Cryst. Res. Technol. 46, No. 1, 79 – 84 (2011) / DOI 10.1002/crat.201000486 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Electrical and optical properties of Co doped TlGaS 2 crystals O. Karabulut*, K. Yilmaz, and B. Boz Department of Physics, Faculty of Arts and Science, Pamukkale University, 20017 Denizli, Turkey Received 16 September 2010, revised 8 November 2010, accepted 9 November 2010 Published online 26 November 2010 Key words ternary compound, doping, TlGaS 2 . In this study, Co doped TlGaS 2 single crystals which belongs to the class of A III B III X 2 VI have been investigated by means of XRD, temperature dependent dark and illuminated conductivity, Space Charge Limited Currents and absorption measurements. The room temperature conductivity and trap concentration values were about 10 -8 (Ω-cm) -1 and 7.5x10 13 cm -3 , respectively. From the temperature dependent conductivity measurements, two activation energies namely 271 and 12 meV have been determined in the high and low temperature regions, respectively. The trap level at 271 meV that was determined by the dark temperature dependent conductivity measurement has also been verified by Space Charge Limited Currents analysis. The absorption measurements have showed that the layered compound had indirect and direct band gaps and the values were determined to be 2.49 and 2.56 eV, respectively. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Thallium chalcogenides known to be layered materials have become increasingly interesting because of applicability to photodetectors and photodiodes etc. [1]. TlGaS 2 compound belongs to the class of wide band gap semiconductors with chemical formula A III B III X 2 VI , where A=Tl, B = Ga or In and X = S, Se or Te. The lattice of these crystals consists of alternating two dimensional layers arranged parallel to the (001) planes [2]. It has been reported that TlGaS 2 may crystallize in various polytypes due to the attachment of their layers. It exhibits an anisotropy of the electronic states near the band edge and is also known to be crystallized with a number of intrinsic defects which strongly influence the electron emission and photoelectric properties. The interaction between successive layers, or interlayer interaction, is of van der Waals type. The interaction forces in the lattice leads to considerable anisotropy of various physical parameters [3]. Because it has high photo- sensitivity in the visible, IR and X-ray ranges, these properties make them useful as photosensitive device applications [4]. In view of its interesting properties such as, anisotropic behaviour, phase transitions, and their quasi-two dimensionality, a great deal of attention has been devoted to the study of the structural, electrical, and photoelectrical properties of TlGaS 2 . Besides the unavailability of data obtained as a result of x-ray diffraction, electrical and optical measurements, it seems that there is an ambiguity of Tl chalcogenides due to different possible versions of layer stacking. Photoconductivity measurements showed that there is a continuous distribution of traps in the energy gap [5]. It was observed from the optical absorption measurements that undoped TlGaS 2 compound have direct and indirect energy gaps. But optical indirect and direct band gap energies show dispercency in literature. While Optical indirect band gap energies were 2.46, 2.38, 2.53, 2.45 eV, the reported direct band gap energies at room temperature were 2.54, 2.53, 2.58, 2.63 eV [6-9]. Transport parameters have been investigated by means of various techniques. Two activation energies at 360 and 240 meV for the high and low temperature ranges have been reported from temperature dependent conductivity measurements [10]. In spite of the fact that Tl compounds belong to the class of A III B III X 2 VI have been investigated intensively in the last decades, doping effect studies on the thallium chalcogenides are very limited. In this study, we report the results of dark electrical conductivity, space-charge limited currents and photoconductivity of Co doped TlGaS 2 as a function of temperature in order to obtain information about the localized levels in the band gap. In addition to this, the effect of Co doping on absorption spectra has been investigated. ____________________ * Corresponding author: e-mail: okarabulut@pau.edu.tr