Sep 19, 2007
An n-n junction two-color nanowire LED
Recently, a collaborative study led by Professor Venkatesh Narayanamurti and myself at Harvard University (http://nanotechweb.org/articles/journal/6/5/5/1) demonstrated that when an n-type semiconductor nanowire is placed on a p-type semiconductor substrate the resulting junction is, in general, a tunnel junction instead of a traditional p-n junction. This has important implications for the behavior of such nanowire-on-substrate devices, especially for light-emitting applications.
Following on that work, the same group of researchers have now shown that one of the consequences of the tunnel junction is the surprising finding that light emission also results when the n-type nanowire (gallium nitride) is placed on an n-type substrate (silicon). Furthermore, because of the interchangeable roles of the semiconductors forming the junction (the nanowire and the substrate), their device emits ultraviolet light from the gallium nitride nanowire for one polarity of the applied voltage, and infrared light from the silicon substrate when the polarity of the voltage is reversed.
A unipolar n-GaN nanowire (NW) on an n-Si substrate light-emitting diode under a positive bias. The scale bar is 5 μm.
These results provide an elegant demonstration that the details of the junction between a nanowire and a substrate, formed not by covalent chemical bonds (as in epitaxially grown junctions) but by van der Waals forces, critically determine the behavior of the device. Interestingly, this device structure could also be used as a novel approach for two-color unipolar LEDs.
About the author
Federico Capasso has been Robert Wallace professor of applied physics in the School of Engineering and Applied Sciences at Harvard University since 2003, after a 27 year career at Bell Laboratories. His group is currently pursuing a broad research agenda, straddling applied and basic research in nanoscale science and technology in such areas as quantum cascade lasers, surface plasmon nanophotonics, novel metamaterials, nanowire optoelectronics, optofluidics and fundamental studies of quantum electrodynamic phenomena, such as the Casimir effect and the vacuum torque.