The Effects of Strain and Confinement on the Optical Properties of Group IV Semiconductor Structures

In this thesis, the effects of strain and confinement on the optical properties of three group IV semiconductor structures, Si/Ge superlattices, Ge-Si alloys and light -emitting porous Si, are examined experimentally and theoretically. Raman scattering is used to study optical phonons in a Si_{12}Ge _4 strained layer superlattice on c-Si that is subjected to hydrostatic pressure. The change in phonon frequency with pressure, domega/dP, for the principle quasi-confined LO mode in the Ge layers is found to be significantly smaller than that for zone -center optical phonons in bulk c-Ge. This difference is shown to be due to the tuning of strain in the Ge layers and the pressure response of the confined mode as hydrostatic pressure is varied. The dispersion relations for optical and acoustic phonons are examined in bulk Si and Ge, Si and Ge strained layers grown on (001) and (111) substrates, and ultrathin Si/Ge superlattices at ambient pressure and under hydrostatic pressure by using a modified Keating model. This model is used to obtain the mode Gruneisen parameters gamma throughout the Brillouin zone for bulk c-Si and c-Ge, explicit analytic expressions for gamma at the zone center and boundaries. The pressure dependence of vibrational Raman scattering in polycrystalline Ge_{rm 1-x }Si_{rm x} alloys is studied up to ~100 kbar across the compositional range, and the mode Gruneisen parameters gamma for the Si-Si, Ge -Ge and Ge-Si optical phonons are determined from the Raman shifts. The dependence of the Raman shift on x and pressure is described by the cellular isodisplacement model, which describes the optical phonons at zone center. Finally, porous silicon that strongly emits in the visible is analyzed by using Raman scattering and photoluminescence spectroscopies. The spectrum shows an asymmetric peak near 508 cm^{-1}, with a width of ~40 cm^ {-1}. Analysis of phonon confinement suggests that the local structure is more like a sphere than a rod and has a characteristic diameter of 2.5-3.0 nm.