Electronic structure of hydrogenated amorphous Si_(1-x)N_x thin films using soft X-ray emission and absorption measurements

摘要

Hydrogenated amorphous silicon nitride thin films (a-Si_(1-x)N_x:H) with x = 0.46, 0.54, 0.55 and 0.58 have been studied. The optical bandgaps of these materials were 2.41, 2.78, 3.29 and 3.85 eV and Urbach edge bandgaps were 2.80, 3.27, 3.85 and 4.04 eV (W. C. Tan et al., J. Mater. Sci., Mater. Electron. 20, S15-S18 (2009) [1]), respectively. The bandgaps determined in this work using soft X-ray spectroscopy (SXS) are 3.92, 4.43, 5.18 and 5.36 ± 0.25 eV. These large bandgaps are most likely due to the surface sensitivity of the SXS measurements and possible oxidation of the films. The conduction and valence band edges are determined using a linear regression fit and the bandgaps show a similar trend to that of the optical bandgap. Using Density Functional Theory (DFT) calculations for the two crystalline phases of silicon nitride the core hole interaction is accounted for in these bandgap determinations. This shows that both N and Si states are involved in bandgap transitions. Comparison of amorphous SXS spectra with that of crystalline phases shows that the Si bonding environment is similar to that of the beta phase while the N bonding environment is similar to that of the gamma phase. This shows there is a degree of short range order in the films similar to that of α, β-Si_3N_4 indicating tetra-hedral SiN_4. The spectra also show that the Si states with 3s-symmetry decrease as a function of nitrogen concentration which is evidence of Si-Si bonds that would increase the Si 3s states. This increase in Si 3s-states effectively decreases the bandgap.