The influence of substrate temperature on the morphology, optical and electrical properties of thermal-evaporated ZnTe Thin Films E. Bacaksiz a, *, S. Aksu b , N. Ozer c , M. Tomakin d , A. O ¨ zc ¸elik d a Department of Physics, Karadeniz Technical University, 61080 Trabzon, Turkey b SoloPower, Inc. 5981 Optical Ct., San Jose, CA 95138, USA c School of Engineering, San Francisco State University, San Francisco, CA 94132, USA d Department of Physics, Rize University, 53100 Rize, Turkey 1. Introduction Due to their unique optical and electrical properties, group II-VI compound semiconductors find a wide range of applications in the fabrication of solid-state devices such as photodetectors, light emitting diodes and thin film solar cells [1]. Zinc telluride is a II-VI semiconductor with a wide and direct band gap (2.26 eV at room temperature) [2] and a low electronic affinity (3.53 eV) [3]. In addition to these attractive characteristics, it is an environmentally benign compound and can be potentially deposited with a low cost [4]. Therefore, ZnTe thin films have recently received an increasing attention for their applications in optoelectronic devices. Since zinc telluride can be easily doped with copper, it has become the most common back contact layer for CdTe in CdTe/CdS hetero- juction solar cells [5]. Polycrystalline ZnTe thin films and related Zn-rich alloy films such as ZnCdTe are used in the fabrication of pure-green and green-yellow light emitting diodes and laser diodes [6]. Such films were also successfully used in tandem solar cell applications [7,8] and quantum well structures [9]. Other applications of ZnTe films include switching devices [10], g-ray detectors [11], terahertz radiation detectors [12,13], transparent heating elements and electromagnetic field coatings [4]. Device-grade thin films of ZnTe have been prepared by a number of deposition techniques, including thermal evaporation [4,14–19], Rf and dc sputtering [20–23], molecular beam epitaxy [24,25], metallorganic vapor epitaxy [2,26], hot wall epitaxy [27], closed space sublimation [28], chemical vapor deposition [29,30], electrodeposition [31], pulsed laser deposition [32] and successive ionic layer adsorption and reaction (SILAR) [33]. While each of these techniques has its own merits and limitations, thermal evaporation has been the most commonly employed method for the preparation of ZnTe thin films due to its simplicity, reproducibility and scalability to deposit onto large area sub- strates. Thermally evaporated ZnTe films present cubic zinc- blende structure when deposited on amorphous substrates kept at room temperature [14]. Applied Surface Science 256 (2009) 1566–1572 ARTICLE INFO Article history: Received 7 July 2009 Received in revised form 7 September 2009 Accepted 7 September 2009 Available online 15 September 2009 Keywords: ZnTe thin films Vacuum evaporation Substrate temperature Annealing temperature Microstructure Optical band gap Electrical resistivity ABSTRACT The structural, morphological, optical and electrical properties of ZnTe films deposited by evaporation were investigated as a function of substrate temperature (at 123 and 27 8C) and post-deposition annealing temperature (at 200, 300 and 400 8C). It was determined that films deposited at both substrate temperatures were polycrystalline in nature with zinc-blende structure and a strong (1 1 1) texture. A small Te peak was detected in XRD spectra for both substrate temperatures, indicating that as-deposited ZnTe films were slightly rich in Te. Larger grains and a tighter grain size distribution were obtained with increased substrate temperature. Scanning electron microscopy (SEM) studies showed that the microstructures of the as-deposited films agreed well with the expectations from structure zone model. Post-deposition annealing induced further grain growth and tightened the grain size distribution. Annealing at 400 8C resulted in randomization in the texture of films deposited at both substrate temperatures. Optical spectroscopy results of the films indicated that the optical band gap value increased from 2.13 to 2.16 eV with increased substrate temperature. Increasing the annealing temperature sharpened the band-edge. Resistivity measurements showed that the resistivity of films deposited at substrate temperatures of 123 and 27 8C were 32 V cm, and 1.0  10 4 V cm, respectively with corresponding carrier concentrations of 8.9  10 15 cm 3 and 1.5  10 14 cm 3 . Annealing caused opposite changes in the film resistivity between the samples prepared at substrate temperatures of 123 and 27 8C. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +90 462 377 25 45; fax: +90 462 325 31 95. E-mail address: eminb@ktu.edu.tr (E. Bacaksiz). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.09.023