Size effects in near-ultraviolet Raman spectra of few-nanometer-thick silicon-on-insulator nanofilms

We have fabricated Si-on-insulator (SOI) layers with a thickness h₁ of a few nanometers and examined them by Raman spectroscopy with 363.8 nm excitation. We have found that phonon and electron confinement play important roles in SOI with h₁ < 10 nm. We have confirmed that the first-order longitudinal optical phonon Raman band displays size-induced major homogeneous broadening due to phonon lifetime reduction as well as minor inhomogeneous broadening due to wave vector relaxation (WVR), both kinds of broadening being independent of temperature. Due to WVR, transverse acoustic (TA) phonons become Raman-active and give rise to a broad band in the range of 100-200 cm^-1. Another broad band appeared at 200-400 cm^-1 in the spectrum of SOI is attributed to the superposition of 1st order Raman scattering on longitudinal acoustic phonons and 2nd order scattering on TA phonons. Suppression of resonance-assisted 2-nd order Raman bands in SOI spectra is explained by the electron-confinement-induced direct band gap enlargement compared to bulk Si, which is confirmed by SOI reflection spectra.