Top-emitting deep-UV LED reaches 6.5% wall-plug efficiency

Deep-ultraviolet LEDs have so far sent their light out through the back of the chip, the substrate. Guo-Dong Hao, Manabu Taniguchi, Lingjie Wei and Shin-ichiro Inoue of Japan's National Institute of Information and Communications Technology (NICT) in Kobe now report a 275 nm device that emits through its top surface and converts up to 6.5% of its electrical input power into ultraviolet light. Their open-access paper appeared in ACS Photonics, volume 13, issue 21, dated 4 November 2026, and has been online since 21 October 2026.

Why this matters

Deep-ultraviolet LEDs (DUV LEDs) are built from aluminium gallium nitride (AlGaN). The authors list surface disinfection, air and water purification, food safety, photolithography, UV curing and solar-blind communication as their fields of use. LED professional has covered one of them in an article on large-scale water disinfection with deep-UV LEDs.

According to the paper, every DUV LED reported to date is a flip-chip: the chip is turned over, mounted on a separate submount and emits through its sapphire substrate. This bottom-emitting construction has two drawbacks. Transferring the chip to a submount makes manufacturing more complex and more expensive. And because sapphire has a lower refractive index (about 1.8) than AlGaN (about 2.2 to 2.6), much of the light is reflected back at the boundary between the two and stays trapped in the semiconductor layers.

Sending the light out through the top would avoid both problems, but the top of the chip is where the current enters. To make a low-resistance (ohmic) contact, manufacturers grow a layer of p-type gallium nitride (p-GaN) on top and cover it with a metal p-electrode. Both absorb deep-UV light strongly. No conductive material that is also transparent at these wavelengths has been available, which is why top-emitting DUV LEDs had not been realized.

What the researchers did

The NICT team did not look for a new transparent contact material. It made the contact semitransparent through its geometry, building on a micro-mesh electrode the group had presented for flip-chip LEDs in 2025.

• Ring-shaped mesa. The mesa, the raised and etched region of the chip that contains the light-emitting layers, is a ring with an inner diameter of 200 µm and an outer diameter of 600 µm. A ring has an inner and an outer sidewall, so light travelling sideways inside the layers reaches an edge sooner and can escape there. The n-electrode is a 180 µm disc in the centre of the ring, where it absorbs little of the light travelling towards the chip edge.
• Concentric micro-ring electrodes. On top of the mesa sit concentric p-electrode rings of nickel and gold, each 2 µm wide with a pitch of 10 µm. Together they cover about one fifth of the mesa. The p-GaN remains only under these rings as the contact layer; between them it is etched away, which opens windows for the light.
• Hafnium oxide nanostructures. The windows are filled with truncated cones (cones with a flat top) of hafnium oxide (HfO2), arranged in a triangular lattice with a spacing of 540 nm. Each cone is about 200 nm high, about 200 nm across at the top and has a sidewall angle of about 60°. These dimensions were optimized with optical simulations. The cones enlarge the escape cone, the narrow range of angles within which light can pass from the semiconductor to the outside.
• Back mirror. An aluminium layer on the back of the sapphire substrate reflects downward light towards the top.

The layers were grown on sapphire by metal organic chemical vapour deposition (MOCVD), the standard growth process for LEDs. The micro-rings were defined by electron-beam lithography. The wafer was diced into chips of 1 mm × 1 mm, which were mounted in TO-can packages (metal can housings) and encapsulated.

As a reference, the team built conventional flip-chip LEDs from the same wafer with an identical electrode geometry, and also compared top-emitting chips with and without the HfO2 cones. All devices were measured in direct-current operation at room temperature without external heat management, with a calibrated integrating sphere for the optical output. The paper does not state how many devices were measured.

Three efficiency terms are used. External quantum efficiency (EQE) is the number of photons leaving the device per electron fed in. Wall-plug efficiency (WPE) is optical output power divided by electrical input power. The authors estimate electrical efficiency by dividing WPE by EQE; it shows how much of the applied voltage is put to use.

Key findings

• A first for this device class. The authors call their work "the first demonstration of top-emitting AlGaN-based DUV LEDs". The emission wavelength is 275 nm, with a spectral width (FWHM) of 10 nm at 10 mA and no parasitic emission.
• Wall-plug efficiency. The packaged LED reaches a peak WPE of 6.5% at 7 mA, about 1.35 times that of the flip-chip reference.
• External quantum efficiency. The peak EQE is 7.5% at 10 mA.
• Electrical efficiency. About 85% at 10 mA.
• Output power. 38 mW at 200 mA.
• Light extraction. Below 50 mA, the top emitter delivers about 1.35 times the output of the flip-chip reference with the HfO2 cones and about 1.13 times without them. Because all devices come from the same wafer, the authors attribute the gain to better light extraction.
• Advantage grows with current. At 200 mA the output is about 2.1 times that of the flip-chip. The authors ascribe this to less self-heating, and pulsed measurements support it. In the top emitter the whole substrate (1 mm²) rests on the package, whereas the flip-chip dissipates heat only through contact pads with an area of 0.06 mm².
• No voltage penalty. Top emitter and flip-chip show the same current–voltage curve, with a turn-on voltage of 4.5 V and 6.5 V at 100 mA, although the p-contact area is reduced.
• Different beam shape. The flip-chip emits in a nearly Lambertian pattern, brightest straight ahead and falling off evenly to the sides. The top emitter sends more light sideways, and with the HfO2 cones it shows strong emission at about ±40° from the surface normal (measured at 5 mA).
• Even current spreading. Near-field images at 50 mA show uniform emission across the ring mesa despite the small central n-electrode, and strong emission from the inner and outer mesa edges of the top emitter.

What it means for the lighting industry

The study shows that a transparent p-contact material is not a precondition for a top-emitting deep-UV LED. An electrode that covers only a fifth of the mesa can carry the current without raising the operating voltage, and the chip no longer needs a submount.

In our view, the thermal result is as relevant for product developers as the efficiency figure. A chip that sits on the package with its entire base loses less output at high current, and in this study the gap to the flip-chip widened from 1.35 times to about 2.1 times at 200 mA. How large this advantage is in practice depends on how the flip-chip being replaced is mounted; the reference here had a thermal contact area of only 0.06 mm².

In our view, developers of UV-C modules should also note the beam shape. Emission peaking at about ±40° instead of a Lambertian pattern changes how reflectors, lenses and reactor chambers need to be laid out, and optics designed for flip-chip emitters cannot be assumed to work unchanged.

The efficiency should be read in context. A peak WPE of 6.5% still means that more than 93% of the electrical input becomes heat, and the peak occurs at 7 mA, far below the 200 mA at which the 38 mW were measured. In our view, the paper demonstrates an architecture and not a record: its own reference list includes a 2017 report of a 275 nm LED with an EQE above 20%.

The authors see a pathway to low-cost devices because the chip transfer step is eliminated. In our view, this remains to be shown. The paper does not quantify cost, and the micro-rings in this work were defined by electron-beam lithography, a slow research technique that would have to be replaced for volume production. For lighting designers the work has no immediate consequences; it concerns the UV-C sources that may later go into disinfection and curing products.

Limitations

The paper presents one chip design at one wavelength and does not state how many devices were measured. The comparison is with the group's own flip-chip LED of identical electrode geometry from the same wafer, which isolates the effect of the architecture but is not a benchmark against the best flip-chip devices. Lifetime, reliability and behaviour at elevated temperature were not studied, and the radiation pattern is reported at a low current of 5 mA only. The cost advantage is argued from the process flow and is not quantified.

Citation

Hao, G.-D., Taniguchi, M., Wei, L., Inoue, S., "High-Efficiency Top-Emitting AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes", ACS Photonics, 13 (21), 2026, pp. 6333–6338. DOI: 10.1021/acsphotonics.6c00589

Authors' affiliation: National Institute of Information and Communications Technology (NICT), Kobe, Japan.

Licence: open access under CC BY-NC-ND 4.0. © 2026 American Chemical Society.

Keywords of the paper: aluminum gallium nitride, deep-ultraviolet emission, light-emitting diode, top-emitting device, light extraction efficiency, high wall-plug efficiency.

Source: ACS Photonics, https://doi.org/10.1021/acsphotonics.6c00589 – read the original paper at ACS Publications