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Rohm Shoots For A Green Diode Laser

Rohm believes that it is impossible to make conventional nitride laser diodes that emit in the green, so it has turned its attention to non-polar equivalents. Is this strategy starting to pay dividends? Richard Stevenson investigates.

After years of sterling service, color TVs with cathode ray tubes and 20-something inch screens are on their way out. Larger and cheaper LCD-based screens are now the biggest seller, according to market researcher iSuppli, and this technology is even eating into the "40 inch plus" market that is served by plasma screens. However, laser-based displays threaten to replace all of these technologies thanks to the delivery of even higher-quality pictures.

These displays require affordable red, green and blue lasers, which makes semiconductor devices the preferred option. Red and blue sources already exist in the form of AlInGaP- and nitride-based lasers. However, no III-V design can emit close to 520 nm, the wavelength required for the green component. Although nitride-based lasers are the most promising candidates to reach this wavelength, no-one has even broken the 500 nm barrier. For example, Nichia, the pioneer of the nitride laser, has been increasing the emission wavelength by just a few nanometers each year and has only just released a 488 nm laser.

Extending a conventional nitride laser diode s emission from around 400 nm is not easy because the intrinsic polarization fields cause a large and unhelpful blue-shift of the lasing wavelength. However, this problem can be avoided by growing the epilayers on m-plane GaN, which produces non-polar devices that are free from strong internal fields.

Rohm has been following this tack since January 2006, after it decided to turn away from developing c-plane laser structures. The company began with the easier task of making a 405 nm device, which contains less strain. Fast progress followed and just over a year later the company published the first ever account of continuous-wave (CW) non-polar lasers in the February 23, 2007, edition of the Japanese Journal of Applied Physics. This would have been the first ever report of any non-polar laser if the University of California, Santa Barbara, hadn t published a paper on a pulsed mode m-plane laser in the same issue.

Once they had built a blue laser, the Rohm engineers turned to developing longer-wavelength equivalents. Unfortunately, epiwafer cracking prevented this structure from being extended beyond 430 nm. At longer wavelengths more aluminum is needed in the cladding layer to maintain the structure s optical confinement and this leads to more strain in the epiwafer.

However, the researchers managed to overcome this issue by switching the guiding layer from GaN to InGaN. With this change, optical confinement could be maintained while reducing epiwafer strain, because less aluminum is required in the cladding.

This has extended emission, and last summer Rohm reported a 452 nm laser driven in pulsed mode, with a threshold current of 22.3 kA/cm2 and a threshold voltage of 11.1 V. Since then the researchers have started to refine this device. Distributed Bragg reflectors have been added to the laser facets, reducing the threshold current and producing CW operation.

Already this year, the team has reported better results in Applied Physics Express. Pulsed and CW threshold current densities for a 400 µm long laser cavity are just 4 and 5 kA/cm2, respectively. Emission wavelengths are also moving towards the green – at 457 nm in pulsed mode and 459 nm for CW operation. This shift in wavelength between the two modes of operation is claimed to result from self-heating, which is caused by a relatively high operating voltage of 6 V.

Kuniyoshi Okamoto, a member of the Rohm team, says that there is room to improve the latest laser s threshold currents, but points out that such values are already low enough for commercialization. Obviously there is still work to do to extend the emission to 520 nm and he believes that changes to the composition of the guiding and cladding layers are needed to produce crack-free epiwafers for green emission.

The CW output also has to increase significantly. The 459 nm laser produced 5 mW and Okamoto believes that more than 30 mW will be required for a 20 inch projection display. This is a tough target, but there is no denying that Rohm is making rapid progress and you would not want to bet against them hitting these goals over the next few years.

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