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Red-shift draws Rohm's GaN lasers to green

25 Feb 2009

Rohm has unveiled a continuous-wave 499.8 nm laser, claimed to be the longest recorded wavelength yet produced by a GaN diode.

Researchers at Rohm believe that they have taken an important step on the way to truly green wavelengths using non-polar diode technology. The key to the laser diode's emission relies on a shift towards the red end of the spectrum of over 10 nm caused by self-heating.

However, the same effect has a great adverse influence on laser diode lifetimes and is therefore not a shortcut straight to widespread green emitter manufacturing.

“This effect can't be used for mass production,” conceded Kuniyoshi Okamoto, Rohm researcher and co-author of the February 18 Applied Physics Letters paper describing the results. He estimates that the self-heating in the record diode leads to an approximate temperature rise of 230K.

The devices showed a threshold current density of 3.1 kA/cm2, with a total threshold current of 46 mA. At 1.02 W input power the diodes emitted 5 mW, 490.5 nm light, while at 3.03 W input power the same devices reached 15 mW output power at 499.8 nm.

“We are [now] trying to fabricate laser diodes emitting greater than 500 nm around the threshold current,” Okamoto told compoundsemiconductor.net.

The Rohm team actually grew two different subsets of diode on m-plane GaN substrates provided by Mitsubishi Chemical. They used low-pressure MOCVD to deposit an InGaN multi-quantum well diode on top of the non-polar material.

The lasers' optical cavities were produced either with 70 percent or 97 percent reflective frontside mirrors, by cleaving the material along its c-plane and sputtering dielectric mirrors onto the resulting surfaces.

Higher reflectivity mirrors were used in the record-breaking diodes, allowing them to exceed the 492.8 nm wavelengths that the second set of devices achieved at 0.83 W input power.

However, the 70 percent reflective mirrors provided devices with a lower threshold current density (2.8 kA/cm2), and a peak output power of 92 mW.

The output power of both types of laser are well ahead of the 5 mW measured from the longest-wavelength diodes grown on polar c-plane GaN substrates: 488 nm devices made by Nichia.

Okamoto says he is “very glad” that Rohm has now surpassed this wavelength and believes that non-polar diode technology will eventually go on to be used commercially.

“I'm not really sure when,” he said, “because the size and cost of the non-polar GaN substrate is an important factor, as well as the characteristics of non-polar GaN laser diodes.”

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