Atomic-layer-resolved bandgap structure of an ultrathin oxynitride-silicon film epitaxially grown on 6H-SiC(0001)

Electronic structures of a silicon-oxynitride (SiON) layer (similar to 0.6 nm in thickness) epitaxially grown on 6H-SiC(0001) were investigated on atomic-layer scale using soft x-ray absorption spectroscopy and x-ray emission spectroscopy (XAS and XES) and first-principles calculations. The SiON layer has a hetero-double-layered structure: an interfacial silicon nitride layer and a silicon oxide overlayer. The element-specific XAS and XES measurements revealed layer-resolved energy-band profiles. Measured gap sizes are 6.3 +/- 0.6 eV at the nitride layer and 8.3 +/- 0.8 eV at the oxide layer. The nitride and oxide layers have almost the same energy of conduction-band minimum (CBM) being similar to 3 eV higher than CBM of the SiC substrate. The energy-band profiles of the SiON layer are qualitatively reproduced by the calculations. The calculations show that broadening of bandgap of the substrate occurs only at an interfacial SiC bilayer.