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7 September 2007


Riken and Saitama University develop UV LED emitting at 227.5nm

A joint research group led by Japan's Institute of Physical and Chemical Research (Riken) and Saitama University has developed a UV LED with an emission wavelength as short as 227.5nm and an output of 0.15mW. Previously, another research group had reported the development of a UV LED with an emission wavelength of 210nm, but its output was only 0.02 µ W.

The latest UV LED was fabricated by first forming an AlN layer on a sapphire substrate, then an n-type AlGaN layer, an AlGaN emitting layer (a triple quantum-well structure), and a p-type AlGaN layer.

The output was enhanced by improving the process for growing the AlN layer on the sapphire substrate using the ‘ammonia pulsed supply multiple layer growth method’, whereby multiple AlN layers are formed by alternately using two different growth methods. First, an AlN layer is formed by continuously supplying the Al material while supplying ammonium gas intermittently (in a pulsed manner). Then another AlN layer is formed by continuously supplying both the Al material and ammonium gas.

The new growth method resulted in: (1) a decrease in the threading dislocation density in the AlN crystal; (2) an increase in the flatness of the crystal layers; and (3) a reduction in the occurrence of cracks due to distortion in the crystal.

Because the quality and flatness of the AlN layer was improved, other layers formed on the AlN layers also resulted in higher quality and flatness, thereby enhancing the emission intensity (output) from the AlGaN emitting layer. The emission intensity was increased to approximately 50 times that obtained by the existing AlN layer formation method.

In addition, the Riken and Saitama University research group has prototyped UV LEDs with wavelengths and outputs, respectively, of 253nm and 1mW, 261nm and 1.65mW, and 273nm and 3.3mW. It is claimed that the outputs of the UV LEDs are equivalent to those of blue, red and white LEDs used in lamps, and are sufficient for use as-is in germicidal lamps.

The research group says that it now intends to enhance the efficiency and output of the latest UV LED further, with a view to applications for sterilization, water purification, medical care and high-speed degrading of pollutants etc.

The latest UV-LED was achieved by Hideki Hirayama, head of the Terahertz Quantum Device Laboratory, Terahertz-wave Research Program, Frontier Research System at Riken, and Norihiko Kamata, professor of the graduate school of science and engineering at Saitama University.

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