Raman spectroscopy on thin film silicon on non-transparent substrates and in solar cell devices

The properties of thin-film silicon strongly depend on the deposition method, conditions and the substrate material. The analysis of the microstructure of thin silicon films requires diagnostic tools which are independent on substrate or device concept. In this contribution Raman spectroscopy as a powerful tool for analyzing the microstructure and material phase of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μSi:H) is discussed. To improve the sensitivity for the bulk properties, a HeNe laser (λ = 633 nm) source is used, due to its longer absorption path length at this wavelength. Partial transmission of the light through the film onto the substrate results in a measured Raman spectrum consisting of the superposition of the spectra of film and substrate. Two methods using different approaches in thin film optics for distinguishing the substrate- and film spectrum from the measurement are discussed. One method to model transmission of the Raman spectrum originating from the substrate is based on thin film interference, adapted to a partial coherent quasi monochromatic light source.[1] This method is applicable in the limiting case of thin a-Si:H films (film thickness: d<;400 nm). For larger thicknesses we demonstrate that absorption is no longer negligible and the transmission needs to be calculated using the transfer-matrix method. The Si-H stretching modes (~1900-2150 cm^-1) provide detailed information on the microstructure of thin films and are generally studied by infrared absorption, demanding the film being deposited on an infrared transparent substrate.[2] We demonstrate an approach in which the Raman spectrum of the substrate is removed from the measured spectrum, resulting in a technique capable of comparing crystalline fraction or microstructure of absorber layers, even in p-i-n solar cell devices.