Unraveling the Origin of Mysterious Luminescence peak at 3.45 eV in GaN Nanowires

TL;DR

This study identifies oxygen-related defect states as the cause of the 3.45 eV luminescence peak in GaN nanowires and demonstrates suppression via AlN encapsulation, improving optoelectronic device performance.

Contribution

It reveals the origin of the mysterious 3.45 eV luminescence peak in GaN nanowires as oxygen-induced defect states and shows how to suppress it using AlN encapsulation.

Findings

Oxygen incorporation causes the 3.45 eV luminescence peak.

AlN encapsulation completely suppresses the UX-band.

Localized excitons at defect centers produce the luminescence.

Abstract

The demand for GaN Nanowires (NWs)-based optoelectronic devices has rapidly increased over the past few years due to its superior crystalline quality compare to their planar counterparts. However, NWs-based devices face significant challenges because of number of surface states, basal plane stacking faults and coalescence related defect states. While the origins of most of the defect states have been identified and mitigated using well-established methods, the origins of few defect states remain unknown, and thus their suppression methods have yet to be explored. One such defect state is the 3.45 eV luminescence peak, known as the UX-band. In this report, we have investigated the origin of this peak in NWs grown on a sapphire substrate by using Plasma Assisted Molecular Beam Epitaxy (PAMBE) tool. We have found that the defect states, generated due to oxygen incorporation, especially when the oxygen atoms substitute the Ga atoms, are the primary cause of this band. Actually the excitons are localized at this defect center and the radiative recombination of localized excitons gives the characteristic UX-band. By protecting the NWs from oxygen incorporation through AlN encapsulation process, we have completely suppressed the 3.45 eV peak and proved further that the peak is caused by oxygen induced defect states. Thus we have addressed an issue that persisted over the last three decades, potentially paving the way for efficiency improvements in optoelectronic devices.