Influence of hydrogen on vibrational and optical properties of a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{H}}_{\mathrm{x}}$ alloys

The effect of hydrogen on the optical gap and network short-range order has been obtained in a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{H}}_{\mathrm{x}}$ alloys by optical-absorption and Raman-scattering measurements. Distinct differences are observed in the role of hydrogen for homogeneous chemical-vapor-deposited (HMCVD) and rf-diode-sputtered (RFS) films. A comparison of the correlation between the Raman TO peak width and the optical gap for films with and without H allows a separation of the roles of H-induced bond-angle ordering and alloy energy-band changes. While the primary effect of H in the HMCVD films is alloy band changes for x>0.05, RFS films indicate comparable structural ordering effects. The effect of hydrogen on the optical gap is found to be ∼3–4 times greater than that in a-${\mathrm{Ge}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{H}}_{\mathrm{x}}$ alloys. In addition to the ordering of bond angles about Si atoms, the Raman spectra indicate changes in the TA-TO and LA-TO ratios due to hydrogen incorporation. These are interpreted in terms of the relative reductions in the number of bond-bending modes as the concentration of triads of Si atoms decreases with hydrogen.