SUVOS targets UV emitters for biosensors and communications
The SUVOS project is expected to run for up to four years and will foster R&D that speeds the introduction of compact high-power UV sources. The program specifically targets wide-bandgap semiconductors such as GaN and its various alloys for use as optical sources that can be integrated into modules and subsystems.
Detecting biopathogens
To address the detection of airborne biological agents, the new project aims to demonstrate novel UV LEDs and lasers that operate from 280 to 340 nm. Recent research has resulted in LEDs with wavelengths as short as 305 nm (Compound Semiconductor March 2002 p29), although these devices typically suffer from low output power and high series resistance.
DARPA is aiming to mitigate these problems and is targeting compact, low-power UV sources to perform tasks such as laser-induced fluorescence spectroscopy of the common constituents in biological agents (figure 1). These constituents include tryptophan, with a peak absorbance at 280 nm, and NADH at 340 nm. The primary focus of the program is to develop the properties of AlGaN materials to provide inexpensive, low-power UV sources tailored to these wavelengths.
Covert communications
The US military also needs a secure means to send messages in the field using low-power communication systems - a requirement that cannot be met with conventional RF radios. Covert communication is made possible by UV sources that exploit the solar blind region located at 280 nm and below. In this region of the spectrum, the terrestrial solar flux is essentially zero, and the very low background can be used for NLOS communications over distances up to 250 m. The strong extinction coefficient of the signal in the UV makes it difficult to detect these emissions from a distance, particularly in the forward direction. The success of such a portable UV frequency communicator unit depends on the availability of a compact, powerful and energy-efficient UV optical source.
"Semiconductor UV emitters offer the potential for compact, inexpensive systems with low power and high reliability, all of which are needed for the systems we require at the Department of Defense," said SUVOS program manager John Carrano. "Current research in the AlGaN/GaN materials system is critical to this effort." By building on the commercial success of visible GaN-based LEDs, DARPA has initiated a program to develop short-wave UV sources. "These sources could be used in a compact unit that includes early warning detection of biopathogens, and that also performs fluorescence spectroscopy with a laser to confirm the diagnosis of the first alert," said Carrano. "We would also like this unit to consume only low power and to be inexpensive so that we can use the units in as many platforms and environments as possible."
DARPA s wish list for UV LEDs and lasers includes high external quantum efficiency, an optical output power in the mW range, high reliability devices with 10,000 hours of life, and a modulation bandwidth of 10 MHz. UV devices should provide room-temperature operation in either pulsed or CW mode.
The SUVOS program recognizes a num-ber of technical challenges that need to be overcome to achieve these goals. According to Carrano, one of the most important issues is attaining sufficient p-type conductivity in high-aluminum-content AlGaN. A doping level of around 5 x 1017/cm3 is needed to achieve sufficient hole injection in the AlGaN active regions and to provide efficient radiative recombination.
"A high aluminum content in the active region is needed to achieve a sufficiently wide electronic bandgap for emission in the UV," explained Carrano. "During the growth of AlGaN with higher concentrations of aluminum, it becomes more and more challenging to achieve p-type doping in the material." A related problem is the suppression of non-radiative recombination mechanisms in the active regions.
The large heterojunction offsets associated with the AlGaN material system also pose a significant problem to carrier transport at heterointerfaces. "To achieve confinement using barrier and cladding layers, we need to control the quality and properties of AlGaN materials with as much as 65% aluminum in the layers," added Carrano.
There are also the additional factors that typically face all LED and laser designers, i.e. the need to efficiently extract light and remove heat, as well as wave-guiding and mirror design for lasers and resonant-cavity LEDs.
"The development of high-quality ohmic contacts is particularly important for the devices envisioned in this program," said Carrano. "As high levels of current injection are needed to drive high-power sources, the design of innovative structures for both LEDs and laser diodes is crucial to meeting SUVOS objectives."
