NIST laboratory researches graphene semiconductors


Non-destructive optical tests give insights into material's electrical properties

Graphene-insulator-semiconductor sample under electrical test

Researchers at NIST's PML (Physical Measurement Laboratory) are using combined optical techniques, IPE (internal photo-emmission), and SE (spectroscopic ellipsometry) to determine graphene's work function and the band alignment of a graphene-insulator-semiconductor structure. The potential impact of the completed study and its published results on future device development is assessed as substantial. Instead of developing a device, destructively measuring what was built in order to determine its electrical properties, the devices can now be engineered, with the known electrical behaviour from the start. While IPE and SE have been around for a long time, scientists have only recently begun to combine the techniques for use in IC device characterization. IPE is used to measure the electrons energy emitted from materials in order to determine binding energies. A light is shone onto a sample, a photocurrent created by the ejected electrons is then measured. In SE, broadband light sources are shone upon a material, and optical properties are ascertained from the reflectivity. "We are the only group in the U.S. who use the techniques full time," says Nhan Nguyen, of the PML's Semiconductor and Dimensional Metrology division. A world-renowned expert in both IPE and SE, he 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," says project leader David Gundlach. "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 successfully determine the energy barrier heights and band structures of MOS devices. Building on that, he hoped to characterize a GIS (graphene-insulator-semiconductor) device in a similarly non-destructive manner opposed to current destructive techniques for cross sectioning and analyzing. "Nhan's technique is extremely valuable in advancing future electronics in the fronts of semiconductor electronics, advanced manufacturing, and nano manufacturing," Gundlach concludes Future studies will use 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 semi-transparent metals used previously. Nguyen intends to stack the graphene layer onto other layers with unknown properties, using the graphene as a key translator to understanding the unknown layers beneath. "This has given us access to measurements that were previously unavailable," he states, critical to the moves beyond CMOS technology. In addition to studying the manipulation of energy levels in a graphene layer, new semiconductor materials used in more complicated device structures and architectures need to be characterized and a non-destructive way to do it has been demonstrated NIST