Properties of nanopillar structures prepared by dry etching of undoped GaN grown by maskless epitaxial overgrowth

Nanopillar structures were prepared by dry etching of maskless epitaxial lateral overgrowth (MELO) GaN samples using a mask of Ni nanoparticles formed upon annealing thin Ni films deposited on top of SiO₂/GaN. Under our experimental conditions the average nanopillars dimensions were close to 170 nm, with the nanopillars density close to 10⁹ cm^-2. The nanopillars formation was random and not correlated with the threading dislocation density in MELO GaN, as evidenced by comparing the size and density of nanopillars in the wing and seed regions of MELO GaN differing in dislocation density by an order of magnitude. After dry etching the luminescent intensity of nanopillars became actually lower than the intensity from the unetched matrix due to the impact of defects introduced in the sidewalls during nanopillars formation. The intensity greatly increased, together with a decrease in the leakage current of Schottky diodes, after rapid thermal annealing of nanopillar structures at 900 °C and further increased after additional etching in KOH solution. These changes are attributed to annealing of radiation defects introduced by dry etching and further removal of the damaged region by KOH etching. The results suggest that, in nanopillar structures produced by dry etching, some increase of internal quantum efficiency alongside improvement of light extraction efficiency are responsible for the observed luminescence intensity changes.