Technical Insight
Osram’s powerful green laser questions the benefits of semi-polar growth
Engineers at Osram Opto-Semiconductors have broken the CW output power record for a true green laser with a polar device delivering 50 mW. Their 524 nm green emitter meets the specs for laser pico projectors, which need a 50 mW source emitting between 515 nm and 535 nm to deliver 10 lm of light on a screen.
The German outfit has also fabricated broad-area test lasers, which have been driven in pulsed-mode and emit at 531.7 nm.
These results, which follow on from Osram’s report in January of a 516 nm laser emitting 50 mW, question the benefits of fabricating lasers on the semi-polar plane.
Back in summer 2009, when Sumitomo reported a 531 nm pulsed laser and 520 nm CW variant on the (2021) plane, it appeared that by far the best way to reach longer wavelengths was to start with a semi-polar nitride plane that reduced the internal electric fields within the device. Researchers at Osram, however, argue that the key to making a green laser is the growth of a high-quality, indium-rich InGaN active layer.
Understandably, they don’t say exactly how they do this. However, they do reveal that they were able to produce longer wavelength lasers by improving epitaxial design and material quality.
Their paper also details the results of calculations that compare the emission wavelength of polar and non-polar quantum wells with indium compositions of 26 to 36 percent and widths of 2.5 nm and 3 nm.
These calculations show that the differences in internal electric field strengths of the two classes of laser mean that a non-polar variant needs an indium content 3-4 percent higher than its polar equivalent if it is to emit at the same wavelength.
Another insight provided by the calculations is the effect of widening the quantum well: Increasing this from 2.5 nm to 3 nm redshifts emission by more than 10 nm, according to Osram’s calculations.
The engineers used MOCVD to produce laser structures featuring AlGaN cladding layers.
A ridge waveguide structure was employed in the 524 nm laser. This has a threshold current of 97 mA and a slope efficiency of 330 mW/A. At an output of 50 mW the wall plug efficiency was 2.3 percent.
Emission at 531.7 nm was realized with the broad area test structures with a laser threshold of 18 kA/cm2. This was driven with 0.4 μs pulses at a duty cycle of 6 percent.
A. Avramescu et. al. Appl. Phys. Express 3 061003 (2010)
