Localized States at the Conduction-Band Edge of Amorphous Silicon Nitride Detected by Resonance Photoemission

The Si $L_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ absorption edge in amorphous silicon nitride ($a-\mathrm{Si}{\mathrm{Ni}}_x:\mathrm{H}$) exhibits a Fano-type antiresonance just below threshold for $x=1.20\mathrm{} \mathrm{and} 1.40$. At the same energy the valence-band photoemission cross section is resonantly enhanced in these samples for those features that carry an appreciable Si-$3s$-derived partial density of states. We argue that the Fano resonance involves Si-Si antibonding states at the bottom of the conduction bands that become localized as their number decreases with increasing nitrogen content compared to that of Si-N bonds.