CVD Graphene

• Research unit: METAS
• Project number: F-5010.30028

Key words (English)

Graphene, electrical metrology, quantum Hall effect

Short description (English)

Graphene was discovered in 2004 by A. Geim and K. Novoselov who both received the No-bel prize in 2010. Soon after the discovery, the quantum Hall effect was observed in this new material. This observation has triggered a considerable interest in the metrology community and with the progress in the device fabrication, graphene was soon identified as a good can-didate to replace the traditional GaAs devices. The major advantage of graphene over GaAs is that it will be possible to make measurement at higher temperature (4.2 K instead of 0.3 K) and lower magnetic fields (2 T instead of 10T). This change of the operational parameters will have a profound impact on the equipment needed to perform the experiment. The whole set up will become simpler and much cheaper to operate.
The goal of the present project is to develop graphene devices which are suitable for metrological applications. The expertise of the Physics department in Basel in the growth of graphene samples using the carbon vapor deposition method (CVD) will be devoted to customize the growth of graphene films to the specific, stringent needs of metrological application: high mobility, low contact resistance, broad sample, large critical current.

Project aims (English)

The objective of this project is to perform a systematic study of the potential of graphene in resistance metrology. In particular, we will focus our attention on graphene devices grown by a CVD method. The major practical outcome of the project will be a set of good quality CVD graphene sample appropriate for high precision resistance measurements. This objective is challenging, up to now CVD has not been able to compete with Si-carbide method in terms of sample quality. However CVD has a huge potential that must be investigated. Since CVD is a simpler and cheaper method, a large variety of parameters can be investigated. Moreover, their impact on the behaviour of the transport properties of the device can determined at cryogenic temperatures.

Abstract (English)

The results can be suimmarized as follows:

• The optimization of the CVD growth was achieved by reducing the methane flow which al-lows growing larger single crystal grains.
• The first precision measurements were performed. However, the low value of the critical current and the poor quality of the contact prevented high accuracy measurements.
• Further improvement of the slow growth method resulted in single crystal grains larger than 1 mm, as shown by Raman spectroscopy. The problem of the shift of the charge neutrality point to large voltages (around 40 V) appeared. It was due to contamination of the graphene film during the transfer process.
• The effect of treating the graphene film with NMP or HMDS (solvents used for polymers in photolithography) after the transfer on the silicon wafer was investigated and successfully brought the CNP closer to zero voltage.
• Then a systematic investigation of the impact of granularity in chemical vapour deposited CVD graphene films was performed by Raman mapping and electrical characterization of single and multi domains graphene. In order to elucidate the quality of the graphene film, its regional variations were studied using large-area Raman mapping and comparison of the G and 2D peak positions of as-transferred (CVD) graphene on SiO2 substrate.
• Even though NMP is known to dissolve a variety of polymers including polymethylmethacrylate (PMMA), polymer residues from the lithographic process still are still present on graphene, which suggests that these polymer residues bind strongly to the graphene surface. The presence of a large amount of polymer residues may be associated with the exposure of PMMA to ammonium persulphate during the etching process, which has an oxidizing effect on PMMA.
• The best results obtained in precision measurements show a quantization of the Hall resistance with an accuracy of 30 parts in 10⁹. The accuracy of these results cannot be compared to state-of-the-art precision measurements of the QHR in GaAs or in SiC graphene where accuracy smaller than 10^-9 can be achieved. However, they are the best results, worldwide, obtained using CVD grown graphene.

Transfer and application (English)

Although the project has delivered the best CVD graphene films, their quality did not reach a level where they can have an impact in resistance metrology. CVD graphene is not yet competitive with GaAs heterostructures or even SiC graphene film. The reproducibility of the film transfer from the Cu foil to the Si wafer must be better controlled in order to make CVD graphene competitive with other quantized Hall resistance standard.

Nevertheless CVD graphene holds promising prospects in a variety of applications. The next step is to develop and to establish a more reproducible manufacturing process.

Publications / Results (English)

Thodkar, K., El Abbassi, M., Lüönd, F., Overney, F., Schönenberger, C., Jeanneret, B. & Calame, M. Comparative study of single and multi-domain CVD graphene using large-area Raman mapping and electrical transport characterisation. Physica Status Solidi (RRL) - Rapid Research Letters 10, 807–811. ISSN: 18626254 (2016).

Stoop, R. L., Thodkar, K., Sessolo, M., Bolink, H. J., Schönenberger, C. & Calame, M. Charge Noise in Organic Electrochemical Transistors. Phys. Rev. Applied 7, 014009 (1 2017).

Thodkar, K., Thompson, D., Lüönd, F., Moser, L., Overney, F., Marot, L., Schönenberger, C., Jeanneret, B. & Calame, M. Restoring the Electrical Properties of CVD Graphene via Physisorption of Molecular Adsorbates, ACS Applied Materials & Interfaces 9. PMID: 28675296, 25014–25022 (2017).

Thodkar, K., Lüönd, F., Overney, F., Schönenberger, C., Jeanneret, B. & Calame, M. High quality CVD graphene for quantum Hall resistance standards. to be submitted to Nature Com. (2018).

El Abbassi, M., Laszlo, P., Makk, P., Nef, C., Thodkar, K., Halbritter, A. & Calame, M. From Electroburning to Sublimation: Substrate and Environmental Effects in the Electrical Breakdown Process of Monolayer Graphene. Submitted (2017).