14 Sep 2007
Researchers from Harvard University have demonstrated electroluminescence from a unipolar LED.
The generation of electroluminescence (EL) in semiconductor nanowires has generally relied on p–n junction light-emitting diodes of different kinds. Mariano Zimmler and colleagues show that it is possible to generate EL if the substrate is also of n-type conductivity – in other words, a unipolar LED (Nanotechnology 18 395201).
The researchers describe the construction of the device, which consists of a gallium nitride (GaN) nanowire on a silicon (Si) substrate, where both semiconductors are n-type.
A novel feature of the device is that by reversing the polarity of the applied voltage the luminescence can be selectively obtained from either the nanowire or the substrate. For one polarity of the applied voltage, ultraviolet (and visible) light is generated in the GaN nanowire, while for the opposite polarity infrared light is emitted from the Si substrate.
The researchers explain this behavior in terms of electron tunneling from the valence band of one semiconductor into the conduction band of the other. For one polarity of the applied voltage electrons can tunnel from the valence band of GaN into the Si conduction band. This process would create holes in GaN, which can recombine with conduction band electrons, generating GaN band-to-band luminescence. A similar process applies under the opposite polarity for Si light emission.
This device structure offers an additional avenue for the experimental study of electroluminescence in single nanowires and represents a novel approach in the construction of two-color light-emitting devices.