NOVELTY - The method comprises: providing vapor phase epitaxy (VPE) precursors for a first composition including a gallium (Ga) precursor as a group III species and a phosphorus (P) precursor as a group V species; performing VPE of the first composition having a hexagonal crystal structure with a direct band gap; and growing a second composition on the nanowires by a vapor-solid (VS) growth method to provide a nanowire structure having a core of the first composition and a shell of the second composition. The VPE comprises growth of nanowires of the first composition using the VPE precursors.    USE - The method is useful for growing wurtzite semiconductor for making an optoelectronic device such as LED, semiconductor lasers, single photon emitters, solar cells, single junction solar cells, multi-junction solar cells, photoelectrochemical cells, and solar-driven photoelectrocatalytic devices (all claimed).    ADVANTAGE - The method fabricates the wurtzite semiconductor with direct band gap thus improving the efficiency of the optoelectronic device.    DETAILED DESCRIPTION - The method comprises: providing vapor phase epitaxy (VPE) precursors for a first composition including a gallium (Ga) precursor as a group III species and a phosphorus (P) precursor as a group V species; performing VPE of the first composition having a hexagonal crystal structure with a direct band gap; and growing a second composition on the nanowires by a vapor-solid (VS) growth method to provide a nanowire structure having a core of the first composition and a shell of the second composition. The VPE comprises growth of nanowires of the first composition using the VPE precursors. The growth of nanowires comprises performing vapor-liquid-solid (VLS) growth of nanowires of the first composition. The VLS growth is performed using metal nanoparticles as a liquid growth catalyst, performed in the presence of a hydrogen chloride gas flow to suppress radial overgrowth of the nanowires and performed at a thermocouple temperature of 460-520 degrees C and a substrate temperature of 395-446 degrees C, and comprises performing an initial nucleation step at the thermocouple temperature and the substrate temperature. The second composition has the same crystal structure as the first composition. A substrate for the VLS growth is a substrate consisting of silicon substrates and III-V semiconductor substrates. The substrate has a (111), (110) or (100) orientation. The first composition includes doping impurities. An INDEPENDENT CLAIM is included for a method for making an optoelectronic device.