Electronic structure of amorphousSiOx:H alloy films studied by x-ray emission spectroscopy: SiK, SiL, and OKemission bands

We present x-ray Si K, Si L, and O K emission bands of hydrogenated amorphous ${\mathrm{SiO}}_{\mathit{x}}$ alloy films covering the concentration range 0≤x≤2.2. All spectral features in the emission bands were identified and attributed to Si 3s, Si 3p, and O 2s, 2p derived states. With increasing x, the shape and the energy position of the main features of the Si K and Si L emission bands change significantly, particularly in the concentration interval 0.5<x<1.5. A comparison with available ultraviolet (UPS) and x-ray (XPS) photoemission spectra demonstrates that x-ray emission bands for a-${\mathrm{SiO}}_{\mathit{x}}$ are more sensitive to sample composition than UPS and/or XPS spectra. The random-bonding model and the random-mixture model of the structure of ${\mathrm{SiO}}_{\mathit{x}}$ alloys are discussed in view of the x-ray emission spectra. For x≊2, the Si K, Si L, and O K spectra are very similar to those observed for α-quartz (c-${\mathrm{SiO}}_2$). This justifies interpretation of all features of a-${\mathrm{SiO}}_2$:H on the basis of c-${\mathrm{SiO}}_2$. We therefore performed self-consistent pseudopotential calculations of the total density of states and the local partial density of states of Si 3s, Si 3p, Si 3d, O 2s, and O 2p of α-quartz. The calculations confirm that due to the high electronegativity of oxygen nonbonding, Si 3d-like states are created which, for high values of x, clearly show up in the Si L emission bands. Finally, we discuss the long-standing so-called d-orbital controversy about ${\mathrm{SiO}}_2$ and give an explanation of it.