Study of the optical and photoelectrical properties of TlGaSeS layered single crystals

Transmission spectra (T) of TlGaSeS crystals in the photon energy (hν) range 1.38-2.38 eV are used to determine the energy gap. The indirect band gap of 2.30 eV was established employing the photon energy dependence of the first derivative dT/d(hν) and the photon energy dependence of absorption coefficient. In order to obtain information about the defect states in the energy gap of TlGaSeS crystals, photoconductivity (PC) measurements are performed in the 140-300 K range. Photoconductivity spectra in the photon energy range of 1.77-3.10 eV show two peaks related to intrinsic and extrinsic excitations. It was revealed that the first peak shifts slightly towards the low energy side with increasing temperature, whereas the second one shifts more significantly to the high energy range. It was assumed that for latter peak the deviating PC originates from the distributed energy levels in the band gap where the photoelectrons arise or where they go.