Characterization of thermal and deposited thin oxide layers by LO-TO excitation in FTIR-transmission measurements

Infrared spectroscopy under oblique incidence was employed as a nondestructive method to characterize layers of amorphous Si02. Transmission spectra of very thin layers (about 10 nm) exhibit a well resolved multiple structure on the high energy side of the main Si-O stretching vibration, which could be analysed in terms of longitudinal and transversal optical exitations. The appearance of an additional LO-TO pair at 1170 cm high minus 1 and 1200 cm high minus 1, aside from the LO-splitting (1254 cm high minus 1) of the distinct TO-mode, is an experimental confirmation for a disorder induced mode coupling in amorphous Si02. This could be verified through a comparison with oxides produced by chemical vapor deposition. Implications of these findings for the characterization of oxides are discussed.