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 T_s 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 T_s >340 °C. The refractive index and energy gap E_g 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 (σ_P/σ_D) of L‐CVD a‐Si:H films having coplanar electrode geometry with A_g electrodes are experimentally determined. The σ_P/σ_D ratio, one rough measure of electronic quality, exhibits a maximum for the L‐CVD a‐Si:H films deposited at T_s =280 °C with σ_P/σ_D =5×10⁵,σ_D =2.5×10⁻¹¹ (Ω cm)⁻¹, and the minimum defect density is 3.1×10¹⁶ cm⁻³.