Origin of the 3.45 eV emission in GaN nanowires traced to oxygen-induced surface defects and suppressed by AlN coating

The demand for GaN nanowires (NWs)-based optoelectronic devices has rapidly increased over the past few years due to their superior crystalline quality compared to their planar counterparts. However, NWs-based devices face significant challenges because of the 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 a 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 unknown defect states (UX band). In this report, we have investigated the origin of this peak in PAMBE grown NWs and provided an in-depth detail of its formation. We have found that the defect states are generated due to oxygen incorporation, especially when the oxygen atoms substitute the Ga atoms. These defect centers capture the free excitons and localized them at the defect center. The radiative recombination of these localized excitons gives the characteristic UX band. By protecting the NWs from oxygen incorporation through the 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.