(The) electrical transport and optical absorption of amorphous Si-Ge alloy film

Abstract

The optical absorption and electrical transport properties of amorphous $Si_xGe_{1-x}$ alloy films prepared by vacuum evaporation were investigated. The substrate temperatures were between 300$^\circ$C and 350$^\circ$C, and the deposition rates of less than 2$\mbox{\AA}$/sec were employed during deposition. Hydrogenation was carried out by diffusing atomic hydrogens produced by D. C. glow discharge decomposition of hydrogen gas into the sample. The electrical conductivitites of a-$Si_xGe_{1-x}$ alloy films showed the remarkable change with X. The room temperature conductivities were obserced to be in the range between $4\times10^{-7}(ohm\,cm)^{-1}$ and $4(ohm\,cm)^{-1}$ with X. The room temperature conductivity of the hydrogenated sample was one tenth of that of non-hydrogenated one deposited under the same conditions. The a-$Si_xGe_{1-x}$ alloy films were found to have a large concentration of localized states in the pseudogap. At low temperatures, phonon-assisted hopping and variable range hopping mechanisms were observed. The activation energy values of a-$Si_xGe_{1-x}$ alloy films for x=0.9 and x=0.95 were 0.451eV and 0.793eV, respectively. The optical absorption edge shifted with the variation of alloy composition between Ge and Si, resulting in the change in the optical band gaps from 0.9eV to 1.6eV.