GaAs-AlGaAs core-shell nanowire arrays: correlating MOVPE growth and luminescence properties

We report on the effects of changing the surface densities of MOVPE-grown free-standing GaAs-AlGaAs core-shell nanowires on the resulting nanostructure size and their photoluminescence (PL) properties. It is demonstrated that decreasing the local density of GaAs nanowires within the array leads to an increase of the overgrown AlGaAs shell thickness and to a substantial redshift of the nanostructure excitonic emission. Application of a vapor mass-transport limited growth model of the AlGaAs shell allows explaining the dependence of shell growth rate on nanowire density. The observed redshift of the nanowire PL emission is then experimentally correlated with these density-induced changes of the nanostructure size, namely with the nanowire shell-thickness to core-radius ratio hs/Rc. To account for a possible contribution of the nanostructure built-in elastic strain to the energy shift of the peak excitonic emission, the strain field in present core-shell nanowires was calculated as function of the nanostructure relevant geometrical parameters, based on a uniaxial elastic energy equilibrium model, and its effect on valence and conduction band shifts of the GaAs core evaluated by means of the Pikus-Bir Hamiltonian. Good agreement is obtained for h(s)/R-c<1, the strain-free excitonic emission being identified at 1.510 eV and ascribed to bound heavy-hole excitons. For h(s)/R-c>1 increasingly larger redshifts (up to similar to 9 meV in excess of values calculated based on the elastic strain model) are observed, and tentatively ascribed to shell-dependent exciton localization effects.