Decomposition of Mixed Phase Silicon Raman Spectra
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Decomposition of Mixed Phase Silicon Raman Spectra M. Ledinský, J. Stuchlík, A. Vetushka, A. Fejfar and J. Kočka Institute of Physics, Academy of Sciences of the Czech Republic Cukrovarnická 10, 162 53 Prague 6, Czech Republic ABSTRACT Series of Raman spectra were measured for microcrystalline silicon thin film with variable crystallinity. Five sets of Raman spectra (corresponding to excitations at 325 nm, 442 nm, 514.5 nm, 632.8 nm and 785 nm wavelengths) were subjected to factor analysis which showed that each set of spectra consisted of just two independent spectral components. Decomposition of the measured Raman spectra into the amorphous and the microcrystalline components is illustrated for 514.5 nm and 632.8 nm excitations. Effect of the light scattering on absolute intensity of Raman spectra was identified even for excitation wavelength highly absorbed in the mixed phase silicon layers. INTRODUCTION Crystalline volume fraction is an important structural property of thin silicon films for solar cells, which influences absorption in the active layer, open circuit voltage VOC and therefore the final efficiency of the photovoltaic cells. Crystallinity is usually determined from the Raman scattering spectra which consist of a broad amorphous band centered at 480 cm-1 and a sharp microcrystalline peak situated around 520 cm-1. Evaluation of crystallinity [1] requires a procedure for decomposition of the spectra into the amorphous and microcrystalline components. In the article [2] we used factor analysis to demonstrate the presence of just two spectrally independent components in mixed phase silicon Raman scattering spectra for excitations 325 nm, 514.5 nm, 632.8 nm and 785 nm. We have also shown that the most frequently used decomposition procedure of Raman spectra into three Gaussian bands has no physical foundation and may lead to inaccurate results. In agreement with [3] we have concluded that the mixed phase silicon Raman spectra have to be decomposed directly into contributions of completely amorphous and fully microcrystalline phases. In this paper we extend the number of analyzed excitation sources by HeCd 442 nm laser and we describe in detail the procedure of decomposition of the measured Raman spectra based on factor analysis. EXPERIMENT All Raman spectra used in this work were measured on a 1 µm thick microcrystalline layer prepared by plasma enhanced chemical vapor deposition. A 4 mm wide permanent magnet placed under the substrate [4, 5] locally changed the nearby plasma and thus influenced the nucleation density and crystallinity of the layer. Raman scattering spectra were measured at 13
predefined positions where the film structure changed from amorphous to microcrystalline. The spectra were recorded using excitation wavelengths 325 nm, 442 nm, 514.5 nm, 632.8 nm and 785 nm. Detailed description of the film deposition conditions and Raman spectra measurements are given in [4, 5].
RESULTS and DISCUSSION Figure 1. shows a representative set of Raman spectra measured on the sample with
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