Abstract

The application of optical second-harmonic (SH) spectroscopy to directly identify the physical origin and nature of various electronic transitions at Si(111)/SiO₂ and Si(111)/Si₃N₄ interfaces is presented. Evidence for bandgap narrowing at the Γ point of the Si(111) interface Brillouin zone (BZ) is given. The occurrence of two SH resonances shifted to lower energies compared with the lowest direct bandgap transitions in silicon indicates an expansion of Si-Si bonds oriented vertically and laterally to the interface normal. In addition, the nonlinear optical susceptibility of the interlayer includes the effect of "mild disorder" at the interface. The term "mild disorder" implies that the bonding between Si atoms is maintained but that the individual bonds are bent and stretched in different ways, thereby causing the perturbation of states out of the Si bulk bands into the gap. The reduction of these gap states after rapid thermal annealing of thermally grown Si(111)/SiO₂ interfaces is demonstrated. In our experiments we used Si(lll) wafers cut with a small offset angle (≃4.5°) towards the [112] direction to get a regular step structure. On these wafers two different interfaces were grown: i) thermal oxides (thickness ≃60 nm) were grown at 850°C in a dry oxygen ambient; ii) nitride films (thickness ≃30 nm) were deposited via a remote plasma enhanced chemical vapour deposition.

© 1995 IEEE