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Research Review: New Plane Promises To Aid Green Lasers

Researchers at the University of California, Santa Barbara (UCSB) and Mitsubishi Chemical claim to have uncovered a superior semi-polar plane for making green lasers.




Last summer Sumitomo won the race for the green laser diode with a device built on the (2021) plane. But corresponding author Po Shan Hsu from UCSB told Compound Semiconductor that even better results should be possible by switching to growth on the (3031) plane.



He says that the (3031) plane offers a much wider laser diode design space.



Theoretical calculations suggest that it is possible to grow In0.06Ga0.94N layers that are 60 percent thicker on this plane than the (2021) one.



The opportunity to grow thicker layers enables the growth of waveguide structures with sufficient refractive index contrast that do not use AlGaN, but just InGaN and GaN.



Eliminating AlGaN cladding layers is a big deal. “Growth of thick AlGaN layers are unfavorable because they can increase device voltage, decrease device lifetime, reduce device yield and reduce reactor stability," says Hsu.



What’s more, AlGaN layers usually require supperlattices, and this increases epiwafer growth times and ultimately manufacturing costs.



The team used MOCVD growth on freestanding GaN substrates provided by Mitsubishi Chemical to fabricate their blue InGaN/GaN laser diodes on the (3031) plane.



These devices produced a threshold current of 5.6 kA/cm2 and a clear lasing peak at 445 nm. Electroluminescence of the laser shifted by just 4 nm when the current density was cranked up from 0.3 kA/cm2 to 6.0 kA/cm2.



This very small shift in wavelength suggests that the device is capable of being driven at high modulation rates. The laser’s operating voltage was 9.9 V, a relatively high value that is probably due to unoptimized p-contacts and doping profiles.



Shifting the emission of these lasers from the blue to the green is one of the next goals for the team. This requires an increase in the indium-content of the active region. The team is looking into this, and investigating how indium incorporation on the (3031) plane compares to that on other semi-polar planes.



 







Po Shan Hsu et al. Appl. Phys. Express 3 052702 (2010)



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