Waveguide Raman Spectroscopy: a non-destructive tool for the characterisation of amorphous thin films

It is shown that waveguide Raman spectroscopy (WRS) can be used to study structural changes occurring in GeO2-SiO2 and Al2O3-SiO2 sol-gel derived planar waveguides as a result of variations both in composition and in annealing temperature. Bands assigned to ring defects characteristic of pure silica are absent in the germano-silicate systems indicating the destabilizing effect of the inclusion of GeO2 in the matrix. In all cases, WRS shows that densification 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 densification process is correlated to the position of the boson peak. The fact that this band shifts towards higher values with annealing temperature, but towards lower wavenumbers for rare-earth doped systems is interpreted in terms of changes in the diameter of cohesive domains and consequently to the degree of the densification process. Finally, for the alumino-silicate system, WRS spectral changes indicate the initiation of crystallization at high temperatures. The position of the low-wavenumber band is used to estimate the average size of the micro-crystallites formed in the matrix.