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Nakamura team claims advance in GaN device performance

Posted: 31 Oct 2005 ?? ?Print Version ?Bookmark and Share

Keywords:nakamura? gan? led? leds? gan devices?

A team led by Shuji Nakamura, inventor of blue LEDs and lasers based on gallium nitride, have developed a new GaN growth technology that enables production of new films with improved or new performance characteristics.

The advance promises to double the emission efficiency in GaN devices.

The team verified various advantages such as high- emission efficiency, emission in a wider range of colors, emission of polarized light, high-mobility, p-type GaN devices and low power consumption.

Nakamura's team succeeded in growing GaN crystal layers on the previously unusable crystal planes of the GaN's hexagonal prism structure. Present GaN devices are grown only on the c-plane, the base of the hexagonal prism. Film growth in this crystal orientation means electrical polarization occurs, and the resulting electric field disturbs efficient recombination of electrons and holes. This limits the performance of GaN devices.

By contrast, GaN films grown on nonpolar or semipolar planes, which are perpendicular or angled relative to c-plane, do not generate an electrical field or significantly reduce the field, thereby enabling electrons and holes to recombine more efficiently.

GaN films grown on nonpolar or semipolar planes were expected to exhibit better performance than present c-plane devices. But films produced on those planes had rough surfaces and many defects, preventing them from functioning as electronic devices.

The Nakamura team succeeded in growing smooth films on the new planes with fewer defects. Twelve patents are pending in connection with the growth technology. The film-growing technique involves existing lateral epitaxial overgrowth technology, but details of the new growth technology were not disclosed.

"The performance outlook is far better than present GaN devices. As a light-emitting device, its output is [still] low at present. But once we clear this issue, nonpolar and semipolar GaN will surely replace all current c-plane GaN devices," said Nakamura, a professor of University of California at Santa Barbara.

Nakamura's team plans to develop GaN devices that can emit light with 200-lumen/W efficiency. The record efficiency for GaN devices is 100 lumen/W, which is comparable to a fluorescent light.

Conventional GaN emits blue light most efficiently but dims at longer wavelengths toward red due to polarization. The new GaN is nearly polarization-free and has the potential to emit bright light in every color range. At about 550 nm of the green wavelength, it is about 10 times more efficient compared to current GaN.

The Nakamura project is funded by the Japan Science and Technology Agency. Masaaki Sakata, research manager of the Nakamura project, said it is seeking partners to help commercialize the technology.

- Yoshiko Hara

EE Times

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