Electron-Phonon Interactions and Quantum Confinement Effects on Optical Transitions in Nanoscale Silicon Films

Theoretical studies have attributed the temperature dependence of the linear optical response (dielectric function) of bulk semiconductors to electron-phonon interactions and thermal expansion of the lattice. However, the role of phonons in the optical properties of nanoscale structures is often overlooked. This thesis systematically investigates the impact of both carrier confinement and electron-phonon interactions using nanoscale films of silicon in crystalline silicon quantum wells (c-Si QW). Spectroscopic ellipsometry (SE) is a linear optical technique used to of extract the dielectric function and thickness of very thin films. X-ray reflectivity (XRR) was used as the complementary thickness metrology method. The dielectric function of c-Si QWs with thicknesses ranging from 2 nm to 10 nm is found to have a significant dimensional dependence. The major differences in the dielectric function with thickness were observed at the critical points (direct gap transitions) in silicon. Critical points (CP) are the vertical band-to-band transitions with maximum probability due to high joint density of states. Specifically, a blue shift (higher energy shift) in the critical point energy (ECP ) and enhanced lifetime broadening (Gamma) of the excitonic E 1 (interband transition in the <111> direction) critical point as a function of thickness was observed. Direct space analysis was used to extract ECP and Gamma from the dielectric function. Low-temperature (Liquid- 4He) SE measurements were performed to study the contribution of electron-phonon interactions on the ECP and Gamma. In contrast to bulk silicon, the E1 direct gap transition of sub-9 nm silicon films becomes more intense than the non-excitonic E2 direct gap transition as temperature decreases. This is evidence of the importance of electron-phonon interactions on the direct gap transitions of nanoscale semiconductor structures. The temperature dependence of the critical point energy was modeled with the Bose-Einstein statistical factor to extract the contribution of acoustic phonon frequency. An elastic continuum theory approach was used to calculate the acoustic phonon energy with a change in the thickness of the c-Si QW and with a change in the dielectric layer on top of the QW. The phonon dispersion of the quantum confined layers was further altered by changing the dielectric layer above the nanoscale silicon. Change in the stress, interface and crystal quality due to sample fabrication steps in the nanoscale silicon films can also alter its dielectric function. High-resolution x-ray diffraction was used to measure the strain and crystal quality, while photoreflectance (a surface sensitive characterization method) was used as the non-linear optical technique to probe the surface of the nanoscale films to measure the energy and lifetime broadening of the E1 critical point.