A comparative study of boron and phosphorus doping effects in SiC : H films prepared by electron cyclotron resonance plasma CVD

Abstract

Hydrogenated silicon carbide films (SiC : H) were deposited using the electron cyclotron resonance chemical vapour deposition (ECR-CVD) technique from a mixture of methane, silane and hydrogen, using diborane and phosphine as doping gases. The effects of changes in the microwave power on the deposition rate and optical bandgap were investigated, and variations in the photo- and dark conductivities and activation energy were studied in conjunction with film analysis, using the Raman scattering technique. In the case of boron-doped samples, the conductivity increased rapidly to a maximum, followed by rapid reduction at high microwave powers. The ratio of photo- to dark conductivity (σ_ph/σ_d) peaked at a microwave power of ~ 600 W. Under conditions of high microwave power, Raman scattering analysis showed evidence of the formation and increase in the silicon microcrystalline and diamond-like phases in the films, the former of which could account for the rapid increase and the latter the subsequent decrease in the conductivity. In the case of phosphorus-doped SiC : H samples, it was found that an increase in the microwave power has the effect of enhancing the formation of the silicon microcrystalline phase in the films; this occurred in correspondence to a rapid increase in the conductivity and a reduction in the activation energy. The conductivity increase stabilized in samples deposited at microwave powers exceeding 500 W, probably as a result of dopant saturation. Results from Raman scattering measurements also showed that phosphorus doping has the effect of enhancing the formation of the silicon microcrystals in the film whereas the presence of boron has the effect of preserving the amorphous structure.