Monolayer di grafene ad alta mobilita' per dispositivi quantistici innovativi

Responsabili di progetto

Stefan Heun, Satoru Suzuki

Accordo

GIAPPONE - JSPS - Japan Society for the Promotion of Science

Bando

CNR/JSPS 2014-2015

Dipartimento

Scienze fisiche e tecnologie della materia

Area tematica

Scienze fisiche e tecnologie della materia

Stato del progetto

Nuovo

Proposta di ricerca

Suppression of backscattering and a very large coherence length are the characteristic properties of edge states in the quantum Hall (QH) regime at the basis of the newly developed quantum electron interferometry. In this field a number of breakthroughs have appeared in recent years, such as the experimental realization of an electronic Mach-Zehnder interferometer [1] which at present appears a sound technology for the implementation of quantum information schemes [2]. All QH interferometers demonstrated so far are based on III-V materials and need to be operated at mK temperatures in order to reach the QH regime. In the last decade, graphene emerged as a promising material for a variety of technological applications [3]. In particular, the QH effect in graphene was measured at much higher temperatures, even at room temperatures [4]. This would allow to implement quantum information devices operating at room temperature. On the other hand, progress was recently made in high-frequency [5] and metrology [6] applications based on graphene. However, the performance of these devices still suffers from a limited carrier mobility in graphene.



In this context, epitaxial graphene grown on SiC(0001) surfaces is an attractive material for future carbon-based electronics, as it combines the exciting properties of exfoliated graphene with a manufacturing-friendly planar structure [7]. Due to the formation of covalent bonds with the substrate, the first carbon layer that grows on SiC(0001) is electronically inactive and therefore called buffer layer. The second carbon layer acts electronically as graphene and is called monolayer graphene (MLG). However, the presence of the buffer layer limits the mobility in epitaxial graphene, and it would therefore be desirable to reduce the influence of the substrate. This has recently been achieved by intercalating Hydrogen under the buffer layer [8]: the intercalation breaks the covalent bonds between the buffer layer and the substrate, converting it into quasi-free standing monolayer graphene (QFMLG). These QFMLG films are an even more appealing candidate for electronic applications.



As an alternative substrate material for graphene, hexagonal boron nitride (h-BN) has recently attracted much attention. h-BN is a layered material and the structural analog of graphene with a very small lattice mismatch. The chemically inert and insulating nature of h-BN with a smooth surface and no dangling bonds makes h-BN suitable as a substrate material for graphene devices. Up to now, excellent electric performance and high mobility has been obtained for graphene/h-BN devices using exfoliated h-BN films as substrates [9]. However, the size of exfoliated h-BN is typically small (~10 um), and the lack of scalability has prevented more common use of h-BN as a substrate. Large-area and high-quality h-BN growth has been proposed by chemical vapor deposition (CVD) and is currently being intensively investigated [10].



We propose to study the structural and electronic properties of graphene films on SiC and on h-BN with a variety of methods, and to exploit these results for the fabrication of high-mobility quantum devices based on graphene. Both groups involved in this proposal (Basic Research Laboratories of the Japanese Telecom NTT in Japan and Istituto Nanoscienze-CNR in Italy) have already started to collaborate in this field (joint development agreement signed on April 1, 2012), see [11,12].



As a first step, we will use scanning tunneling microscopy (STM) and low energy electron microscopy (LEEM) to investigate and compare the atomic structure of graphene layers on SiC and on h-BN. A combination of micro-Raman and atomic force microscopy (AFM) analysis will provide spatially resolved information on their thickness and quality. STM will allow atomic resolution in structural investigations of the graphene layers, while scanning tunneling spectroscopy (STS) will provide details on their electronic properties. LEEM will complement these studies because it allows to follow growth of graphene on SiC in situ and to spatially resolve the number of graphene layers, as demonstrated by the NTT group [13].



In a second step we will build on these results to study electron transport in Hall bar devices made on optimized QFMLG samples and on monolayer graphene on h-BN. Transport properties (e.g. differences in mobility) will be correlated to structural information (e.g. defect density). The transport measurements in NTT will be complemented by scanning gate microscopy (SGM) experiments by the CNR group on samples with quantum point contacts (QPC) realized on Hall bars [14], and we will study coherent electron flow in graphene [15], similar to our previous studies in III-V quantum well samples [16]. These measurements will be complemented by theoretical analysis and will prepare the stage for SGM experiments on graphene in the quantum Hall regime, a field in which we have considerable experience [17].




Y. Ji et al., Nature 422, 415 (2003).
V. Giovannetti et al., Phys. Rev. B 77, 155320 (2008).
K. S. Novoselov, Rev. Mod. Phys. 83, 837 (2011).
K. S. Novoselov et al., Science 315, 1379 (2007).
Y.-M. Lin et al., Science 327, 662 (2010).
A. Tzalenchuk et al., Nature Nanotech 5, 186 (2010).
C. Berger et al., Science 312, 1191 (2006).
C. Riedl et al., Phys. Rev. Lett. 103, 246804 (2009).
C. R. Dean et al., Nature Nanotechnol. 5, 722 (2010).
K. H. Lee et al., Nano Lett. 12, 714 (2012) ; W. Yang et al., Nature Mater. (2013), doi:10.1038/nmat3695.
T. Mashoff et al., Appl. Phys. Lett. 103, 013903 (2013).
A. Iagallo et al., arXiv.
H. Hibino et al., Phys. Rev. B 77, 075413 (2008).
S. Nakaharai et al., Phys. Rev. Lett. 107, 036602 (2011).
R. Yang et al., Phys. Rev. B 84, 035426 (2011).
N. Paradiso et al., Physica E 42, 1038 (2010).
N. Paradiso et al., Phys. Rev. B 83, 155305 (2011).

Obiettivi della ricerca

QFMLG obtained by hydrogen intercalation from buffer layers grown: Optimization of the sample preparation protocols, i.e. measure, understand, and optimize the structural and electronic properties of QFMLG. Understand the doping mechanisms involved.

Graphene on h-BN substrates: Successful growth of high-quality films of h-BN by CVD and transfer to Si/SiO2 substrates has already been demonstrated by the NTT group. The first objectives are to optimize the cleaning procedure for the h-BN surface and the transfer process of exfoliated graphene on h-BN.

Optimize device fabrication protocols. The fabrication of Hall-bars with Ohmic contacts might require different approaches for the two types of graphene samples, but the NTT group has already acquired substantial experience in the fabrication of Hall-bars on epitaxial graphene on SiC, while the CNR group has experience with Hall-bars based on exfoliated graphene.

Determine the mobility of samples as a function of fabrication protocol, and correlate sample mobility with structural parameters of the films obtained by microscopic methods. The objective is to identify the mobility-limiting defects in these films and to find ways to reduce their density, thereby maximizing mobility.

Demonstration of high-mobility graphene devices. Measure coherent electron flow in graphene. Thorough theoretical analysis of these data. Furthermore, study the properties of edge channels in graphene samples in the quantum Hall regime.

Ultimo aggiornamento: 29/07/2025