Thermal and optical properties of polycrystalline CdS thin films deposited by the gradient recrystallization and growth (GREG) technique using photoacoustic methods M.L. Albor-Aguilera a , M.A. González-Trujillo b , A. Cruz-Orea c , M. Tufiño-Velázquez a, ⁎ a ESFM-IPN, Depto. de Física, Edif. 9, UPALM, 07738 México, D. F., Mexico b ESCOM-IPN, Depto. de Ciencias Básicas, UPALM, 07738 México, D. F., Mexico c CINVESTAV-IPN, Depto. de Física, Av. IPN No. 2508, 07360 México, D. F., Mexico abstract article info Available online 8 November 2008 Keywords: CdS Thin films Thermal properties CSVT Open photoacoustic cell In this work we report the study of the thermal and optical properties of polycrystalline CdS thin films deposited by the gradient recrystallization and growth technique. CdS films were grown on pyrex glass substrates. These studies were carried out using an open photoacoustic cell made out of an electret microphone. From X-ray diffraction, atomic force microscope and photoluminescence measurements we observed polycrystalline CdS films with good morphology and crystalline quality. We obtained a thermal diffusivity coefficient of our samples with values ranging from 3.15 to 3.89×10 − 2 cm 2 /s. For comparison, we measured a value of 1.0×10 − 2 cm 2 /s for the thermal diffusivity coefficient of a CdS single crystal. We measured an energy gap value of 2.42 eV for our samples by using a photoacoustic spectroscopy system. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Extensive research has been done in the last two decades on cadmium sulphide (CdS) thin films, mainly due to its applications in large area electronic devices like thin film field-effect transistors [1] and solar cells [2]. Investigations on thin film optical integrated circuits grown on single crystal substrates have started during recent years, and CdS films are also a good candidate for this application [3]. For these purposes CdS films have been deposited by a wide variety of techniques, including Laser Ablation (LA), Close Space Vapor Transport (CSVT), Chemical Bath Deposition (CBD), Sputtering (Sp), and others. The properties of CdS thin films often depend on the deposition technique and the growth parameters for each technique. Further- more, a variety of deposition methods show good reproducibility with uniform films and good adherence on different substrates, which are easily adapted to solar cell module manufacturing. In this sense, CdS is one of the most important semiconductors for the electronics industry today, and it has become essential as the heterojunction partner to CdTe and Cu(In,Ga)Se 2 for the development of thin film polycrystalline solar cells. Therefore, the investigation of polycrystalline CdS thin films is very relevant nowadays. Due to the different type of applications and the extreme weather conditions in which CdS-based devices may operate, it is important to study how their properties change with temperature. It is of great interest for us to study the thermal and optical properties of polycrystalline CdS thin films, as this material has been used with polycrystalline CdTe films to obtain CdS/CdTe heterojunction solar cells with a conversion efficiency record of 16.5% [4], processing the solar cell by the CSVT technique. Compared with similar studies performed in single crystal devices, research on polycrystalline thin films becomes of great interest, as we have the possibility to change several parameters such as film thickness, grain size, stoichiometry, and others, by changing the growth parameters. In this work we obtained polycrystalline CdS thin films deposited by the gradient recrystallization and growth (GREG) technique [5], which is an improved version of the Close Spaced Vapor Transport (CSVT) process. In this technique the properties of the films depend upon the temperature gradient between the source and the substrate, the gas used in the chamber and the gas pressure. Thermal and optical properties of these thin films samples were obtained by using photoacoustic methods. Complementary studies by using X-ray diffraction, atomic force microscope and photoluminescence mea- surements were done in order to observe morphology and crystalline quality of these films. 2. Experimental The deposition of CdS thin films by the GREG technique was made using high purity CdS powder (99.99%, Balzers). CdS films were grown on pyrex glass substrates using the following deposition parameters: 450–550 °C for the substrate temperature and 650–750 °C for the Thin Solid Films 517 (2009) 2335–2339 ⁎ Corresponding author. Escuela Superior de Física y Matemáticas, IPN, U. Prof. “A. López Mateos” Zacatenco, Edif. 9, Col. Lindavista, 07738 México, D. F., Mexico. Tel.: +52 55 5729 6139; fax: +52 55 5586 2957. E-mail address: mtufinovel@yahoo.com.mx (M. Tufiño-Velázquez). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.11.006 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf