Sterilising with far UVC

A revolutionary short-period superlattice holds the key to high-performance LEDs emitting deep within the UVC.

BY WILLIAM LEE AND LIAM ANDERSON FROM SILANNA

One of the lasting legacies of the pandemic is an increased interest in establishing efficient ways to deactivate harmful micro-organisms. Of the many options available, one that has received a tremendous amount of attention is UV sterilisation, especially in the form of the UV LED. This source offers a compact form factor, a chemical-free nature and high germicidal efficiency. However, UV LEDs come with a severe health warning: sterilisation based on most of these sources is inherently dangerous for everyday consumer use, due to the damaging effect of UVA, UVB and UVC radiation on skin and eyes.

Fortunately, there’s a solution, involving heading further into the UVC. According to a number of recent studies, when UV light is shorter than 240 nm, it is fully absorbed by our stratum corneum, the outermost layer of skin, which is 10-40 µm-thick and made up of dead cells. Due to this, light in the far UVC deactivates viruses and bacteria, but does not penetrate the shield of dead cells, so is ideal for sterilisation, because it leaves healthy cells unharmed and safe from damage.

Drawing on this insight, in 2020 The American Conference of Governmental Industrial Hygienists issued a Notice of Intended Change, which proposed an increase in the exposure limit to 8 hours, for skin and eyes subjected to UV light with a wavelength less than 240 nm. This move propels far-UVC LEDs into the spotlight, as successful commercialisation of high-power sources based on this device would enable effective sterilisation tools in homes, on public transport and on planes. Implement this strategy and when the next pandemic arrives, far UVC LEDs could provide the best weapon in our defence.

Making far-UVC LEDs
Unfortunately, stretching the emission of the LED to the far-UVC is far from easy. Ever since Nobel prize winner Shuji Nakaruma generated p-type doping in GaN and demonstrated the blue LED, researchers have been developing LEDs that emit at shorter and shorter wavelengths. Realised by increasing the aluminium content in AlGaN ternary alloys, this has led to the manufacture and commercialisation of LEDs emitting in the UVA, the UVB, and more recently, the UVC. However, there is a roadblock at 260 nm, due to an inherent and unique property of the AlGaN material system that is hampering the production of high-power, efficient LEDs emitting in the far-UVC.

The inherent drawback of AlGaN comes from its bandstructure at high aluminium content. For this alloy, the valence band is split into the heavy hole, light hole, and spin split-off sub-bands, with an ordering that depends on various physical parameters of the ternary alloy. For a low aluminium content, the heavy-hole band is the top-most band, with the majority of electron-hole recombination taking place between the conduction band and the heavy-hole band. When this happens, this transition is polarised in a way that favours vertical optical emission, corresponding to transverse electric polarisation, an orientation that assists the coupling of light out of the structure.