18 May 2006
The shortest-wavelength LED ever seen has been fabricated by a research team at NTT in Japan.
NTT researchers have made an aluminum nitride (AlN) LED operating at 210 nm - the shortest wavelength at which such a semiconductor device has ever been shown to emit light. (Nature 441 325)
Although extremely inefficient and with a hefty operating voltage of 25 V, the LED made by Yoshitaka Taniyasu and co-workers at NTT's Basic Research Laboratories represents a crucial first step towards the development of very low wavelength emitters that could be used to detect or destroy harmful biological species.
Reporting their work in this week's Nature, the NTT group describes how it used a refined doping strategy to make the PIN LED and grew the device epilayers using MOCVD.
Until now, researchers had been unable to control the doping of the n-type and p-type layers of AlN precisely enough to demonstrate an LED with the material, which has the widest direct bandgap among semiconductors.
"By reducing the dislocation density and finely controlling the silicon doping level, we were able to boost the room-temperature electron mobility," wrote Taniyasu. According to III-N expert Asif Khan at the University of South Carolina in the US, this dopant control is the critical part of the work. It means that sufficient conductivity can be imparted to the layers of AlN to form both p-type and n-type layers, which sandwich the undoped AlN layer in the device. As a result, enough electrons and holes can recombine to produce the deep-UV photons.
In the past, AlGaN-based LEDs have been manufactured that emit down to 245 nm. In fact, Khan's own research group at USC has made such devices using a technique called migration-enhanced MOCVD developed in collaboration with Sensor Electronic Technology.
But doping AlN has proved much more troublesome. "Unfortunately, as the aluminum fraction increases, so too does the difficulty of the doping," wrote Khan in Nature. "It is hardest of all for AlN - which is, in fact, an insulator."
At present, the 210 nm LED reported by the NTT team is nowhere near good enough for any real-world applications such as water or air purification systems. The primary challenge facing the researchers now is to increase device efficiency and improve on the tiny output power - just 0.02 µW.
Reducing the number of dislocations in the structure is the most effective way to do this. Since the reported LED was fabricated on a SiC substrate, the lattice mismatch between the two materials yielded relatively high dislocation densities of 109cm-2.
That level could be greatly reduced by using native AlN substrates, which have recently become available on the open market.
Although this should increase the internal conversion efficiency of the LED by two orders of magnitude, Khan reckons that much more will be required before any commercial application becomes plausible, and warns that it will not be a straightforward development path.
"For high-Al compounds one can surely benefit from AlN substrates," said Khan. "For [AlGaN-based] 245 nm devices, efficiency can potentially be doubled by reducig defects by a factor of ten."
"However, for AlN LEDs the problem will be to find a material with a bandgap larger than AlN that is compatible with the AlN material system," he added. "This will be needed to form quantum wells for efficient carrier capture."
Two key areas require huge improvements for commercial applications. First, an increase in efficiency by a factor of at least a million; and second, a reduction in operating voltage to well below 25V.
While switching to a native substrate will help, further increasing the room-temperature conductivity of the AlN layers will also be required - probably by developing more efficient doping methods.
Taniyasu suggests that this could be achieved partly by using elements such as carbon, beryllium, zinc or cadmium instead of magnesium to dope the p-type layer, and partly though the use of carrier confinement structures such as quantum wells.
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