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Blue semi-polar laser catches the performance of incumbents

Dec 13, 2013
A blue-emitting laser grown on the (2021) plane delivers efficient emission at high current densities, highlighting the potential of laser-based solid-state lighting

The UCSB blue-emitting laser, driven in pulsed mode, produces a current-limited output of more than 2 W. 

Researchers at the University of California, Santa Barbara (UCSB), have produced a blue-emitting semi-polar laser that is claimed to deliver a similar level of performance to state-of-the-art, conventional equivalents. And in future, according to these researchers, this class of laser should exceed the performance of the conventional lasers grown on the c-plane, thanks to a trimming of the polarization-related electric fields that hamper efficiency in the active region.

The team’s 454 nm laser, which is built on a (2021) GaN substrate provided by Mitsubishi Chemical Corporation, emits 2.15 W of optical power at an external quantum efficiency of 39 percent.

Another attractive attribute of this laser is that it can operate efficiently at a very  high current density of 28.1 kA cm-2. This value underlines the potential of low-cost, laser-based solid-state lighting using chips that are driven hard and emit at very high power densities.

“We believe that lasers will play a strong role in future display and directional lighting applications such as headlights,” argues team member Jim Speck.

Fabricating lasers on semi-polar planes, rather than the c-plane, opens up the possibility to use thicker quantum wells, due to a reduction in internal electric fields within the heterostructure.

High modal confinement is then possible without the need for AlGaN cladding layers, which limit laser reliability and the level at which catastrophic optical mirror damage occurs. The laser fabricated by the team is free from an AlGaN cladding layer. Sandwiched between 13.5 nm-thick barriers, it has four In0.2Ga0.8N wells that are not particularly thick – they are just 4.5 nm wide.

“In our future research, we will definitely explore wider quantum wells, as well as many other aspects of the active region design,” says Speck. “InGaN quantum wells on semi-polar planes, such as (2021), have very low electrical field at blue emission wavelengths, as demonstrated in our LED work on (2021) and by simple Schrodinger-Poisson/drift-diffusion solvers.”

According to first-principles calculations by Chris Van de Walle’s group as UCSB, the biggest contribution to modal loss in InGaN-based lasers is phonon-assisted absorption by acceptor-bound holes.

This loss increases with magnesium doping density, and to reduce its impact on the semi-polar laser, the team has moved the locations for high magnesium doping away from the centre of the optical mode.

Magnesium doping is just 7.5 x 1017 cm-3 in the 60 nm-thick In0.06Ga0.94N waveguide and 1.5 x 1018 cm-3 in the 200 nm-thick cladding. A 400 nm-thick cladding with a 7.5 x 1018 cm-3 magnesium doping level sits on top of this, followed by a 20 nm-thick magnesium-doped contact (1 x 1020 cm-3).
Reactive ion etching formed 900 µm-long ridge waveguide lasers with a cavity width of 8 µm. Polishing these chips created smooth facets, before a high-reflectivity coating based on eight quarter-wavelength-thick layers of SiO2 and Ta2O5 was applied to the back facet and the front-facet received an anti-reflection coating based on the same pairing of materials.
Measurements of laser performance were taken with an on-wafer probe, with the device driven with a 1 percent duty cycle – pulses with a 1 µs width, provided at a repetition rate of 10 kHz.

Maximum output power for the laser, 2.15 W, occurred at a drive current of 2.02 A. Even higher powers are possible, since the current delivered by the power supply limited the laser’s output, and the increases in current applied to the device produced a monotonic increase in its external quantum efficiency.

One weakness of the laser is its operating voltage: Threshold is 9.0 V, rising to 18.7 V at peak output power. These high voltages are a result of the high series resistance – after turn-on it is 6 Ω – and they stem from a high p-contact resistance. The team say that its laser could be improved by trimming its electrical resistance and cutting internal loss.
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