Sony and Pioneer join blue-laser race

HIROSHIMA, Japan — Two major disk-player suppliers, Sony Corp. and Pioneer Electronic Corp., have joined the race to develop a blue laser based on gallium nitride. Both trail the current leader, Nichia Chemical Industries Ltd. Blue lasers or shorter wavelength violet lasers will be in big demand for next-generation storage and communications systems.

Sony has so far achieved laser emission with continuous-wave (CW) operation in room temperature (RT), a stage that only a few companies, including Nichia, have reached. Pioneer has reported a pulsed emission.

The three companies detailed their progress at the autumn conference of the Japan Society of Applied Physics earlier this month.

"Nichia is far above other competitors in this field," said Masao Ikeda, project leader of N Project at the Sony Research Center, who ascribed Nichia's lead to crystal growth. So Sony focused, he said, on the crystal-generating process as the key to a short-wave laser.

Sony had been developing its blue laser based on a II-VI zinc selenide compound. But after successive, dramatic achievements with wide-gap III-V nitride devices, the company shifted to the GaN compound in 1994.

Since GaN lasers emit in the wavelength range of about 390 to 410 nm, which is violet, Sony's blue laser is actually violet. A blue laser's wavelength is about 50 nm.

Looking for crystal growth, Sony developed a raised-pressure metal organic chemical vapor deposition (MOCVD) system. Nichia developed a proprietary two-flow MOCVD method in which two flows — one from the side and another from the top down — generate GaN layers. Because GaN breaks down at about 1,000°C, the temperature needed for fabrication, Ikeda said, Nichia went to the two-flow MOCVD system while Sony used its raised-pressure approach.

The elements and structure of the laser are similar to what Nichia is pursuing, said Ikeda. The laser developed in 1.6 atm emits with CW operation from room temperature to 80°C. Threshold current is 150 mAh and voltage is at 7.4 V. The emission forms a peak at 401.2 nm.

Ikeda said an expected lifetime is under evaluation but declined further comment. "When we get viable data with Nichia's result, then we will announce it," he said.

Pioneer, taking an approach opposite to Sony's, used a low-pressure metal organic chemical vapor deposition system. "MOCVD is usually used with low pressure. If we establish a technology with low pressure, it will have [an] advantage in volume production. We optimized the process under 300 Torr," said Hiroyuki Ota, manager of Pioneer's R&D division device laboratory.

Using an existing MOCVD system, Pioneer researchers generated an aluminum-nitride buffer layer on a sapphire substrate. On the aluminum nitride, a separate confinement heterostructure (SCH) laser diode including InGaN, a multiquantum well active layer was formed. The AlN buffer layer, which prevents cracks, is a Pioneer characteristic.

Except for the buffer, the structure is similar to those of Pioneer's competitors. The laser diode emitted with pulsed operation in room temperature in June, about three years after it started a GaN R&D project. With the threshold current at 820 mA, driving voltage at threshold is 34 V. "The emission still has multiple peaks. The central peak is about 411 nm," said Yoshinori Kimura, chief of the project at Pioneer's R&D division.

Nichia Chemical last April reported the development of a blue laser with a 10,000-hour lifetime and 2 mW output power in accelerated tests; those lasers were still emitting after an actual 4,000 hours of use. With commercialization at hand, Nichia researchers are working to give higher power and durability in high ambient temperature to violet lasers.

Nichia is changing from a sapphire substrate and etching method used in the laboratory to a GaN substrate and cleaving for volume production.

Nichia has used a conventional hydride vapor-phase epitaxy system to speed GaN layer growth.

The laser diode with cleaved mirror facets was reported to have an output power at 160 mW under RT-CW operation.

"We are planning to begin volume production by the end of this year," said Shuji Nakamura, senior researcher of Nichia.