Evidence for disorder-induced vibrational mode coupling in thin amorphous SiO2 films

Infrared spectroscopy was employed to characterize thermal SiO2 layers down to 15 nm thickness. From these measurements, performed in transmission mode under a 30° angle of incidence, we obtained a multiple resonance structure on the high-energy side of the main Si-O stretching vibration at 1075 cm−1. Aside from a longitudinal optical (LO) mode at 1254 cm−1, we found experimental evidence for two additional vibrations, a transverse optical mode at 1200 cm−1, and an LO mode at 1165 cm−1. These features could be explained by assuming disorder-induced vibrational coupling effects. Beyond that, an ion damage study and a comparison with IR spectra of oxide layers deposited in various chemical vapor deposition processes clearly confirmed the idea of disorder-induced mode coupling.