One word that is special to scientific research is serendipity. Prof. Shuji Nakamura knows the meaning of the word better than anyone else. He is credited with the inventions of blue, green and white light-emitting diodes (LEDs) and the blue laser diode. The importance of his inventions is marked by the six international awards he received between 2002 and 2010 which include the Benjamin Franklin Medal in Physics from the Franklin Institute, Finland’s Millennium Technology Prize, Prince of Asturias Award for Technical and Scientific Research and the Harvey Prize from the Technion in Israel. We cannot rule out a future Nobel Prize in Physics for this researcher from Japan who now teaches in USA!
Shuji Nakamura was born in Japan and it seemed like he was destined for a career in obscurity. Nakamura graduated from the University of Tokushima in 1977 with a degree in electronic engineering, and obtained a master’s degree in the same subject two years later, after which he joined the Nichia Corporation, also based in Tokushima. It was while working for Nichia that Nakamura invented the first high brightness gallium nitride (GaN) LED whose brilliant blue light, when partially converted to yellow by a phosphor coating, is the key to white LED lighting.
Story of LED
Electroluminescence was discovered in 1907 by the British experimenter H. J. Round of Marconi Labs, using a crystal of silicon carbide. Thereafter, several others observed light emission from various semiconductor materials mostly in the infrared region. However, the first practical visible-spectrum (red) LED was developed in 1962 by Nick Holonyak Jr., while working at General Electric Company. Holonyak is seen as the “father of the light-emitting diode”. M. George Craford, a student of Holonyak, invented the first yellow LED and improved the brightness of red and red-orange LEDs by a factor of ten in 1972. In 1976, T. P. Pearsall created the first high brightness, highly efficient LEDs for optical fibre telecommunications by inventing new semiconductor materials specifically adapted to optical fibre transmission wavelengths.
Like a normal diode, the LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side to the n-side, but not in the reverse direction. Charge carriers - electrons and holes - flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon.
The wavelength of the light emitted, and thus its colour, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect band gap materials. The materials used for the LED (such as GaAs, InGaAsP, GaAsP, etc.) have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.
Until 1968, visible and infrared LEDs were extremely costly, on the order of US $200 per unit, and so had little practical use. Costs reduced considerably as companies started mass production and with the arrival of planar fabrication techniques.
The Blue LED
Scientists were able to produce colours such as red and yellow from available materials but not blue or violet which required a higher band-gap material. A suitable choice was GaN using which many scientists tried hard but did not manage to make a marketable GaN LED in the 1960s. The principal problem was the difficulty of making strongly p-type GaN.
Nakamura was somewhat luckier than other workers in that another Japanese group led by Professor Isamu Akasaki published their method to make strongly p-type GaN by electron-beam irradiation of magnesium-doped GaN. However, this method was not suitable for mass production and its physics was not well understood. Nakamura managed to develop a thermal annealing method which was much more suitable for mass production. In addition, he and his co-workers worked out the physics and pointed out the problem was hydrogen, which passivated acceptors in GaN.
Nakamura was also fortunate that Nobuo Ogawa, the founder of Nichia, was willing to support his GaN project. At the time, many considered creating a GaN LED too difficult. Ogawa strongly believed in Nakamura and supported him for ten years even though everyone else wanted the project to be wound up.
And not only did he produce new semiconductors that could produce blue light, he also invented diodes that could glow bright green and, most useful of all, LEDs that can produce white light. Today, Nakamura is at the University of California Santa Barbara and those green LEDs are used around the world in traffic lights. The white LEDs provide the light for the colour screens in cell phones. And if that’s not enough, Nakamura also invented the blue laser diode that is already being exploited in the next generation of DVD player called the blue-ray disk.
Display devices and printing machines mostly rely on a combination of primary colours. The simple method is to use a combination of red, green and blue (RGB) which is used in display devices. Another technique is use a combination of cyan, yellow, magenta and black (CYMK) that is a norm in colour printing devices.
There are two primary ways of producing high intensity white-light using LEDs. One is to use individual LEDs that emit three primary colours —red, green, and blue—and then mix all the colours to form white light. The other is to use a phosphor material to convert monochromatic light from a blue or ultraviolet (UV) LED to broad-spectrum white light, much in the same way a fluorescent light bulb works.
The importance of Nakamura’s work lies in here. Before his pivotal invention of the high brightness blue and green LEDs scientists were unable to produce white light using the RGB technique. Nakamura’s work gave the key to produce both blue and UV LEDs and when this is used to irradiate yellow phosphor, scientists were able to produce white LEDs that became commercially available very soon afterwards. The technique was nothing but a “YAG”, phosphor coating to mix yellow (down-converted) light with blue to produce light that appears white. One can easily notice that the white light produced by these devices are not as pure as a fluorescent lamp because of this method.
White LEDs could provide a sustainable, low-cost alternative to light bulbs, especially in developing countries.
The Present and Future of Energy Efficient Lighting
The world is craving for energy efficient systems as we are certain that our drive for more energy using conventional methods has degraded the environment. We are reeling under global warming, ozone layer depletion and pollution larger than ever. Developing energy efficient lighting systems can ease the pressure on existing energy systems. Recently our government was driving a movement to replace all the existing incandescent lamps with CFL lamps. Even though CFLs offer energy efficiency, their production and recycling or dumping involves mercury pollution. LED lighting systems coupled with solar panels offer an eco-friendly alternative for lighting up the future world if we are able to surmount the twin hurdles of low efficiency and high production cost.
The development of LED technology has caused their efficiency and light output to rise exponentially, with a doubling occurring about every 36 months since the 1960s, in a way similar to Moore’s law. The advances are generally attributed to the parallel development of other semiconductor technologies and advances in optics and materials science. This trend is normally called Haitz’s Law after Dr. Roland Haitz.
Solid state devices such as LEDs are subject to very limited wear and tear if operated at low currents and at low temperatures. Many of the LEDs made in the 1970s and 1980s are still in service today. Typical lifetimes quoted are 25,000 to 1,00,000 hours but heat and current settings can extend or shorten this time significantly. The most common symptom of LED (and diode laser) failure is the gradual lowering of light output and loss of efficiency. Sudden failures, although rare, can occur as well. Early red LEDs were notable for their short lifetime. With the development of high-power LEDs the devices are subjected to higher junction temperatures and higher current densities than traditional devices. This causes stress on the material and may cause early light output degradation. Like other lighting devices, LED performance is temperature dependent. Most LEDs made today are for an operating temperature of 25°C. LEDs used outdoors, such as traffic signals or in-pavement signal lights, and that are utilized in climates where the temperature within the luminaire gets very hot, could result in low signal intensities or even failure.
LED light output actually rises at colder temperatures. Consequently, LED technology may be a good replacement in uses such as supermarket freezer lighting and will last longer than other technologies. Because LEDs emit less heat than incandescent bulbs, they are an energy-efficient technology for uses such as freezers. However, because they emit little heat, ice and snow may build up on the LED luminaire in colder climates. This lack of waste heat generation has been observed to cause sometimes significant problems with street traffic signals and airport runway lighting in snow-prone areas, although some research has been done to try to develop heat sink technologies to transfer heat to other areas of the luminaire.
With the development of high efficiency and high power LEDs it has grown possible to use LEDs in lighting and illumination. Replacement light bulbs have been made, as well as dedicated fixtures and LED lamps. LEDs are used as street lights and in other architectural lighting where colour changing is used. The mechanical robustness and long lifetime is used in automotive lighting on cars, motorcycles and on bicycle lights.
LED street lights are employed on poles and in parking garages. In 2007, the Italian village Torraca was the first place to convert its entire illumination system to LEDs. LEDs are used in aviation lighting. Airbus has used LED lighting in their Airbus A320 Enhanced since 2007, and Boeing plans its use in the 787. LEDs are also being used now in airport and heliport lighting. LED airport fixtures currently include medium intensity runway lights, runway centreline lights and obstruction lighting.
LEDs are also suitable for backlighting for LCD televisions and lightweight laptop displays and light source for DLP projectors. Screens for TV and computer displays can be made thinner using LEDs for backlighting. LEDs are used increasingly commonly in aquarium lights. Particularly for reef aquariums, LED lights provide an efficient light source with less heat output to help maintain optimal aquarium temperatures. LED-based aquarium fixtures also have the advantage of being manually adjustable to emit a specific colour-spectrum for ideal coloration of corals, fish, and invertebrates while optimizing photo-synthetically active radiation (PAR) which raises growth and sustainability of photosynthetic life such as corals, anemones, clams, and microalgae. These fixtures can be electronically programmed to simulate various lighting conditions throughout the day, reflecting phases of the sun and moon for a dynamic reef experience.
The lack of infrared or heat radiation makes LEDs ideal for stage lights using banks of RGB LEDs that can easily change colour and decrease heating from traditional stage lighting, as well as medical lighting where IR-radiation can be harmful.
LEDs are small, durable and need little power, so they are used in hand held devices such as flashlights. LED strobe lights or camera flashes operate at a safe, low voltage, instead of the 250+ volts commonly found in xenon flash lamp-based lighting. This is especially useful in cameras on mobile phones, where space is at a premium and bulky voltage-raising circuitry is undesirable. LEDs are used for infrared illumination in night vision uses including security cameras. A ring of LEDs around a video camera, aimed forward into a retro reflective background, allows chroma keying in video productions.
LEDs are used for decorative lighting as well. Uses include but are not limited to indoor/outdoor decoration, cars, cargo trailers, conversion vans, cruise ships, boats, automobiles, and utility trucks. Decorative LED lighting can also come in the form of lighted company signage and step and aisle lighting in theatres and auditoriums.
Light can be used to transmit broadband data, which is already implemented in IrDA standards using infrared LEDs. Because LEDs can cycle on and off millions of times per second, they can be wireless transmitters and access points for data transport. Lasers can also be modulated in this manner for optical communication.
Efficient lighting is needed for sustainable architecture. A 13 watt LED lamp emits 450 to 650 lumens which is equivalent to a standard 40 watt incandescent bulb. A standard 40 W incandescent bulb has an expected lifespan of 1,000 hours while an LED can continue to operate with reduced efficiency for more than 50,000 hours, 50 times longer than the incandescent bulb.
One kilowatt-hour of electricity will cause 610 grams of CO2 whereas a bulb will cause 89 kg of CO2 emission per year. The 13-watt LED equivalent will only cause 29 kg of CO2 over the same time span. A building’s carbon footprint from lighting can be reduced by 68% by exchanging all incandescent bulbs for new LEDs in warm climates. In cold climates, the energy saving may be lower, since more heating is needed to compensate for the lower temperature.
LEDs are also non-toxic unlike the more popular energy efficient bulb option: the compact fluorescent (CFL) which contains traces of harmful mercury. While the amount of mercury in a CFL is small, introducing less into the environment is preferable.
LED light bulbs could be a cost-effective option for lighting a home or office space because of their very long lifetimes. Consumer use of LEDs as a replacement for conventional lighting system is currently hampered by the high cost and low efficiency of available products. The high initial cost of the commercial LED bulb is due to the expensive sapphire substrate which is important in the production process. The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted.
In 2008, a materials science research team at Purdue University succeeded in making LED bulbs with a substitute for the sapphire components. The team used metal-coated silicon wafers with a built-in reflective layer of zirconium nitride to lessen the overall production cost of the LED. They predict that within a few years, LEDs produced with their revolutionary new method will be competitively priced with CFLs. The less expensive LED would not only be the best energy saver, but also a low cost bulb.
Receiving the Millennium Technology award, Professor Nakamura said: “I hope the award of this prize will help people to understand that this invention makes it possible to improve quality of life for many millions of people. This is not just a source of light that makes enormous energy savings possible; it is also an innovation that can be used in the sterilisation of drinking water and for storing data in much more efficient ways.”
As LEDs are more robust than traditional light bulbs and use relatively little power they can easily be combined with solar panels to provide lighting in remote areas of developing countries.
A solid-state light consumes just four watts to produce as much light as a 60‑watt incandescent bulb today. The catch right now is the price. These lights cost about $50 a piece. But there’s hope the price will fall once they are mass produced. Nakamura believes they will someday provide light to people in the developing world. He along with several others are still working towards finding answers to the problem of efficiency loss in LED lighting devices with increased operating current. It is high time that every Indian physics student dreams to join this high cause that can provide a developing country like ours some chance to survive an energy drain. Let us hope that powerful, efficient and cheap solid state lighting systems become a common reality along with highly efficient solar energy systems.
http://en.wiktionary.org/wiki/LED “The Blue Laser Diode: The Complete Story,” Shuji Nakamura, Gerhard Fasol, and Stephen J Pearton, Springer Verlag, (2000). Four Solid State Lighting Trends for 2010, Keith Scot, Greentech Media (Jan. 2010). http://www.ecse.rpi.edu/~schubert/Light-Emitting-Diodes-dot-org
 Refers to a natural inclination for making fortunate discoveries while looking for something unrelated.
 Materials whose conduction band minimum does not lie directly above the valence band maximum.
 An empirical law put forward by Gordon Moore suggesting that the number of transistors that can be fabricated in a single chip will double every eighteen months.