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Industry  LEDs T


he LED is, in general, getting bigger and bigger. Until recently, the most common dimensions for


an LED were 300 µm by 300 µm, a size that can generate enough light for backlighting the displays of handsets, laptops and other screens. But the killer applications for this decade and beyond, general lighting, requires far higher light levels, and this means chips with sides of at least 1 mm.


Making chips bigger, however, is not the only route to increasing the number of applications that LEDs can serve. New markets also beckon when the device’s dimensions are reduced substantially, so that its sides are less than 100 µm. Some markets can be addressed with single emitters of this size, but most of the opportunities require a battalion of them that form an LED array. These can be used to expose resists, confine and manipulate cells, play a role in measurements of fluorescent lifetimes, aid the acquisition of depth- resolved microscopic images and help to build direct- write photolithography systems.


One of the most exciting opportunities of all is in optogenetics, a nascent field in neuroscience that involves using high-speed optical methods to probe and control genetically targeted cells with intact neural circuits. Miniature LED arrays could help 1.5 billion sufferers of neurological disorders, such as Alzheimers, Parkinsons, depression and chronic pain. In addition, there are opportunities for miniature LEDs in visible light communications. This technology has already realized data transmission rates of up to 1 Gbit/s from a single pixel at 450 nm, using on-off keying non-return to-zero modulation.


At mLED, which is headquartered in Glasgow, UK, and was founded in July 2010, we are starting to tap into the many exciting opportunities associated with miniature LEDs. One of our core strengths is our exclusive licence that allows us to exploit patented research from Martin Dawson’s group at the Institute of Photonics (IoP), University of Strathclyde. The IoP has been at the forefront of research on micro-pixellated LEDs for more than a decade, and during that time it has demonstrated many technical achievements in this area, in collaboration with partners at several UK universities, including Edinburgh, Glasgow, St. Andrews and Imperial College London. In particular, the IoP group has pioneered the use of these micro-pixellated sources in optical microsystems.


Our mission is to create a range of industry-leading, high-brightness micro-displays that can be controlled by computer to provide pattern-programmable sources. We aim to work alongside system integrators, first developing prototype capabilities and then scaling up to production volumes. This way, we can provide bespoke designs that address specific applications and enable products to get to market fast. To make this happen, efforts have focused


Figure 1. Graphical user interface for matrix


addressable 64x64


demonstrator array, permitting simple


installation and turn-key operation for the customer’s specific application


on taking a patented technology and turning it into a robust commercial product. This quest has been aided by securing access rights to facilities at the West of Scotland Science Park in Glasgow. Here we can access a range of equipment dedicated to III-V manufacturing and development. This allows us to reduce variations in our processes and minimize capital expenditure. In addition, this approach reduces the risk of cross-contamination and process instability, while the standard process flows and building blocks that we have established give us the opportunity to scale to high-volume production.


Shrinking the size of LEDs At the heart of our displays are high-density arrays of miniature GaN LEDs, which can span the ultraviolet through to the blue and green (recent research at the IoP, however, shows that it is also possible for GaN LEDs to even emit in the green-yellow and amber – see the box “Stretching GaN beyond the green”). These arrays are the foundation of a high-brightness monochromatic GaN microdisplay or ‘pico-projector’ technology, which can be used to form full-colour micro- displays with the addition of pixel colour conversion


The LEDs in our arrays are typically just 2 µm to 80 µm in diameter. To form these devices, we use a series of processing steps that were developed at the IoP. These involve the growth of a GaN LED epiwafer on sapphire,


Figure 2.A number of device configurations highlighting the flexibility of the technology


centre-to-centre spacing of 23 µm.(b) 200 µm x 200 µm checkerboard device with pixel edge separation of 2 µm and (c) a magnified view of a single pixel in operation


January/February 2012 www.compoundsemiconductor.net 33


.(a) 128 microLED stripes each 20 µm x 500 µm on a


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