Non-Stoichiometry and the Electronic Structure of Amorphous Silicon Nitride

Abstract

AbstractWe have measured core-, valence-, and conduction band densities of states of amorphous hydrogenated (a-SiNx:H) and unhydrogenated (a-SiNx) silicon nitride with x varying between 0 and 2. From an analysis of the Si 2p core level spectra in terms of five chemically shifted components the number of Si-N bonds is calculated and compared to the total nitrogen concentration. Above x ≈ 0.8 the average silicon coordination of nitrogen starts to deviate from three. The addition of hydrogen increases this deviation because N-H bonds are favored over N-Si bonds accounting thus for the excess nitrogen concentration (x ≥ 1.33) found in hydrogenated samples. A band of N2p lone pair states is identified at the top of the valence bands in stoichiometric Si3N4. This band determines the character and position of the valence band 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 percolation-like transition to Si-N antibonding states occurs which is accompanied by a sharp recession of the CBM. The position of the Fermi level within the gap is investigated as a function of x and 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-Si3N4. 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.