Solid-state electronics has been transforming our lives for many decades by bringing us increasingly small, cheap and efficient devices and appliances. Our modern life-styles have come to be defined by our access to an increasingly wide assortment of technologically advanced equipment for personal and group use. From smart phones to personal music players and from satellite navigators to tablet computers the benefits of modern electronics are all around us. In this continuing tradition, solid-state lighting now seems set to revolutionise the way we light our surroundings – indoors and outdoors. Its impact is already being felt globally and in the coming years it will further entrench its position as one of the defining technologies of the twenty-first century.
Lighting technology has changed remarkably little since its inception more than a hundred years ago. We still illuminate our homes and offices with lamps that bear a striking resemblance to Thomas Edison’s invention in the late nineteenth century. Indeed the basic design of incandescent light bulbs has remained essentially the same over the years and with that their efficiencies have also changed very little. The development of tungsten-halogen lamps in the nineteen-fifties did raise the efficiency somewhat but it still remained woefully low. The later development of fluorescent lighting resulted in a big improvement in efficiency but even that is now considered insufficient in our increasingly energy-conscious world. Moreover, their use of toxic and environmentally hazardous mercury has always remained a cause for concern. The emergence of highly efficient diode-based solid-state lighting over the past decade has, therefore, been widely welcomed and acknowledged as the next logical step in the evolution of lighting technology.
Solid-state light emitters were invented in the nineteen sixties as semiconductor pn-junction diodes capable of emitting coloured light. These light-emitting diodes (LEDs) were made from materials such as gallium arsenide and gallium arsenide phosphide. For many years, LEDs only served as small indicator lights for electronic equipment. They were ubiquitous in everything from portable transistor radios to televisions and telephones. Epoxy-packaged low-power LEDs are still around in essentially the same form in which these devices have been used for several decades. In later years, LEDs were also used to build dot-matrix displays that found particular favour in countries of the far-east. Even today a visitor to such places as Hong Kong, Singapore or Tokyo cannot escape the overwhelming concentration of advertising LED bill boards in city centres. Traffic light is another application where LEDs made an early appearance. A typical LED-based traffic light utilising clusters of low-power red, amber and green LEDs is shown in figure 1.
LEDs were traditionally available in most colours except blue which made it impossible to build full-colour displays using a combination of red, green and blue light emitters. A blue-emitting LED was a long sought after goal and, therefore, it caused much excitement when a practical blue LED was reported by a researcher at a small Japanese electronics company. Figure 2 shows an LED wafer with many individual blue LEDs, undergoing testing on an assembly line. Shuji Nakamura’s invention of the blue LED at Nichia Corporation resulted in the proliferation of LEDs in all kinds of applications. Its development also gave rise to the white LED which consists of a blue LED chip coated with a light-
emitting material called a phosphor. The phosphor gets excited by the blue light from the LED chip and converts a large amount of the blue light into yellow light. The resulting light – a combination of yellow emission from the phosphor and the residual un-converted blue light – appears white to our eyes. The availability of white LEDs soon started people thinking about the possibility of using these devices for illumination purposes.
Early LEDs were low-power devices, capable of running at no more than a quarter of a watt of power dissipation. While this was adequate for use as indicator devices and even for multi-colour dot-matrix displays, space lighting demanded higher power devices. This was a formidable problem once because high power LEDs have to use larger chips that also produce much more heat than the tiny chips used in conventional low-power LEDs. It took several years for device packaging technology to advance to the point where half watt LEDs could become commercially available. Companies such as Philips and General Electric spearheaded these developments, resulting in the eventual availability of watt-class white LEDs. Once these devices became available, systems designers set thinking about designing lighting systems that could take advantage of the many benefits offered by LED-based luminaires.
A radical departure from conventional means of generating light, LEDs have features that make them especially suited for lighting applications. Their small size, extreme efficiency in converting electrical energy to light, availability in many colours (including white) and absence of any environmentally harmful substance that might pose a problem during disposal make them ideal as light sources for any conceivable application. Little wonder then that LED-containing lighting systems are finding increasing acceptance all over the world. The market for LEDs and solid-state lighting systems has been growing at close to 25% per annum for the past several years and by all indications will continue to do so for the foreseeable future.
The first luminaires to be designed with high power white LEDs were shaped to resemble traditional tungsten filament light bulbs. These so-called retrofit bulbs have standard screw or bayonet bases to fit in existing lamp sockets. The argument was that this was the quickest way to market for LED lamp makers as it required no modification of existing lighting infrastructure. In spite of their significantly higher cost, the sales of retrofit LED light bulbs have been rising over the past five years. Manufacturers cite their very long lifetimes as the feature that offsets their purchase price – a typical LED light bulb can last for 10,000 to 20,000 hours before needing replacement. Compare this with the typical 800 hours lifetime of a tungsten incandescent bulb and the higher cost of an LED bulb doesn’t seem too onerous. The increased cost of these bulbs results from the need to incorporate a complete power supply inside every bulb, as LEDs only operate with low voltage DC power. The power supply is also the most vulnerable part of any LED bulb because the failure of any of its components can render the bulb useless. The actual LEDs themselves are much less prone to failure and are the reason manufacturers are able to quote such ambitious figures for their products. Figure 3 shows the interior of an 8 watt bulb containing 6 surface mount power LEDs. With prolonged use, LEDs tend to grow dimmer and a bulb’s useful life is considered over once its LEDs drop to half of their initial brightness. The fall in brightness is caused by a slow degradation of the LED chip and the colour conversion phosphor. The fact that LED bulbs do not fail abruptly like incandescent bulbs also reduces chances of untoward accidents.
In addition to a miniature switch-mode power supply, retrofit bulbs also incorporate a good heat sink to dissipate heat generated by the LEDs. Keeping the LEDs cool during operation with good thermal management can vastly increase their useful lifetime. This heat sink is usually a prominent feature of any LED bulb and designers try to make it as aesthetically appealing as possible (see figure 4). Over the years, manufacturers have been freely experimenting with the form factor of LED bulbs and this shows up in the many shapes and sizes now available on supermarket counters.
During the first few years after the appearance of LED light bulbs their consumer acceptance was mainly limited by the inferior quality of their light output. To many users their light appeared too cold and markedly different from that given off by incandescent lamps. This remained an issue for several years until LED manufacturers developed better LED phosphors. These luminescent materials consist of a crystalline host, such as an oxide, sulphide or nitride, doped with one or more transition metal or rare-earth element. Typical dopants include manganese, copper, cerium and europium. A typical widely used phosphor is cerium-doped yttrium aluminium garnet (Ce:YAG). Phosphors are inert organic materials and thus present no dangers to their use in LEDs. Thanks to advanced phosphors, present day LED bulbs produce light that is virtually indistinguishable from that emitted by tungsten filament lamps. In fact, LED technology has advanced to such an extent that these devices can even produce full-spectrum light containing all visible colours in more or less equal proportions. Such balanced spectrum light sources are useful for all lighting applications where there is a need for improved colour rendering. This development highlights the intrinsic superiority of LEDs over thermal light sources which can only generate one type of light. Figure 5 here shows the spectra of commercial LEDs from Electrospell that mimic the light from incandescent lamps (left) and produce full-spectrum white light (right).
Lighting systems utilising balanced full-spectrum white light can reproduce colours of real life objects exactly as seen with natural daylight. This type of illumination is, therefore, of great utility for retailers, photographers and museum goers. A pair of 3 watt flat-white LEDs from Electrospell appear here in figure 6.
LED lighting is, of course, not limited to just indoor lighting applications. Automotive lighting has also been benefiting from developments in LED technology. Vehicle interiors and dashboards have been illuminated with LEDs for several years and now it is the turn of exterior lights to go solid-state. Parking and running lights as well as brake and reversing lights are now increasingly implemented with high power LEDs. Headlights are next on the cards and indications are that major automobile manufacturers will begin to introduce LED-based headlights by the middle of next year.
Display lighting is yet another area where LEDs are making great inroads. It was six years ago when LED-backlit liquid crystal display (LCD) televisions and computer monitors first appeared on the market and now all flat-screen LCD displays are being made with only LED backlighting. LEDs offer several performance benefits over the older technology of cold cathode fluorescent lamp (CCFL) sources for LCD display backlighting. Their use significantly cuts down on power usage, causes less heating, provides better lighting quality and enables much larger screen sizes than was possible with edge-lit CCFL-based backlights. Furthermore, use of LEDs can also lead to better picture contrast through use of a technique called dynamic dimming where LEDs, directly behind dark scenes, are dimmed in intensity to accentuate the overall contrast. Major TV manufacturers are also competing with each other in bringing out ever thinner TV sets and here too LEDs can provide a helping hand. This is due to their capability to produce customised angular emission patterns that enable the distance between the backlight and the LCD panel to be reduced, resulting in significantly thinner televisions. In order to achieve this, an array of shallow holes is etched on the surface of LEDs, as can be seen in the atomic force microscope image in figure 7. This array forms what is known as a photonic crystal – an ordered periodic structure that is essentially a two-dimensional diffraction grating. The photonic crystal serves two purposes. It modifies the spatial pattern of light exiting the LED and it also makes the LED brighter by extracting more light from deep inside the chip. Both effects are beneficial for LCD backlighting; making TVs both thinner and more power efficient.
An important emerging application of LEDs is in indoor plant growth. Incandescent lamps, previously used for horticultural use, are ill-suited to the purpose as these are extremely inefficient and produce little radiation that is actually used by plants. LEDs provide the ideal solution by providing light at exactly those wavelengths that are efficiently absorbed by plants. In contrast, thermal sources produce most radiation in the yellow-green portion of the visible spectrum which is precisely the light that is reflected rather than absorbed by most plants (this is the reason plants appear green in colour). Modern broadband red and blue LEDs efficiently generate light that matches exactly with plants’ spectral requirements. LEDs are thus being increasingly used all over the world for indoor cultivation of a wide variety of plants. Experiments on LED-powered crop cultivation have also been carried out on the international space station.
The rising popularity of LED-based solid-state lighting systems is backed by cutting-edge research to support consumer expectations. A great deal of R&D on LED materials, processing techniques and device design is being carried out in academic, industrial and government laboratories around the world. Some of it is aimed at further improving the already impressive efficiency of these devices and bringing down their manufacturing cost while other work explores the quality of light generated by LEDs and ways of further prolonging their useful lifetime. Besides these, highly specialised LEDs for niche applications are also under development so that we will see their uses proliferate even more in the coming years.
Perhaps most industrial R&D effort at this time is being directed at reducing the cost of production of blue-emitting gallium nitride LEDs. The main focus of attention is on replacing the sapphire substrate, on which gallium nitride LED device layers are grown, with silicon. If this can be done successfully then there will be great cost benefits to the LED industry. This is because sapphire substrates are significantly more expensive than silicon wafers and are also mostly available as only two inch diameter wafers. Much larger silicon wafers, with diameters as large as twelve inches, are commercially available at low cost, thanks to the silicon integrated circuit industry that uses such wafers for making a wide variety of analogue and digital chips. If gallium nitride can be grown on large diameter silicon wafers then each wafer will yield vastly more LED devices than the usual two inch diameter sapphire wafer. This is the principal driver for lowering the cost of LED chips. The main impediment in growing device-quality gallium nitride on silicon is the propensity for the gallium nitride layer to crack due to mismatch in the lattice parameter (distance between atoms) of gallium nitride and silicon (see figure 8 – left). Progress has been made in solving this problem by a variety of means such as growing gallium nitride on pre-patterned silicon wafers where the growth stress is accommodated in the ridges, leaving pristine gallium nitride in between (see figure 8 – right). There are several other benefits that result from changing the substrate from sapphire wafers to silicon wafers. Readily available equipment originally designed for processing silicon wafers can be used for fabricating LED wafers. In contrast to sapphire, the silicon base can be easily removed from gallium nitride-on-silicon wafers, freeing the actual LED structure which can then be placed on a metal substrate for much improved heat removal. This way much higher brightness LEDs can be easily produced. Several major LED manufacturers, such as Lumileds, Bridgelux and Lattice Power have active programmes in this area with expectations of large scale market introduction next year.
Other prominent research themes include work on zinc oxide – a promising alternative to gallium nitride for making blue LEDs. Making p-type zinc oxide is still a problem and thus research continues on new ways for doping zinc oxide to produce good p-type material. Several research groups are also working on the growth of ultra-narrow columns of gallium nitride called quantum wires that exhibit higher efficiency in light emission. Scientists from Osram, in particular, have reported significant advances in their quest for quantum wire LEDs. Yet other research collaborations have been exploring the incorporation of colour-converting cadmium selenide quantum dots with gallium nitride LEDs to produce narrow-band LEDs of high colour purity. Several research laboratories are also busy improving the structure of LED device layers in an effort to boost their efficiency by eliminating mechanisms that lead to non-radiative energy loss in LEDs.
With so much being staked on LEDs and the promise of many further developments in the near future, the prospects for solid-state lighting are brighter than ever before. Half a decade from now, LED bulbs and luminaires will be as commonplace as incandescent lamps are at the time of this writing. In another few years after that, traditional tungsten filament lamps will seem as quaint as vacuum tubes; completing the revolution to an era dominated by semiconductor-based light sources.
The author, Faiz Rahman, is a technology officer at Electrospell Ltd, UK.