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Industry  LEDs


which is processed into an array of pillars with a combination of photolithography and etching with an inductively coupled plasma.


Figure 3.8 x 8 individually addressable microLED array demonstrator unit operating at 450 nm


As with conventional LEDs, a flip-chip architecture holds the key to far higher light extraction efficiency. This is realised by adding several layers to the processed epiwafer: An n-contact; a silicon dioxide insulator layer; and then a highly reflective, common p-contact. After the devices are formed, chemical mechanical polishing reduces the thickness of the sapphire substrate.


Efforts at the IoP demonstrate that it is possible to make thousands of miniature LEDs into arrays that are collectively addressable and have a total active area equivalent to that of one ‘conventional’ LED chip. With this passive matrix approach, only 2N contacts are required for an N x N matrix. Using this, back in 2004 IoP demonstrated a 128 x 96 passive matrix device with 12 µm x 16 µm pixels on a 20 µm center-to-centre spacing. Occupying an active area of 3 mm x 2 mm, displays based on blue and green devices were produced with a luminance in excess of 30,000 Cd/m2 (This is brighter than today’s state-of-the-art micro- displays based on organic LEDs, a technology described in the box “What are the alternatives?”).


We have improved the performance of these displays Stretching GaN beyond the green


Researchers at the IoP, in collaboration with colleagues at Peking University and Epilight, Shanghai, have recently fabricated GaN LED micro-arrays emitting in the yellow-green and amber.


Producing GaN LEDs operating in this spectral range is very challenging, because the high indium content required in InGaN quantum wells emitting in the yellow and amber tends to produce material imperfections that hamper light emission. To overcome these issues, the researchers have produced LEDs containing an electron reservoir layer that increases radiative recombination and cuts electron overflow.


Yellow-green and amber LED epiwafers were grown by MOCVD on sapphire substrates. The shorter-wavelength variant features a two-period electron reservoir layer with interlaced 3 nm-thick In0.12


Ga0.88 Ga0.7 N and 10-nm


thick GaN, and an active region with a six period multiple quantum well – 3 nm-thick In0.3


N wells and 10-nm thick GaN barriers. The design for the amber LED had a 65 nm-thick, 5 period electron reservoir with interlacing layers of In0.18


Ga0.82 quantum well with 2.5 nm-thick, In0.4


N and GaN, and a five-period multi- Ga0.6


N wells.


Epiwafers were processed into LED arrays with 10 by 10 pixels, each with a diameter of 40 µm. A typical pixel in the yellow-green LED array emits at 560 nm, has a turn-on voltage of 3.2 V and a peak output power of 100 µW. This corresponds to a maximum power density of 80 mW mm-2


. In comparison, the 600 nm amber LED pixels turn-on at 3.8 V and have a typical peak output power of 55 µW, corresponding to a maximum power density of 44 mW mm-2


. 34 www.compoundsemiconductor.net January/February 2012


Figure 4.Colour conversion process for ultraviolet microLED emitter array using direct inkjet processing


with matrix-addressing schemes that are suitable for high-resolution applications. All pixels along one column share a common anode and all pixels along one row share a common cathode. We now offer a turn-key 64 x 64 demonstrator unit. This light engine is supplied with a simple-to-use graphical user interface, offering the ability to interface easily with a host of system applications.


It is also possible to address every pixel individually. We can do this with our bespoke CMOS backplane technology and flip-chip bonding techniques, which combine to control the output power of microLEDs. By employing a novel device arrangement, heat-sinking improves, opening the door to increased current density handling. Thanks to the possibility to use emission through the polished sapphire substrate (inert window), devices can be used in close proximity to an object, or the microLED emitter plane can be optically relayed/ imaged. Alternatively, aligned microlenses can be monolithically integrated and formed in the sapphire substrate.


Driving the arrays To drive these pixels, we use a CMOS design based on standard low-voltage 3.3 V logic. However, we have developed a technique for the driver array that enables biasing above or below ground prior to an excitation signal being applied by the driver. This means that for higher output power density applications each diode can be biased at voltages greater than 5 V to allow the LED to be driven at high currents and therefore provide higher output power. LEDs can be driven continuously, or with excitation pulse widths that can be as short as just 300 ps (FWHM). A collaboration involving the IoP and other universities integrated such devices with arrays of single-photon avalanche diodes, and in 2010 the partnership claimed that it had made the smallest reported solid-state microsystem for fluorescence decay analysis.


Flexibility of the lithographic mask design enables the manufacture of various structures, including micro-disc, micro-stripe and chequerboard arrangements. Applications requiring structured illumination can be catered for with a stripe configuration, and high fill factors in excess of 98 percent are possible with chequerboard structures. We currently offer a turn-key 8


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