Nichia eyes new markets with expanded laser-diode portfolio

Over a decade has passed since Nichia first demonstrated pulsed emission from an InGaN laser diode. In the intervening years the lifetime and output of these violet emitters has been improved, to such an extent that the devices are finally set to enter volume production for HD DVD and Blu-ray Disc players, as well as Sony s PlayStation 3 games console.

Nichia is obviously keen to grab a large share of this market, but it has also been developing laser diodes that span from the ultraviolet to the aquamarine region of the spectrum. This could dramatically increase the number of addressable markets for the company s high-power diodes, as 365 nm ultraviolet emitters can be used for curing applications that have previously been served by high-pressure mercury lamps, while blue lasers can feature in full-color laser displays, and aquamarine diodes can offer a more compact alternative to argon-ion lasers that are often used for spectroscopy.

All of these laser diodes have the same basic structure, which is shown in figure 1. They were first grown on sapphire substrates using the epitaxial layer over growth (ELOG) technique to reduce defect density, but more recently they have been fabricated with a mixture of Nichia s own and commercially available n-type free-standing GaN substrates. This switch has produced several benefits for high-power laser diodes, including lower resistance and more straightforward processing, as laser cavities can be formed by cleaving instead of reactive ion etching.

The benefits of using native substrates can be seen in the improved performance of many of Nichia s lasers diodes, including the company s 365 nm emitters. These ultraviolet sources have trailed the performance of their 405 nm cousins, because the additional aluminum content that is needed to shift the emission wavelength increases the strain in the epiwafers, generating cracks and dislocations.

The latest UV lasers feature a buried-ridge geometry, reflective coatings on the front and rear facets, and an n-type electrode that is formed by evaporation onto the bottom of the n-type substrate. The devices produce 20 mW continuous-wave output under a drive current of 76 mA and an operating voltage of 5 V. The lifetime, which is defined as the time taken for the diode s operating current to reach 1.3 times its initial value, has been estimated at more than 10,000 h following device operation lasting 1500 h. In comparison, the GaN-on-sapphire devices had a lifetime of only 2000 h when delivering 3 mW at 25 °C.

Nichia has also been improving the performance of its blue laser diodes. These devices are needed to complete the line-up in full-color laser display systems that can feature high-power red emitters, and green lasers created through second-harmonic generation that are already commercially available. Until recently, blue laser diodes operating in the 440-460 nm window were unable to deliver the output power and reliability for this application, but Nichia has recently demonstrated 445 nm emitters producing 200 mW and it will release lasers operating at this wavelength later this year. Encouragingly for this emerging application area, surface-emitting laser developer Novalux has sealed a supply deal with display giant Epson. Its red, green and blue chips will be used to power the Japanese company s market-leading LCD projectors.

Nichia has been developing both single-transverse-mode and multi-transverse mode formats of its 445 nm emitters. The singlemode devices have a 2 × 1 μm aperture, can deliver a continuous-wave output of 50 mW at room temperature, and have an estimated lifetime of more than 10,000 h. Their multimode counterparts have the same estimated lifetime, but have 7 × 1 μm apertures and produced 300 mW under the same conditions.

The threshold current density of these blue-emitters is strongly dependent on the lasing wavelength (see figure 2). At longer wavelengths the threshold current density is higher because the additional indium content in the InGaN quantum wells that is required to increase the lasing wavelength also produces dislocations in the epilayers. However, these problems can be addressed by modifying the growth conditions and the device structure, which has allowed Nichia to extend the emission wavelength and make lasers that it describes as aquamarine. These efforts have produced 5 mW room-temperature emission at 482 nm, which is believed to be the longest wavelength ever reported for III-V GaN current-injection laser diodes.

Nichia is also continuing to develop its violet laser diodes operating at 400-410 nm, and is focusing on increasing the output power of these devices so that they are suitable for high-speed recording in high-density optical-disk systems and industrial applications. The company has already fabricated single-transverse-mode 405 nm lasers producing 200 mW at room temperature, and will release a commercial version of this device next year. These diodes can operate at up to 80 °C, and were lifetime-tested at this temperature when delivering 160 mW output in pulsed-mode (a 30 ns pulse width and a 50% duty cycle). This examination revealed a steady degradation in the operating current, which increased by typically 7-8% over 1000 h.

The company has also built multimode versions of the violet-emitting devices that are targeting industrial applications such as the processing of printed-circuit boards. These lasers have 7 × 1 μm apertures and also show a steady degradation of the operating current when subjected to a lifetime test. In this case the diodes showed a typical increase in operating current of 20% over the 3000 h of operation.

Nichia s development program for high-power laser diodes emitting in the ultraviolet, violet, blue and aquamarine has equipped the company with a portfolio of devices delivering between tens and hundreds of milliwatts. And while the Japanese outfit will expect to reap the rewards of a ramp in sales of consumer electronics deploying 405 nm emitters, the company is also now in the position to chase business from many other types of application.