Optical properties of UV laser photolytic deposition of hydrogenated amorphous silicon (a-Si:H)

Abstract

The effects of substrate temperature on the optical properties of the laser assisted chemical vapor deposited (L-CVD) thin films of amorphous-silicon–hydrogen alloy (a-Si:H) are investigated herein. A controlled substrate temperature Ts from 35 to 350 °C was used to vary the hydrogen content in the films. The spectrophotometric transmittance characteristics of the L-CVD a-Si:H films are used to evaluate the optical parameters, including: refractive index n(E); extinction coefficient k(E); and the thickness d of the samples. The L-CVD a-Si films are obtained for Ts >340 °C. The refractive index and energy gap Eg of the L-CVD a-Si films are found to be 3.4 and 1.55 eV, respectively. The optical absorption spectra α vs E of the L-CVD a-Si:H films exhibit Tauc, Urbach, and Urbach tail (shoulder) regions. The ratio of photoconductivity/dark conductivity (σP/σD) of L-CVD a-Si:H films having coplanar electrode geometry with Ag electrodes are experimentally determined. The σP/σD ratio, one rough measure of electronic quality, exhibits a maximum for the L-CVD a-Si:H films deposited at Ts =280 °C with σP/σD =5×105,σD =2.5×10−11 (Ω cm)−1, and the minimum defect density is 3.1×1016 cm−3.