Cryst. Res. Technol. 40, No. 9, 893 – 895 (2005) / DOI 10.1002/crat.200410452 © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Optical anisotropy in GaSe A. Seyhan* 1 , O. Karabulut 1,2 , B. G. Akınoğlu 1 , B. Aslan 1,3 , and R. Turan 1 1 Department of Physics, Middle East Technical University, 06531, Ankara, Turkey 2 Present Address: Department of Physics, Faculty of Arts and Science, Pamukkale University, 20017 Denizli, Turkey 3 Present Address: Institute for Microstructural Sciences, National Research Council Canada, Ottawa K1A 0R6, Canada Received 18 August 2004, accepted 15 September 2004 Published online 15 August 2005 Key words GaSe, photoluminescence, optical anisotropy. PACS 78.55.Hx, 78.20.Ek Optical anisotropy of the layer semiconductor GaSe has been studied by photoluminescence (PL) and Fourier Transform Infrared Spectroscopy (FTIR). The PL spectra are dominated by two closely positioned emission bands resulting from the free exciton and the bound exciton connected direct band edge of GaSe. Photoluminescence and transmission spectra of GaSe crystals have been measured for two cases in which the propagation vector k is perpendicular (k⊥c) and parallel to the c-axis (k//c). Peak position of the PL emission band and the onset of the transmission have been found to be significantly different for these two cases. This observed anisotropy is related to anisotropic band structure and the selection rules for the optical absorption in layered GaSe. FTIR transmission spectrum is in good agreement with PL results. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Being a member of the group III-VI, GaSe is a layer semiconductor whose c-axis is perpendicular to the layer planes. The sandwich layer consists of four covalently bound sheets of hexagonal close-packed atoms, arranged in the sequence Se-Ga-Ga-Se. In literature, four different periodic stacking sequences of the layers have been reported as β, γ, ε, and δ [1]. The ε and β- phases are 2H Hexagonal polytypes, and the modification γ and δ have a 3R trigonal structure. The layers are bound by weak Van der Walls forces while intralayer – bonding forces are primarily ionic-covalent in nature [2]. GaSe is a highly anisotropic material due to its layer structure. The structural anisotropy results in anisotropy in the electrical and optical properties of GaSe. Optical anisotropy of GaSe have been investigated by transmission, absorption or reflectivity measurements, where it was found that optical constants (e.g. absorption coefficient or refractive index) are different for E//c and E⊥c [3-8]. This difference was attributed to the selection rules related to the anisotropic band structure of GaSe. In this work, we present the results of PL and transmission spectroscopy for GaSe samples that we have grown by the Bridgman method with emphasis to the optical anisotropy in GaSe. This paper is a complementary work to our previous publications [9, 10] where we reported electrical transport properties of the same structure. The optical anisotropy in the PL emission is clearly demonstrated with the dependence of the PL emission on the k vector of the exciting photons. 2 Experimental procedure The GaSe single crystals were grown by the Bridgman technique and they were found to have hexagonal structure having lattice parameters in accordance with the literature. Preparation of the ampoules and growth processes were explained in our previous works [9, 10]. The samples used in this study were prepared by cleaving an ingot parallel to the layers. Typical sample dimensions were about of 1cm 2 and 0.5 to 2.0 mm of thickness. For PL measurements, each sample was mounted to a cold finger and placed into a closed cycle helium cryostat which provides a temperature variation from 10 K to 300 K. Sample was exposed to a laser with a wavelength of 337.1 nm and an average power of 7 mW. The luminescence emitted was focused by two convex lenses onto the entrance slit of Oriel model 77700a monochromator which is equipped with four ____________________ * Corresponding author: e-mail: ayseyhan@metu.edu.tr