Raman Scattering Analysis of Structural Transformations Due Precision Engineered Si, 6H-SiC and beta-Si3N4

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Title: Raman Scattering Analysis of Structural Transformations Due Precision Engineered Si, 6H-SiC and beta-Si3N4
Author: Walter, Jennifer Jean Huening
Advisors: R.J. Nemanich, Committee Chair
H. D. Hallen, Committee Member
L. Bergman, Committee Member
R. O. Scattergood, Committee Member
Abstract: This thesis explores the local residual structural transformations that result from indentation and ductile material removal of machined Si, 6H-SiC, and β-Si₃N₄. During the machining process, very high pressures can be achieved at the contact interface between the machining tool and the surface of a material. The induced pressures can cause transformations through a series of phases, and upon releasing the pressure, the material can again be transformed to a new phase. Many of the high-pressure phases of Si, and some of the phases of SiC and β-Si₃N₄ have been defined experimentally for hydrostatic pressure conditions in diamond anvil cell experiments, and have also been predicted theoretically. However, the complexity of plastic deformation complicates the transition process from non-hydrostatic pressure causes the response of the material to be difficult to define for precision engineered and machined surfaces. In these cases, the transmission of applied force can be dependent on nonhydrostatic structural changes due to plastic flow, and structural transitions can be induced at lower pressures than would occur, if at all, for hydrostatic pressures. The Raman technique is employed as a nondestructive structure sensitive probe to investigate the structural and vibrational properties of indented and machined Si, 6H-SiC, β-Si₃N₄. Visible Raman scattering of Si indentations indicate that localized regions are transformed into metastable phases. On machined Si surfaces, a layer of amorphous material is observed, and the depth of this layer has been found to be dependent on the machining conditions. For the wide bandgap materials, 6H-SiC and β-Si₃N₄, the short absorption depth of UV light allows for accurate probing of the surface, and the transparency to visible light allows analysis of the bulk material. The studies on indentations made on the 6H-SiC (0001) surface indicate biaxial stress from point pressure. Machined 6H-SiC basal plane edges of (0001) wafers indicate that the near surface structure is changed from single crystal to polycrystalline. Studies conducted on nm and μm precision-machined β-Si₃N₄ reveals transformation of a surface layer into an amorphous phase. Probing the depth dependence through wavelength selection provides information on the depth of the amorphous zone. In addition, the chips that are produced during machining are found to transform to an amorphous phase.
Date: 2004-12-01
Degree: MS
Discipline: Physics
URI: http://www.lib.ncsu.edu/resolver/1840.16/2806

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