research review
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 red- shifts 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)
SET makes brighter ultraviolet chips
A US collaboration led by Sensors Electronic Technology (SET) claims to have raised the bar for power output from a single ultraviolet LED chip. The team, which includes researchers from Rensselaer Polytechnic Institute and the US Army Research Laboratory, has produced a 273 nm chip emitting 30 mW, and a 247 nm version delivering 6 mW.
SET’s president, Remis Gaska, says that the longer wavelength chip could be used for water sterilization and disinfection. Higher output powers enable an increase in water flow rate, and could lead to a cut in the cost of treatment per liter.
“Although DNA absorption peaks at 265- 268 nm, the combination of DNA absorption and wavelength-dependent UV light attenuation in the water shifts the maximum germicidal efficiency to 270-275 nm,” says
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www.compoundsemiconductor.net July 2010
Gaska. His 273 nm chip is in the center of this range.
The shorter wavelength, 247 nm LED is a promising source for water treatment and it could be combined with hydrogen peroxide for chemical decontamination.
The US team realized record output powers from its UV LED chips by reducing built-in strain, improving n-type and p-type doping, and minimizing dislocations and point defects. Thanks to these advancements, the researchers could increase the area of their chips without sacrificing quantum efficiency at low current densities.
Epitaxial structures were produced on sapphire via a combination of MOCVD and migration-enhanced MOCVD, and processed into devices with junction areas of 0.5 mm2 and 1 mm2.
LED chips emitting at 273 nm that have a junction area of 1 mm2 showed an increase in external quantum efficiency as the drive current ramped to several hundred milliamps. At 700 mA, the CW output was limited by the heat dissipation from the junction of the TO-3 package. 247 nm LEDs with a chip area of 0.5 mm2 produced a peak CW output of 6 mW at 275 mA.
The researchers have also studied the reliability of their LEDs. The output power of a 273 nm , 0.5 mm2 chip driven at 400 mA (400 µs pulses, 10 percent duty cycle) halved after about 5000 hours of operation. Device degradation was caused by a change in the electrical properties of the p- type layers, which hampered electrical injection, leading to a cut in output power.
M. Shatalov et al. Appl. Phys. Express 3 062101 (2010)
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