Determining the material structure of microcrystalline silicon from Raman spectra

Abstract

An easy and reliable method to extract the crystalline fractions in microcrystalline films is proposed. The method is shown to overcome, in a natural way, the inconsistencies that arise from the regular peak fitting routines. We subtract a scaled Raman spectrum that was obtained from an amorphous silicon film from the Raman spectrum of the microcrystalline silicon film. This subtraction leaves us with the Raman spectrum of the crystalline part of the microcrystalline film and the crystalline fraction can be determined. We apply this method to a series of samples covering the transition regime from amorphous to microcrystalline silicon. The crystalline fractions show good agreement with x-ray diffraction (XRD) results, in contrast to crystalline fractions obtained by the fitting of Gaussian line profiles applied to the same Raman spectra. The spectral line shape of the crystalline contribution to the Raman spectrum shows a clear asymmetry, an observation in agreement with model calculations reported previously. The varying width of this asymmetrical peak is shown to correlate with the mean crystallite size as determined from XRD spectra.

Polarized Raman Spectroscopy and Chemometric Analysis of Micro-crystalline Silicon for Solar Cells

“…As these peaks result from the crystalline phase, they are most pronounced when the sample has a large crystalline component. The second order peaks overlap the regions assigned purely to the amorphous spectra and used to calculate the scaling factor in [3]. Therefore the correct scaling factor cannot be found using the least fit analysis described in [3] due to this overlap and leading to errors as shown in Fig.…”

“…Two peaks fitting assigns one Gaussian peak to the amorphous component and a second Lorentzian peak to the crystalline component of the μc-Si:H spectrum. Furthermore, the amorphous component is not represented by a single Gaussian function but rather a combination of optical and acoustic phonon bands [3], therefore the Gaussian function does not adequately fit the spectra without significant error. Adding a further peak, intermediate between the amorphous peak and the crystalline peak as shown in Fig.…”

“…where I a , I m and I c are the amorphous, intermediate and crystalline Raman integrated intensities, and γ is a wavelength dependent scaling factor taken to be on average 0.88 [3]. It is extremely difficult to reliably and repeatably fit the amorphous and intermediate peaks.…”

“…Smit et al [3] proposed an alternative technique in which the shape of the amorphous component was considered constant across samples of varying crystallinity and the change in the spectra was primarily due to changes in the crystalline peak. All peaks surrounding the first order scattering between approximately 200 and 440 cm -1 are assigned to the amorphous spectrum.…”

“…I a is the TO component of the amorphous spectrum, the TO component can be found approximately by fitting with three Gaussian curves (see [3] for details). This was significantly more reliable than the two and three peaks fitting approaches.…”