Researchers based in China and the USAhave used a patterned combination ofsapphire substrate and silica (SiO2) layer

to improve the performance of ultraviolet light-emitting diodes (UVLEDs) [Hongpo Hu et al,Nano Energy, vol69, p104427, 2020]. Theenhanced light output was due to a combinationof improved crystal quality of the III–nitridelight-emitting material and a boost in light extrac-tion efficiency (LEE) from reducing internal reflec-tion and absorption of the generated photons.The team from Wuhan University, HC SemiTek

Corp, University of Science and Technology ofChina, Huazhong University of Science andTechnology in China and University of Michigan inthe USA see the technology as having potentialfor increasing the performance of LED-basedsolid-state UV light sources for curing, waterpurification, sterilization and phototherapy.The researchers used three types of sapphire

substrate: flat (FSS), patterned (PSS) and patterned with silica array (PSSA). The pointedcones of the PSS were created using thermalreflow of patterned photoresist and inductivelycoupled etching. With the PSSA the pointedcones were etched into a 2µm plasma-enhancedchemical vapor deposition (PECVD) silica layer.The etch was continued down into the sapphiresubstrate.The effect of the patterned sapphire was to redirect

dislocations in aluminium gallium nitride (AlGaN) growth,bending their path so that they annihilate rather thanthreading through the indium gallium nitride (InGaN)light-emitting active layers. Such dislocations becomecenters for electron-hole recombination without theemission of UV photons, sapping efficiency. The patterned sapphire technique is not as effective

in AlGaN, compared with GaN-based devices. This isrelated to the stronger sticking of Al, compared withGa, in the growth process. This generates misorientedcrystal growth. One effect of the silica array layer is toreduce misoriented crystal regions.The nucleation for III–N deposition consisted of 20nm

sputtered AlN. Further growth was by metal-organicchemical vapor deposition (MOCVD) with trimethyl-metaland ammonia precursors. The n- and p-type doping

were via silane (SiH4) and bis(cyclopentadienyl)mag-nesium (Cp2Mg), respectively. Hydrogen and nitrogenwere used as carrier gases.The final epitaxial structure (Figure 1) was completed

with a 720°, 20 minute thermal anneal to activate theMg doping.The material was fabricated into LEDs with a 1.25µm

mesa etch to expose the n-AlGaN contact layer. The p-GaN contact layer was covered with an annealedindium tin oxide (ITO) transparent conductor current-spreading layer. Chromium/platinum/gold were evaporated to create both the n- and p-electrodes ofthe diode structure. The wafer was thinned to 120µmby grinding and polishing. The 254µmx685µm chipswere then singulated. The threading dislocation density (TDD) was assessed

using x-ray diffraction (XRD) and scanning transmis-

Technology focus: UV LEDs

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Researchers improve aluminium gallium nitride crystal quality and light extraction efficiency to boost output power.

Patterned sapphire silicasubstrate for ultraviolet LEDs

Figure 1. UVLED epitaxial structure.

sion electron microscopy (STEM). The TDD in III–Nmaterial grown on flat sapphire is typically of the orderof 1010/cm2. The estimate from the STEM analysisgave values for the PSS and PSSA samples of5.63x108/cm2 and 2.80x108/cm2, respectively. Theseresults were consistent with the less reliable XRD data.The STEM studies also suggested that the dislocations

formed at the sapphire interface became bent and ter-minated at the cones (Figure 2). The PSSA furtherreduced the upward propagation of dislocations. ThePSSA sample also suffered less from parasitic crystalformation on the cones. The researchers comment:“The AlGaN epilayer grown on PSSA had a lower TDDthan that grown on PSS by taking advantages of themore preferred vertical growth in the 3D growth stageand reduced misfit at the coalescence boundary.”Under photoluminescence the peak wavelength was

368nm. Electroluminescence from the FSS, PSS andPSSA UVLEDs resulted in light output powers of 68.3,153.8 and 193.9mW, respectively, at 150mA injection(Figure 2). The researchers attribute the enhancedperformance of the PSSA device to “improved crystalquality and higher LEE”. The corresponding peak exter-nal quantum efficiencies were 23.1%, 44.9% and50.4%. The researchersclaim the 50.4% is “betterthan those reported in pre-vious publications”. The cur-rent–voltage performanceof the devices was similar.The LEE factor in the

improved performance wasexplored using numericalsimulations of the lightreflection at interfaces. With the FSS substrate theflat interface tends to reflectmost of the light back intothe diode due to the refractive index contrasts,drastically reducing lightextraction. Reflectivity

measurements suggested improved reflectivity andtransmittance at 368nm over FSS diodes of 10.2% forPSS devices and 15.8% for PSSA. ■https://doi.org/10.1016/j.nanoen.2019.104427Author: Mike Cooke

Technology focus: UV LEDs

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Figure 2. Cross-sectional STEM images of AlGaNgrown (a) on PSS and (b) PSSA. (c) Enlarged STEMimages of the marked areas in (a) and (d) that in (b).

Figure 3. (a) I–V and (b) L–I characteristics ofUVLEDs grown on FSS,PSS and PSSA. (c) EQEsof UVLEDs grown onFSS, PSS and PSSAversus current curves.(d) State-of-the-art inEQE for UVLEDs.