Microfacet Laser Displays 0.3-mW, 401-nm Operation

Daniel S. Burgess

Group III nitrides such as AlN, GaN and InN feature wide, direct bandgaps that make them particularly useful for ultraviolet and visible laser diodes. However, the conventional mirror-fabrication techniques tend to produce facets that are neither smooth, parallel nor vertical to the substrate. Now a team at NTT Basic Research Laboratories in Atsugi, Japan, has combined dry etching and a subsequent overgrowth process to construct an InGaN laser with smooth, vertical mirror facets.

The researchers employed electron cyclotron resonance dry etching and photolithography to inscribe a hexagonal trench through the layers of the multiple-quantum-well device to the SiC substrate. They deposited a 900-nm-thick layer of P-type GaN over the top and etched the sides of the stack by metallorganic vapor phase epitaxy, yielding atomically flat facets vertical to the substrate and forming a ring cavity.

The laser produced a maximum output of approximately 0.3 mW of pulsed 401-nm light at room temperature under pulsed current injection. Tetsuya Akasaka, a research scientist at NTT and a member of the team, noted that this performance reflected scattering by a corner of the structure and that the actual output power would be much greater. The output of a completed device would be extracted either by directly or evanescently coupling a waveguide to the ring cavity.

DVDs and micromachining

The design provides several advantages for group-III-nitride lasers that Fabry-Perot or vertical-cavity surface-emitting geometries do not have, Akasaka said.

Unlike the edge-emitters, there is virtually no reflection loss, and the nonradiative surface recombination of injected carriers is minimized because the active region is not exposed. Moreover, the lattice mismatch between AlGaN and GaN leads to cracking in the distributed Bragg reflectors of III-nitride-based vertical-cavity surface-emitting lasers.

The technique could be used to fabricate edge-emitting lasers that would be combined with external micromirrors for next-generation DVD systems, Akasaka said. The hexagonal lasers may find a place as micron-size blue and UV light sources for micromachining applications.