Raman scattering characterization of well-aligned IrO₂ nanocrystals grown on sapphire substrates via reactive sputtering

Raman spectroscopy (RS) was used for the characterization of well-aligned IrO₂ nanocrystals (NCs) grown on sapphire substrates with different orientations via reactive magnetron sputtering. Three Raman-active modes, E _g, B_2g and A_1g, were observed in the range 500-800 cm^-1. The intensity of certain modes depended on the orientation of NCs and followed the selection rules reasonably well. The result indicates the possibility of determining the preferred growth direction of NCs using RS. Raman spectra show red shifts and peak broadening of the IrO ₂ signatures with respect to those of the bulk counterpart. The red shifts and asymmetric broadening of the Raman line shapes of the NCs were analyzed by a modified spatial correlation (MSC) model, which included a factor of stress-induced shift. The MSC model showed that the effects of stress and nanometric size could be separated by analyzing the observed Raman features. The usefulness of experimental RS together with the MSC model analysis as a structural and residual stress characterization technique for NCs is demonstrated.