Evaluation of phonon confinement in ultrathin-film silicon-on-insulator by Raman spectroscopy

Abstract

Raman spectroscopy is a practical evaluation technique for the quantum effect of phonons in a microcrystalline structure. It is very sensitive to fluctuations of crystalline potential or localized atomic geometry. Phonon confinement is observed as a broadening and desymmetrization of the Raman spectrum. However, Raman spectra also include information on crystal quality, strain, and thermal influence caused by the excitation source. Because these factors have an effect similar to that of phonon confinement on spectra, distinction of the factors is essential for accurate evaluation of the phonon confinement effect. The influence of these effects in the utrathin-film silicon-on-insulator (SOI) was investigated by Raman spectroscopy. Marked broadening and desymmetrization of Raman spectra were confirmed for the SOIs with thickness less than 5 nm. The crystalline quality and strain in the SOI layer were investigated by X-ray diffraction. We developed a precise simulation technique for phonon confinement with consideration of thermal and strain effects. By comparing the simulation with the results of Raman spectroscopy, an exact evaluation of phonon confinement effects in utrathin-film SOI was achieved.