Low frequency plasma deposition and characterization of Si1−xGex:H,F films

Si–Ge alloys have been widely studied because of their wide spread applications. RF discharge is a conventionally adopted technique for the deposition of Si–Ge films. Little is known about the deposition and properties of Si–Ge films in low frequency plasmas. The present work concerns an investigation of growth, structure and electronic properties of a-Si1−xGex films deposited by low frequency PECVD, where x is the Ge content in the gas phase, from SiH4 and GeF4 as source gases, and H2 and Ar for dilution. There is no deposition of the film without dilution, while in both cases of H- and Ar-dilution, the growth is observed over the entire x range. However, the deposition rate depends on x and decreases faster in the case of Ar dilution. Both, SIMS and Raman spectra data, demonstrate that incorporation of Ge in the solid phase reaches saturation at x≈0.5. In Raman spectra, the scattering related to Si crystalline phase (line at k=520 cm−1) increases and new lines in vicinity of k=480 cm−1 appear with x. It suggests a change in the local structure around Si atoms. The electrical properties were studied by measurements of the temperature dependence of the conductivity. The reduction of the activation energy from 0.86 to 0.39 eV and the increase of room temperature conductivity from 1 × 10−9 to 2.1 × 10−4 Ω−1 cm−1 were observed with the changes of x from 0 (Si) to 1(Ge). No correlation between pre-exponent factor of conductivity σ0 and activation energy was observed in the samples studied in contrast with a-Si–Ge films deposited from mixtures with GeH4.