2D heteroepitaxial BN/ epigraphene /SiC films for electronics – BONNEG

Dies from monocrystalline 4H-SiC wafers are graphitized on the Si (0001) face by high temperature decomposition of the SiC surface in vacuum (Confinement Controlled Sublimation method). Samples are then transferred to a metalorganic vapor phase epitaxy (MoVPE) semi-industrial system to produce BN films epitaxially grown on epigraphene.

All the samples are systematically characterized by Atomic Force microscopy (AFM), Raman spectroscopy, Scanning Electron Microscopy, (SEM) before and after BN deposition. Sample mapping is performed to ensure that the graphene and BN coating are uniform.

Two series of monolayer graphene samples were produced and characterized at Georgia Tech – Atlanta (collaborator), for a total of 29 samples. All are at least 90% monolayer. Monolayer, bilayer and buffer layer areas and their respective coverage ratio were identified by Raman spectroscopy mapping (30µmx30µm) and spectra taken at ten random locations on the surface. These results are confirmed by local AFM measurements (contact/friction and non-contact mode).
Two series of BN films were epitaxialy grown by MOVPE on epigraphene at IRL 2958. The experimental conditions for epi growth were optimized to produce 3 and 6nm thick BN films respectively.

Structural studies of the BN films by AFM and SEM shows a complete graphene coverage by BN with extended areas exhibit a pleated morphology, as expected for an unstrained two-dimensional coating. Lines of BN dots are also observed, prominently at step edges. These grains reveal a large BN nucleation rate. The BN film thickness is that expected for the deposition parameters as confirmed by AFM line profile of a scratch.

Meetings: fours consortium meetings were organized
- A kick-off meeting where each group presented their activities and contribution to the project. We discussed detailed implementation and first steps. (11 participants from 4 laboratories: CNRS/GT – IRL, CNRS/ONERA-LEM, CNRS-Institut Néel, GT-Atlanta). online. Meeting.
- three progress meetings (Dec 12 2020, May 26 2021, June 21 and 2021).

Personal: A PhD student was hired for the duration of the ANR project, and a postdoc was recently recruited (Covid related delays).

Best BN films will be examined by cross sectional transmission electron microscopy at LEM to confirm the BN layer thickness, their epitaxial relational to graphene and their structural order.
The next series of films will be produced on micronmeter scale Hall bar patterned epigraphene samples. These structures are most appropriate for transport studies to determine graphene quality and doping level, and the efficiency of the BN coating as a top gate transistor dielectric.

Important remark: The project suffered important delays due to the Covid pandemic.
Adding to the lab closure, recruiting was difficult due to travel and visa restrictions.

These results have not been published yet

This proposal addresses the two major roadblocks in the development of graphene for high-performance nano-optoelectronics, namely how to efficiently and reliably integrate them in pristine conditions in electronic devices, and how harness the exceptional properties of graphene. Specifically, proof of principle of ultra-thin body tunnel field effect transistors (UB-TFET) are proposed consisting of two-dimensional (2D) all epitaxial graphene/boron nitride heterostructures with a viable large scale integration scheme. Tunnel transistors are an efficient alternative to standard field effect transistors designs that are inefficient for graphene because of the lack of a bandgap. Importantly UB-TFET should overcome the thermal limitation of thermioic sub-threshold swing in common transistors. The TFET will be based on epitaxial graphene on SiC (epigraphene, or EG)/BN structures; the most advanced implementation will utilize the recently discovered exceptional conductance properties of epigraphene nano-ribbons that are quantized single channel ballistic conductors at room temperature. But having excellent graphene is far from having a device and the active component has to be integrated. This project is based on the fundamental realization that only (hetero)-epitaxial growth can provide the required atomic control for reliable devices. Epitaxial growth insures clean interfaces and precise orientation of the stacked layers, avoiding trapped molecules and the randomness inherent to layer transfer. However, despite this absolute requirement, very little progress has been made up to now to grow large 2D dielectric on graphene; most dielectric deposition needs chemical modification of the graphene surface for adhesion, which invariably compromises the graphene electronic performance.
Hexagonal boron nitride (h-BN) layers is considered the best substrate for graphene, but only micron size BN flakes are available, making the integration tedious, unreliable and impossible at large scale. In this proposal we will grow h-BN epitaxialy on epigraphene by metalorganic vapor phase epitaxy (MOVPE). As demonstrated in preliminary work by this three-team partnership, this technique provides exceptional unmatched graphene/h-BN epitaxial interfaces as required for high performance electronics, and immediate upscaling capabilities. The SiC/EG/h-BN heterostructure will give access to graphene properties in an exceptionally reproducible and clean environment, not otherwise available. Growth conditions will be investigated to produce ultra thin h-BN on epigraphene, which have not been achieved yet. This proposal will then follow two tracks to build UB-TFETs, demonstrating proof of principle of vertical and lateral BN/EG-based FETs. Our ultimate goal is to combine ballistic epigraphene nanoribbons in tunneling devices to enable a new generation of electronic devices. This is an extremely promising alternative to the standard FET paradigm that can enable ultra-high frequency operation as well as low power operation.
This project is a tight well-focused partnership between three teams with a history of highly successful collaboration and perfect complementarity: CNRS-Institut Néel (Grenoble), CNRS/ONERA-Laboratoire d’Etude des Matériaux (Châtillon), and CNRS/Georgia Institute of Technology -UMI 2958 (Metz, in collaboration with GT Atlanta). We will build up on the important milestone of epitaxial h-BN growth on EG, towards critical development including ultra-thin BN and fabrication of tunnel transistors devices. IN will be in charge of providing epigraphene, will design and realized transistor devices and perform transport measurements; the UMI team will produce the BN epitaxial film and provide basic structural study for rapid optimization of the growth process; LEM will perform advanced structural and optical studies, in particular HR-TEM studies, critical to the layer characterization of ultra thin 2D films.