Technical note Comparative study of the p + layer created in CdTe films by nitric–phosphoric etching and chemical-deposition methods Osvaldo Vigil a , Ramo ´ n Ochoa-Landı ´n b,c , Jesu ´ s Fandin ˜o d,n , Francisco Cruz-Gandarilla a a Escuela Superior de Fı ´sica y Matema ´ticas-Instituto Polite´cnico Nacional (IPN), C.P. 07738, Me ´xico D.F., Me´xico b Centro de Investigacio ´n y Estudios Avanzados del IPN. Unidad Quere´taro. Apdo. Postal 1-798, 76001 Quere´taro, Quere´taro, Me´xico c Departamento de Fı ´sica, Universidad de Sonora. Apdo. Postal 5-88, Hermosillo, Sonora 83190, Me ´xico d Universidad Auto ´noma de la Ciudad de Me´xico. Avenida la Corona 320, Col. Loma la Palma, Gustavo A. Madero 07160, Mexico D.F., Me´xico article info Article history: Received 17 March 2009 Received in revised form 6 September 2009 Accepted 20 October 2009 Keywords: A. thin films A. semiconductors D. electrical properties abstract The physical properties of the etched CdTe surfaces obtained during the process of formation of a p+ region on CdTe surface films using (i) a nitric/phosphoric acid mixture and (ii) a chemical Te-deposition method involving thermal annealing have been compared in this study. This study suggests the chemical-deposition method as an alternative to the chemical-etching methods for use in back-contact technology to increase the efficiency of CdTe solar cells. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction Creating a good and reliable back contact is probably one of the most challenging issues in the fabrication of high efficiency CdS/CdTe thin film solar cells. Back-contact formation in these devices is complicated by the high work function and low doping levels of CdTe. One approach to solve this problem is to generate a Te-rich layer by an etching step, sometimes followed by the formation or deposition of another Te-rich intermediate layer. A wet-chemical etch with nitric and phosphoric acids (NP) has been successfully used so far. However, this etching process can result in voids and shunt paths between the crystal grains when thin absorber films are used. Other alternative methods for back- contact formation are dry etching in Ar plasma [1] and the deposition of a Te layer by physical methods, such as thermal vacuum deposition with post-deposition annealing [2], described as being successful in the literature. In this study, we propose the deposition of Te using a chemical bath (chemical bath deposition, CBD) as a method for back-contact formation and compare the results obtained with that of the traditional NP chemical-etching process. 2. Experimental details CdTe thin films were deposited onto SnO2:F substrates using the close-spaced vapour transport hot-wall (CSVT-HW) technique. Details about the growth processes are explained elsewhere [3]. Consequent to the growth processes, CdTe layers of about 4 mm thickness were obtained. A 200 nm CdCl 2 layer was deposited onto CdTe films using the CSVT-HW technique. Post-thermal treatment consisted of anneal- ing these films at 450 1C for 30 min in an Ar atmosphere. Next, the p+ regions were formed by the following two procedures: (a) by immersing CdCl 2 -treated CdTe films in a solution containing 0.4% nitric acid, 29% phosphoric acid and 70.6% water for 30 s and (b) by immersing CdTe films in a solution containing Te ions [4], the latter prepared according to the process reported in the study by Sotelo-Lerma et al. [5]. The CdTe films were removed from the solution after 60 min and dried with nitrogen. In both cases, the samples were annealed in air at 300 1C for 30 min. The thickness of the Te film was about 1 mm, as measured by profilometry. Photoluminescence (PL) measurements were obtained at a temperature of 10 K, using an Ar+ laser (l =514.5 nm) focused on the sample through a spherical lens. Scanning electron microscopic (SEM) examinations of the surface morphology and the composite layer above it were carried out using a Philips X-20 SEM equipment. An energy dispersive X-ray (EDAX) detector was used to identify the chemical species on the surfaces of the samples. For the electrical characterization, Au contacts (300 ˚ A thick) were evaporated on CdTe and thermally ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jpcs Journal of Physics and Chemistry of Solids 0022-3697/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2009.10.010 n Corresponding author. E-mail address: jfand72@yahoo.com (J. Fandin ˜o). Journal of Physics and Chemistry of Solids 71 (2010) 404–406