Double strain state in a single GaN/AlN nanowire: Probing the core-shell effect by ultraviolet resonant Raman scattering

We report the demonstration of an ultra-sensitive Raman probing of single GaN/AlN nanowires (NWs). The high sensitivity of the Raman scattering by longitudinal optical phonon is achieved by using ultraviolet resonant excitation near the energy band-gap of GaN. Structural variations within one single nanowire are evidenced very accurately by strong LO phonons shifts in the UV Raman spectra recorded on different regions of the NW. They are interpreted as a fine probing of the double strain state experienced by GaN, due to the formation of an AlN shell in the bottom part of the NW. The core-shell structure has been confirmed in a statistical way by measuring the average strain in the NWs ensemble thanks to the Raman scattering excited in the visible range. Data have been comprehensively accounted for by considering an axial strain in GaN NW part covered by AlN shell, in the elastic regime, while the top GaN is relaxed.