LEDs Get Ready To Take To The Skies
Solid-state lighting is making an impact. Niche applications are emerging that can enjoy the benefits of high-brightness LEDs where the overall advantages of solid-state lighting outweigh concerns over luminous efficacy and long-term color control.
One area where LEDs are poised to make significant inroads is on aircraft, and our team from the UK is working towards that goal through a project entitled "Novel LEDs for efficient lighting solutions (Novelels)". This aims to advance GaN LEDs and demonstrate this technology in a new generation of cockpit displays and high-reliability external lighting units for aircraft. We are backed with Â£1.7 million ($2.54 million) from the UK s Technology Strategy Board and are led by LED lighting system manufacturer Enfis Technology. Other partners in this three-year effort that kicked off in summer 2007 include end-users Agusta-Westland and Airbus UK, a pairing that is responsible for defining and qualifying cockpit displays and external lighting units. GE Aviation and Enfis have the role of developing new lighting systems featuring the University of Brunel s novel phosphors and optical coupling technology created by Exxelis Ltd. Underpinning all of this effort are innovations in GaN LED technology and epitaxy, pioneered by the University of Bath, IQE and Enhanced Photonics, a new spin-out company from the University of Southampton.
One of our primary motivations for the project is to improve the performance of LCD displays, which are widely used in aircraft cockpits. All aircraft, and especially helicopters, demand displays with rugged backlighting, which generally takes the form of a "serpentine" tube and diffuser, covering the active display area.
Serpentine lamps are not ideal – they introduce a single-point failure for the entire display, unless two or more are employed together. There are other weaknesses too. Drive electronics can introduce electromagnetic interference close to sensitive navigational equipment, due to the high voltages required by the lamp; and the inefficiency of the system increases the fuel payload and exhaust emissions.
LED backlights can overcome these problems, while delivering improvements in the clarity of the display. By selectively dimming and brightening areas of the backlight in response to the displayed image, it is possible to increase the display s apparent contrast range. And sequential strobing of the backlight allows moving images to appear sharper, while also easing detection of moving objects.
Applications of solid-state lighting for aircraft are not confined to displays – LEDs are already providing navigation lighting (the red, green and white lights deployed on the aircraft s wing tips and tail). In addition, LEDs could be used in taxi lights, landing lights and searchlights, where they could deliver weight savings, greater robustness and a cut in airline maintenance costs. LEDs could also allow aircraft designers to apply novel approaches to exterior lighting, such as beam pattern alteration and steerable lighting, which does not require moving parts or bulky optics. Aircraft aerodynamics could be improved, alongside a reduction in weight. Finally, the compactness of the LED could allow it to be deployed in novel exterior lighting applications that are out of bounds for conventional lighting, due to installation restrictions. However, these opportunities will require improvements in LED technology, from epitaxy all the way through to packaging. Achieving aircraft external lighting based on such improvements in LED technology is the other main aim of the Novelels project.
One of the LED s biggest limitations arises from the high refractive index of the chip. The large refractive index difference with air causes most of the light generated in the active layers to be trapped in the device by total internal reflection, before it is ultimately reabsorbed.
A primary objective of our project is to incorporate photonic crystal structures into LEDs, which increase device output by extracting light that would otherwise be confined to waveguide modes inside the chip. Photonic crystal structures that are etched into the LED s surface can act as a two-dimensional diffraction grating, scattering light that would otherwise be trapped in guided modes.
Scattering occurs at several zenith and azimuth angles, which are determined by the Bragg condition. (The zenith angle is the angle made with respect to the vertical axis. The azimuth angle is essentially equivalent to the angle of longitude on a globe). While zenith-angle light dependence is an advantage for LEDs serving displays and other aerospace applications, any dependency on the azimuth angle is a drawback because it produces uneven illumination of the display.
To overcome this, we have developed photonic quasicrystal (PQC) structures with a square-triangular tiling pattern that lacks short-range symmetry but exhibits long-range order. These structures reduce the influence of the azimuth angle on the scattered light intensity, but retain the desirable zenith-angle dependence of the light output. Enhanced Photonics and the University of Bath are leading the investigation of technologies to optimize the PQC design to meet the aerospace requirements.
PQC structures are formed by defining a square-triangular tiling pattern of circles with electron-beam lithography, and etching down into the p-type layer of a GaN-based LED with a Cl2/Ar plasma (figure 1). Altering the pitch of the PQC tiling adjusts the zenith angle of peak emission intensity.
LEDs with a PQC structure have been fabricated by members of the Novelels team. These devices feature a 730 nm pitch PQC etched into the surface. The 12-fold symmetry of these structures can be seen in the light power output variations with the azimuth angle at a given wavelength (figure 2). While apparent for a single light wavelength, this variation is washed out when the light power is integrated over the whole emission band, illustrating the advantage of PQC structures over periodic photonic crystals.
Even without optimization of the PQC or the LED, these emitters produce a 60% enhancement in light extraction efficiency. Simulations predict that even higher extraction efficiencies should be possible with PQC structures, and research continues to identify the factors that inhibit the performance of the LEDs produced in the lab. These simulations can also calculate the shape of the emitted light intensity profiles, which are a close match to experimentally observed values.
Another early milestone for our project was the demonstration of thick GaN layers grown by HVPE. These form the basis of freestanding GaN templates, a promising alternative to the sapphire substrates that are used to produce the vast majority of today s LEDs, which have a 13% lattice mismatch with GaN. This mismatch gives rise to a very high density of misfit dislocations that act as non-radiative recombination centers, limiting the device s internal quantum efficiency.
Switching the growth platform to SiC or silicon does not alleviate the high misfit dislocation density in the nitride film, unless steps are taken to mediate lattice mismatch. Even then, reducing the dislocation density by a factor of 2–3 orders of magnitude – the level that s probably needed for ultra-efficient LED lighting – is a tough proposition. Freestanding GaN templates, however, could overcome this technology-limiting problem. By growing thick layers of GaN, dislocations on inclined crystallographic planes meet and mutually annihilate, leaving a low-defect-density platform for LED growth. These templates could lead to a reduction in parasitic resistances, if electrically conducting GaN can be deposited.
LED manufacturers targeting solid-state lighting have strived to improve device efficacies at high current density. Efficacy falls as drive currents are ramped up but innovative designs can combat this droop. The cause of droop is being intensely debated within the research community, but a consensus appears to be emerging that Auger recombination – probably assisted by phonon interaction – is the root cause of efficiency droop in the best quality material, which has low dislocation densities.
Combating LED droop
Our team, led by a partnership between IQE and researchers at the University of Bath, is developing InGaN/GaN heterostructures that mitigate the effects of Auger recombination and have a very low dislocation density. One advanced design under development incorporates a wide InGaN well just below the active multi-quantum-well region. This well acts as a reservoir for electrons, leading to improved carrier injection efficiency into the active region. However, the well also has a secondary benefit – it enables the growth of InGaN barriers with an alloy composition closer to that of the well, which decreases the lattice mismatch between these two layers. This reduces the polarization field across the quantum wells, brings the electrons and holes closer together, to increase the radiative recombination rate. The upshot is a lower carrier density, reduced Auger recombination and more-efficient LEDs.
Lateral LEDs measuring 1 mm × 1 mm have been produced with this design that have side contacts to the n-type region. These devices emit at 460 nm and have a higher efficacy than a commercial equivalent at drive currents of up to 1 A (figure 3). Given the known limitation of non-uniform current injection in side-contacted LEDs, these results are highly encouraging because the commercial device has a roughened upper surface on the p-layer to enhance light output. In comparison, the Bath LED was grown on a patterned substrate, but had no upper surface roughening. Devices have also been fabricated by another member of our team, IQE, which were produced on a smooth substrate, had a smooth upper surface, and were therefore not optimized for high light extraction yet delivered a superior performance to the commercial LED used for comparison. Efforts at IQE and Bath are continuing, with the focus on improving epitaxial designs for high-brightness LEDs.
Members of our team are also trying to increase the LED output by reducing light trapped by total internal reflection. One common technique for increasing LED light extraction centers on the growth of nitride layers on roughened or patterned substrates, because this cuts total internal reflection. A possibly superior approach is to insert a photonic crystal structure at the substrate interface, which can direct the light output, just like a PQC in the upper surface of the chip. This potentially reduces absorption loss in the highly imperfect GaN-sapphire interface layer, as well as diffracting the light upwards.
We have recently started to develop patterned substrates, which are formed by a low-cost nanoimprinting technique. Reactive ion etching has created an array of 300 nm long, 130 nm wide and 100 nm deep holes in sapphire, with a period of 450 nm. Efforts at Bath have revealed that reductions in the dislocation density result from the creation of these nanostructures at the buffer-substrate interface.
Our portfolio of promising results obtained from the Novelels project will help to drive improvements in GaN LED technology. In addition, through GE Aviation and Agusta-Westland, our team has demonstrated prototypes of new generations of cockpit displays. Researchers at Brunel are playing their part by developing novel phosphors for converting blue light to white light, and engineers at Exxelis are encouraging new designs for display optics. As this project passes its halfway stage the research results are encouraging and it is on track to deliver its primary objectives – a new baseline for cockpit displays and low-cost exterior lighting on aircraft.
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