X-ray induced photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and electron microprobe analysis (EMA) were used to study semi-insulating polycrystalline silicon layers obtained by chemical vapor deposition fromSiH4andN2Ogases. A mean “bulk” oxygen concentration determined by EMA ranged from 16 to 50 at. &percent;. Photoelectron spectra excited by Mg K&agr;and Al K&agr;radiation were used to find surface composition and bonding information of as-received layers. Beneath the native oxide with a stoichiometry close to theSiO2,there is a heterogeneous material consisting of silicon (Si–Si) and silicon oxide (Si–O) regions. A drop in the Si2ppeak position differences with the mean bulk oxygen concentration indicates a differential charging of the silicon islands surrounded by a silicon oxide phase. A spectral band of an asymmetric Si–O–Si stretching vibration mode around1030 cm−1was used to characterize the samples. An assumption that the samples were a homogeneousa-SiOxphase leads to a mean oxygen content much higher than that determined by the EMA technique. On the other hand, oxygen concentration deduced from the number of (Si–O) bonds, calculated by the normalized integrated absorption intensity, were very close to the results of EMA. This leads us to the conclusion that the samples consist of two phases,a-SiOrand Si. The values of the refractive index obtained by the spectrophotometric method from reflectance spectra and the results of XPS measurements strongly support the two-phase model. ©1997 American Institute of Physics.