to 0.01 ps(-1), b similar to 6 x 10(-19) cm(3)center dot ps(-1) and c similar to 7 x 10(-40) cm(6)center dot ps(-1)) compared with 4H-SiC (a similar to 0.001 ps(-1), b similar to 3 x 10(-18) cm(3)center dot ps(-1) and c similar to 2 x 10(-36) cm(6)center dot ps(-1)), i.e., tau similar to 100 ps and tau similar to 1 ns at the limit of relatively low injection, when the contribution from Auger and interband radiative recombination is small. The thermal annealing of as-grown ZnGeP2 crystals and the electron irradiation reduced the charge carrier lifetime, while their doping with 0.01 mass % of Sc increased the charger carrier lifetime and reduced mobility. It was found that the dark terahertz complex conductivity of the measured crystals is not fitted by the Drude-Smith model with reasonable parameters, while their terahertz photoconductivity can be fitted with acceptable accuracy.