Sergey Kubatkin is a Full Professor at Chalmers University of Technology in Gothenburg, Sweden. He graduated from the Moscow Institute for Physics and Technology and defended his PhD thesis at P.L. Kapitza Institute for Physical Problems (Moscow, USSR). His research interests include, among other things, electron transport in low-dimensional systems, scanning probe techniques, search for the sources of decoherence in superconducting quantum devices.
With the advent of the era of graphene and related 2D materials, the task of the carrier density control in these materials, while preserving their high electronic quality, has become extremely important. Since 2D materials are essentially surface, their direct chemical doping is problematic. So far, the most successful way to dope them was though van der Waals contact the with other high quality dielectric crystal, like hBN, and then, using electrostatics with the conducting gate on top for tuning the carrier concentration. This approach is not scalable, since there are no large enough hBN crystals at the wafer scale, it suffers from the contaminations, inevitably introduced during the van der Waals bonding; yet, it is widely used for research purposes, since there was no alternatives.
With our background in a wafer scale epitaxial graphene on SiC and its application for the quantum resistance metrology1,2, we have faced a problem of uniform carrier density reduction from the initial concentration of 1013 cm-2 down to the level of 1011 cm-2 – a convenient concentration range for the observation of quantum Hall effect in reasonable magnetic fields. Charge transfer between graphene and the SiC substrate 3, making our graphene so nice for metrology 4, made all available gating schemes 5,6 ineffective. Recently we have found a simple and effective way of controlling carrier density in epitaxial graphene on SiC 7. The method is wafer-scale-compatible, produces the charge inhomogeneity in graphene at the levels, usually achieved in the hBN-encapsulated samples. We believe, that our approach, tested successfully on epitaxial graphene, can be useful for doping other 2D materials and, possibly, topological insulators. Two practical applications of developed doping technique will be discussed: A. The samples with our doping are currently under evaluation in several metrological laboratories as quantum resistance standards; the preliminary results are results are encouraging. B. We have also found recently, that graphene devices with low carrier density, produced by our method, work very well as sensors of THz radiation.
1.Tzalenchuk, A. et al., Nat. Nanotechnol. 5, 186 (2010).
2. Janssen, T. et al., 2D Mater. 2, 35015, (2015).
3. Kopylov, S. et al., Appl. Phys. Lett. 97, 11 (2010).
4. Alexander-Webber, J.A. et al., Scientific Reports 6, 30296 (2016)
5. Lara-Avila, S. et al., Advanced Materials 23, 878, (2011).
6. Lartsev, A. et al., Appl. Phys. Lett. 105, 063106 (2014)
7. He, H. et al., Nat. Commun. 9, 3956 (2018)