17 Jan 2007
The demonstration of zinc-oxide nanowires with p-type conductivity could pave the way for LEDs that are cheaper and more efficient than those based on gallium nitride.
Scientists at the University of California, San Diego (UCSD), and Peking University in China have reported a low-cost technique for fabricating p-type zinc-oxide (ZnO) nanowires (Nano Letters 6 2768). According to UCSD researcher Deli Wang, the nanowires could be used in a new generation of LEDs that can emit light from ultraviolet wavelengths to the visible part of the spectrum.
Zinc oxide is a very good light emitter with a band gap of 3.37 eV, and scientists have long been interested in its potential for producing highly efficient LEDs. However, engineers have found it difficult to achieve p-type doping in wide-bandgap semiconductors such as ZnO, which has limited its use in electronic applications.
More recently, p-type conductivity has been demonstrated in ZnO thin films, allowing ZnO-based LEDs to be fabricated for the first time. Just a few months ago, optics.org reported on MOXtronics, a US start-up that has produced a ZnO LED with a range spanning from UV to visible light.
The UCSD-Peking University project is the first time that p-type conduction has been replicated at the nanoscale. Using a simple form of chemical vapor deposition (CVD), the team doped the zinc oxide with phosphorous to make p-type ZnO-P nanowires.
"In our process, zinc and phosphor vapors travel down to the substrate and condense to form the nanowires of desired dimensions," says Wang. Because of its simplicity, the method could be cheaper than the metal-organic chemical vapour deposition (MOCVD) process that is used to fabricate GaN semiconductors.
Nanowires made in this way resemble a thick patch of grass, composed of pillar-shaped structures projecting up like spikes. Such a geometry would provide any LED with a large junction area, which in turn translates to higher efficiency.
"The LEDs would chiefly have their utility in the ultraviolet spectrum, but can be adapted for use in lighting, display signs and backlights for LCD monitors," says Wang. Lasers based on the nanowires could also serve as high-efficiency light sources for optical data storage, imaging and chemical sensing applications.
Wang has filed a provisional patent for the fabrication process and is cheerful about its future prospects. "We are now trying to manufacture the nanowires from the solution-phase itself, as this would be cheaper than the current vapour-based process," he says.