Optical characterization of hydrogenated silicon thin films using interference technique

This work introduces an application of an "interference spectroscopy techni que" (IST) for determination of absorption coefficient and refractive index spectra of amorphous silicon (a-Si:H) and related thin film materials. The technique is based on computer analysis of measurements of optical transmi ssion and specular reflection (T & R) of thin films (including the films on substrates) over a wide range of the incident photon energies (0.5-2.8 eV) using carefully controlled spectrometer conditions. IST is used to investi gate the absorption spectrum in the sub-gap energy range (0.8-1.6 eV) of in trinsic and phosphorous-doped a-Si:H, "polymorphous-Si:H," and microcrystal line silicon films. The enhanced sensitivity of the technique over conventi onal analysis of T & R data results from utilization of interference to obt ain absorption coefficient values at the maxima of transmission. The factor s limiting the accuracy of the calculated absorption coefficient are discus sed in detail. Measurement on films of thickness ranging from 0.1 to 5 mu m identifies that the sub-gap absorption in these films arises from the bulk rather than the surface. A set of samples prepared under widely different conditions that appear to have overlapping (alpha=20 cm(-1)) sub-gap absorp tion spectra measured using photo-thermal deflection spectroscopy (PDS), re veal significant differences (alpha=10 to 100 cm(-1)) using IST. Changes (f actor of 2) in sub-gap absorption spectra due to light soaking are also cle arly observable using IST. (C) 2000 American Institute of Physics. [S0021-8 979(00)04116-5].