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Subgap absorption spectroscopy in microcrystalline silicon thin films
Intrinsic hydrogenated microcrystalline silicon thin films prepared by VHF-PECVD and HW-CVD methods under different deposition conditions have been investigated using steady state photoconductivity method (SSPC), photothermal deflection spectroscopy (PDS) and dual beam photoconductivity (DBP) method, and transmission spectroscopy. Absorption spectra of the investigated thin films were measured in a wide energy region using PDS and DBP. A procedure, for the firs time, was used to calculate fringe free absolute absorption coefficient of thin films from DBP yield spectrum and simultaneously measured transmission signal. The results were found to be in agreement with those of PDS above the bandgap energy. However, there are differences between below the bandgap energy in the spectra of both methods. The differences are discussed to be consistent with the underlying physics of these methods. For some of investigated thin films there are remaining fringes in the .(h.) spectra measured using both methods. This is a strong indication of inhomogeneity present in the films in growth direction. DBP measurements were also performed for ac monochromatic light incident from substrate side in order to investigate the effect of inhomogeneous microstructure of the material on the absorption spectrum. It is found that some films have remaining fringes on their spectra for back ac measurements both for VHF-PECVD and HW-CVD grown thin films, whereas there is no remaining fringes observed for front ac measurements or vice versa. These findings are discussed to be an indication of inhomogeneity in growth direction which is already reported from TEM and Raman study. Sub-bandgap absorption coefficients .(0.8 eV) were correlated with the silane concentration, which is main parameter to change the microstructure of these films. It is found that the thin films that deposited in the transition region, where a transition from a fully amorphous growth to full microcrystalline growth occurs, have smaller absorption coefficients indicating that the thin films deposited at transition region have less defect density. However, thin films deposited at the highly crystalline region have the highest defect density due to etching effect of H during the deposition. These results are also consistent with reported ESR studies.