Enhanced radiative efficiency in GaN nanowires grown on sputtered TiN$_{\boldsymbol{x}}$: effects of surface electric fields

TL;DR

This study demonstrates that surface electric fields, rather than defect density, dominate nonradiative recombination in GaN nanowires grown on TiN_x substrates, leading to enhanced radiative efficiency.

Contribution

It reveals that exciton field ionization due to surface band bending controls nonradiative processes, independent of growth temperature or defect density.

Findings

Decay time correlates with nanowire diameter.

Surface electric fields influence exciton recombination.

Nonradiative processes are dominated by surface effects.

Abstract

GaN nanowires grown by molecular beam epitaxy generally suffer from dominant nonradiative recombination, which is believed to originate from point defects. To suppress the formation of these defects, we explore the synthesis of GaN nanowires at temperatures up to 915 ${\deg}C$ enabled by the use of thermally stable TiN$_x$/Al$_2$O$_3$ substrates. These samples exhibit indeed bound exciton decay times approaching those measured for state-of-the-art bulk GaN. However, the decay time is not correlated with the growth temperature, but rather with the nanowire diameter. The inverse dependence of the decay time on diameter suggests that the nonradiative process in GaN nanowires is not controlled by the defect density, but by the field ionization of excitons in the radial electric field caused by surface band bending. We propose a unified mechanism accounting for nonradiative recombination in GaN nanowires of arbitrary diameter.