Physics & Astronomy ETDs


An-Shyang Chu

Publication Date



The fabrication and optical analysis of well-defined Si quantum walls (periodic slabs) are presented. Using a unique combination of interference lithography and wet-­chemical KOH etching, vertically-standing, periodic Si rectangular slabs are fabricated with a slab thickness ranging from a few nm to ~ 200 nm. Each slab has its three principal axes in the <110>, <112> and <111> direction, and each slab surfaces is extended in the <110>-<112> plane and normal to the <111> direction. These samples are used to study the effects of the size confinement on the crystal properties of Si. Raman scattering experiment is carried out because the scattering of the phonons is highly dependent on the details of the crystal properties, and provides us a sensitive tool for detecting changes in the structures (i.e. size confinement effects). This is the first time that such a long range (in size) systematic optical study is carried out using such well-defined structures.

The observed first order Raman data can be classified into four very distinctive regimes depending on the thickness (w) of the slab: a standard Lorentzian lineshape (w > 250 nm), an enhanced scattering intensity (20 nm < w < 250 nm), an asymmetric lineshape (10 nm < w < 20 nm) and a splitting of the degenerate phonon modes ( w < 10 nm). Using a momentum relaxation model, the asymmetric lineshape is identified as the relaxation in the ∆k=O selection rule due to the size confinement. Consequently, the scattered phonons have an energy spread and an asymmetric spectrum. The splitting of the degenerate r25 optical phonons in Si is attributed to the distortion of the crystal lattice due to the structure relaxation in a nano-structure. Using a lattice dynamics model and a valence-force model, the splitting of the phonon modes is verified.

Using an exact Model analysis, the field intensity inside the grating is calculated for our specific structures to verify the observed Raman scattering enhancement. Qualitatively, the analysis shows some very important physics in the same size range as the enhancement data,. For the first time to our knowledge, the existence of anti-guided modes that can be supported by a dielectiric grating has been clearly identified. These anti-guided modes propagate with low-loss in the grating region. The resonant coupling of the incident field with these modes alters the electromagnetic responses of a dielectric grating greatly. Using the propagating phase factor of the anti-guided mode, the Fabry-­Perot resonant conditions are demonstrated. This analysis did not show quantitative agreement with the experimental enhancement data. There are, however, additional evidences correlate with the enhancement that are not yet fully understood.

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Level of Degree


Department Name

Physics & Astronomy

First Committee Member (Chair)

Steven R. J. Brueck

Second Committee Member

Sudhakar Prasad

Third Committee Member

K. J. Malloy



Document Type