Raman scattering in large single indium nitride dots: Correlation between morphology and strain

We present an experimental work on the Raman analysis of single InN dots grown by metal-organic vapor phase epitaxy on GaN buffer layer. InN islands of controlled sizes have been fabricated by taking advantage of the Stranski-Krastanov growth mode. Atomic force microscopy and micro-Raman spectroscopy have been employed to investigate their morphology and internal strain. The usual shape of the dots corresponds to truncated pyramids with a hexagonal base as revealed by atomic force microscopy measurements. The E-2 phonon frequency, detected in micro-Raman spectra recorded from single dots of sizes ranging from 480 nm down to 30 nm in height, allows a rough evaluation of the residual strain field. Careful analysis of these data reveals that the islands are weakly strained, which is likely due to the formation of dislocations near the InN/GaN interface. Nevertheless, the reduction of the size of InN islands leads to an increasing strain field. Finally, we observed a surprisingly small strain increase when the dots are capped with a thin GaN top surface layer. This demonstrates the major role played by the plastic strain relaxation.