Monday, May 9, 2022
Kubos Semiconductors Ltd., a developer of LEDs based on cubic gallium nitride, has identified in collaboration with researchers at Manchester and Cambridge Universities the potential for Kubos’s proprietary material to deliver LEDs capable of switching at faster speeds than those produced in the hexagonal crystal phase.
Kubos is working on improving its cubic GaN technology to make more efficient green, amber, and red LEDs, as well as red microLEDs, for a variety of lighting and display applications. According to Kubos, its potential to drastically cut carbon emissions associated with solid-state lighting is well-recognized, and this might save up to 120 megatons of CO2 emissions on an annualized basis, which is comparable to the emissions of 32 coal-fired power plants. However, the ability to play a secondary role in offering quicker switching speeds in communications applications is revolutionary.
In an interview with Power Electronics News, Caroline O’Brien, CEO of Kubos Semiconductors, has highlighted as Kubos has demonstrated all the key elements of cubic GaN material, including the elimination of the quantum-confined Stark effect (QCSE), which limits the efficiency in hexagonal GaN LEDs.
“The GaN market is growing very fast and gaining significant traction in the power device markets,” said O’Brien. “As we know, it is already dominant in lighting applications and has largely replaced conventional light bulbs across the globe. Kubos’s cubic GaN technology has the potential to deliver further efficiency improvements in lighting and unlock the potential of microLEDs for displays and will therefore contribute to the further adoption of GaN technologies. Kubos plans to license its technology to LED and epitaxial manufacturers to ensure that the technology can be adopted as quickly as possible. We believe cubic GaN can remove the roadblock caused by the poor efficiency of InGaAlP red microLEDs, which is currently holding back the augmented and virtual reality [AR/VR] display markets.”
O’Brien pointed out that solutions based on non- and semi-polar hexagonal GaN are the main strategies that have been pursued to overcome the limitations in conventional GaN LEDs. Other approaches such as nano column devices based on hexagonal GaN have also seen lots of interest.
“The key differentiator for cubic GaN is that rather than having to work around the limitations of conventional GaN, it actually removes or reduces them,” said O’Brien. “There are no internal electric fields inherent in its structure, the narrower bandgap naturally shifts emission toward longer wavelengths, and higher hole concentrations and mobilities are possible, all of which contribute to improving performance.”
GaN for high-speed communications
According to Kubos, carrier lifetimes in cubic GaN quantum wells have been recorded at 0.5 ns, which is more than 20× quicker than standard c-plane hexagonal GaN LEDs (10 ns). These shorter carrier lifetimes, together with other advantageous features of cubic GaN, open up the possibility of developing LEDs that can switch at very high rates (>1 GHz) over the visible spectrum.
The communication sector of the LED market is an application area that is gaining momentum. In this regard, Li-Fi is an emerging market that could develop with such technology in an interesting way by improving communication speeds. The Li-Fi system is a two-way multi-user communication system and could be classified as a nano-wave communication system. The Li-Fi communication system is different from the visible light communication (VLC) system because VLC is a point-to-point communication system, while Li-Fi is a wireless network system that supports point-to-multipoint communication. In a Li-Fi system, the data rate can be correlated with LED properties, so LED selection plays a key role. Parameters such as LED size, on/off switching speed, and the number of LEDs used in an application can affect the data rate of the communication model. The data rate is inversely proportional to the LED size, meaning that the smaller the LED size, the higher the data rate will be.
“The improved efficiency along with the demonstrated short carrier lifetime available in cubic GaN LEDs means that it will be possible to make LEDs that can be switched faster and deliver more light than conventional LED technologies,” said O’Brien. “This will translate directly to increased data transmission speeds in Li-Fi and other visible light communication applications.”
According to O’Brien, the manufacturing has been already demonstrated on 150-mm silicon wafers, and she believes the technology can be readily scaled to 200-mm wafers and beyond. “The main challenge is controlling the wafer bow of larger wafers, and we are currently implementing strategies to achieve this by applying the well-understood wafer-scaling techniques that have been developed for GaN and other compound semiconductors over many years,” she said. “It is also noteworthy that our cubic GaN material is manufactured on industry-standard compatible MOCVD reactors, which means it can be adopted without modification into established large-volume production lines.”
Because silicon wafers may be up to 450 mm in diameter and are compatible with the vast silicon foundry infrastructure, GaN-on-silicon is widely regarded as the ideal platform for low-cost, high-volume manufacture. According to O’Brien, this is why GaN-on-silicon power devices may now be manufactured on 200-mm wafers. “Kubos’s technology is tailored to this trend, as it makes use of conventional CMOS  silicon wafers to allow the platform to scale as needed,” she said.
O’Brien noted that the focus of the next stage of development is to improve the internal quantum efficiency of our LED structures and, as we go forward, demonstrate that cubic GaN can exceed the performance of the current solutions in the market for green LEDs and red microLEDs.
“With regard to costs of the cubic GaN technology, preliminary studies we have undertaken show that with larger wafer sizes, combined with the fact that we are on a silicon platform, we can be at cost parity with conventional LED solutions, and as wafer sizes are scaled further, we have the potential to become more price-competitive,” O’Brien concluded.
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