Abstract
Three-dimensional core-shell nanostructures could resolve key problems existing in conventional planar deep UV light-emitting diode (LED) technology due to their high structural quality, high-quality nonpolar growth leading to a reduced quantum-confined Stark effect and their ability to improve light extraction. Currently, a major hurdle to their implementation in UV LEDs is the difficulty of growing such nanostructures from Al xGa1- xN materials with a bottom-up approach. In this paper, we report the successful fabrication of an AlN/Al xGa1- xN/AlN core-shell structure using an original hybrid top-down/bottom-up approach, thus representing a breakthrough in applying core-shell architecture to deep UV emission. Various AlN/Al xGa1- xN/AlN core-shell structures were grown on optimized AlN nanorod arrays. These were created using displacement Talbot lithography (DTL), a two-step dry-wet etching process, and optimized AlN metal organic vapor phase epitaxy regrowth conditions to achieve the facet recovery of straight and smooth AlN nonpolar facets, a necessary requirement for subsequent growth. Cathodoluminescence hyperspectral imaging of the emission characteristics revealed that 229 nm deep UV emission was achieved from the highly uniform array of core-shell AlN/Al xGa1- xN/AlN structures, which represents the shortest wavelength achieved so far with a core-shell architecture. This hybrid top-down/bottom-up approach represents a major advance for the fabrication of deep UV LEDs based on core-shell nanostructures.
Highlights
I t is well-known that the III-nitride semiconductors are an important class of materials for devices emitting in the ultraviolet (UV) with applications, including UV curing,[1] medical diagnostics, phototherapy,[2] optical sensing,[3] security, communications,[4] sterilization, and water and air purification.[5−7] These applications are made possible thanks to the widely tunable bandgap energy of AlGaN alloys ranging from 3.4 eV for GaN to 6.2 eV for AlN
This is significantly lower than the 80% external quantum efficiency (EQE) achieved for GaN-based blue light-emitting diode (LED).[11,12]. This gap in terms of efficiency between deep UV and visible LEDs is due to several factors, such as the high density of point and extended defects, which can act as nonradiative recombination centers in AlN and AlxGa1−xN materials,[13,14] the low-carrier injection and difficulties in achieving efficient p-doping in AlN-rich AlxGa1−xN alloys,[15,16] the poor light extraction,[17−19] and the internal electric field in quantum wells,[20,21] which are exacerbated compared to GaN due to the much larger spontaneous polarization properties of AlN-based materials.[22]
Coupled plasma (ICP) etching with a chlorine chemistry was used to transfer the metal mask into the 2 inch AlN template, following the optimum etching conditions presented in our previous work and detailed in the Methods section.[38,47]
Summary
The AlGaN/AlN nonpolar core−shell nanorod array was fabricated using a hybrid approach composed of top-down etching and subsequent bottom-up MOVPE regrowth. The line spectrum extracted from the top to the bottom of the nanorod in Figure 5c confirms the improvement of the AlGaN SQW emission centered around 229 nm (5.41 eV): it has a similar intensity to the AlN NBE and a good emission uniformity across the whole height of the nanorod. Further improvement for AlGaN SQW 3 could be due to higher carrier confinement and/or reduced carrier leakage due to the thicker QW These results demonstrate that optimizing the growth conditions enables tuning of the emission wavelength and significantly improves the intensity while maintaining a high or relatively high emission uniformity from the whole height of the nanorods. Since the hybrid top-down/bottom-up approach can be transferred to any III-nitride template, both binary and ternary alloys, its application to state of the art n-type AlGaN layers represents the most promising solution for electrically injected core−shell structures emitting in the deep UV
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