Infrared ellipsometric characterization of silicon nitride films on textured Si photovoltaic cells
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1123-P02-02
Infrared ellipsometric characterization of silicon nitride films on textured Si photovoltaic cells M. F. Saenger1, M. Schädel3, T. Hofmann1, J. Hilfiker2, J. Sun2, T. Tiwald2, M. Schubert1 and J. A. Woollam1,2 1
Department of Electrical Engineering, and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, NE, 68588-0511, USA. 2 J. A. Woollam Co. Inc., Lincoln, NE, 68508-2243, USA. 3 Q-Cells AG, Thalheim, 06766, Germany. ABSTRACT
We present an infrared spectroscopic ellipsometry investigation of SixNy films deposited on textured Si substrates employed for photovoltaic cells. A multiple-sample data analysis scheme is used in order to determine the SixNy dielectric function and thickness parameters regardless of the surface morphology of the substrate. We observe changes in the dielectric function of the silicon nitride film which suggest variations in the chemical composition of the films depending on the substrate morphology. INTRODUCTION Surface texturing is a commonly used technique used to reduce reflection losses and thus increase the quantum efficiency of silicon solar cells. Typical texturing methods include mechanical processes or chemical reactions involving alkaline or acidic etchants producing different surface morphologies [1-4]. In order to further suppress reflection losses, an antireflecting (AR) thin film is commonly added to the textured surface, making AR films an important feature in a high efficiency solar cell design. SixNy is a common material used for the AR film on silicon solar cells due to its passivation, adhesive, and optical properties. The film thickness together with the optical constants determine how effectively the reflected light is suppressed for an optimal design wavelength, usually around 550 nm for photovoltaic applications. Due to the striven low reflectance and high scattering, the optical characterization of the thin film thickness and optical constants parameters renders as a difficult task, such that advanced techniques or measurement geometries are necessary to analyze them [5]. Previous investigations in the near-infrared to ultraviolet spectral range showed that the optical constants of the SixNy films on textured silicon wafers can be well approximated by an effective medium approximation (EMA) that combines the optical constants of a reference film with a void fraction which varies depending on the substrate morphology [5]. However, it is not clear if the changes have a pure physical origin or if they are also due to variations in the films chemistry. Investigations in the infrared (IR) spectral range provide an option to overcome the low reflection problem and also access to the film thickness parameter values. Furthermore, it is possible to study chemical variations in the film by analyzing the corresponding phonons or chemical bond signatures of silicon nitride detectable in the IR spectral range. The vibration frequencies for the N-Si3, Si-N, Si-O-Si and N-H modes are in the range between 300 and 1200 cm-1 and thus t
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