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

We present a comprehensive core-level and valence-band photoemission study of hydrogenated and unhydrogenated amorphous silicon nitride ($a-\mathrm{Si}{\mathrm{N}}_x:\mathrm{H}$ and $a-\mathrm{Si}{\mathrm{N}}_x$). 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 $\mathrm{Si}-{\mathrm{Si}}_{4-n}{\mathrm{N}}_n(n=0,\dots ,4)$ bonding configurations. The chemical shift per Si—N bond is between 0.62 ($x\le 0.6$) and 0.78 eV ($x\simeq 1.3$). 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 $x\simeq 0.8$ 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 $x=1.1\mathrm{}$. Below $x=1.1\mathrm{}$ Si—Si bonding states mark the VBM. The conduction-band minimum (CBM) is determined by Si-Si antibonding states up to $x=1.25\mathrm{}$ and its position relative to the core levels is virtually unaffected by the presence of nitrogen or hydrogen. Above $x=1.25\mathrm{}$, 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±0.3) eV above the VBM. Plasmon energies vary between 17 eV in $a$-Si and 22 eV in $a$-${\mathrm{SiN}}_{1.5}$.