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Inner Stripe Boosts GaN Laser Output

Switching from a buried ridge-waveguide structure to an inner stripe design can increase the output power of single mode lasers to a record-breaking 1000 mW, says NEC. Richard Stevenson investigates the novel approach.

Blue–violet laser diodes are finally starting to have an impact on the life of the home-entertainment buff. Blu-ray and HD DVD players and recorders incorporating these devices are now in stores and, although prices aren t cheap, there is a growing selection of movie titles in both formats. Gamers are also benefiting, since the PlayStation3 console from Sony features a Blu-ray player.

Despite these recent successes, further improvements in 405 nm laser performance are needed for these edge-emitting devices to serve a broader range of applications. For example, if they are to offer faster writing and recording speeds, an increase in laser output power is required – in conjunction with a high-quality beam profile.

Unfortunately, it is very difficult to address these weaknesses with a conventional ridge-waveguide laser design (see figure 1 (a)), which can only maintain single mode emission at low output powers. Above an output power that is referred to as the "kink level", emission switches to a higher order mode that is incompatible with the focusing optics used to read from, and write to, the disc.

Narrowing the waveguide is one way to overcome this problem. However, it is difficult to use dry etching to fabricate stripe widths of 1.4 μm or less, which are needed for "kink-free" operation up to 200 mW. And even if a laser with such a narrow waveguide could be produced, it would suffer from a high operating voltage and temperature that would impair performance.

The drawbacks of dry etching have led researchers at NEC Corporation in Japan to develop an alternative design that features an "inner-stripe" and a regrown AlN optical-confinement and current-blocking layer. It s a structure that delivers three key advantages, says Masaki Ohya, assistant manager of the company s system devices research laboratories: a narrow waveguide with dimensions that can be precisely defined by crystal growth and photolithography; a low electrical resistance that stems from lateral current flow in the AlGaN/GaN superlattice; and a relatively low operating temperature thanks to the width of the p-type cladding layer and contact metal.
A wet etching approach

To produce these high-quality inner-stripe structures first requires growth of crack-free AlN on GaN. This is followed by formation of a damage-free stripe in the material and planar regrowth of the AlGaN/GaN superlattice cladding. The NEC researchers have approached the task by using a very low growth temperature of 400 °C. This creates an amorphous, crack-free AlN film (confirmed by X-ray diffraction measurements), which can then be wet-etched with phosphoric acid to produce a high-quality stripe. "With dry etching you have no selectivity, and you don t get sharp interfaces," explained Ohya. Ramping the reactor temperature for subsequent growth crystallizes the remaining AlN, allowing high-quality growth of the p-type structure.

Ohya and colleagues have made lasers with 650 μm-long cavities, fabricated from material with a low-reflection-coated front facet and a high-reflection-coated rear facet. A typical 1.4 μm-wide stripe produces a kink-free continuous wave (CW) output power of more than 200 mW over the 20–90 °C range, with a threshold current and voltage of 32 mA and 4.1 V at 25 °C.

Variants of this design with a 1.0 μm-wide stripe produce more than 400 mW in CW mode over the same temperature range, while in pulsed mode, with a 50% duty cycle, this increases to more than 600 mW. When the duty cycle is cut to 0.03%, kink-free output surpasses 1000 mW at up to 70 °C. "To our knowledge, that s the highest single mode output power for blue–violet laser diodes," said Ohya.

The researchers have also started to assess the reliability of their new design. When pulsed with a 50% duty cycle at 80 °C over the course of 1000 h, four 1.4 μm cavity lasers required a 10% increase in operating current to maintain a 200 mW output. Transmission electron microscopy images reveal that these tests did not produce defects near the regrown interfaces, and suggest that this type of laser is a strong contender for deployment in tomorrow s high-density optical disc systems.

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