(Invited) Large Area Synthesis of 2D Metal Dichalcogenides By Van Der Waals Molecular Beam Epitaxy
Authors: Dimoulas Athanasios , Tsoutsou Dimitra , Tsipas Polichronis , Fragkos Sotiris , Sant Roberto , Alvarez Carlos , Okuno Hanako , Renaud Gilles 
Published: February 2020

A large number of 2D Transition Metal Dichalcogenide (TMD) materials on micron-sized flakes exfoliated from bulk have been studied extensively showing exciting new physical properties and good prospects to impact a wide range of applications [1]. On the other hand, large area growth on suitable substrates is urgently needed in order to make the transition from lab-based research to real world applications. Molecular beam epitaxy (MBE) is traditionally used to achieve van der Waals epitaxy as first conceptualized by Koma and co-workers[2]. The epitaxial growth of single or few layer 2D TMDs is very challenging because it depends critically on the substrate and surface preparation methodology. Although Si as one of the most technologically important substrates is preferred for TMD growth, the film quality is not adequate. Engineered substrates such as epi-AlN(0001)/Si and epi-InAs(111)/Si available in large wafer sizes (200 and 300 mm) could be good candidates in the effort to improve the thin film quality.
In this work, we will first review our work on MoSe2 [3], HfSe2 [4] WSe2[5], SnSe2[5] and HfTe2 [6] and their van der Waals heterostructures [5,7] grown by MBE on AlN/Si focusing on the microstructure, thin film morphology and homogeneity. Then we will show more recent results on metallic or semimetallic telluride TMDs (MoTe2, TiTe2, ZrTe2, HfTe2) grown on InAs/Si substrates with much improved epitaxial quality. Using synchrotron XRD, high resolution STEM and in-situ STM, excellent rotational commensurability, quasi van der Waals gap at the interface and Moire pattern are observed indicating good registry between the 2D TMD epilayer and the substrate through weak van der Waals forces. We will show that MBE grown 1ML MoTe2 on InAs(111) adopts the metastable distorted 1T’ crystal structure which is metallic and predicted to be a 2D tolological insulator (or Quantum Spin Hall state). Using in-situ ARPES complemented by DFT calculations we show that the valence and conduction bands of few layer HfTe2 [6] and ZrTe2 cross at the Fermi level exhibiting abrupt linear dispersions at the zone center. The latter indicates massles Dirac fermions which are maintained down to the 2D limit suggesting that single layers of epitaxial HfTe2 and ZrTe2 could be considered as the electronic analogue of graphene. Finally, we show that in 1 ML TiTe2 it is possible to substitute the top Te row by Se to form the ordered Janus ternary 2D material Te-Ti-Se similar to S-Mo-Se material recently reported [8,9]. These materials are predicted to have exiting new physical properties and are expected to open a new field in the wide area of 2D materials and devices.
Acknowledgements: This work is financially supported by the LANEF Chair of Excellence program of U. Grenoble Alpes and CEA (A.D.), the Greek State Scholarships Foundation (IKY) Program for the strengthening of postdoctoral research (P.T.), The FLAG-ERA project MELoDICA (A.D.) and the French state funds ANR-10-LABX-51-01 (Labex LANEF du Programme d'Investissements d'Avenir) and Equipex ANR-11-EQPX-0010 (G.R.)
References: [1] W. Choi, et al., Materials Today, 20(3), 116-130, (2017); [2] A. Koma, J. Cryst. Growth, 201/202, 236-241 (1999); [3] E. Xenogiannopoulou, et al., Nanoscale, 7, 7896-7905, (2015); [4] K. E. Aretouli, et al., Appl. Phys. Lett. 106, 143105 (2015) ; [5] K. E. Aretouli et al., ACS Appl. Mater. Interfaces 8, 23222-23229 (2016); [6] S. A. Giamini et al., 2D Mater. 4, 015001 (2017); [7] D. Tsoutsou, et al., ACS Appl. Mater. Interfaces 8, 1836-1841 (2016).