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Industry  LEDs


x 8 microLED demonstrator unit for system integration. The IoP has also fabricated arrays with colour conversion, realised by integrating photocurable nanocomposites and polymer blends onto the micro-LEDs. These are added by ink-jet printing, and convert ultraviolet or blue output of the array into a ‘RGB’ display. This team of researchers has also demonstrated self-aligned direct writing and colour conversion with a colloidal quantum- dot nanocomposite. In this case, the ultraviolet microLED cures the nancomposite in registry with the underlying pixels, and is then down-converted by this composite film.


Many applications have minimum levels for brightness, and if the lumen output exceeds this figure, so much the better. Our LEDs are outstanding in this regard, with power densities in excess of 3250 mW/mm2


per pixel


produced by 8 x 8 individually addressable 14 µm emitters operating at 450 nm. In comparison, conventional high- brightness LEDs are typically 700 mW/mm2


.


when driven in DC; that’s two orders of magnitude higher than that for high-brightness LEDs. Operating at this very high current density does not require any specific heat-sinking arrangements, and junction temperatures are low, thanks to the architecture of our arrays. What’s more, there is still room for further improvement in the performance of arrays of miniature LEDs: We are currently developing more efficient light extraction techniques that will boost lumen output and enable this technology to target the general lighting market.


Our miniature LEDs have operated at a current density of 18 kA/ cm2


One area where our novel LEDs could soon start making an impact is the field of optogenetics. Neurological disorders affect more than one in five people across the world, and the total bill for treatment exceeds $1 trillion per year. Drugs, neuromodulation, surgery and talk therapy are all used today either to improve or control a patient’s condition. However, in future, optogenetics may be added to that list. Although optogenetics research is at an early stage,


“What are the alternatives?”


The market for micro-displays is well established, with several technologies competing in this arena. This includes spatial light modulators (SLMs) based on digital micro-mirror devices (DMDs), which have proven to be very successful for projection and now pico-projection systems. They have some highly desirable attributes, being able to form high-resolution full-colour micro-displays and operate at high modulation frequencies of several kHz. However, there are certain applications where micro displays based on DMDs - as well as those incorporating LCDs and organic LEDs – are not ideal.


DMDs, like other SLM technologies such as LCDs, are based on switching matrices that require external light sources and associated optics. Thus, a key drawback of the DMDs and LCDs is low efficiency, which stems from image patterning created by redirecting unwanted light out of the excitation field (micro-mirrors can only switch between an ‘on’ or ‘off’ state). According to mLED, there are several applications where, on average, only 20 percent or less of the pattern is illuminated. In other words, 80 percent of the incident light is ‘lost’. These losses result in unwanted heating, plus the need for bulky cooling systems requiring battery power. The upshot is a limit for maximum power that can be transmitted through DMDs, which is particularly stringent for ultraviolet applications, as absorption at the mirror causes local heating issues.


Displays made with organic LEDs are significantly different from those based on DMDs and LCDs, because they use an emissive technology. Some of their strengths include a flexible, lightweight display, and the promise of low-cost, high-volume manufacture. However, organic LED displays can only serve a narrow range of applications, due to several limitations. These include low levels of brightness; short product lifetimes, due to short lifetimes of the intrinsic materials; and poor reliability, because the materials can oxidize and are sensitive to ultraviolet light.


rapid progress is being made. Our devices have a great deal to offer here, because they can deliver light of the required wavelength at sufficiently high power densities using very high switching speeds. We plan to launch specific products for this growing market over the next 24 months, and also investigate other opportunities for LED arrays. Their success will highlight that making LEDs smaller, just like making them bigger, opens the door to new and lucrative applications for these solid-state emitters.


© 2012 Angel Business Communications. Permission required.


Figure 5.Advanced processing equipment used in the formation of microLED arrays.


Further reading Choi et al,IEEE Electron Device Lett.25277 (2004) J.McKendry et al. IEEE Photonics Technology Lett.21811 (2009) B.R.Rae et al.IEEE Transactions on Biological Circuits and Systems 4437 (2010) J.McKendry et.al.IEEE Photonics Technology Lett.221346 (2010) K.Deisseroth et.al.Journal of Neuroscience 2610380 (2006) D.E.Moorman et.al.Nature,News & Views 458980 (2009)


January/February 2012 www.compoundsemiconductor.net 35


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