Study of the annealing effect in optical properties for phosphorus-doped a-Si x C1− x :H films deposited by PECVD

Abstract The optical properties of phosphorus-doped hydrogenated amorphous silicon carbide (P-doped a-Si x C 1− x :H) thin films are studied. Films were deposited by plasma-enhanced chemical vapor deposition at 110 kHz of frequency and pressure of 1.5 Torr, with silane (SiH 4 ) and methane (CH 4 ) as precursor gases. Hydrogen (H 2 ) and phosphine (PH 3 ) were used as diluent and dopant gases, respectively. The impact of the gases flow rate and thermal annealing on the optical properties is evaluated. A concordance is observed between the atomic content of Si, C, and P in films, determined by Energy Dispersive X-ray Spectroscopy, as gases flow rate increases. The composition of the films is established by identifying the vibrational bonding modes using Fourier transform infrared spectroscopy measurements. Si−H, Si−C, and C−H bonds were identified and their density was obtained. Characterization with UV–Vis spectroscopy shows that the optical band gap ( E gopt ), oscillation energy ( E o ), Urbach energy ( E U ), and iso-absorption energy ( E 04 ) increase with the doping level but decrease with the annealing treatment. However, values of dispersion energy ( E d ), dielectric constant ( ϵ ) and the refractive index ( n ) generally become smaller as the phosphorus and carbon content grows, but with a tendency to fall during the annealing. The refractive index by UV–Vis is compared with measurements made with ellipsometry, resulting in a similarity between both techniques. High values of E gopt obtained in P-doped a-Si x C 1− x :H films can be related to better values of the temperature coefficient of resistance for flow sensor applications.