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Dilute nitride closes in on telecom demands

Perfect annealing conditions hold the key to fabricating room-temperature single-mode GaInNAs lasers operating at 1.5 microns.

Optimized rapid thermal annealing is the secret behind a new continuous-wave (CW) GaInNAs laser that emits at 1.5 µm in single mode, and at room temperature (Elec. Lett. 44 737).

The laser s developers, who work at the University of Wuerzburg in Germany and spin-off company Nanoplus, claim that the dilute nitride emitters can now target commercial 1.55 µm telecom applications currently served by single mode InP-based lasers.

GaInNAs structures offer several advantages over the incumbent technology, including the use of cheaper GaAs substrates and a higher conduction band offset that promises to improve high-temperature performance.

This is not the first time that the German team has claimed success at this wavelength with a dilute nitride laser. In 2004, they fabricated a 1.5 µm GaInNAsSb edge-emitter in collaboration with Stanford University, but the addition of antimony is an undesirable complication to growth of the active region (Elec. Lett. 40 1487).

The team produced its latest laser structure by solid-source MBE on n-doped GaAs substrates. An Applied Epi UNI-Bulb radio-frequency plasma source supplied the nitrogen, while arsenic was delivered from a valved cracker cell.

The laser structure features a 7 nm-thick Ga0.62In0.38N0.045As0.955 quantum well sandwiched in a 380 nm-thick undoped waveguide, two 1400 nm-thick Al0.4G0.6As cladding layers, and a heavily doped GaAs cap.

Annealing at 700 °C in a mixture of 95% argon and 5% hydrogen improved the active region's optical quality, and photoluminescence intensity measurements showed that an 8 minute anneal produced the best results.

After the annealing step, 2.5 µm-wide ridge waveguides were formed through photolithography and reactive ion etching. Electron beam lithography followed, to define lateral distributed feedback gratings on either side of the ridge, and finally the team added metal contacts and high-reflectivity dielectric facet coatings.

The 800 µm x 2.5 µm laser chips formed by this process produced 1486 nm single-mode CW emission with a side-mode suppression ratio in excess of 45 dB.

Threshold currents and external efficiencies were 44 mA and 0.06 W/A, and the laser produced a maximum CW output power of just over 3 mW per facet.

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