Atomic-layer-resolved bandgap structure of an ultrathin oxynitride-silicon film epitaxially grown on6H-SiC(0001)

Electronic structures of a silicon-oxynitride (SiON) layer ($\ensuremath{\sim}0.6\text{ }\text{nm}$ in thickness) epitaxially grown on $6H\text{-SiC}(0001)$ 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 $6.3\ifmmode\pm\else\textpm\fi{}0.6\text{ }\text{eV}$ at the nitride layer and $8.3\ifmmode\pm\else\textpm\fi{}0.8\text{ }\text{eV}$ at the oxide layer. The nitride and oxide layers have almost the same energy of conduction-band minimum (CBM) being $\ensuremath{\sim}3\text{ }\text{eV}$ 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.