review research Novel nitride lasers take the upper hand
KEY PERFORMANCE characteristics of non-polar and semi-polar lasers are now eclipsing those of their conventional cousins.
University of California, Santa Barbara spin- off Soraa is claiming output power and efficiency records for it blue, continuous wave single-mode lasers. What’s more, Japanese material specialist Sumitomo has pushed nitride lasers further into the green with a semi-polar laser emitting at 534 nm, and it has also revealed that the threshold current of this type of device is two to three times lower than that of an equivalent one built on the c-plane.
James Raring, Soraa’s VP of laser engineering, says that the advantages of turning to semi-polar and non-polar planes include a higher gain and a lower effective mass for the holes, attributes that increase design flexibility.
“We believe that this is enabling us to get low threshold currents and high slope efficiencies without catastrophic facet damage,” says Raring.
This west coast start-up, which is based in Goleta, California, has produced a 447 nm edge-emitting laser with a 900 µm cavity that delivers an output of more than 800 mW, and a shorter version with a 600 µm cavity that has a peak wall plug efficiency (WPE) of 23.2 percent. In addition, the company has made a 521 nm laser that delivers more than 60 mW and produces a WPE in excess of 1.9 percent.
All the green and blue lasers have been built on undisclosed, non c-plane orientations of GaN using conventional semiconductor processing technology for this wide bandgap material. The devices were 1.5- 2.5 µm wide and featured a surface ridge laser architecture designed for single lateral mode operation.
Soraa’s most powerful blue laser had a threshold current and voltage of 45 mA and 3.8 V. Slope efficiency was 1.55 W/A at a case temperature of 20 °C. The shorter variant had a threshold current and voltage of 30 mA and 3.8 V, a slope efficiency of 1.68 W/A, and a WPE of 23.2 percent at a laser output of 180 mW.
At an output of 100 mW, the efficiency of these laser chips is about 40 percent higher
paper we thought we’d be conservative.”
Green lasers with a 600 µm cavity that are fabricated by the start-up had a threshold voltage and current of 7.3 V and 130 mA, and a slope efficiency of 0.3 W/A.
Soraa’s latest family of non c-plane lasers include a 521 nm device delivering a 60 mW continuous output
than those reported by Osram Opto Semiconductors at last year’s Photonics West meeting. Soraa says that this highlights the attractiveness of non c-plane lasers for applications requiring high efficiency, such as next generation displays.
Reliability testing of five blue lasers revealed a mean lifetime of 10,000 hours. These tests involved initially driving lasers at 60 mW and recording the time it took for the output power to fall by 20 percent.
According to Paul Rudy, general manager of Soraa’s laser division, the approach that they adopt to evaluate device lifetime is widely used in the high-power laser diode industry that serves biomedical and industrial applications.
“In the display community lots of folks use 30 percent and some folks use 50 percent,” says Rudy. “But for the purposes of the
The 60 mW, CW output of this device compares favorably with the 50 mW output of Osram’s 524 nm laser, which the German outfit reported in summer 2010.
Meanwhile, researchers at Sumitomo have been comparing the threshold current of green lasers built on the c-plane and the (2021) plane.
The Japanese outfit produced a series of semi-polar, 600 µm cavity lasers with dielectric coatings on the facets that realized threshold current densities of just 5.4 kA/cm2
, 5.2 kA/cm2 and 4.3 kA/cm2
at
emission wavelengths of 533.6 nm, 527.2 nm and 525.6 nm, respectively. In comparison, the threshold currents of c- plane green nitride lasers recently reported by Nichia and Osram are far higher: 18 kA/cm2
, 9 kA/cm2 and 3.8 kA/cm2 at
531.7 nm, 524 nm and 518 nm, respectively. A high threshold current is a major impediment to the production of powerful diode lasers. The standard route to cranking up laser power is to reduce reflectance at the facets, a step that increases threshold current and junction temperature. If the temperature gets too high, it hampers performance and cuts laser lifetime.
Sumitomo’s 525.5 nm laser has a threshold voltage of 6.38 V, a slope efficiency of 0.15 W/A and a maximum continuous-wave output of 36.4 mW. The team believes that a higher output is possible through refinements in the growth and fabrication processes used to make the laser, and optimization of its architecture.
Soraa also expects to take the power of its lasers to a new level and improve their efficiency. “This small group is working hard on blue and green,” says Rudy. “We still don’t think that we’ve turned all the knobs and optimized everything.”
J. Raring et al. Appl. Phys. Express 3 112101 (2010) M. Adachi et al. Appl. Phys. Express 3 121001 (2010)
January / February 2011
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