Waveguide Raman spectroscopy: a non-destructive tool for the characterization of amorphous thin films

It is shown that waveguide Raman spectroscopy CNRS) can be used to study st ructural changes occurring in GeO2-SiO2 and Al2O3-SiO2 sol-gel derived plan ar waveguides as a result of variations both in composition and in annealin g temperature. Bands assigned to ring defects characteristic of pure silica are absent in the germano-silicate systems indicating the destabilizing ef fect of the inclusion of GeO2 in the matrix. In all cases, WRS shows that d ensification resulting from thermal treatment is accompanied by T-O-T (T = Si, Ge or Al) angle changes which cause an appreciable rearrangement of the glass network. Data for the low-wavenumber region show that the densificat ion process is correlated: to the position of the boson peak. The fact that this band shifts towards higher values with annealing temperature, but tow ards lower wavenumbers for rare-earth doped systems is interpreted in terms of changes in the diameter of cohesive domains and consequently to the deg ree of the densification process. Finally, for the alumino-silicate system, WRS spectral changes indicate the initiation of crystallization at high te mperatures. The position of the low-wavenumber band is used to estimate the average size of the micro-crystallites formed in the matrix. (C) 1999 Else vier Science B.V. All rights reserved.