Understanding exciton recombination in GaN nanowires

27.05.2021

A strong asset of bottom-up nanowires and related nanowire heterostructures is the enhanced surface strain relaxation that delays the onset of plastic relaxation. As a matter of fact, GaN nanowires essentially free of dislocations can be directly grown on technologically relevant substrates such as silicon or metals. Yet, in spite of their high structural perfection – on par with bulk GaN – the electron-hole recombination in these nanoscopic structures remains predominantly nonradiative, even at cryogenic temperature. Here, we provide first evidence that the efficient nonradiative channel does not take place at point defects located at the nanowire surface or in the bulk, but stems instead from field-ionization of the exciton in the surface electric field.

In this work, we explore exceptionally high growth temperatures for the synthesis of GaN nanowires by molecular beam epitaxy with the initial aim to reduce the formation of point defects. At these temperatures, conventional substrates for GaN nanowire ensemblesdegrade during growth and induce a spurious background doping. We circumvent this issue by establishing GaN nanowire growth on sputtered TiN_x, a material known for its extreme refractory properties. On this novel substrate, GaN nanowires with unprecedented radiative efficiencies are obtained, nearly matching the performance of bulk GaN. However, the key factor in this breakthrough turns out to be not the increase in the growth temperature but instead the reduction in the average nanowire diameter. We explain this counterintuitive fact by considering field-ionization of the exciton in the surface electric field as the trigger for nonradiative recombination of holes at surface states. The exciton decay times modeled within this framework as function of the nanowire diameter are reproducing the experimental results obtained here and also in all previously published studies, thus providing a unified understanding of nonradiative recombination in GaN nanowires of arbitrary diameters.

The understanding of the role of surface electric fields in GaN nanowire is of fundamental importance for the design of GaN nanowire-based devices. It provides guidance for optimizing the performance of the device, whether efficient radiative recombination of the electron-hole pair is desired (such as in light-emitters) or not (such as in solar cells and photoelectrochemical cells). The field-ionization of the exciton evidenced here will also occur in other prominent nanowire systems (GaAs, ZnO, etc.), with its strength depending on the respective exciton binding energy. Last but not least, our work documents the benefit of using sputtered TiN_x as a platform for the high-temperature synthesis of GaN nanowires, which is particularly relevant for the development of deep ultraviolet emitters based on (Al,Ga)N and AlN nanowires.

Author: T. Auzelle , M. Azadmand , T. Flissikowski , M. Ramsteiner , K. Morgenroth , C. Stemmler , S. Fernández-Garrido , S. Sanguinetti , H. T. Grahn , L. Geelhaar , O. Brandt
Title: Enhanced radiative efficiency in GaN nanowires grown on sputtered TiNx
Source: ACS Photonics , 8 , 1718 ( 2021 )
DOI: 10.1021/acsphotonics.1c00224