Terahertz imaging of graphene paves the way to industrialisation

 

Graphene Flagship researchers have developed a new measurement standard for the analysis of graphene and layered materials that could accelerate production and optimise device fabrication. Credit: Graphene Flagship

https://phys.org/news/2021-02-terahertz-imaging-graphene-paves-industrialisation.html

X-ray scans revolutionized medical treatments by allowing us to see inside humans without surgery. Similarly, terahertz spectroscopy penetrates graphene films allowing scientists to make detailed maps of their electrical quality, without damaging or contaminating the material. The Graphene Flagship brought together researchers from academia and industry to develop and mature this analytical technique, and now a novel measurement tool for graphene characterization is ready.

The effort was possible thanks to the collaborative environment enabled by the Graphene Flagship European consortium, with participation by scientists from Graphene Flagship partners DTU, Denmark, IIT, Italy, Aalto University, Finland, AIXTRON, UK, imec, Belgium, Graphenea, Spain, Warsaw University, Poland, and Thales R&T, France, as well as collaborators in China, Korea and the US.

Graphene is often 'sandwiched' between many different layers and materials to be used in electronic and photonic devices. This complicates the process of quality assessment. Terahertz spectroscopy makes things easier. It images the encapsulated materials and reveals the quality of the graphene underneath, exposing imperfections at critical points in the fabrication process. It is a fast, non-destructive technology that probes the electrical properties of graphene and layered materials, with no need for direct contact.

The development of characterisation techniques like terahertz spectroscopy is fundamental to accelerating large-scale production, as they guarantee that graphene-enabled devices are made consistently and predictably, without flaws. Quality control precedes trust. Thanks to other developments pioneered by the Graphene Flagship, such as roll-to-roll production of graphene and layered materials, fabrication technology is ready to take the next step. Terahertz spectroscopy allows us to ramp up graphene production without losing sight of the quality.

"This is the technique we needed to match the high-throughput production levels enabled by the Graphene Flagship," explains Peter Bøggild from Graphene Flagship partner DTU. "We are confident that terahertz spectroscopy in graphene manufacturing will become as routine as X-ray scans in hospitals," he adds. "In fact, thanks to terahertz spectroscopy you can easily map even meter-scale graphene samples without touching them, which is not possible with some other state-of-the-art techniques." Furthermore, the Graphene Flagship is currently studying how to apply terahertz spectroscopy directly into roll-to-roll graphene production lines, and speed up the imaging.

Terahertz spectroscopy penetrates graphene films allowing scientists to make detailed maps of their electrical quality, without damaging or contaminating the material. Credit: Peter Bøggild (Graphene Flagship / DTU)

Collaboration was key to this achievement. Graphene Flagship researchers in academic institutions worked closely with leading graphene manufacturers such as Graphene Flagship partners AIXTRON, Graphenea and IMEC. "This is the best way to ensure that our solution is relevant to our end-users, companies that make graphene and layered materials on industrial scales," says Bøggild. "Our publication is a comprehensive case study that highlights the versatility and reliability of terahertz spectroscopy for quality control and should guide our colleagues in applying the technique to many industrially relevant substrates such silicon, sapphire, silicon carbide and polymers." he adds.

Setting standards is an important step for the development of any new material, to ensure it is safe, genuine and will offer a performance that is both reliable and consistent. That is why the Graphene Flagship has a dedicated work-group focused on the standardization of graphene, measurement and analytical techniques and manufacturing processes. The newly developed method for terahertz spectroscopy is on track to become a standard technical specification, thanks to the work of the Graphene Flagship Standardization Committee. "This will undoubtedly accelerate the uptake of this new technology, as it will outline how analysis and comparison of graphene samples can be done in a reproducible way," explains Peter Jepsen from Graphene Flagship Partner DTU, who co-authors the study. "Terahertz spectroscopy is yet another step to increase the trust in graphene-enabled products," he concludes.

Amaia Zurutuza, co-author of the paper and Scientific Director at Graphene Flagship partner Graphenea, says: "At Graphenea, we are convinced that terahertz imaging can enable the development of quality control techniques capable of matching manufacturing throughput requirements and providing relevant graphene quality information, which is essential in our path towards the successful industrialisation of graphene."

Thurid Gspann, the Chair of the Graphene Flagship Standardization Committee, says, "This terahertz [spectroscopy] technique is expected to be widely adopted by industry. It does not require any particular sample preparation and is a mapping technique that allows one to analyze large areas in a time efficient way."

Marco Romagnoli, Graphene Flagship Division Leader for Electronics and Photonics Integration, adds, "The terahertz spectroscopy tool for wafer-scale application is a state-of-the-art, high TRL system to characterize multilayer stacks on wafers that contain CVD graphene. It works in a short time and with good accuracy, and provides the main parameters of interest, such as carrier mobility, conductivity, scattering time and carrier density. This high-value technical achievement is also an example of the advantage of being part of a large collaborative project like the Graphene Flagship."

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds, "Yet again, Graphene Flagship researchers are pioneering a new characterisation technique to facilitate the development of graphene technology. This helps us progress steadily on our innovation and technology roadmap and will benefit the industrial uptake of graphene in a wide range of applications."

Abstract-Non-contact mobility measurements of graphene on silicon carbide

Patrick R.Whelan,  Xiaojing Zhao, Lwona Pasternak, Wlodek Strupinski, Peter U.Jepsen, Peter Bøggild,

https://www.sciencedirect.com/science/article/abs/pii/S0167931719300644

Non-invasive measurement techniques are of utmost importance for characterization of atomically thin materials to speed up the measurement process while avoiding mechanical damage or contamination of the fragile materials. Terahertz time-domain spectroscopy (THz-TDS) provides non-contact measurement of the frequency dependent conductivity of thin films. Here, we expand the applicability of THz-TDS by spatially mapping the carrier density and mobility of epitaxial graphene grown on silicon carbide. The extracted values are compared to Hall measurements and agrees well for homogeneously conducting samples.

Abstract-Conductivity mapping of graphene on polymeric films by terahertz time-domain spectroscopy

Patrick R. Whelan, Deping Huang, David Mackenzie, Sara A. Messina, Zhancheng Li, Xin Li, Yunqing Li, Timothy J. Booth, Peter U. Jepsen, Haofei Shi, Peter Bøggild,

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-26-14-17748

Fast inline characterization of the electrical properties of graphene on polymeric substrates is an essential requirement for quality control in industrial graphene production. Here we show that it is possible to measure the sheet conductivity of graphene on polymer films by terahertz time-domain spectroscopy (THz-TDS) when all internally reflected echoes in the substrate are taken into consideration. The conductivity measured by THz-TDS is comparable to values obtained from four point probe measurements. THz-TDS maps of 25x30 cm2 area graphene films were recorded and the DC conductivity and carrier scattering time were extracted from the measurements. Additionally, the THz-TDS conductivity maps highlight tears and holes in the graphene film, which are not easily visible by optical inspection.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Quality assessment of terahertz time-domain spectroscopy transmission and reflection modes for graphene conductivity mapping

David M. A. Mackenzie, Patrick R. Whelan, Peter Bøggild, Peter Uhd Jepsen, Albert Redo-Sanchez, David Etayo, Norbert Fabricius, and Dirch Hjorth Petersen

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-7-9220

We present a comparative study of electrical measurements of graphene using terahertz time-domain spectroscopy in transmission and reflection mode, and compare the measured sheet conductivity values to electrical van der Pauw measurements made independently in three different laboratories. Overall median conductivity variations of up to 15% were observed between laboratories, which are attributed mainly to the well-known temperature and humidity dependence of non-encapsulated graphene devices. We conclude that terahertz time-domain spectroscopy performed in either reflection mode or transmission modes are indeed very accurate methods for mapping electrical conductivity of graphene, and that both methods are interchangeable within measurement uncertainties. The conductivity obtained via terahertz time-domain spectroscopy were consistently in agreement with electrical van der Pauw measurements, while offering the additional advantages associated with contactless mapping, such as high throughput, no lithography requirement, and with the spatial mapping directly revealing the presence of any inhomogeneities or isolating defects. The confirmation of the accuracy of reflection-mode removes the requirement of a specialized THz-transparent substrate to accurately measure the conductivity.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Robust mapping of electrical properties of graphene from terahertz time-domain spectroscopy with timing jitter correction

Patrick R. Whelan, Krzysztof Iwaszczuk, Ruizhi Wang, Stephan Hofmann, Peter Bøggild, and Peter Uhd Jepsen
obust mapping of electrical properties of graphene from terahertz time-domain spectroscopy with timing jitter correction

We demonstrate a method for reliably determining the electrical properties of graphene including the carrier scattering time and carrier drift mobility from terahertz time- domain spectroscopy measurements (THz-TDS). By comparing transients originating from directly transmitted pulses and the echoes from internal reflections in a substrate, we are able to extract electrical properties irrespective of random time delays between pulses emitted in a THz-TDS setup. If such time delays are not accounted for they can significantly influence the extracted properties of the material. The technique is useful for a robust determination of electrical properties from THz-TDS measurements and is compatible with substrate materials where transients from internal reflections are well-separated in time.

© 2017 Optical Society of America

Full Article  |  PDF Article

Abstract-Terahertz wafer-scale mobility mapping of graphene on insulating substrates without a gate

Jonas D. Buron, David M. A. Mackenzie, Dirch. H. Petersen, Amaia Pesquera, Alba Centeno, Peter Bøggild, Amaia Zurutuza, and Peter U. Jepsen
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-23-24-30721

We demonstrate wafer-scale, non-contact mapping of essential carrier transport parameters, carrier mobility (µdrift), carrier density (Ns), DC sheet conductance (σdc), and carrier scattering time (τsc) in CVD graphene, using spatially resolved terahertz time-domain conductance spectroscopy. σdc and τsc are directly extracted from Drude model fits to terahertz conductance spectra obtained in each pixel of 10 × 10 cm2 maps with a 400 µm step size. σdc- and τsc-maps are translated into µdrift and Ns maps through Boltzmann transport theory for graphene charge carriers and these parameters are directly compared to van der Pauw device measurements on the same wafer. The technique is compatible with all substrate materials that exhibit a reasonably low absorption coefficient for terahertz radiation. This includes many materials used for transferring CVD graphene in production facilities as well as in envisioned products, such as polymer films, glass substrates, cloth, or paper substrates.

© 2015 Optical Society of America

Full Article  |  PDF Article

Illuminating the electronic properties of graphene

http://phys.org/news/2015-07-illuminating-electronic-properties-graphene.html

Danish researchers have for the first time mapped the carrier mobility and density of large sheets of graphene with electromagnetic radiation.
For the last decade, the usual way of measuring the electronic properties of graphene – in particular the carrier mobility and carrier density, which together give the sheet conductance – has been to fabricate a transistor-like device and electronically measure how the conductance changes as a function of applied electrostatic gate voltage. This all-electronic approach is best when dealing with small pieces of graphene, such as the microscopic flakes produced by micromechanical cleavage (also known as the 'scotch-tape method') – however, advances in graphene production techniques now allow us to continuously produce large areas of graphene meters across. Producing and measuring thousands or millions of microscopic devices from such sheets would be impractical and would reduce the useful area of graphene for the intended application. We need to be able to check the electronic properties of such large regions without destroying them in the process.

Researchers at the Technical University of Denmark (DTU) have shown that both the carrier mobility and the carrier density of graphene can be measured in a spatially resolved and non-destructive way – providing 'maps' of the electronic properties critical for the successful use of graphene in photovoltaics, electronics, spintronics and optics – using terahertz (THz) radiation and doing away with the need to fabricate devices. Using a procedure known as THz time-domain spectroscopy, Jonas Buron and colleagues from DTU research teams led by Peter Uhd Jepsen and Peter Bøggild measured the carrier mobility and carrier density at tens of thousands of points in a centimetre sized single layer of graphene.

A key enabling step in these first contact-free measurements of the electronic properties of graphene was the realisation that the graphene conductance could be tuned during the measurements using a back-gate, which is transparent to THz radiation. "While we still need to transfer the graphene to a special substrate with the THz-invisible gate, it is far easier and less destructive than conventional techniques... and much, much faster", says Jonas Buron. For many electronic applications of graphene, the fabrication of a back-gate is a necessary step anyway. "With some optimisation we could potentially map the carrier mobility and density of a graphene-coated 4-inch wafer in minutes."

The maps of the electronic properties of graphene are already providing insight and surprises about the origin of their spatial variation – in one sample, the researchers observed twice as much variation in mobility as in carrier density. Variations in conductance are usually ascribed to carrier density changes due to doping variations, but the researchers proved that here this was not the case. "We have often noted such slow variations of the conductivity across many centimeters in THz measurements." Peter Bøggild explained. "But since graphene is so easily doped due to its extreme surface-to-volume ratio, we always expected these to be related to local doping level variations. In this case, we have the exact opposite situation, and this is puzzling. Without this mobility mapping technique we would never have known."

The THz-TDS technique has a strong potential, adds Peter Uhd Jepsen. "It is already surprising how deep information we can extract from transmitting radiation through a just 0.3 nm thin sheet of carbon atoms, which is supported by a 1.5 million times thicker piece of silicon. We are still learning how to characterise the electric properties of graphene without electric contacts, and there seem to be excellent options for improving and speeding up the technique."

 Explore further: A single-sheet graphene p-n junction with two top gates

More information: www.nature.com/articles/srep12305

Read more at: http://phys.org/news/2015-07-illuminating-electronic-properties-graphene.html#jCp