Correlation of optical and structural properties of GaN/AlN core-shell nanowires

We analyze the correlation between the structural and optical properties of GaN/AlN coherent core-shell nanowire heterostructures, with different AlN shell thicknesses. The presence of an AlN shell induces a nearly uniaxial compressive strain in the core epsilon(c)(zz) oriented along the nanowire axis, which translates into a blueshift of the photoluminescence. The dependence of the photoluminescence shift on the uniaxial strain is experimentally established on the basis of correlated microphotoluminescence and high-resolution scanning transmission electron microscopy on a large number of single nanowires. The results confirm the crossing of X(A) and X(C) exciton energies at e(zz)(c) = 0.35%-0.5%, with a shell-to-core volume ratio V(AlN)/V(GaN)=0.1-0.15, as predicted by the elastic theory for purely uniaxial strain.