RIKEN Environmental Report 2009

Deep UV light opens up new eco markets Many breakthroughs in growing high quality crystals

Hideki HIRAYAMA, Team leader Terahertz Quantum Device Team

Deep ultraviolet (deep-UV) light is light with a shorter wavelength than normal ultraviolet light. This light, invisible to human eyes, has huge potential. For example, in one practical application a deep-UV light emitting diode (LED) could be used for high-speed degradative treatment for harmful chemical substances which are traditionally difficult to deal with. Other possible applications in the fields of health and the environment include water purification and soil improvement. RIKEN surprised the world by announcing the first-ever deep-UV LED with high output, which was achieved by growing crystals on an aluminum gallium nitride (AlGaN) based semiconductor.

Application in the fields of health, safety and pollutant management

Deep-UV, with a wavelength range of 200nm to 350nm, is invisible to the naked eye.
　How can this light be used? Hideki Hirayama, leader of the Terahertz Quantum Device Team explains,
　‘The prototype announced in July 2008, with a wavelength of 282 nm and output of 10 mW, can kill 99% of coliforms within one minute when emitted from a distance of approximately 20 cm. This can be applied to medical use for sterilization, replacing a bulky mercury lamp’.
　The high degradative ability of deep-UV light means it may be useful in factories that need to clean up their effluent. Hirayama continues,
　‘Using a large number of arrays of 260 to 320 nm semiconductor UV light sources, a high-speed degradative treatment facility can be developed for pollutants such as dioxin, PCB and endocrine-disrupting chemicals, which are difficult to break down safely using traditional methods. And this means deep-UV light could be used to purify rivers, soil, sewage and smoke.
　Isn’t deep-UV light harmful to the human body and other life forms? Hirayama explains,
　‘The shorter the light wavelength, the more power it has to alter substances. Ultraviolet light in the wavelength range 315 nm – 400 nm is found in sunlight and you get sunburn from it. Yet it is also beneficial for us because vitamin D is only absorbed into our body with the help of sunlight, and energy from ultraviolet light is needed to generate vitamin D3. Deep-UV light in the wavelength range of 200 nm to 280 nm, on the other hand, is so powerful that it can destroy DNA. That’s why this range of UV can be used as a germicide and obviously it has to be used with rigorous safety management.’
　Why has it been difficult to deal with deep-UV light up to now?
　‘In order to create UV light in the 250 to 270 nm wavelength range, large and expensive equipment such as gas and excimer lasers were required. And the light emitting efficiency was as low as 0.01％, with an operating life of 1000 hours. With the new development at RIKEN, we can now look forward to an emitting efficiency of 50 to 80% with an operating life of more than one year’, Hirayama enthuses.

World’s first high output deep-UV light with a wavelength of 280 nm

Deep-UV light is expected to be applicable in a wide range of fields; in biochemistry such as with DNA analysis, in industrial chemistry such as with UV curing, and in other areas such as with sensors, high color rendering light and high-density optical memory lasers.
　‘If we can get the wavelengths that have not been utilized so far, we should be able to obtain higher resolutions for substance analysis. This in turn leads to advantages in acquiring new information. The United States is very keen in this field and we have been engaging in fierce technological competition for some time’.
　The United States has been seriously engaged in research on high output deep-UV since 2000 at the Defense Advanced Research Projects Agency (DARPA) under the US Department of Defense, and has been producing results close to RIKEN in the 260 nm to 280 nm range.
　‘We often clash in a flurry of sparks at academic conferences, but the existence of a good rival makes the technology go further’.
　The trigger for the escalation of this technological race was the research result achieved by the Hirayama team at RIKEN.
　The Hirayama team, which had been working on LED and laser diode (LD) using an aluminum gallium nitride (AlGaN) based semiconductor, succeeded in prototyping the world’s first high efficiency UV light emissions, at 340 nm in 2001 and 280 nm in 2008.

Growing high-quality semiconductor crystals with
high-output emission of deep-UV rays

What is AlGaN which is used to emit deep-UV rays? Hirayama explains.
　‘AlGaN is a crystal mixture of aluminum nitride (AlN) and gallium nitride (GaN), semiconductor materials suitable for emitting light. By altering the ratio of AI and Ga in a crystal mix, deep-UV light emission of long-lasting 200 nm to 350 nm wavelengths can be achieved with high efficiency . This mixture contains no elements harmful to the environment, such as arsenic, lead or mercury’.
　The wavelength of the blue LED, which is commercially used in displays and lighting, is approximately 450 nm, which is within human visibility. Commercially available blue LED is mostly made of an indium gallium nitride (InGaN) mixture, with smaller emitting power than AIGaN.

To use AlGaN to emit high output deep-UV rays, the Hirayama team needed to solve many problems. He explains,
　‘The most important point was the fabricating technology needed to grow the high quality crystal which forms the substrate. AlN crystal is used as a layer between AlGaN with a negative electrode and a sapphire substrate, but since AlGaN and sapphire have different crystal lattices, they don’t go well together. If the quality of the crystals is poor, such as by having non-flat formation of layers, light emitting efficiency deteriorates very easily.’
　To overcome this problem, Hirayama devised a multi-layer growth method in which an AlN layer is formed by continuously supplying aluminum (Al) gas while also supplying ammonium (NH3) gas in a pulsed manner. Using this method Hirayama succeeded for the first time in the world in growing very high quality crystals on a sapphire substrate, with very few threading dislocations or cracks. As a result, the emission intensity from the AlGaN emitting layer was increased approximately 50 times. That was in September 2007. This breakthrough led to the development of LEDs with a high output of approximately 2mW brightness achieved with a 227.5 nm wavelength, the shortest AlGaN type at the time, and a highly germicidal 260 nm wavelength.
　Then in July 2008, Hirayama achieved, as has already been mentioned, what was at that time the highest LED output of 10 mW at a deep-UV wavelength of 280 nm. This was achieved at room temperature and continuously for more than just a few seconds.
　It is believed that deep-UV LED output can be made even stronger. Hirayama says, ‘In future we want to develop high power deep-UV LEDs with 1000 times greater output than that of today.’