Multiplet Theory for Conduction Band Edge and O-Vacancy Defect States in SiO₂, Si₃N₄, and Si Oxynitride Alloy Thin Films

This article, the second of a two-part sequence, combines X-ray absorption spectroscopy (XAS) and many-electron theory to develop a model for intrinsic bonding defects in non-crystalline and nanocrystalline thin films used primarily as gate dielectrics for field effect transistors and related devices. In SiO₂ these defects are O-atom vacancies and their spectral features in O K pre-edge spectra are assigned to multiplet transitions based on a d² model for the electrons in a neutral O-vacancy site. This model is applied to (i) non-crystalline (nc-) SiO₂, and extended to (ii) Si oxynitride alloys, (nc-SiO₂)_1-x(Si₃N₄)_x, and nc-Hf Si oxynitrides. As in Part I, this relies on Tanabe–Sugano (TS) energy diagrams to identify spin-allowed X-ray transitions and negative ion states. Differences between band edge states in (i) nc-SiO₂, and (ii) nc-Si₃N₄ explain quantitative differences in trapping and trap-assisted tunneling. Preliminary results for nc-GeO₂ films prepared by remote-plasma-enhanced chemical vapor deposition shows these films exhibit surface stability similar to nc-SiO₂, suggesting that local nc-GeO₂ bonding involves the same 4-fold Ge, and 2-fold O coordination as Si and O in nc-SiO₂.