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По умолчанию 'Diamond LED' Can Be Used at 400 Deg C

Mar 26, 2009 20:22 Tetsuo Nozawa, Nikkei Electronics

Light emission of the diamond LED was demonstrated by AIST. The light seen in the picture is not UV light but visible light generated by a lattice defect, etc. The LED actually emitted deep UV light with an intensity (per unit wavelength) nearly 20 times higher than that of visible light, according to AIST.

The structure of the diamond LED

The temperature dependency of the emission spectrum with a supply current of 50mA. The emission intensity continues to increase even when the temperature is 421°C.

Japan's National Institute of Advanced Industrial Science and Technology (AIST) developed a deep UV LED that uses diamond semiconductor.
AIST plans to unveil the details at the 56th Spring Meeting, 2009 of the Japan Society of Applied Physics, which will take place from March 30 to April 2, 2009, in Ibaraki Prefecture, Japan.

The deep UV LED is composed of a 2mm-square diamond substrate on which a p-i-n-structured diamond semiconductor is stacked. It emits deep ultraviolet light with an emission wavelength of 235nm. The output is 30μW when a 320mA current is supplied.

"The LED is very close to practical use," said Satoshi Yamasaki, principal research scientist of the Energy Technology Research Institute of AIST and a professor at Graduate School of Pure and Applied Sciences at University of Tsukuba.

According to AIST, the new LED has two main characteristics. First, its luminous efficiency continues to increase even when a large current is supplied. Second, it has an excellent resistance to high temperatures.

Specifically, when a current with a density exceeding 2,000A/cm2 is applied through an electrode with a diameter of 120μm, the luminous efficiency continues to increase without being saturated, according to AIST. At present, the operating current density of a deep UV LED using AlGaN-based semiconductors, which may rival the new LED, is only 500A/cm2 at a maximum, AIST said.

In regard to the excellent heat resistance, even when temperature rises from room temperature to 420°C, the light emission intensity does not degrade but continues to increase, AIST said.

Unlike general LEDs, the diamond LED emits light because of the generation of "excitons." An exciton is an electron-hole pair that behaves like a kind of particle. In general, it is vulnerable to heat and often deteriorates quickly.

However, the excitons generated in the diamond LED are "very stable and will not break until 600°C," said Toshiharu Makino, researcher of the Energy Enabling Technology Group at the Energy Technology Research Institute of AIST. This is the major reason for the excellent high-temperature resistance.

Significant cost reduction expected in near future

Currently, there are some problems in the commercialization of diamond LEDs. Not only is diamond used for the substrate very expensive, but available substrates are usually several square millimeters in size. Regarding these problems, Makino and others commented as follows.

"We are currently developing a technique to stack a polycrystalline diamond semiconductor film on a Si wafer, and the prototype formed by using the new technique has an efficiency that is only an order of magnitude lower than that of the LED we announced this time," they said. "Because it only requires quite common materials such as silicon and methane, when the technique becomes practical, diamond LEDs can be produced at a very low cost."

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