Voshadhi Amarasinghe

Oxidation induced stress in SiO2/SiC structures

Physical stress in SiO2/SiC stacks formed by the thermal oxidation of SiC is studied experimentally through both room temperature ex-situ and variable temperature (25–1150 °C) in-situ investigations. Mechanisms giving rise to the stress are a thermal component, associated with differences in thermal expansion coefficients of the oxide and the substrate, and an intrinsic component associated with the different atomic densities and structure of the film and substrate. Ex-situ results show a ∼108 Pa compressive stress in the SiO2 film in a SiO2/SiC stack with a strong crystal face dependence (C face(000ī) and Si face (0001)) and processing (temperature, growth rate) dependence. Real-time stress determination demonstrates that at temperatures above ∼900 °C, the total intrinsic stress and a portion of the thermal stress may be relieved. On the basis of these findings, a viscous model is proposed to discuss the stress relaxation.

Excitation of long-wavelength surface optical vibrational modes in films, cubes and film/cube composite system using an atom-sized electron beam

Using spatially resolved Electron Energy-Loss Spectroscopy, we investigate the excitation of long-wavelength surface optical vibrational modes in elementary types of nanostructures: an amorphous SiO2 slab, an MgO cube, and in the composite cube/slab system. We find rich sets of optical vibrational modes strongly constrained by the nanoscale size and geometry. For slabs, we find two surface resonances resulting from the excitation of surface phonon polariton modes. For cubes, we obtain three main highly localized corner, edge, and face resonances. The response of those surface phonon resonances can be described in terms of eigenmodes of the cube and we show that the corresponding mode pattern is recovered in the spatially resolved EELS maps. For the composite cube/substrate system we find that interactions between the two basic structures are weak, producing minor spectral shifts and intensity variations (transparency behaviour), particularly for the MgO-derived modes.

Effect of nitrogen passivation on interface composition and physical stress in SiO2/SiC(4H) structures

The electron density and physical stress at the thermally oxidized SiC/SiO2 interface, and their change with nitrogen incorporation, were observed using x-ray reflectivity, Raman scattering, and in-situ stress measurement. There is no evidence for residual carbon species at the SiO2/SiC. Instead, a ∼1 nm thick low electron density layer is formed at this interface, consistent with interfacial suboxides (SiOx, 0.3 < x < 2), along with high interfacial stress. Nitrogen passivation, a known process to improve the interface state density and electronic properties, eliminates the low density component and simultaneously releases the interface stress. On the basis of these findings, a chemical interaction model is proposed to explain the effect of the nitrogen in terms of both stress reduction and elemental control of the dielectric/SiC interface, resulting in a higher quality gate stack on SiC.