SUVOS pushes UV LEDs and lasers to shorter wavelengths

11 Jun 2004

In just two years, DARPA's Semiconductor Ultraviolet Optical Sources (SUVOS) program has made 280 nm LEDs a reality, demonstrated prototype UV biosensor and communication systems, and yielded lasers operating below 350 nm. Tim Whitaker reports.

Semiconductor light sources operating in the ultraviolet (UV) region are required for a number of applications, including solid-state lighting, biological agent detection, covert communication and sterilization. Various groups around the world are attempting to achieve shorter-wavelength operation, while at the same time improving the output power and extraction efficiency of their devices.

In the US, this effort is being coordinated by DARPA's Semiconductor Ultraviolet Optical Sources (SUVOS) program, which currently consists of 17 projects with a total of more than 20 participants. The program is about halfway through its four-year cycle, and by the time it finishes in spring 2006, DARPA will have invested a total of around $50 million.

The original goal of SUVOS was to develop device structures operating down to 340 nm during Phase I of the program, which lasted for 18 months and has now finished. In Phase II, the participants were due to investigate shorter-wavelength devices operating down to 280 nm, as well as integrating the sources into system test beds and prototypes.

In fact, SUVOS has exceeded expectations, according to program manager John Carrano. "The program has already demonstrated UV LEDs operating down to 275 nm, with output powers in the milliwatt range under DC conditions," he said. "Accelerated lifetime testing is under way, and we hope to reach 1000 h lifetimes very shortly."

Sterilization and decontaminationCarrano says that the "wildly successful" results achieved for LEDs during Phase I of SUVOS have opened the door to further progress. "We are now intending to expand the goals of the program to get all the way down to 265 nm LEDs," he said.

Crucially, 265 nm represents the sweet spot for water and air purification as well as surface decontamination. While there is a military requirement for such functionality, there is also a large commercial market; water treatment facilities around the world use UV lamps to kill bacteria. They currently use mercury vapor lamps, which are inefficient and offer little directional control. However, semiconductor emitters operating at 265 nm should be able to provide considerable improvement.

Another application is the sterilization of water in remote areas. Hydro-Photon, Inc of Blue Hill, ME, sells a product called Steri-Pen, a pocket-sized device containing a mercury vapor tube emitting at 253.7 nm, which can be used to sterilize a beaker of water in around one minute (see picture). The company is investigating the use of solid-state sources as an alternative, and owns a patent entitled "Hand-held water purification system using solid-state devices" (US patent number 6,579,495). The company has several other patents pending in this area.

"At the moment we are working under contract with the Office of Naval Research and with the help of DARPA to develop a portable UV LED-based water purification system for troops in the field," said Hydro-Photon's Miles Maiden. "Our chief subcontractor on this project is the materials group headed by Shuji Nakamura and Steve DenBaars at the University of California, Santa Barbara."

Biosensors

Detecting biological warfare agents is of particular concern to the US military. One technique for discriminating between these agents and harmless particles or organisms is to excite the sample with a UV source and measure the resulting fluorescence. Tryptophan, a common constituent of biological organisms, has an absorption peak at 280 nm, while the biochemical compound NADH has an absorption peak at 340 nm.

Carrano says that three years ago, when the SUVOS program was being put together, the prospect of a viable 280 nm LED seemed very remote. "Now we have devices that are good enough to use in real applications," he said. One such application is the world's first and only dual-wavelength LED-based biosensor. Using 340 and 280 nm LEDs, the system, which is currently about the size of a large notebook PC, can detect single particles of a species that simulates anthrax. It can also, to a certain extent, discriminate between potential hazards and other particles such as dust, dandruff, coffee creamer and smoke particles. Discrimination is enabled by the combination of two excitation sources and several detectors tuned to different fluorescence bands.

Laser-based systems have also been demonstrated. While these will ultimately provide higher performance, LED-based systems will be sufficient for certain biosensor applications. "In future, we will need a suite of sensors - there won't be just one that will fit all customers and requirements," said Carrano. "The LED-based systems will be deployed sooner, and will also be smaller and considerably cheaper than the laser-based systems."

SUVOS participants have made good progress developing short-wavelength lasers as well as LEDs. Cree recently announced that it had made a 348 nm CW laser operating under electrical injection at room temperature. This is the shortest wavelength reported for a semiconductor laser diode. Carrano says that similar devices operating at a slightly longer wavelength had a lifetime in excess of 100 h and produced many milliwatts of optical power.

Covert communicationsAnother application for UV LEDs that is of particular interest to DARPA is non-line-of-sight (NLOS) communications. For example, UV sources will enable ultra-low-power, covert systems that can be used in urban terrain or to provide a network of unattended sensors. Systems operating at around 280 nm are able to exploit the solar-blind region of the spectrum, a gap in which the terrestrial component of the sun's radiative flux is essentially nil. This low background enables excellent signal-to-noise, so low-power optical sources can be used, but the high extinction coefficient in the UV means that the range is limited to around 250 m, and hence there is a low probability of jamming or interception.

A prototype NLOS communication system using deep-UV LEDs has been demonstrated in broad daylight. UV radiation from a cluster of six LEDs operating at 275-280 nm was scattered by the atmosphere and detected by a NLOS receiver about 15 m away. The NLOS digital link had a data rate of 200 bit/s, so there is obviously room for improvement. However, at its current rate of progress the SUVOS program seems set to yield high-performance NLOS communication systems, as well as biosensors and sterilization systems, before it finishes in two years' time.

University of South Carolina and Sensor Electronic Technology develop commercial-grade 280 nm LEDs

Armed with funding from the SUVOS program, the University of South Carolina (USC), in collaboration with start-up company Sensor Electronic Technology Inc (SET), has recently demonstrated AlGaN-based deep-UV LEDs at 280 nm with record power levels. The organizations, both based in Columbia, SC, have reported peak powers of 1.53 mW for CW operation at 450 mA and 24 mW for pulsed operation at 1.5 A. These power values translate to a record external quantum efficiency of 0.36% and a wall plug efficiency of 0.12%.

"These results bolster our confidence that deep-UV LEDs are capable of producing milliwatt power levels at wavelengths well below 300 nm," said USC's Asif Khan in an invited presentation at the MRS Fall 2003 meeting in Boston. In 2001, Khan's USC group was the first to report quaternary AlInGaN-based UV LEDs with emission at 340 nm. Within one year they succeeded in demonstrating the world's first AlInGaN multiple quantum well (MQW) deep-UV LEDs emitting at 325, 305 and 285 nm with sub-mW power levels.

Khan's USC group has been responsible for several key innovations that have led to recent advances in deep-UV light sources. The first is an AlN/AlGaN short-period superlattice for strain management, which was first proposed and used by the USC team in 2002 to enable the growth of thick crack-free n⁺-AlGaN layers. These buffer layers help to reduce the defect density, and they minimize the effect of lateral current crowding in devices on insulating sapphire substrates. The USC team also uses a p-AlGaN/p⁺-GaN heterostructure to increase hole injection due to a polarization-field-induced hole accumulation at the heterointerface. This enables deep-UV LED operation at cryogenic temperatures.

The USC/SET deep UV LEDs are 100 µm and 200 µm square geometry devices on sapphire substrates with Al_0.44Ga_0.56N/Al_0.48Ga_0.52 N MQW active regions. They are flip-chip mounted on AlN carriers and then bonded to gold-plated headers. The flip-chip package increases the output powers by a factor of three to five, thanks to better light collection and more efficient heat sinking. The improved heat sinking also enables CW operation at higher currents without power saturation.

"These innovations have allowed us to keep advancing and make great progress in the deep-UV LED area," said Khan, adding that every group reporting high-power UV LEDs uses one or more of the USC developments.

Now, by further improving the materials, USC has succeeded in obtaining high-mW-power LEDs at 269 nm with an external quantum efficiency of 0.4%. Sub-mW LEDs with emission at 245 nm have also been obtained. With further optimization and new device topologies such as micro-pixels, mW-power LEDs emitting at 250 nm (the pump wavelength used for the fluorescent lamps) are believed to be within reach, added Khan.

Commercial UV LED productsThe close collaboration between USC and SET has served as a vehicle for accelerated technology development and scale-up, with SET aggressively pursuing commercialization of deep-UV emitters. The company has developed proprietary growth tools that combine conventional MOCVD growth technology of AlInGaN-based wafers with novel migration-enhanced (ME) MOCVD, which is expected to bridge the gap between MOCVD and MBE. Importantly, these growth tools are designed to handle large amounts of highly reactive aluminum required for deep-UV LED production.

"We are in the final stages of prototyping fully packaged UV LEDs emitting in the 270-340 nm range," said Remis Gaska, president and CEO of SET. "This will be followed by the pilot production of evaluation kits containing a few LEDs with various emission wavelengths." The company has already started sampling 340 nm wavelength UV LEDs, called UVTOP-340, to potential customers, and plans to start shipping evaluation kits in the second or third quarter of 2004.

Further readingJ Zhang et al. 2002 Crack-free thick AlGaN grown on sapphire using AlN/AlGaN superlattice for strain management Appl. Phys. Lett. 80(19) 3542.
A Chitnis et al. 2002 Low-temperature operation of AlGaN SQW LEDs with deep UV emission at 285 nm Appl. Phys. Lett. 81(16) 2938.