Natural and actual valence-band discontinuities in the a-Si/a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{C}}_{\mathrm{x}}$:H system: A photoemission study

We have measured core-level and valence-band spectra of a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{C}}_{\mathrm{x}}$ and a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{C}}_{\mathrm{x}}$:H (0≤x≤1), alloys of 2H-SiC, and of the a-Si/a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{C}}_{\mathrm{x}}$ and a-Si/a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{C}}_{\mathrm{x}}$:H interfaces. From these measurements we determine the valence-band offsets at the interface Δ$E_v$, the overlayer-induced band bending, and the ‘‘natural’’ band discontinuities Δ$E_v^0$. The latter quantity is defined as the difference in the valence-band maximum of a-Si and a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{C}}_{\mathrm{x}}$:H when both are referred to the chemically unshifted Si 2p core-level component. It is found that Δ$E_v^0$ agrees with Δ$E_v$ for x<0.5 both for the hydrogenated and for dehydrogenated (annealed at 650°) alloy interfaces. In this range Δ$E_v$ is rather insensitive to x and amounts to 0.8 (with H) and 0.3 eV (without H), respectively. Above x=0.5, Δ$E_v$ rises in both cases linearly with x and reaches values of 2.0 (with H) and 1.5 eV (without H), respectively. These values are smaller than expected for the ‘‘natural’’ offsets and the differences can be accounted for by invoking an interface dipole density of 3.4×${10}^{14}$ eÅ/${\mathrm{cm}}^2$ or a charge transfer of about 0.01 electrons per atom pair across the interface from silicon to the more electronegative ${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{C}}_{\mathrm{x}}$ or ${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{C}}_{\mathrm{x}}$:H.