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

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.