Abstract:We demonstrated for the first time the formation and study of semi-polar AlGaN multiple-quantum-wells (MQWs) grown on highly regular hexagonal AlN nanopyramids. The AlN nanopyramids were obtained by a metal-organic chemical vapor phase deposition regrowth method on a well-ordered AlN nanorod array prepared by a top-down etching process. The growth mechanism of the AlN nanopyramids was ascribed to the slow growth of the (101[combining macron]1) semi-polar plane, which resulted from hydrogen passivation. Beneath… Show more
“…Additionally, at 20 mbar, the c ‐plane growth rate is increased which almost leads to a full pyramid with six (10‐11) semi‐polar planes. It is also observed that inclined planes are formed in the upper part of the nanorod sidewalls, which could be associated with ( n 0‐ n ‐1) semipolar crystal planes, where n ≥ 1 …”
Core–shell nanostructures are predicted to highly improve the efficiency of deep‐UV light emitting diodes (LEDs), owing to their low defect density, reduced quantum‐confined Stark effect, high‐quality non‐polar growth and improved extraction efficiency. In this paper, we report on the nanofabrication of high‐quality AlN nanorod arrays using a hybrid top‐down/bottom‐up approach for use as a scaffold for UV LED structures. We describe the use of Displacement Talbot Lithography to fabricate a metallic hard etch mask to allow AlN nanorod arrays to be dry etched from a planar AlN template. In particular, we investigate the impact of etching parameters on the nanorod etch rate, tapering profile and mask selectivity in order to achieve vertical‐sided nanorod arrays with high aspect ratios. AlN facet recovery is subsequently explored by means of regrowth using Metal Organic Vapor Phase Epitaxy. Low pressure and high V/III ratio promote straight and smooth sidewall faceting, which results in an improvement of the optical quality compared to the initial AlN template. The promising results open new perspectives for the fabrication of high‐efficiency deep‐UV‐emitting core–shell LEDs.
“…Additionally, at 20 mbar, the c ‐plane growth rate is increased which almost leads to a full pyramid with six (10‐11) semi‐polar planes. It is also observed that inclined planes are formed in the upper part of the nanorod sidewalls, which could be associated with ( n 0‐ n ‐1) semipolar crystal planes, where n ≥ 1 …”
Core–shell nanostructures are predicted to highly improve the efficiency of deep‐UV light emitting diodes (LEDs), owing to their low defect density, reduced quantum‐confined Stark effect, high‐quality non‐polar growth and improved extraction efficiency. In this paper, we report on the nanofabrication of high‐quality AlN nanorod arrays using a hybrid top‐down/bottom‐up approach for use as a scaffold for UV LED structures. We describe the use of Displacement Talbot Lithography to fabricate a metallic hard etch mask to allow AlN nanorod arrays to be dry etched from a planar AlN template. In particular, we investigate the impact of etching parameters on the nanorod etch rate, tapering profile and mask selectivity in order to achieve vertical‐sided nanorod arrays with high aspect ratios. AlN facet recovery is subsequently explored by means of regrowth using Metal Organic Vapor Phase Epitaxy. Low pressure and high V/III ratio promote straight and smooth sidewall faceting, which results in an improvement of the optical quality compared to the initial AlN template. The promising results open new perspectives for the fabrication of high‐efficiency deep‐UV‐emitting core–shell LEDs.
“…The curved facet evolves between the (0111) plane and the (0112) plane, because (0111) and (0112) are the most energetically favorable planes for the growth of AlGaN and GaN respectively. [45,46] As shown in Figure 1c, it is important to mention that the intensity of the bottom four AlGaN layers is relatively weaker compared with the intensity of the top four AlGaN layers and that the apparent width of the layer is also larger for the bottom InGaN dots than the top ones, all factors which can be explained by the curved nature of the bottom AlGaN layers. Given the complex nature of this heterostructure, it is clear that the combination of APT technique with the information from TEM are the only combined approaches that can reveal this slightly curved nature of the quantum layers and the crystallographic nanosized structure within the heterostructures.…”
Due to the increasing desire for nanoscale optoelectronic devices with green light emission capability and high efficiency, ternary III‐N‐based nanorods are extensively studied. Many efforts have been taken on the planar device configuration, which lead to unavoided defects and strains. With selective‐area molecular‐beam epitaxy, new “Russian Doll”‐type InGaN/AlGaN quantum wells (QWs) have been developed, which could largely alleviate this issue. This work combines multiple nanoscale characterization methods and k∙p theory calculations so that the crystalline structure, chemical compositions, strain effects, and light emission properties can be quantitatively correlated and understood. The 3D structure and atomic composition of these QWs are retrieved with transmission electron microscopy and atom probe tomography while their green light emission has been demonstrated with room‐temperature cathodoluminescence experiments. k∙p theory calculations, with the consideration of strain effects, are used to derive the light emission characteristics that are compared with the local measurements. Thus, the structural properties of the newly designed nanorods are quantitatively characterized and the relationship with their outstanding optical properties is described. This combined approach provides an innovative way for analyzing nano‐optical‐devices and new strategies for the structure design of light‐emitting diodes.
“…A comprehensive analysis of the polarization effect in an Al 0.5 Ga 0.5 N template has been reported. The calculation results indicated that the piezoelectric polarization field becomes nearly zero for nonpolar and semipolar orientations 31 . Therefore, MQWs grown on the m -plane facet naturally have a high internal quantum efficiency.…”
Section: Resultsmentioning
confidence: 95%
“…This result confirms the previous study of Tian et al . 31 . Figure 6 TEM images of regrown AlGaN/AlN MQWs.…”
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...
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