Another research Highlighted the graphene grain boundaries
Courtesy Justin Koepke, Joe Lyding |
Using
graphene -- either as an alternative to, or most likely as a complementary
material with -- silicon, offers the promise of much faster future electronics,
along with several other advantages over the commonly used semiconductor.
However, creating the one-atom thick sheets of carbon known as graphene in a
way that could be easily integrated into mass production methods has proven
difficult. When graphene is grown, lattices of the carbon grains are formed
randomly, linked together at different angles of orientation in a hexagonal
network. However, when those orientations become misaligned during the growth
process, defects called grain boundaries (GBs) form. These boundaries scatter
the flow of electrons in graphene, a fact that is detrimental to its successful
electronic performance.
Beckman Institute researchers Joe Lyding and Eric Pop and their
research groups have now given new insight into the electronics behavior of
graphene with grain boundaries that could guide fabrication methods toward
lessening their effect. The researchers grew polycrystalline graphene by chemical
vapor deposition (CVD), using scanning tunneling microscopy and spectroscopy
for analysis, to examine at the atomic scale grain boundaries on a silicon
wafer. They reported their results in the journal ACS Nano.
"We
obtained information about electron scattering at the boundaries that shows it
significantly limits the electronic performance compared to grain boundary free
graphene," Lyding said. "Grain boundaries form during graphene growth
by CVD, and, while there is much worldwide effort to minimize the occurrence of
grain boundaries, they are a fact of life for now.
"For
electronics you would want to be able to make it on a wafer scale. Boundary
free graphene is a key goal. In the interim we have to live with the grain
boundaries, so understanding them is what we're trying to do."
Lyding
compared graphene lattices made with the CVD method to pieces of a cyclone
fence.
"If
you had two pieces of fence, and you laid them on the ground next to each other
but they weren't perfectly aligned, then they wouldn't match," he said.
"That's a grain boundary, where the lattice doesn't match."
The
research involved Pop's group, led by Beckman Fellow Josh Wood, growing the
graphene at the Micro and Nanotechnology Lab, and transferring the thin films
to a silicon (Si02) wafer. They then used the STM at Beckman
developed by Lyding for analysis, led by first author Justin Koepke of Lyding's
group.
Their
analysis showed that when the electrons' itinerary takes them to a grain
boundary, it is like, Lyding said, hitting a hill.
"The
electrons hit this hill, they bounce off, they interfere with themselves and
you actually see a standing wave pattern," he said. "It's a barrier
so they have to go up and over that hill. Like anything else, that is going to
slow them down. That's what Justin was able to measure with these spectroscopy
measurements.
"Basically
a grain boundary is a resistor in series with a conductor. That's always bad.
It means it's going to take longer for an electron to get from point A to point
B with some voltage applied."
Images from the STM reveal grain boundaries that suggest two
pieces of cloth sewn together, Lyding said, by "a really bad
tailor."
In the
paper, the researchers were able to report on their analysis of the orientation
angles between pieces of graphene as they grew together, and found "no
preferential orientation angle between grains, and the GBs are continuous
across graphene wrinkles and Si02 topography."
They reported that analysis of those patterns "indicates that
backscattering and intervalley scattering are the dominant mechanisms
responsible for the mobility reduction in the presence of GBs in CVD-grown
graphene."
Lyding
said that the relationship between the orientation angle of the pieces of
graphene and the wavelength of an electron impinges on the electron's movement
at the grain boundary, leading to variations in their scattering.
"More
scattering means that it is making it more difficult for an electron to move
from one grain to the next," he said. "The more difficult you make
that, the lower the quality of the electronic performance of any device made
from that graphene."
The
researchers work is aimed not just at understanding, but also at controlling
grain boundaries. One of their findings -- that GBs are aperiodic -- replicated
other work and could have implications for controlling them, as they wrote in
the paper: "Combining the spectroscopic and scattering results suggest
that GBs that are more periodic and well-ordered lead to reduced scattering
from the GBs."
"I
think if you have to live with grain boundaries you would like to be able to
control exactly what their orientation is and choose an angle that minimizes
the scattering," Lyding said.
Source: Beckman Institute for Advanced Science and
Technology
Leave Your Comments!
Share What’s Going on
in your brain about the Topic. We need Your Response . Feel free to leave comments!
Posted by Unknown
on Saturday, January 19, 2013.
Filed under
Chemistry and Physics
.
You can follow any responses to this entry through the RSS 2.0