Posted: 3:00 p.m. EST, 4/9/98

TOKYO — High-power gallium-nitride laser diodes that emit light in the violet region of the spectrum have been developed by Nichia Chemical Industrial Co. Ltd. (Anan, Tokushima). By operating at a shorter wavelength than blue, the laser diodes could allow optical disks to store greater amounts of data, and may find application in other areas that require blue laser light, such as flat-panel displays. Nichia researchers described their work at the recent Spring Meeting of the Japan Society of Applied Physics and Related Societies.

Writable power
Researchers at Nichia observed single-mode stable emission of up to 80-mW output in the diodes. "DVD applications require 30- to 40-mW power; thus, our lasers can be used for DVDs," said Shuji Nakamura, senior researcher at Nichia. "We are targeting a laser with writable power from the beginning." Nichia has made the commercialization of a short-wavelength laser diode a high priority. "At present, the wavelength at around 400 nm shows the lowest threshold current," Nakamura said, "so a [violet] laser with around 400-nm wavelength will naturally come first."

With plans to sample the parts this year, Nichia said its laser diodes promise record performance. They operate at room temperature with continuous -wave operation, and emit in the range of 380 nm to 440 nm. The lowest threshold current reached by an individual diode was 16 mA, the lowest threshold current density was 1.2 kA per square centimeter, and the lowest operating voltage was 4 V. The highest output power reached was 400 mW, and the highest operating temperature was over 100 ^o C. These extreme figures were not all realized in the same device, however.

The latest very short-wavelength lasers add to a growing portfolio of promising laser diodes based on gallium nitride that Nichia has been developing. The conference revealed a large gap in the race for short -wavelength lasers between Nichia and other potential competitors as leading-edge laser achievements were presented at the gallium-nitride sessions. In contrast to Nichia, other groups are still working with pulsed lasers. Nichia engineers reported the actual lifetime of its lasers at room temperature has passed the 4,000-hour mark, with 2-mW output power. Two major changes — adoption of modulation-doped strained-layer superlattices to form a thicker cladding layer for better light confinement and the use of an epitaxially laterally overgrown GaN (ELOG) as the substrate of the laser diode to reduce defects — contributed to extending the life of Nichia's lasers.

For commercial production, Nichia plans to adopt cleaving and pure gallium-nitride (GaN) substrate technology. GaN is a promising material for shortwave lasers. But the lack of lattice-matching substrates for epitaxial growth has been a big setback. A sapphire substrate is popularly used for the III-V gallium-nitride lasers, but its lattice does not match with that of GaN. This causes high defect density in the shortwave lasers grown on the substrate and results in short operating lifetimes. The GaN substrate would provide perfect lattice match and its thermal conductivity is about three times higher than sapphire's. Furthermore, it is easily cleaved, eliminating the need to etch processes to form fac ets. But it is small and very expensive because of production difficulties.

Matsushita Electric Industrial Co. Ltd. and CBL Technologies Inc. (Redwood, Calif.) last July announced a challenging development effort to produce a large pure GaN wafer, with sampling targeted for this year. Nichia's approach is to avoid difficult GaN wafer development by using a sapphire substrate. First, a 100-micron-thick GaN layer is formed on the sapphire substrate, then the sapphire substrate is removed by polishing to leave an 80-micron-thick GaN substrate.

"The lasers formed on the GaN substrate have not yet shown good performance," Nakamura said. "The threshold current, for example, is twice higher than that of lasers formed on an ELOG layer. But we are optimistic that we can improve. I don't think lifetime depends on output power, [but] rather on heat."

Nakamura's group intends to improve lifetime further even under high temperature and with high output power. High thermal conductivity increas es the importance of the gallium-nitride wafers, since heat production is very high for short-wavelength laser diodes, affecting lifetime. "We are going to use the GaN substrate structure for commercial production," he said.

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