Effect of substrate temperature on spontaneous GaN nanowire growth and optoelectronic properties

The catalyst-free growth and the optoelectronic properties of GaN nanowires (NWs) grown on (1 1 1) Si substrates by plasma-assisted molecular beam epitaxy have been investigated. At constant N/Ga flux ratio, the NW morphology, density and growth rate are controlled by the substrate temperature, which affects the gallium adatom diffusion length before desorption. An increase in substrate temperature results in lower growth rate and smaller diameter of NWs with lower areal density NWs. Low-temperature photoluminescence spectra at 20 K revealed that PL intensity ratio of donor-bound exciton peak (D°X at 3.470 eV) with defect-related peak (Y2 at 3.424 eV) increased with increase in substrate temperature. Micro-Raman spectra showed that the GaN NWs are completely stress free irrespective of the growth conditions.