Researchers at the University
of Illinois have found a new way to make transistors smaller and faster.
The technique uses self-assembled, self-aligned, and defect-free nanowire channels
made of gallium arsenide.
In a paper to appear in the IEEE (Institute of Electrical and Electronics Engineers)
journal Electron Device Letters, U. of I. electrical and computer engineering
professor Xiuling Li and graduate research assistant Seth Fortuna describe the
first metal-semiconductor field-effect transistor fabricated with a self-assembled,
planar gallium-arsenide nanowire channel.
Nanowires are attractive building blocks for both electronics and photonics
applications. Compound semiconductor nanowires, such as gallium arsenide, are
especially desirable because of their better transport properties and versatile
heterojunctions. However, a number of challenges - including integration
with existing microelectronics - must first be overcome.
"Our new planar growth process creates self-aligned, defect-free gallium-arsenide
nanowires that could readily be scaled up for manufacturing purposes,"
said Li, who also is affiliated with the university's Micro and Nanoelectronics
Laboratory and the Beckman Institute. "It's a non-lithographic process
that can precisely control the nanowire dimension and orientation, yet is compatible
with existing circuit design and fabrication technology."
The gallium-arsenide nanowire channel used in the researchers' demonstration
transistor was grown by metal organic chemical vapor deposition using gold as
a catalyst. The rest of the transistor was made with conventional microfabrication
While the diameter of the transistor's nanowire channel was approximately 200
nanometers, nanowires with diameters as small as 5 nanometers can be made with
the gold-catalyzed growth technique, the researchers report. The self-aligned
orientation of the nanowires is determined by the crystal structure of the substrate
and certain growth parameters.
In earlier work, Li and Fortuna demonstrated they could grow the nanowires
and then transfer-print them on other substrates, including silicon, for heterogeneous
integration. "Transferring the self-aligned planar nanowires while maintaining
both their position and alignment could enable flexible electronics and photonics
at a true nanometer scale," the researchers wrote in the December 2008
issue of the journal Nano Letters.
In work presented in the current paper, the researchers grew the gallium-arsenide
nanowire channel in place, instead of transferring it. In contrast to the common
types of non-planar gallium arsenide nanowires, the researchers' planar nanowire
was free from twin defects, which are rotational defects in the crystal structure
that decrease the mobility of the charge carriers.
"By replacing the standard channel in a metal-semiconductor field-effect
transistor with one of our planar nanowires, we demonstrated that the defect-free
nanowire's electron mobility was indeed as high as the corresponding bulk value,"
Fortuna said. "The high electron mobility nanowire channel could lead to
smaller, better and faster devices."
Considering their planar, self-aligned and transferable nature, the nanowire
channels could help create higher performance transistors for next-generation
integrated circuit applications, Li said.
The high quality planar nanowires can also be used in nano-injection lasers
for use in optical communications.
The researchers are also developing new device concepts driven by further engineering
of the planar one-dimensional nanostructure.