Deep-Ultraviolet AlGaN/AlN Core-Shell Multiple Quantum Wells on AlN Nanorods via Lithography-Free Method

Abstract

We report deep ultraviolet (UVC) emitting core-shell-type AlGaN/AlN multiple quantum wells (MQWs) on the AlN nanorods which are prepared by catalyst/lithography free process. The MQWs are grown on AlN nanorods on a sapphire substrate by polarity-selective epitaxy and etching (PSEE) using hightemperature metal organic chemical vapor deposition. The AlN nanorods prepared through PSEE have a low dislocation density because edge dislocations are bent toward neighboring N-polar AlN domains. The core-shell-type MQWs grown on AlN nanorods have three crystallographic orientations, and the final shape of the grown structure is explained by a ball-and-stick model. The photoluminescence (PL) intensity of MQWs grown on AlN nanorods is approximately 40 times higher than that of MQWs simultaneously grown on a planar structure. This result can be explained by increased internal quantum efficiency, large active volume, and increase in light extraction efficiency based on the examination in this study. Among those effects, the increase of active volume on AlN nanorods is considered to be the main reason for the enhancement of the PL intensity.Aluminum nitride (AlN) based devices operating in the deep ultraviolet (UVC) regime (200-280 nm) have attracted considerable attention owing to their wide range of potential applications such as medical therapy, UV curing, biochemical sensing, water or air purification, and disinfection 1-3 . However, AlN based UVC devices still suffer from low quantum efficiency and output power. Although the external quantum efficiency of UVC light-emitting diodes (LEDs) has increased dramatically lately, most of the reported efficiency values remain at a few percent of the commercially required level. The low internal quantum efficiency due to a high dislocation density 4,5 and polarization coefficient 6,7 is the bottleneck for high-performance UVC applications.Such critical obstacles can be surmounted by introducing three-dimensional (3D) nanostructures because of their high stress relaxation, low dislocation density, increased active volume, and high extraction efficiency 8,9 . However, AlN based nanostructures for UVC light emitters have seldom been investigated because of the difficulty of using patterned masks. Selective area growth is strongly inhibited because Al atoms have very high sticking coefficient. Zhao et al. recently reported that these difficulties can be addressed by employing nitrogen (N)-polar Al(Ga)N nanowires grown on a Si substrate using a lithography-free process 10 . They developed self-organized AlN nanowire LEDs operating at 210 nm, the shortest wavelength ever reported for a nanostructure, and also reported an AlN nanowire laser diode fabricated by radio-frequency plasma-assisted molecular beam epitaxy (MBE) 9,11 . However, AlN or sapphire substrates are preferable for devices operating in the UVC regime because they are transparent to UVC emission. The polarity of the AlN layer can be controlled by varying the preflow conditions before AlN growth and the annealing temperature...

UV Emission from GaN Wires with m-Plane Core–Shell GaN/AlGaN Multiple Quantum Wells

“…Wires with AlN core and core-shell AlGaN/AlN QWs emitting at 260 nm exhibit a 40times enhancement of the photoluminescence (PL) intensity with respect to planar structures. 21 The same core-shell structures with AlN core emitting at 229 nm have also been demonstrated. 11 However, no electroluminescence (EL) has been reported so far, due to the difficult electrical injection through the AlN inner core.…”

Review of nanophotonics approaches using nanostructures and nanofabrication for III-nitrides ultraviolet-photonic devices

“…Recently, AlN NRs were prepared on a sapphire substrate by a catalyst/lithography-free method, which is polarity-selective epitaxy and etching. 92 Core-shell-type AlGaN/AlN MQW emitting at 260-nm wavelength and grown on these AlN NRs show an improvement by 40 times in the PL intensity compared with the same structure grown on planer AlN template.…”

Creation of regular arrays of faceted AlN nanostructures via a combined top-down, bottom-up approach

“…Nanopatterning of sapphire/AlN templates has been shown to be effective at lowering threading dislocation densities [10] [11] [12]. 3D nanostructures also allow for stress relaxation, increased active volume [13] and higher extraction efficiencies [14]. In the case of higher extraction efficiencies, utilising nanostructures has been found the be very effective in allowing the escape of TM-polarised ra760@bath.ac.uk (R. Armstrong) ORCID(s):…”

“…Recently, a catalyst-and lithography-free method for the fabrication of AlN nanorods by polarity selective epitaxy and etching has also been demonstrated [14]. Here, an AlN layer of mixed polarity is grown and wet etching is used to selectively etch away N-polar regions of material [6].…”