Sub-meV linewidth in GaN nanowire ensembles: absence of surface excitons due to the field-ionization of donors

TL;DR

This study combines experimental photoluminescence measurements and theoretical modeling to show that surface electric fields in GaN nanowires cause donor ionization, preventing surface exciton formation and resulting in ultra-narrow spectral lines.

Contribution

The paper provides a combined experimental and theoretical analysis demonstrating how surface electric fields lead to donor ionization in GaN nanowires, explaining the absence of surface excitons.

Findings

Narrow donor-bound exciton lines observed at 0.4 meV in GaN nanowires.

Surface electric fields above 10 kV/cm ionize donors in nanowires smaller than 100 nm.

Surface donor ionization explains the absence of surface excitons and the narrow linewidths.

Abstract

We observe unusually narrow donor-bound exciton transitions (0.4 meV) in the photoluminescence spectra of GaN nanowire ensembles grown on Si(111) substrates at very high (> 850 degrees Celsius) temperatures. The spectra of these samples reveal a prominent transition of excitons bound to neutral Si impurities which is not observed for samples grown under standard conditions. Motivated by these experimental results, we investigate theoretically the impact of surface-induced internal electric fields on the binding energy of donors by a combined Monte Carlo and envelope function approach. We obtain the ranges of doping and diameter for which the potential is well described using the Poisson equation, where one assumes a spatially homogeneous distribution of dopants. Our calculations also show that surface donors in nanowires with a diameter smaller than 100 nm are ionized when the surface electric field is larger than about 10 kV/cm, corresponding to a doping level higher than 2 x 10^16 cm^-3. This result explains the experimental observation: since the (D+,X) complex is not stable in GaN, surface-donor-bound excitons do not contribute to the photoluminescence spectra of GaN nanowires above a certain doping level, and the linewidth reflects the actual structural perfection of the nanowire ensemble.