Jul 5, 2005
Nanowire device suits microwave detection
Researchers have made a nanowire device that can detect microwave radiation up to 110 GHz at room temperature. The team, from the University of Manchester, UK, VTT Information Technology, Finland, University of Würzburg, Germany, University of Salamanca, Spain, and Lund University, Sweden, believes the devices could have applications in detecting terahertz radiation and in plastic electronics.
"This is the simplest diode in the world, as well as the quickest novel electronic nanodevice to date," Aimin Song of the University of Manchester told nanotechweb.org. "The self-switching device (SSD) is made in only one nanolithography step and is planar: both electrodes and the active semiconductor are within a two-dimensional plane. This is in great contrast to a traditional diode, which is made using up to 10 steps and is a complex three-dimensional vertical structure."
Song and colleagues made nanowire devices with intentionally broken symmetry by etching L-shaped trenches in a modulation-doped InP/InGaAs/InP quantum well wafer. This defined nanowires in between the trenches. The wires were 1.2 µm long and 60-100 nm wide.
"The device image is like two 'L's, back to back," said Song. "When connected into an array, the 'L's become 'U's. And indeed, to make such a device or a circuit based on SSDs we just need to 'write' insulating lines on a semiconductor layer."
The scientists say this simplicity significantly reduces production costs. The planar layout also makes the structure extremely suitable for detecting incoming electromagnetic radiation. What's more, the device has a threshold voltage that's tunable from zero to about 10 V.
"Therefore, it allows us to detect extremely weak signals, showing a very high sensitivity even when no external bias circuit is used," said Song. "A normal diode always has a threshold voltage, typically around 0.7 V, which is fixed and determined by the semiconductor used. If the applied signal is below the threshold, the diode will not operate at all."
A device containing 18 parallel nanowires produced roughly 75 mV of DC output for every mW of nominal input power of a 110 GHz signal. That's despite the fact that only about 0.4% of the microwave power was effectively transferred to the structure because of an impedance mismatch. And the detection sensitivity of the devices was pretty much stable over a frequency range of 100 MHz - 110 GHz.
"From the frequency dependence, it is virtually certain that the device will also operate well in the terahertz (1000 GHz) frequency regime, in which a very broad range of applications have been envisioned," said Song. "It could dramatically improve or even revolutionize technologies such as local oscillators and radar arrays for astronomy, high bandwidth and wireless networks for communication, environmental monitoring, industrial tomography for atmosphere control, drug delivery, food analysis for process technology, cancerous tissue imaging, and spectroscopy for medical applications."
The researchers reckon their SSDs could be ideal terahertz detectors, because the devices are small, ultra-sensitive, broadband, scalable and easy to couple. They could also prove useful in plastic electronics.
"The key difficulty [with today's plastic electronics technology] is that the speed of organic devices is too low, typically around or below kHz, whereas most applications need at least MHz," said Song. "Because the self-switching device has been shown to work to at least 110 GHz, the physical rule of transistor speed scaling ensures that organic self-switching devices will be able to perform at not only tens of MHz, but also most likely in the hundreds of MHz range."
The researchers reported their work in Nano Letters.
About the author
Liz Kalaugher is editor of nanotechweb.org.