ICULTA 2023: Expanding horizons for deep-UV LEDs

Tremendous progress is underway with the deep-UV LED, in terms of its use in healthcare and it gains in output power, efficiency and lifetime, in both the UVC and far UVC.

BY MICHAEL KNEISSL FROM TU BERLIN AND THE FERDINAND-BRAUN-INSTITUT

Devices that attract a great deal of attention tend to have breakthroughs on many fronts. As well as a hike in performance, they often undergo improvements in reliability, along with increasing interest from more applications, opened up by superior characteristics. That’s certainly the case for the deep-UV LED. Its increasing power and lifetime in both the UVC and the far UVC is attracting interest for fighting hospital germs and airborne viruses.

Reports of progress in all these areas lay at the heart of the third International Conference on UV Light Emitting Technologies and Applications, known as also ICULTA 2023. Held in Berlin, Germany, from 23-26 April 2023, this year’s meeting, which attracted more than 220 attendees from 27 countries, was organised by the recently established Advanced UV for Life e.V. association, together with Ferdinand-Braun-Institut (FBH), Berlin, and in collaboration with the International UV Association (IUVA).

Building better devices
Two solid-state lighting heavyweights helping to drive progress in the UVC are ams Osram and Nichia. At ICULTA 2023 representatives from both these chipmakers unveiled their latest developments in UVC-LED technologies, showcasing peak powers in the disinfection band between 260 nm and 280 nm.

Speaking on behalf of ams Osram were Project Manager Marc Hoffmann and Director of Product Management Christian Leirer. As well as outlining the company’s advances, this duo delivered an excellent overview on the latest advances in UVC-LEDs, as well as their use in industrial and consumer applications. The application space is now moving from consumer to industrial applications, influencing the requirements for UVC-LEDs. To support this trend, ams Osram is improving the performance of its UVC LEDs. Current products are delivering output powers of up to 100 mW when driven at 250 mA, and have a lifetime of 10,000 hours, judged in terms of L70, which is the time it takes for the output power to fall to 70 percent of its initial value. According to the speakers, the third generation of devices from ams Osram are producing up to 200 mW at a wall-plug efficiency (WPE) of 10 percent.

The company has set itself a very ambitious goal, targeting a WPE of 20 percent by 2026, a L70 lifetime of 40,000 hours, and a pricing level that’s just ten times above that of the current cost of the mercury lamp. Such a level of improvement would be revolutionary, ensuring that the cost of a UVC-LED based system is lower than that of a conventional mercury-lamp based system. This rival to the incumbent would also deliver energy savings, alongside enhanced product safety and reliability, thanks to the combination of UVC-LED and UV sensors. Note that this forecast of progress lies within the range that I gave in my presentation (see Figure 1).

Figure 1. Wall-plug efficiencies for production level UVC-LEDs and engineering prototypes in the 250 nm - 280 nm and 225 nm – 235 nm wavelength bands. Credit: Michael Kneissl/TU Berlin

Representing Nichia Europe, Managing Director Ulf Meiners offered an interesting comparison between the chronology of WPE increases for the blue and the UVC-LED, noting that both technological developments appear to be on a similar fast-paced trajectory. Meiners didn’t present an extended outlook, but forecast that the WPE of Nichia’s devices will hit 7.5 percent in 2024, with output power at 280 nm reaching 150 mW.

While many make a direct comparison between UVC-LEDs and mercury lamps using individual performance parameters, Meiners argued that this is not very meaningful. He emphasised the importance of studying concrete use cases that account for the specific properties of UVC-LEDs. For example, it is important to be aware of completely new, more efficient system designs that cannot be realised with conventional UV sources. Merits of deep-UV LEDs that can increase the light utilisation efficiency of the system include point source emission, which enables a tailoring of the source to fit the application, and the opportunity to trim energy consumption – UVC-LEDs are easily dimmable, and can be turned on-and-off rapidly, allowing activation only when it’s needed and at the required power level. This point of view was reinforced by Alexander Wilm, Senior Key Expert Applications at ams Osram, who described several use case scenarios in which UV-C LEDs are already outperforming conventional UV-C lamps despite their lower WPE.

An interesting difference between the two heavyweights is the peak wavelength of their UVC LEDs. Nichia is sticking with its 280 nm LED technology platform, reasoning that the reliability of the UVC-LED is higher at the slightly longer wavelength, while ams Osram is focusing on LEDs centred near the germicidal efficiency peak that’s around 265 nm.

Competition for the production of class-leading devices is coming from a handful of other suppliers, including Crystal IS, a subsidiary of Asahi Kasai. Delegates at ICULTA were given an update on the capability of these devices by Rajul Randive, Director, Applications Engineering, Crystal IS. Randive revealed record performance levels for the company’s 265 nm LEDs, with single-chip output power now as high as 160 mW at 500 mA and maximum WPE reaching 6.5 percent. Unlike most UVC-LED products, those made by Crystal IS are grown on internally produced, low-defect-density bulk AlN substrates. Threading dislocation densities in bulk AlN are many orders of magnitude lower than those found in the heterostructures of UVC-LEDs grown on sapphire. The company attributes the far lower level of threading dislocations to superior device lifetimes. The L70 lifetimes of Crystal IS devices are already reaching 25,000 hours at a drive current of 350 mA.

Left: Some of the key people responsible for organizing ICULTA 2023 were (from left to right): Martin Strassburg (ams Osram, Chairman of ICULTA 2023); Neysha Lobo-Ploch (Ferdinand-Braun-Institut, Program Chair of ICULTA 2023); Michael Kneissl (TU Berlin & Ferdinand-Braun-Institut, Chairman of Advanced UV for Life e.V.) and Sven Einfeldt (Ferdinand-Braun-Institut, Chairman of ICULTA 2021). Photo: T. Rosenthal

Far UVC: Improving health…
Interest in far-UVC emitters has been boosted by recent, very encouraging studies on in-vivo inactivation of multi-drug resistant bacteria, fungi and viruses. Reports of efforts on this front were provided at the latest ICULTA meeting by those working at Columbia University, New York, and Charité – Berlin University of Medicine.

According to David Brenner, Director at Columbia Medical Centre, Centre for Radiological Research, while the effects of the Covid-19 pandemic are fading, there are many other airborne pathogens causing significant infections and death every year, including influenza and tuberculosis. His group has shown that a dose level of just 2 mJ cm^-2 at 222 nm reduces the viral load by several orders of magnitude for SARS-Cov-2 and other pathogens.

Several experts attending ICUTA 2023 stressed that antimicrobial resistance is one of the biggest health challenges facing humanity. In the US alone 2.8 million people are infected by multi-drug resistant germs every year, and for 35,000 of them this is fatal. Globally, the number of deaths is estimated to be around 1.2 million per year and rising.

The deep-UV LED industry is well-positioned to help save lives. Martina Meinke, Head of Experimental Skin Physiology at Charité – Berlin University of Medicine, explained that far-UVC LEDs emitting at around 233 nm can be an effective and safe tool in the fight against MRSA, MSSA, and other germs.

Meinke’s team has already realised very encouraging results from clinical trials on volunteers. Typical dose levels of 40 mJ cm^-2 show only minor DNA damage in human skin, equivalent to less than 0.1 MED (Minimal Erythema Dose), and there are no signs of DNA damage after 24 hours. Spurred on by these very promising results, Meinke and colleagues are extending the scope of their investigations to additional hazardous germs, including fungi like candida albicans. The group is also starting to investigate the treatment of other parts of the human body, in particular the oral and nasal mucosa.

Additional work at improving healthcare includes an innovative UVC phototherapy platform for decontaminating chronic non-healing wounds. Involved in this pioneering project is Mark Gerber from Spectrum Medical Technologies. He explained that when they used 235 nm UVC-LEDs from Asahi Kasai to deliver a dose level of 80 mJ cm^-2, this led to a 95 percent reduction of MRSA on human skin. Gerber claimed that 235 nm emitters are better than 222 nm excimer lamps for biofilm inactivation due to the slightly deeper penetration depth, while still not causing significant skin damage. Initially, the technique will be used in nursing homes, hospitals and doctor’s offices.

Figure 2. Schematic of a 234 nm LED heterostructure with a distributed polarization doped (DPD) AlGaN hole-injection layer. (Source: T. Kolbe et al. Appl. Phys. Lett. 122 191101 (2023))

… and increasing output power
A number of presentations at this year’s ICULTA outlined options for improving the performance of deep-UV emitters. They included a talk by Leo Schowalter, co-founder of Crystal IS and currently a designated professor at Nagoya University, who discussed the pseudomorphic growth of AlGaN heterostructures on low-defect-density bulk AlN substrates. Schowalter made the case for the native substrate, pointing out that despite the significant lattice mismatch between AlN and Al_1-xGa_xN, which increases with gallium concentration, threading dislocation densities in the active device region of UVC LEDs grown on high-quality bulk AlN substrates are well below 10⁶ cm^-2. The lower density of threading dislocations aids all forms of light emitter, with Schowalter arguing that it is behind major performance improvements in far-UVC LEDs, and is to thank for the demonstration of continuous-wave laser diodes emitting near 272 nm. He also mentioned recent advances in the efficiency of sub-230 nm LEDs, with EQE now as high as 0.5 percent. This work has just been published in Applied Physics Letters (see H. Kobayashi et al. Appl. Phys. Lett. 122 101103 (2023)).

One of the challenges with deep-UV LEDs is the injection of holes into the device. An approach that addresses this and has been adopted by the Ferdinand-Braun-Institut is to employ a distributed polarization doped, hole-injection layer. Discussing devices with this feature, spokesman Tim Kolbe explained that the foundation for their LED heterostructures are MOCVD-grown 2-inch diameter AlN-on-sapphire templates with double growth and double annealing. The team estimates that the threading dislocation density in these templates is around 2.1 x 10⁸ cm^-2, based on values for the full-width at half-maximum for (00.2) and (10.2) X-ray rocking curves. On the annealed templates the team has grown LED structures that feature an AlGaN multiple-quantum-well active region, a 2 nm thin AlN electron blocking layer, and an AlGaN distributed polarization doped hole-injection layer with an aluminium mole fraction gradient ranging from 100 percent to 82 percent (see Figure 2).

Compared with their visible cousins, light extraction is considerably more challenging with the far-UVC LEDs. Offering a new approach to this are researchers at the Ferdinand-Braun-Institut, who are using microLED arrays to enhance the extraction of far-UVC emitters. Involved with this work is Jens Rass, who told delegates that the team applied precisely tailored plasma etching and metallisation steps to create two-dimensional arrays of UV emitters with diameters down to 1.5 µm and a spacing as short as 2 µm (see Figure 3). Since the emitted light can be redirected by the inclined mesa sidewalls, shrinking the emitter diameter leads to tremendous gains in light extraction efficiency. By carefully selecting and optimising mesa diameters, insulator materials, and mesa sidewall angles, Rass and colleagues have demonstrated record external quantum efficiencies of 1.6 percent for far-UVC microLEDs that incorporate micro-pixels with 1.5 µm diameters and a SiO₂ insulator for sidewall passivation. These 233 nm emitters are capable of output powers of 1.7 mW and 3.5 mW at 20 mA and 50 mA, respectively.

Figure 3: Scanning electron micrograph image of UV microLED array with 2 µm pitch. Credit: Ferdinand-Braun-Institut (FBH), Berlin, Germany. Source: https://www.fbh-berlin.de/en/research/research-news/arrays-of-ultraviolet-micro-leds-for-production-sensing-and-communication

One of the newcomers in the field of far-UVC LEDs is the Australian company Silanna UV. Its team leader for MBE processing, William Lee, spoke to those attending ICULTA 2023 about the company’s development of high-power, far-UVC LEDs emitting at 235 nm. The MBE tool at Silanna UV is capable of producing epiwafers up to 6 inches in diameter, and is used to grow LED heterostructures that feature short-period superlattices for the silicon-doped current-spreading layers as well as for the active region. These superlattices, comprised of alternating AlN and GaN layers, extend the spectral range of the transverse-electric-dominant emission, as well as lowering the activation energy of donors. Resistivity can be as low as 0.015 Ω.cm for an n-type short-period superlattice that’s equivalent to Al_0.8Ga_0.2N. Drawing on this innovation, the team at Silanna UV has fabricated 235 nm LEDs. Driven at 200 mA, these devices deliver an output power of up to 7 mW at a WPE of 0.4 percent. Dialling back the drive current to 20 mA ensures an L70 lifetime of 2,800 hours.

Torsten Jenek from Heraeus Noblelight, the second Program Chair of ICULTA 2023, presented prizes for the best student poster. The winner was Tim Achenbach from the Technical University, Dresden, for the paper Novel UV dosimetry for surfaces based on the interplay of oxygen and room-temperature phosphorescence (award collected by colleague Linda Steinhäußer, FEP Dresden, far right). Sharing second place were Iman Roqan (second from the right) from KAUST (Sub-quantum well effect on the carrier dynamics of Al-rich AlGaN/AlGaN multiple-quantum-well DUV LEC structure grown on AlN substrate) and Silke Lohan (far left) from Charité – Berlin University of Medicine (Radical formation in skin at different wavelengths: from UVC to NIR). Photo: T. Rosenthal

The advances reported at this year’s ICULTA underscore the great rate of progress in UVC and far-UVC LEDs, as well as medical treatments based on their emission. Further breakthroughs will follow, with many sure to be reported at ICULTA 2025, which is planned to be held again in Berlin.