Optical properties and layer microstructure of CdS films obtained from an ammonia-free chemical bath deposition process M.G. Sandoval-Paz a , M. Sotelo-Lerma b , A. Mendoza-Galvan a , R. Ramírez-Bon a, ⁎ a Centro de Investigación y Estudios Avanzados del IPN, Unidad Querétaro, Apdo, Postal 1-798, 76001 Querétaro, Qro., México b Centro de Investigación en Polímeros y Materiales, Universidad de Sonora, Apdo, Postal 130, 83190 Hermosillo, Son., México Received 27 May 2006; received in revised form 15 July 2006; accepted 14 September 2006 Available online 23 October 2006 Abstract In this work we report the optical and structural properties of chemically deposited CdS films obtained from an ammonia-free chemical bath deposition process. The deposition process is based in the substitution of ammonia by sodium citrate as the complexing agent in the reaction solution. We applied spectroscopic ellipsometry and optical transmission and reflection spectroscopy to determine the thickness, optical constants and energy band gap of a set of CdS samples obtained by this process at different deposition times. Additionally, the spectroscopic ellipsometry analysis provided information about the microstructure and kinetic growth process of the CdS films. We found that experimental data fit quite well with a model of two different layers, corresponding to different growing stages, to simulate the CdS films. The first layer, formed at the earlier growing stages by means of the ion by ion deposition process, was assigned to a pure compact CdS layer and their optical properties were simulated by a Kramers–Kronig consistent model. The second external layer composed by a mixture of CdS and voids was modeled by the Bruggeman effective medium approximation. The results were compared to those obtained for a set of CdS samples obtained from a deposition process which employs ammonia in the reaction solution. © 2006 Published by Elsevier B.V. Keywords: Ammonia; Cadmium sulfide; Deposition process; Optical properties 1. Introduction Nowadays CdS is a very important technological semicon- ductor material which has been studied for decades. CdS thin films have been applied in a variety of devices such as photodetectors, piezoelectric transducers, bolometers, solar cells [1–8], etc. Since the first reports in the early 90's on the fabrication of high efficiency solar cells based on CdS/CdTe and CdS/CuInSe 2 thin film heterostructures [1,9], the interest on this material has increased. CdS polycrystalline thin films with an energy band gap around 2.4 eV are up to now the best window layers for the p-type absorbing materials CdTe and CuInSe 2 in these heterojunction solar cells. The chemical bath deposition (CBD) is the deposition technique most widely employed to obtain CdS films for this specific application. The CBD process is a simple and inexpensive technique to obtain homogeneous, hard, adherent, transparent and stoichiometric CdS thin films. Typically, chemically deposited CdS films are formed from the reaction between a cadmium salt and thiourea in an ammoniacal alkaline solution. The main role of ammonia in the CBD process is as complexing agent for the cadmium ions in the reaction solution. It is clear that the fabrication of CdS by CBD for large scale solar cell production represents a serious environmental problem because the employment of large amounts of ammonia, which is toxic, is highly volatile and harmful for the environment [10,11]. Some efforts have been dedicated to the investigation of CBD processes for the deposition of good quality CdS films, which reduce this environmental problem. One of the main approaches is the substitution of ammonia as the complexing agent of cadmium ions in the CBD process [10,12]. In recent papers we reported the properties of CdS films deposited by an ammonia-free CBD process employing sodium citrate as the complexing agent substituting ammonia [13–15]. We have found that good quality highly oriented CdS films can be obtained from a cadmium chloride, thiourea and sodium citrate alkaline solution. The CdS films obtained from this ammonia-free process showed properties comparable and even better than those of CdS films obtained from an ammonia based CBD process. In this work we have applied optical spectroscopies to determine Thin Solid Films 515 (2007) 3356 – 3362 www.elsevier.com/locate/tsf ⁎ Corresponding author. Tel.: +52 442 4414906; fax: +52 442 4414937. E-mail address: rrbon@qro.cinvestav.mx (R. Ramírez-Bon). 0040-6090/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.tsf.2006.09.024