Future challenges in microLEDs

Isotropic ALE is able to remove sidewall damage, and has potential benefits in many applications, including microLEDs.

MicroLEDs, LEDs with dimensions from 1 µm to a few hundreds of microns, are currently receiving a lot of attention. With much higher light output and lifetime, and lower operating powers than OLEDs, if two or three challenges can be overcome, microLEDs could empower the next generation of displays in areas including AR/VR, automotive, wearables, phones and large displays.

GaN LED mesa structures defined by plasma etch, shown at increasing magnification (images: Oxford Instruments Plasma Technology).

The main challenges revolve around cost, though it has been predicted that they will be at a similar cost to the (2024) cost of OLEDs by 2026. There are also challenges around the fabrication of red LEDs and reducing the loss of light output as dimensions shrink.

It is known that the plasma etch process, that is used to define GaN LED mesa structures, damages the sidewalls and creates non-radiative recombination (NRR) states, which reduce the light output, usually quantified as external quantum efficiency (EQE). Oxford Instruments Plasma Technology has an active programme in producing a low damage mesa etch, but also an etch capable of removing the damage in the mesa sidewalls. Early results show promising increases in luminescence of bulk material, and further work on LED structures is planned. The images below show the effect of isotropic ALE on the (Cathodo-) luminesence of a damaged n-GaN mesa. In Fig. 1, the mesa top, which is protected by the mask during the etch, shows no damage and gives a bright luminescence, whilst there is damage in the GaN around it, hence giving out low luminescence. Fig. 2 shows that after treatment with isotropic ALE, which removes material from the top of the GaN and the sides, the signal is much brighter and the undamaged top is indistinguishable from the previously damaged backgrounds

Figure 1. (Cathodo-)luminescence after mesa formation by plasma etch (scale bar 5 µm).

Figure 2. (Cathodo-)luminescence after additional isotropic ALE damage removal (scale bar 5 µm).

A further challenge is in the manufacture of red-emitting LEDs. The green and blue GaN-based types are well known and achieve EQEs near to 50%, but for red, the record is <10%. Consequently, red is often made using AlGaInP, as opposed to the alloys of Al, In and GaN used for the other colours. There is an advantage, not least, in the fabrication of monolithic three-colour LEDs (i.e. each LED element has red, green and blue generating structures operating simultaneously). Here, the basic material system has to be GaN and its alloys. InGaN, which is the best candidate for red LEDs, is very difficult to make, though this is at an early stage.

To find out more about ALE and ALD technologies, visit plasma.oxinst.com

This was a sponsored post by Oxford Instruments (email: Grant.BALDWIN@oxinst.com)