Anharmonic effects on Raman -active phonons.

摘要

This dissertation explores anharmonic properties of semiconductor materials associated with strain and phonon lifetime using Raman spectroscopy. In recent years, extensive research and development of strain engineered advanced complementary metal-oxide-semiconductor devices utilizing high-k dielectrics and metal gate technology has been conducted to meet the challenges imposed by fundamental limits of device scaling. From a development and manufacturing viewpoint, the metrology required to drive these new technologies is critical to their success. In particular, UV-Raman spectroscopy has been extensively used to measure wafer and device strain due to the high spatial and spectral resolution coupled with an ultra-short optical penetration depth in Si. However, the strain-shift coefficients reported in the literature, which correlate the shift in Raman frequency with strain, have typically been measured in the visible portion of the spectrum and appear to differ from their UV counter-parts. This work presents a detailed measurement of the strain-shift coefficients in the UV at 325 and 364nm for Si, Ge, and Si:C and SiGe alloys.;In addition, the temperature dependence of the frequencies and linewidths of the Raman-active longitudinal-optic (LO) phonons in GaAs and AlAs III-V semiconductor compounds is presented. Contrary to early theoretical predictions, the low temperature lifetime of the LO phonon is similar for the two materials with tau = 9.5 ps and 9.7 ps in GaAs and AlAs, respectively. The discrepancy between theory and experiment is caused by the accidental degeneracy between the AlAs LO phonon frequency and a Van Hove singularity in the two-phonon density of states. A new expression, based on the frequency dependence of the phonon self-energy, is derived to model the phonon lifetime.