Anisotropy in Hydrogenated Amorphous Silicon Films as Observed Using Polarized FTIR-ATR Spectroscopy

We used polarized attenuated total reflection (ATR) measurements together with Fourier transform infrared (FTIR) spectroscopy to investigate the vibrational spectra of hydrogenated amorphous silicon (a-SiH_x) films 0.5-1.0 microns in thickness. We deposited the films using hot-wire or plasma-enhanced chemical vapor deposition methods (HWCVD or PECVD, respectively) on crystalline silicon and cadmium telluride substrates. Our ATR technique gave a spectral range from 2100-400 cm^-1, although the Si-H wagging mode absorption band at 640 cm^-1 was somewhat distorted in the a-SiH_x/Si samples by impurity and lattice absorption in the silicon ATR substrates. We report the identification of a Si-O-C impurity band with maximum intensity at 1240-1230 cm^-1. The assignment of this band to a Si-O-C vibration is supported by secondary-ion mass spectrometry (SIMS) measurements. Our polarized FTIR-ARR spectra of HWCVD and PECVD a-SiH_x films on <111] Si ATR substrates show that the impurity dipoles are oriented strongly parallel to the film growth direction. The wagging mode absorbance band is more intense in the film plane. This trend is less pronounced for the Si-H stretching vibrations. These observations are consistent with some degree of anisotropy or medium-range order in the films. The anisotropy in the Si-H bands may be related to residual stress in the films. Our scanning electron microscopy (SEM) analyses of the samples offer additional evidence of bulk structural anisotropy in the a-SiH_x/Si films. However, the Si-O-C impurity band was not observed in the polarized ATR-FTIR spectra of the a-SiH_x/CdTe samples, thus indicating that the Si substrates influence formation of the impurity in the a-SiH_x/Si films.