Doping dependence of optical properties of solid-phase crystallized evaporated poly-Si thin films on glass

Boron and phosphorus doped polycrystalline silicon films with moderate and heavy doping of ~5times10¹⁷cm^-3 and ~1times10¹⁹ cm^-3 respectively are investigated to observe the impacts of doping on the optical properties, with "intrinsic" evaporated poly-Si films for comparison. The films are prepared by solid-phase crystallization (SPC) of evaporated amorphous silicon films on borosilicate glass. Tauc-Lorentz models with one or two oscillators are applied for the fitting of both reflection and transmission data collected by a spectrophotometer. Ultraviolet (UV) reflectance shows that the crystal quality of the films is improved by phosphorous doping, while boron has a negligible impact on the crystal quality. The refractive indices of "intrinsic\´ and moderately doped poly-Si films are similar to the values of crystalline silicon (c-Si), while heavy doping causes a decrease of the refractive indices for wavelengths longer than 1000 nm. The doped poly-Si films exhibit greater absorption than c-Si for visible wavelengths. This absorption is attributed to defects in the films. This "defect absorption" is greatest for the "intrinsic" and boron doped films and least for the phosphorus doped films. There is a strong correlation between the crystal quality of the films determined by UV reflectance and the enhanced absorption at visible wavelengths.