Optical properties of phosphorus-doped polycrystalline silicon layers

The index of refraction and the absorption coefficient of low-pressure chemical vapor deposited polycrystalline silicon (poly Si) implanted with various doses of phosphorus were obtained by means of transmittance and reflectance measurements. The refractive index of poly Si is relatively insensitive to doping in the visible region of the spectrum and agrees quite well with published values for single-crystal Si. The absorption coefficient of these films decreases with doping in the visible. Nevertheless, the absorption coefficient corresponding to the highest-doped film (N≊3×1020/cm3) is almost twice the published values for lightly doped single-crystal Si. In the infrared, the index of refraction decreases systematically and the absorption coefficient increases systematically with doping. This dependence of the optical parameters on doping is attributed to the presence of free carriers. A free-carrier dispersion/absorption model is applied to the measured refractive index in the region 1.2–1.8 μm and to the reflectance measurements in the region 2.5–7.5 μm. The model gives carrier concentrations for the two types of measurements which differ by 50%. Possible explanations for this difference are discussed. The carrier concentrations and mobilities obtained from optical and Hall-effect measurements are compared.