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Blue-violet laser diodes roll for next-gen DVD

Posted: 02 Apr 2007 ?? ?Print Version ?Bookmark and Share

Keywords:blue-violet laser diode? non-polar diodes? GaN devices?

Nakamura: GaN-based laser diodes run longer.

A team of University of California, Santa Barbara (UCSB) researchers led by Shuji Nakamura, a pioneer of laser diode science, has created a low-threshold-current, non-polar blue-violet laser diode that it says could replace the c-plane violet laser diodes used by Blu-ray and HD DVD.

The team's initial results indicate that the gallium nitride (GaN)-based non-polar laser diodes "require lower operating power and have longer lifetimes" than c-plane violet laser diodes, Nakamura said.

Indeed, he said, "it has been known for a long time that non-polar diodes are theoretically superior to c-plane-based laser diodes, specifically because they are free from polarization." But the years of effort invested in growing the crystalline material on non-polar planes have only recently yielded effective results, Nakamura said.

Nakamura's team from the Solid State Lighting and Display Center (SSLDC) at UCSB's College of Engineering reported the pulsed emission of a 405nm laser beam from a non-polar, m-plane laser diode. Rohm Co. Ltd immediately followed with an announcement that continuous lasing had been achieved at 404nm on a device grown on the m-plane.

Red LED replacement?
Growing the crystal on non-polar planes allows the GaN devices to yield much higher performance and to emit light in a much wider range of colors, including red, according to the UCSB researchers. That means GaN devices covering the full range of visible colors might one day replace current-generation red LEDs, whose manufacture requires the use of hazardous materials such as arsenic.

The UCSB laser diode displayed threshold current densities as low as 7.5kA/cm?. "In the early days of GaN laser development, back to the mid-90s, the densities of c-plane laser diodes were 20-50kA/cm?. Now, about 10 years later, those densities have been lowered to around 2kA/cm?," Nakamura said. While the experimental non-polar laser comes in above that target, its good performance out of the chute suggests a short learning curve toward far lower current-threshold densities, he said. "It may have the potential to operate with one-fifth the current of present laser diodes. If a laser operates with low current, the lifetime of the laser would be extended, or it can emit high power with high current."

Nakamura said he is confident that "m-plane laser diodes will replace c-plane laser diodes in a fairly short time."

The UCSB team has been working on m-plane crystalline growth since 2000, but until recently, the crystalline material was defect-ridden and the resultant diodes did not exhibit laser oscillation. The breakthrough was the development of a GaN bulk substrate made by an ammonothermal growth method, Nakamura said. The university team has also reported record high performance for LEDs based on the same substrate.

The research was funded by the SSLDC and the Japan Science & Technology Agency's Exploratory Research for Advanced Technology program, which is headed by Nakamura. Application development will be handed over to 10 SSLDC corporate partners.

Crystalline growth
Rohm, meanwhile, says it has succeeded in achieving continuous laser oscillation at room temperature using an m-plane laser diode. Since the UCSB team reported pulsed rather than continuous oscillation for its experimental device, Rohm's achievement of continuous oscillation suggests superior crystalline growth.

As a precursor to the laser achievement, the company showed an m-plane-based LED at Ceatec in Japan last October. The vapor-phase crystalline-growth process used to produce the continuous-oscillation laser is proprietary technology that eliminates defects in the crystal, a Rohm spokesman said.

The prototype laser has a waveguide 1.5?m wide and 600?m long, which is typical for current c-plane laser diodes. Yet the Rohm device exhibits a superior 28mA oscillation threshold, according to the spokesman.

Rohm is now developing a prototype blue laser with a 460nm wavelength. It also expects to develop a 532nm green laser before the year is out, addressing a gap in the market for green semiconductor lasers for commercial use.

- Yoshiko Hara
EE Times

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