Wide-gap hydrogenated amorphous silicon carbide and silicon for solar cell applications

The deposition, basic material properties, opto-electronic characteristics, metastable behaviour, and Schottky barrier performance of wide gap amorphous silicon carbide and silicon have all been studied to yield a better understanding for their solar cell applications. 19:1 H2:(SiH4 + CH 4) was a typical gas dilution used for depositing the a-SiC:H films. Hydrogen content was found to have the primary role in determining the optical gaps of these materials. Increasing the plasma electron energy during deposition decreased the carbon content of the thin film materials. From FTIR, light-soaking, and subgap photoconductivity studies there was indication that dihydride has influence in the mid-gap states, and monohydride influences the valence band tail. There was also a strong dependence of valence band tail slope in the annealed and light soaked states on the total hydrogen content, which may support the weak bond breaking model for the Staebler-Wronski effect. Wide gap a-Si:H showed slightly better quality characteristics than a-SiC:H material of comparable optical gap. In Schottky barrier structures the short wavelength quantum efficiency was improved by over 30% by leaving the native oxide between the metal Schottky barrier contact and the semiconductor. Both the mutau product and hole mobility were found to be poor for materials having optical gaps >1.9 eV. This may be one reason that the value obtained for the mobility gap in this wide gap material is closer to the value of the optical gap than it is in standard device quality a-Si:H.