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 C1−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 (SiH4) and methane (CH4) as precursor gases. Hydrogen (H2) and phosphine (PH3) 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 (Eo ), Urbach energy (EU ), and iso-absorption energy (E 04) increase with the doping level but decrease with the annealing treatment. However, values of dispersion energy (Ed ), 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 C1−x :H films can be related to better values of the temperature coefficient of resistance for flow sensor applications.