Effects of substrate temperature and post-deposition anneal on properties of evaporated cadmium telluride films E. Bacaksiz a, ⁎ , B.M. Basol b , M. Altunbaş a , V. Novruzov c , E. Yanmaz a , S. Nezir d a Department of Physics, Faculty of Arts and Sciences, Karadeniz Technical University, 61080 Trabzon, Turkey b ASM NuTool Inc, Department Proc and Technol, Milpitas, CA 95035, USA c Department of Physics, Rize Faculty of Arts and Sciences, Karadeniz Technical University, Rize, Turkey d Department of Physics, Faculty of Arts and Sciences, Kırıkkale University, Kırıkkale, Turkey Received 4 July 2005; received in revised form 1 August 2006; accepted 29 August 2006 Available online 3 October 2006 Abstract The effects of substrate temperature and post-deposition heat treatment steps on the morphology, structural, optical and electrical properties of thin film CdTe layers grown by vacuum evaporation were investigated. Scanning electron microscopy and X-ray diffraction (XRD) techniques were employed to study the structural changes. It was observed that the grain sizes and morphologies of as-deposited layers were similar for substrate temperatures of - 173 °C and - 73 °C. However, CdTe films produced at a substrate temperature of 27 °C had substantially larger grain size and clearly facetted morphology. Annealing at 200–400 °C in air did not cause any appreciable grain growth in any of the films irrespective of their growth temperature. However, annealing at 400 °C reduced faceting in all cases and initiated fusing between grains. XRD studies showed that this behavior after annealing at 400 °C coincided with an onset of a degree of randomization in the originally strong (111) texture of the as- grown layers. Optical band gap measurements showed sharpening of the band-edge upon annealing at 400 °C and a band gap value in the range of 1.46–1.49 eV. Resistivity measurements indicated that annealing at 400 °C in air forms a highly resistive compensated CdTe film. All results point to 400 °C to be a critical annealing temperature at which optical, structural and electrical properties of CdTe layers start to change. © 2006 Elsevier B.V. All rights reserved. Keywords: Cadmium telluride; Evaporation; Optical properties; Annealing; X-ray diffraction; Scanning electorn microscopy 1. Introduction Thin film polycrystalline CdTe is one of the most promising materials for the fabrication of optoelectronic semiconductor devices. Specifically, near-optimum band gap of about 1.45 eV and high optical absorption coefficients for visible light make this material suitable for low cost terrestrial solar cell applications [1]. Some of the commonly used low-cost growth techniques for CdTe thin film production include electrodepo- sition [2], spray pyrolysis [3] and close-spaced sublimation [4]. Irrespective of the growth process, the grain size and surface morphology of CdTe films are two of the important parameters that affect the performance of active devices such as solar cells made on such layers. For example, fabrication of high efficiency solar cells requires films with columnar grains to minimize grain boundary crossings by light-generated minority carriers. Therefore, understanding the microstructure and morphology evolution in polycrystalline CdTe films is important to further develop an understanding of the performance of devices employing these layers. Polycrystalline films are used in a wide variety of applications in which their average grain size, distribution of grain size and distribution of grain orientations affect their performance and reliability. These grain structure characteristics are often defined by grain formation and growth during the film formation period. However, they may also be modulated through post-deposition process steps involving high temperature annealing [5]. Interest in the properties of CdTe films grown at low temperatures has increased in recent years especially for low- cost solar cells applications [6–11]. The power conversion Thin Solid Films 515 (2007) 3079 – 3084 www.elsevier.com/locate/tsf ⁎ Corresponding author. E-mail address: eminb@ktu.edu.tr (E. Bacaksiz). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.08.026