Ultraviolet Raman scattering of GaN nanocrystallites: Intrinsic versus collective phenomena

Resonant Raman scattering in wurtzite structured GaN nanocrystallites of various morphologies were studied. The polar mode A1(LO) exhibited Fröhlich-type resonant Raman scattering whose characteristics were found to depend weakly on the morphology of the crystallites. In contrast, the UV-laser heating and heat retention in the porous media of a crystallite ensemble were discovered to drastically modify the Raman properties: A Raman thermal redshift was observed that might mask any redshift due to the confinement effect. The thermal redshift was found to depend on the laser power and on the ensemble size. An ensemble temperature on the order of 550K was inferred from the electron–phonon interaction model, a result that was verified via Raman scattering experiments at the elevated temperature regime. For a small ensemble that contains ∼10–20 crystallites and with nominal laser-heating effect, the Raman line shape was found to have mainly a Lorentzian component indicative of phonon-lifetime broadening mechanism. The line shape had no pronounced Gaussian component and had similar characteristics to the line shape of GaN film, implying that a collective of a few crystallites still reflects the Raman properties of an individual crystallite. Complementary photoluminescence investigations concur with the Raman findings.

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