Nhan Nguyen demonstrates how he performs optical measurements on a graphene-insulator-semiconductor sample structure. |
That
graphene is the hot new material in the world of future electronics
manufacturing is well known. With its high carrier mobility and low
noise, graphene is seen as a possible candidate to ultimately replace
silicon in integrated circuits. Finding a way to fully characterize new
materials such as graphene is critical to the ultimate goal of
successful engineering and manufacturing of next-generation devices.
Researchers at NIST’s Physical Measurement Laboratory have brought us
one important step closer to this goal with the determination of
graphene’s work function and the band alignment of a
graphene-insulator-semiconductor structure by using the combined optical
techniques of internal photoemission (IPE) and spectroscopic
ellipsometry (SE).
While
IPE and SE have been around for a long time, only recently have
scientists begun combining the techniques for use in integrated circuit
device characterization. IPE is used to measure the energy of electrons
emitted from materials in order to determine binding energies.
Essentially, a light is shone onto a sample and a photocurrent created
by the ejected electrons is measured. In SE, broadband light sources are
shone upon a material, and optical properties are ascertained from the
reflectivity. Both techniques are truly crafts. Only a skilled
practitioner can perform the measurements precisely.
“We
are the only group in the U.S. who use the techniques full time,”
explains Nhan Nguyen, of the PML’s Semiconductor and Dimensional
Metrology Division. Nguyen, a world-renowned expert in both IPE and SE,
brings a wealth of experience to the state-of-the-art facilities at
NIST.
“Nhan
is one of, arguably, two photoemission specialists world-wide that have
a tremendous depth and experience in that measurement technique,”
states David Gundlach, Nguyen’s Project Leader. “As far as ellipsometry,
there are relatively few ellipsometric specialists that have the
spectral range that he can cover with the measurement apparatuses that
he has available to him at NIST.”
Nguyen
originally used the combined measurement techniques to determine
successfully the energy barrier heights and band structure of
metal-oxide-semiconductor (MOS) devices. Building on that study, his
hope was that he could characterize a graphene-insulator-semiconductor
(GIS) device in a similarly non-destructive manner. Current methods for
characterizing such a device employ destructive techniques for cross
sectioning and analyzing. These methods not only destroy the device, but
also potentially compromise the very electronic properties that are
being measured.
Band
alignment is important in GIS devices because the correct band offsets
are necessary to prevent undesirable leakage currents in device
applications. In other words, if the layers are not lined up in a
precise way, the device will behave differently than anticipated,
perhaps even failing entirely. This information is critical for the
successful engineering and reproducible manufacturability and
reliability of such devices. Yet, until now, no detailed study on the
band alignment of these devices had been reported.
Nguyen
and his team investigated a structure that consisted of a graphene film
grown by chemical vapor deposition (CVD), a degenerately doped p-type
silicon substrate, and a 10 nm thick thermal SiO2 layer. The graphene
film, a continuous one-atom layer, had the necessary properties (i.e.,
extremely thin, robust, continuous, and semi-transparent) to enable
excellent optical transmission allowing electrical measurements well
beneath the surface.
A graphene-insulator-semiconductor sample under electrical test |
Using
a combination of IPE (setup included a 150 W broadband Xenon light
source and a quarter-meter Czerny Turner monochromator to tune the
incident light with photon energy) and SE, Nguyen was able to view the
whole picture of the structure’s band alignment. IPE revealed the offset
between bands and how they aligned with respect to each other, but only
on one side of the device. SE measurements allowed the calculation of
the band gaps, which led to the determination of the entire band
structure.
“In devices,” Nguyen explains, “we want band offsets large
enough so that you don’t have noise or leakage. If they are too close,
the electrons can jump across. With IPE, you can really look deeper
below the surface of the material without changing the properties of the
interface.”
Nguyen
was also able to determine the work function of the graphene layer,
which can vary greatly depending on what the layer is placed upon and
other environmental factors. Future studies will focus on the
possibility of reproducibly controlling the energy properties of the
graphene layer based on the needs of the end device.
The
potential impact of this completed study and published results* on the
development of future devices is substantial. Instead of developing a
device and destructively measuring what was built afterwards to
determine its electrical properties, devices can be engineered with
known electrical behavior from the start. “Nhan’s technique is extremely
valuable in advancing future electronics in the fronts of semiconductor
electronics, advanced manufacturing, and nano manufacturing,” Gundlach
concludes.
In
addition to studying the manipulation of energy levels in a graphene
layer, future studies will utilize graphene’s unique properties to study
other materials. Since graphene can be applied in a very thin and
continuous layer, it allows for much better optical transmission than
the semi-transparent metals previously used. Nguyen intends to stack the
graphene layer onto other layers with unknown properties, using the
graphene as a key to understanding the unknown layers beneath.
“This has given us access to measurements that were previously unavailable,” Nguyen states.
This
is critical as the industry moves beyond CMOS technology. New
semiconductor materials used in more complicated device structures and
architectures need to be characterized. And now Nguyen and colleagues
have demonstrated a non-destructive way to do it.
Source: NIST
