Electronic structure of hydrogenated and unhydrogenated amorphousSiNx (0≤x≤1.6): A photoemission study

Abstract

We present a comprehensive core-level and valence-band photoemission study of hydrogenated and unhydrogenated amorphous silicon nitride ($a\ensuremath{-}\mathrm{Si}{\mathrm{N}}_{x}:\mathrm{H}$ and $a\ensuremath{-}\mathrm{Si}{\mathrm{N}}_{x}$). Position, width, and shape of the Si $2p$ line as a function of $x$ are interpreted in terms of a superposition of five chemically shifted components which correspond to the possible $\mathrm{Si}\ensuremath{-}{\mathrm{Si}}_{4\ensuremath{-}n}{\mathrm{N}}_{n}(n=0,\dots{},4)$ bonding configurations. The chemical shift per Si---N bond is between 0.62 ($x\ensuremath{\le}0.6$) and 0.78 eV ($x\ensuremath{\simeq}1.3$). From the intensities of the chemically shifted Si $2p$ components the number of Si---N bonds is calculated and compared with the total nitrogen concentration. Above $x\ensuremath{\simeq}0.8$ the average number of N---Si bonds per N starts to deviate from three. The addition of hydrogen increases this deviation because N---H bonds are favored over N---Si bonds. A band of N $2p$ lone-pair states is identified at the top of the valence bands in nearly stoichiometric $a$-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$. This band determines the position of the valenceband maximum (VBM) above $x=1.1$. Below $x=1.1$ Si---Si bonding states mark the VBM. The conduction-band minimum (CBM) is determined by Si-Si antibonding states up to $x=1.25$ and its position relative to the core levels is virtually unaffected by the presence of nitrogen or hydrogen. Above $x=1.25$, a transition to Si-N antibonding states occurs which is accompanied by a sharp recession of the CBM. The position of the Fermi energy within the gap is investigated as a function of $x$ and the hydrogen content. Si---H and N---H bonding states are identified at 6.3 and 9.8 eV below the VBM in nearly stoichiometric $a$-${\mathrm{Si}}_{3}$${\mathrm{N}}_{1}$: H. Si---Si bonding defect states lie 0.5 to 1.0 eV above the VBM and the corresponding antibonding states (3.0\ifmmode\pm\else\textpm\fi{}0.3) eV above the VBM. Plasmon energies vary between 17 eV in $a$-Si and 22 eV in $a$-${\mathrm{SiN}}_{1.5}$.