Structural studies of chloride-treated RF sputtered Cd 1-x Mn x Te films
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Structural studies of chloride-treated RF sputtered Cd1-xMnxTe films S. L. Wang*, S. H. Lee, A. Gupta, and A. D. Compaan Department of Physics & Astronomy, University of Toledo, Toledo, OH, 43606, USA ABSTRACT Cd1-xMnxTe alloy films with band gaps of 1.6 ~ 1.8 eV have been deposited by RF magnetron sputtering for solar-cell applications. The films have been treated by chloride vapors to improve the photovoltaic performance. These as-deposited and chloride-treated CdMnTe films have been investigated by Raman spectroscopy, x-ray diffraction (XRD) and scanning electron microscopy (SEM). Raman results indicate that Te and/or TeO2 exists in the annealed samples depending on anneal conditions.
INTRODUCTION A practical conversion efficiency of 25% has been predicted for a two-junction polycrystalline thin-film tandem cell with an energy band gap of 1.14 eV for bottom cell and 1.72 eV for top cell [1]. CdTe-based ternary alloy materials, such as CdZnTe and CdMnTe are attractive due to their flexibility of controlling band gap with composition and have been considered as attractive wide-band-gap top cell candidates for such tandem cells [2, 3]. CdMnTe has been studied for several decades as a well-known dilute magnetic semiconductor [4-6]. However, there are few reports of this material on photovoltaic applications [3, 7]. We have previously reported [3] limited chloride annealing efforts and fabrication of CdS/CdMnTe cells. The cells were of poor efficiency maybe due to a shift in band gap towards lower energy, and nonuniformities and damage on the surface. Since chloride treatment is very important and essential in the fabrication of high efficiency CdTe solar cells [8, 9], in this paper we investigate the chloride treatment of RF sputtered CdMnTe films over a range of temperatures with various ambients. Raman, XRD and SEM are employed to characterize the properties of as-deposited and treated films. EXPERIMENTAL DETAILS CdMnTe films were magnetron sputtered on glass from a target composed of 13% MnTe and 87% CdTe. The composition of the as-deposited CdMnTe films was determined by the optical absorption edge from the transmission spectra. The CdMnTe film thickness was typically 1µm. To obtain p-type CdMnTe films, a post deposition treatment was carried out on a small piece of sample (2 cm × 3 cm) in a one inch diameter quartz tube. There were two approaches to the post deposition treatment, one was to directly carry out vapor chloride treatment on the films, and another was a two-step process in which a high annealing was carried out in a temperature range of 400 to 500 oC and then followed by the vapor chloride treatment. The high annealing process was under different ambients, including 2% H2 / Ar, nitrogen and dry air. A chloride source plate of either CdCl2 or a 10% CdCl2 + 90% MnCl2 (Cd(Mn)Cl2) mixture was used in the vapor chloride treatments. The source was made by evaporating a methanol solution of the chlorides
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on a warm glass plate. The sample was placed on the source plate with the film side
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