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 degrees angle of incidence, the author 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, the author 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.