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GaN VCSEL delivers electrically pumped lasing

GaN VCSELs are now producing electrically pumped lasing thanks to superlattice structures in the n-type mirror and indium tin-oxide coating of the aperture.

Shing-Chung Wang and colleagues from National Chiao Tung University, Taiwan, have produced the first ever electrically pumped GaN VCSEL.

The laser, which has the potential to be used in high-density optical storage and laser printing applications, produced continuous-wave 462 nm emission with a linewidth of 0.15 nm at 77K.

Details of the device, the culmination of eight years of VCSEL development, were reported in the 7 April issue of Applied Physics Letters (App. Phys. Lett. 92 141102).

However, Wang told compoundsemiconductor.net that the team had previously announced its success at the "International symposium on VCSELs and integrated photonics". This meeting was held in Tokyo on 17 and 18 December, 2007, to celebrate the first 30 years of the VCSEL.

The development of electrically pumped GaN VCSELs has been held back by the quality of the distributive Bragg reflectors (DBRs) that form the mirrors and the low efficiencies of carrier injection into the active region.

GaN and AlGaN are the most common pairing for producing GaN-based DBRs due to their high difference in refractive index. However, this combination suffers from large differences in lattice mismatch and thermal expansion coefficient, which ultimately cause epiwafer cracking.

Wang's team has overcome this issue by inserting a superlattice of 5.5 pairs of AlGaN/GaN, which has a thickness that is equivalent to half of the lasing wavelength, into every four pairs of a AlN/GaN quarter-wavelength stack.

The additional layers reduce in-plane tensile stress, according to X-ray diffraction measurements.

Although the team's DBR is free from the cracking that occurs in standard DBRs, it shares the poor electrical conductivity that hampers currents injection into the active region.

Intra-cavity contacts can overcome this, and Wang's team has developed a novel design that features a transparent indium tin oxide (ITO) layer in the aperture and a Ta2O5/SiO2 dielectric DBR top mirror.

Annealing the ITO layer, which has a thickness that matches the resonance phase condition of the microcavity, reduces its contact resistance and increases transparency.

Individual VCSELs were formed by dicing 120 µm by 150 µm chips from the epiwafers and inserting them into TO cans.

The laser chosen for device testing had a threshold voltage of 4.1V at 77K. This low voltage shows that the ITO has formed a good electrical contact, and supports the team's choice for current injection. Lasing from the 10 µm diameter aperture device kicked in at 1.4 mA.

The aperture produces non-uniform emission at 1mA, which the researchers put down to variations in indium composition.

"One potential remedy is to reduce the indium concentration in the multiple quantum well," remarked Wang. "In fact, the lowering of the indium concentration is also in line with our future effort to develop shorter-wavelength 405 nm VCSELs."

However, the team's first priority is to develop a continuous-wave blue GaN VCSEL operating at room-temperature.

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