+44 (0)24 7671 8970
More publications     •     Advertise with us     •     Contact us
 
Loading...
Technical Insight

Magazine Feature
This article was originally featured in the edition:
Volume 30 Issue 3

UV LEDs go micro

News

Shrinking UV LEDs to a micron or so delivers multiple benefits, including higher efficiencies and opportunities in new applications.

BY JENS RASS FROM FERDINAND-BRAUN-INSTITUT FBH, BERLIN

Many applications can be served by the UV LED, including disinfection, sensing, and material processing. For this class of device, sources emitting within the far-UVC at wavelengths below 240 nm are attracting considerable attention, as they have the potential to deactivate pathogens without damaging human skin. However, the uptake of the UV LED is being held back by its efficiency – it is far lower than its siblings that emit visible light.

For those visible emitters, over the last few years much effort has been directed at scaling dimensions, with the diameter of the LED reduced to below 100 mm, and sometimes shrunk to less than 10 µm. Motivating this miniaturisation is the creation of light-emitting pixels for display technologies, such as augmented- and virtual-reality headsets.

Our research team at the Ferdinand-Braun-Institut FBH, Berlin, has brought these two worlds together by developing UV microLED arrays. This format offers two substantial advantages over conventional UV LEDs with large emitter areas: a higher efficiency, thanks to enhanced light extraction; and the opportunity to create individually addressable emitter arrays, which can generate temporarily and spatially modulated UV radiation patterns. A number of applications could benefit from these arrays, including material processing, mask-free photolithography, sensing, non-line-of-sight communication, and UV-fluorescence microscopy.

One issue that plagues LEDs emitting in the far-UVC is a fairly low emission efficiency. Typical values for external quantum efficiency (EQE) and wall-plug-efficiency are below 1 percent, due to a number of factors that include strong non-radiative recombination of charge carriers, a low injection efficiency, and a low light-extraction efficiency. For the latter metric, values are 10-20 percent for LEDs emitting in the UVB and UVC, and fall to about 5 percent for those that emit in the far-UVC.

The main reason for the poor light extraction efficiency is the high degree of total internal reflection at interfaces. The trapped light is subsequently absorbed in various layers of the LED chip (see Figure 1).


Figure 1. UV LEDs have a low light extraction efficiency (LEE), due to the trapping of photons in the chip and subsequent absorption within the device structure, including layers of GaN, metals, and insulators. Light trapping is more severe in far-UVC LEDs, since more transverse-magnetic polarised light is travelling in the quantum well plane.

For UVB and UVC LEDs, the light that’s generated in the AlGaN-based layer structure is emitted through the transparent substrate. In most cases this is sapphire, which has a refractive index in the UVC of 1.8 - 1.9. As the AlGaN stack has a refractive index of approximately 2.7 and the surrounding air a value of 1, only radiation within a small angular range can leave the chip. Consequently, most of the photons that hit the AlGaN-sapphire and sapphire-air interfaces do so at angles beyond the critical angle for total internal reflection, causing them to be trapped inside the chip. Since many of the materials typically used to make the LED are inevitably absorbing – light can be absorbed at the metal contacts, the GaN p-side contact layer, and sometimes the insulators – there is a low probability for photon emission after several reflections.

Unfortunately, the likelihood that light is trapped within an LED increases at very short UV wavelengths. Reaching this spectral domain requires an increase in the aluminium mole fraction in the active region of the AlGaN quantum wells. This leads to a re-ordering of the valence subbands, resulting in a switching of the optical polarisation of the generated photons from dominantly transverse-electric, with the electric field vector lying in the chip surface and the photon emission perpendicular to it, to transverse magnetic. Since light polarised in the transverse magnetic direction is emitted parallel to the chip surface, the majority of photons traverse with a direction outside the light escape cone, and thus have a very high probability of undergoing total internal reflection and subsequent absorption.

Figure 2. Light extraction from the UV microLED is enhanced by internal reflection at the mesa side wall and redirection towards the chip back surface. This requires small mesas with slanted and highly reflective side walls and small diameters.

Increasing the light-extraction efficiency is a common goal for all developers of LEDs. For devices emitting in the visible and UV, improvements come from introducing surface patterning, reflective contacts, and encapsulation techniques. However, it’s far from easy to enjoy success with these approaches with the UVC LED. Progress is held back by strong absorption and instability of materials under irradiation with high photon energies.

Getting the right angle
Our approach is to pattern the surface of the AlGaN-based LEDs into arrays of individual micro pixels with a slanted mesa side wall. With this architecture, photons emitted parallel to the chip surface are redirected towards the chip backside where they can be extracted (see Figure 2). To optimise performance, the micro pixels need to have a small diameter and smooth, reflective sidewalls with an angle near 45 °.

To fabricate these emitters, we began by growing LED layer structures made of AlGaN on 2-inch sapphire substrates by MOCVD. The active region in these epiwafers is designed to deliver single peak emission at 233 nm.

We patterned the wafer surface by photolithography and employed chlorine-based plasma etching to create circular mesas with diameters between 1.5 µm and 100 µm and a sidewall angle of 45 ° to 50 ° (see Figure 3). While this step proved to be very challenging, it is crucial for guaranteeing a high efficiency through the re-direction of transverse-magnetic polarised photons. Our work involved having to fine-tune the parameters for the photolithography process on the transparent and heavily bowed wafers, and developing suitable plasma etch conditions, including the gas mixture, plasma power, and pressure.


Figure 3. The mesa definition is a crucial step in the production process, in particular the adjustment of the sidewall angle, as well as the alignment of the following layers.

Following etching, we deposited and annealed p- and n-metal contacts. In order to realise a high reflectivity at the tilted mesa surface, we then deposited a SiO2 layer. This electrically insulating oxide is fully transparent in the entire UV range, and offers a high refractive index contrast to AlGaN, enabling it to act as a reflector. We capped the SiO2 with aluminium, also acting as a reflector, before adding thick metal pads for an electrical contact.

The final processing steps involved dicing LED chips from the processed wafer and mounting them on ceramic submounts in a flip-chip geometry. For comparison, we also produced LEDs that incorporate a different insulator, by replacing SiO2 with SiNx. Since SiNx has a smaller refractive index contrast to AlGaN and absorbs in the UVC range, LEDs with this insulator should not provide a significant enhancement in light extraction.

Miniature marvels
Measurements of the electroluminescence characteristics of our devices, using an integrating sphere, reveal that the micro pixel architecture produces a very strong increase in output power, and therefore efficiency. Moving from a conventional design to a pixel diameter of just 1.5 µm produces a fourfold hike in peak EQE (see Figure 4). Scaling to such small dimensions is needed to benefit from miniaturisation – there is no benefit for only reducing the LED diameter to 5 µm. Note that this is not the case with visible microLEDs, where benefits are already realised for devices with diameters of 10 µm or even more. What’s more, for our devices the peak EQE occurs at current densities between 70 A cm-2 and 200 A cm-2, while for LEDs in the literature, the record peak EQE tends to occur at very low current densities that are unsuitable for applications.