Optical and transport studies on thin microcrystalline silicon films prepared by very high frequency glow discharge for solar cell applications

The initial growth stage of phosphorus doped microcrystalline silicon films prepared by plasma enhanced chemical vapor deposition with different plasma excitation frequencies in the range 13.56–116 MHz was studied by Raman and infrared spectroscopy, optical transmission and reflection, and conductivity measurements. The sensitivity of Raman spectroscopy and optical reflection on Si crystallites in the initial growth regime is compared and optical reflection at 4.5 eV is proposed as an easy and reliable tool for this investigation. While the crystallite formation on amorphous silicon substrates at 13.56 MHz is delayed in comparison with glass, SiO2 and chromium substrates, nucleation of the crystalline phase on amorphous silicon is found to be greatly enhanced at higher plasma excitation frequencies. On the other hand, for deposition on glass, SiO2, and chromium at frequencies equal to or higher than 70 MHz, increased porosity is found in the initial growth region. The results are interpreted within a model that suggests a conelike initial formation of the silicon crystallites and a higher etching rate of disordered material at high plasma excitation frequencies. In addition, the extension of the process of crystallite formation from the film-plasma interface into a growth zone more than 10 nm deep is proposed. The application of the microcrystalline silicon layers prepared at high plasma excitation frequency is demonstrated in amorphous silicon based tandem solar cells.