Influence of environmental conditions and surface treatments on the photoluminescence properties of GaN nanowires and nanofins

Abstract

Due to their intrinsically large surface-to-volume ratio, nanowires and nanofins interact strongly with their environment. We investigate the role of the main air constituents nitrogen, oxygen and water on the efficiency of radiative recombination in GaN nanostructures as a function of different surface treatments and at temperatures up to 200 °C. Oxygen and water exposures exhibit a complex behavior as they can both act quenching and enhancing on the photoluminescence intensity dependent on the temperature. For oxygen, these characteristics are already observed for low concentrations of below 0.5% in nitrogen. While the photoluminescence intensity changes induced by oxygen occur independently of illumination, the influence of water is light-induced: it evolves within tens of seconds under ultraviolet light exposure and is heavily influenced by the nanostructure pre-treatment. In contrast to observations in dry atmospheres, water prevents a recovery of the photoluminescence intensity in the dark. Combined measurements of the electrical current through GaN nanofins and their photoluminescence intensity reveal the environmental influence on the interaction of non-radiative recombination processes and changes in the surface band bending of the nanostructures. Several investigated solvents show an enhancing effect on the PL intensity increase, peaking in c-hexane with a 26-fold increase after 6 min of light exposure. Stabilization of the PL intensity was achieved by a passivation of the GaN surface with Ga x O y , and ZnO shells. Surprisingly, Al2O3 coatings resulted in a highly instable PL intensity during the first minutes of illumination. Our findings reveal the high importance of controlled environmental conditions for the investigation of nanostructures, especially when aimed at their applications in the fields of environmental sensing, photo-catalysis and light-emitting diodes.

Environmental sensitivity of GaN nanofins grown by selective area molecular beam epitaxy

“…Thus, we investigate whether the ambient conditions have a qualitatively different influence on the electric properties of a-and m-plane NFs. It is known from previous studies that adsorbed molecules are able to change the sbb of GaN NSs [20,24]. An increased sbb reduces the width of the conductive channel in the NFs, which decreases the conductivity of the structure.…”

“…It has been shown in the literature that the current quenching through GaN NFs due to oxygen exposure can be enhanced by increasing temperature and under high-intensity UV excitation [24]. Thus, we want to study the influence of UV and visible light excitation on the environmental sensitivity of GaN NSs with moderate excitation intensities as well as at different temperatures in the dark.…”

“…The ambient temperature significantly influences the adsorption kinetics of molecules on GaN surfaces [24]. Figure 3(c) shows the influence of dry SA on the current through GaN NFs at different temperatures in the dark.…”

“…Molecules from the ambient induce a charge transfer between these adsorbates and the GaN, thereby influencing the charge carrier concentration and surface properties of the NSs [18]. As a prominent example, the ambient environment has a pronounced impact on the overall intensity of the photoluminescence (PL) of GaN NSs [19][20][21] as well as their chemical stability [22] and electrical properties [23,24]. Nevertheless, there is still a lack of understanding the underlying dynamics of the adsorption mechanisms.…”

“…In addition, results and interpretations in the literature are sometimes contradicting each other, and a generally valid explanation for all experimental observations has not yet been found. A recent study investigated the influence of the main air constituents on the radiative recombination efficiency of GaN NSs and found that oxygen can act either quenching or enhancing on their PL intensity, depending on the temperature [24]. By combining electrical and PL measurements, they conducted that oxygen gives rise to two fundamentally different adsorption states.…”