Fig. 1: growth of hBN under ultra-high vacuum, formation of triangular domains of several aligned planes
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hBN: the best ally of graphene
Graphene is a material famous for its exceptional electronic properties.
However, if these properties are to be exploited in real components, it is essential to reduce electronic interaction with the environment as much as possible. Hexagonal boron nitride (hBN), another 2D material characterized by a very wide band gap (~6 eV), is very promising for this purpose. It can be used both to isolate graphene from the substrate and as a gate dielectric material. The study of the growth of hBN by molecular beam epitaxy started at IEMN in 2017 (see figure 1), in an ultra-high vacuum growth chamber that also allows the growth of graphene.
Fig. 2: 2DHetero project, a) complete heterostructure obtained by growth,
b) heterostructure requiring a transfer step and c) test component
The first results have notably led to the 2DHetero project, which has just been selected in the framework of the Flag-Era 2019 call. The aim of this project is to explore the direct growth of complete hBN/graphene/HBN heterostructures using coupled molecular beam epitaxy and chemical vapor deposition techniques (see figure 2a), avoiding as much as possible any transfer/report step of a part of the heterostructure (figure 2b). IHP Frankfurt/Oder (Germany) and the University of Namur are the other members of the 2DHetero consortium.
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And with a forbidden band…
For more than fifteen years, the exceptional properties of graphene have stimulated materials research and led to numerous proposals for applications. Nevertheless, the absence of band gap remains an obstacle to the use of graphene in many micro and optoelectronic devices and thus to the development of 2D electronics. Other 2D materials and among them transition metal dichalcogenides (DMTs), with a band gap of 1 to 2 eV, can then take over. These materials offer properties that are not found in usual semiconductors: absence of hanging surface bonds, nature of the band gap varying with thickness, strong optical absorption, valleytronics,… It is in this perspective that to develop its activity on 2D materials, the IEMN has equipped itself at the beginning of 2020 with a frame for molecular jet epitaxy (MJE) of heterostructures of DMTs. This Vinci Technologies system is connected under ultra-high vacuum to a surface analysis chamber and a III-V semiconductor EJM frame. It is equipped with a valve cracker cell as Selenium source and a 6-crucible electron gun and Knudsen effusion cells for metal evaporation. It will be dedicated to the epitaxy of DMT heterostructures but also to the growth of hybrid III-V/2Ds heterostructures.
Fig. 3: Example of DMT heterostructure
for the realization of Esaki diodes
