Contents lists available at ScienceDirect Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp Study of the optical and photoelectrical properties of TlGaSeS layered single crystals S. Delice a, ⁎ , N.M. Gasanly b,c a Department of Physics, Hitit University, 19030 Çorum, Turkey b Department of Physics, Middle East Technical University, 06800 Ankara, Turkey c Virtual International Scientific Research Centre, Baku State University, 1148 Baku, Azerbaijan ARTICLE INFO Keywords: Semiconductors Optical properties Photoconductivity ABSTRACT 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. 1. Introduction Improving of optoelectronic devices gains great momentum with the optical and photoelectrical investigations of semiconductor materi- als. Thallium based ternary compounds have been attracted tremen- dous interest by researchers for decades and have been studied for different purposes serving to development of science and technology. Particularly, the ternary TlGaS 2 and TlGaSe 2 crystals have been abundantly investigated in terms of their electrical and optical proper- ties [1–5]. In the visible region of spectrum, they exhibit high photosensitivity and possess wide transparency range varying between 0.5 and 14.0 μm [6]. The studies on TlInS 2 crystals which have anisotropic physical properties revealed the suitability to many appli- cations such as X-ray, gamma and neutron detectors, visible and infrared sensitive receivers and piezoelectric photo resistors [7,8]. TlGaSe 2 crystals have also been inspected to investigate phase matched second harmonic generation effect [9]. The quaternary compounds being obtained from TlGaS 2 and TlGaSe 2 crystals by substituting constituent elements can also be promising for optoelectronic applica- tions. The band gap of the quaternary TlGaSe x S (2-x) (x=0.5, 1.0, 1.5) can be varied by replacing the selenium (sulfur) atoms with sulfur (selenium) atoms. This gives opportunity to obtain higher radiant efficiency at broader wavelength region. The quaternary TlGaSeS layered crystals have two-dimensional layers which are parallel to the (001) plane. Interlayer bonding occurs between Tl and S(Se) atoms and intralayer bonding is created between Ga and Se(S) atoms. Since TlGaSeS crystal exhibits remarkable physical properties, they promise for technological applications, especially in the areas that concerns with the development of photosensitive and photoresponsive devices. The temperature-dependent electrical resistivity and Hall mobility of TlGaSeS crystal (160–350 K) were reported in Ref. [10]. The study allowed to determine the energy of acceptor centers as 230 and 450 meV. Alharbi [11] investigated the electrical conductivity and Hall effect for TlGaSeS crystals over a wide temperature range of 278–563 K. As a result of these measurements, the position of the acceptor level was evaluated to be 250 meV. The transmittance and reflectance investigations of TlGaSeS crystals in the wavelength range of 400–1100 nm have been carried out and reported in Ref. [12]. The optical indirect transitions with the band gap energy of 2.27 eV were found by means of the analysis of the absorption data at room temperature. Moreover, TlGaSeS crystals have been studied in the low temperature range of 10–180 K by the help of thermoluminescence measurements [13]. Two distinctive glow peaks were observed around peak maximum temperatures of 39 and 131 K. Applications of the curve fitting method revealed the presence of trap levels having activation energies of 16 and 97 meV. This paper reports the results of the study on the optical properties of TlGaSeS layered single crystals in the photon energy range of 1.38– http://dx.doi.org/10.1016/j.mssp.2017.02.005 Received 23 August 2016; Received in revised form 21 January 2017; Accepted 5 February 2017 ⁎ Corresponding author. E-mail address: serdardelice@hitit.edu.tr (S. Delice). Materials Science in Semiconductor Processing 63 (2017) 72–75 1369-8001/ © 2017 Elsevier Ltd. All rights reserved. MARK