In this article, we present a fab-compatible metal–organic chemical vapor deposition growth process, realized in a hydrogen ambience, of two-dimensional (2D) layered GaSe on 200 mm diameter Si(111) wafers. Atomic scale characterization reveals initial stages of growth consisting of passivation of the H–Si (111) surface by a half-monolayer of GaSe, followed by nucleation of 2D-GaSe from the screw dislocations located at the step edges of the substrate. We, thus, demonstrate that by using a Si wafer that is slightly misoriented toward [1̄1̄2], the crystallographic orientation of 2D-GaSe can be step-edge-guided. It results in a coalesced layer that is nearly free from antiphase boundaries. In addition, we propose a sequential process to reduce the density of screw dislocations. This process consists in a subsequent regrowth after partial sublimation of the initially grown GaSe film. The local band bending in GaSe near the antiphase boundaries measured by Kelvin probe force microscopy emphasizes the electrical activity of these defects and the usefulness of having a nearly single-orientation film. Such a low defectivity layer opens up the way toward large-scale integration of 2D-optical transceivers in Si CMOS technology.

The magnetic order associated with the degree of freedom of spin in two-dimensional (2D) materials is subjected to intense investigation because of its potential application in 2D spintronics and valley-related magnetic phenomena. We report here a bottom-up strategy using molecular beam epitaxy to grow and dope large-area (cm$^2$) few-layer MoSe$_2$ with Mn as a magnetic dopant. High-quality Mn-doped MoSe$_2$ layers are obtained for Mn content of less than 5 % (atomic). When increasing the Mn content above 5 % we observe a clear transition from layer-by-layer to cluster growth. Magnetic measurements involving a transfer process of the cm$^2$-large doped layers on 100-micron-thick silicon substrate, show plausible proof of high-temperature ferromagnetism of 1 % and 10 % Mn-doped MoSe$_2$. Although we could not point to a correlation between magnetic and electrical properties, we demonstrate that the transfer process described in this report permits to achieve conventional electrical and magnetic measurements on the doped layers transferred on any substrate. Therefore, this study provides a promising route to characterize stable ferromagnetic 2D layers, which is broadening the current start-of-the-art of 2D materials-based applications.

Two dimensional (2D) layered structures, such as graphene and boron nitride, have been attracting much attention as substrate for the growth of other materials. Direct growth of van der Waals heterostructures combining different types of 2D monolayers is an exciting way to explore exotic 2D building blocks with new functionalities [1]. 2D layers are also used as a van der Waals substrate to grow other bulk materials. For instance, a strain free growth of GaN is expected using graphene substrate. However these 2D materials have no covalent bonding on their surface and the epitaxial correlation between the 2D substrate and the grown materials has not been clearly understood.

Large-area growth of continuous transition metal dichalcogenides (TMDCs) layers is a prerequisite to transfer their exceptional electronic and optical properties into practical devices. It still represents an open issue nowadays. Electric and magnetic doping of TMDC layers to develop basic devices such as p-n junctions or diluted magnetic semiconductors for spintronic applications are also an important field of investigation. Here, we have developed two different techniques to grow MoSe2 mono-and multi-layers on SiO2/Si substrates over large areas. First, we co-deposited Mo and Se atoms on SiO2/Si by molecular beam epitaxy in the van der Waals regime to obtain continuous MoSe2 monolayers over 1 cm(2). To grow MoSe2 multilayers, we then used the van der Waals solid phase epitaxy which consists in depositing an amorphous Se/Mo bilayer on top of a co-deposited MoSe2 monolayer which serves as a van der Waals growth template. By annealing, we obtained continuous MoSe2 multilayers over 1 cm(2). Moreover, by inserting a thin layer of Mn in the stack, we could demonstrate the incorporation of up to 10% of Mn in MoSe2 bilayers.

Epitaxial SnTe (111) is grown by molecular-beam epitaxy on Bi2Te3 substrates. Structural evaluation indicates that SnTe deviates from cubic due to in-plane compressive strain, which induces significant changes in the electronic band structure. More specifically, a pair of gapless crossings between the two uppermost valence bands occurs in k space along the out-of-plane Gamma Z direction of the Brillouin zone, associated with a band inversion, thus defining topological three-dimensional Dirac nodes. Combined first-principles calculations and angle-resolved photoelectron spectroscopy reveal an overtilted Dirac cone indicating that the crossing is a topological type-III Dirac node at the borders between type-I and type-II Dirac nodes.

The group IVB 2D transition metal dichalcogenides are considered to be stable in the high symmetry trigonal octahedral structure due to the lack of unpaired d ‐electrons on the metal site. It is found that multilayer epitaxial TiTe 2 is an exception adopting a commensurate 2 × 2 × 2 charge density wave (CDW) structure at room temperature with an ABA type of stacking as evidenced by direct lattice imaging and reciprocal space mapping. The CDW is stabilized by highly anisotropic strain imposed by the substrate with an out‐off‐plane compression which reduces the interlayer van der Waals gap increasing the coupling between TiTe 2 layers. A weaker 2 × 2 CDW is also confirmed at room temperature for epitaxial monolayer TiTe 2 . The addition of epitaxial strained TiTe 2 to the family of CDW materials will enable real world applications that take advantage of a CDW ground state at room temperature.

Controlled substitutional doping of two-dimensional transition-metal dichalcogenides (TMDs) is of fundamental importance for their applications in electronics and optoelectronics. However, achieving p-type conductivity in MoS2 and WS2 is challenging because of their natural tendency to form n-type vacancy defects. Here, we report versatile growth of p-type monolayer WS2 by liquid-phase mixing of a host tungsten source and niobium dopant. We show that crystallites of WS2 with different concentrations of substitutionally doped Nb up to 1014 cm-2 can be grown by reacting solution-deposited precursor film with sulfur vapor at 850 °C, reflecting the good miscibility of the precursors in the liquid phase. Atomic-resolution characterization with aberration-corrected scanning transmission electron microscopy reveals that the Nb concentration along the outer edge region of the flakes increases consistently with the molar concentration of Nb in the precursor solution. We further demonstrate that ambipolar field-effect transistors can be fabricated based on Nb-doped monolayer WS2.

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 MoSe 2 [3], HfSe 2 [4] WSe 2 [5], SnSe 2 [5] and HfTe 2 [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 (MoTe 2 , TiTe 2 , ZrTe 2 , HfTe 2 ) 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 MoTe 2 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 HfTe 2 [6] and ZrTe 2 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 HfTe 2 and ZrTe 2 could be considered as the electronic analogue of graphene. Finally, we show that in 1 ML TiTe 2 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).

MnCoGe thin films were produced using simultaneous magnetron sputtering of Mn, Co, and Ge on SiO2, followed by non-diffusive reaction. The MnCoGe compound begins to form at similar to 588 K, and structural characterizations show that the obtained MnCoGe film is polycrystalline with the hexagonal Ni2In-type structure. This structure is found to be stable from 873 K down to room temperature, the expected hexagonal/orthorhombic structural transition being prevented. The film exhibits a lower average Mn composition than the standard MnCoGe stoichiometry. Furthermore, small clusters(<3 nm) forming planar distributions parallel to the sample surface are observed. They are regularly located every similar to 11 nm in the specimen depth. They mainly contain Mn and O atoms. Magnetic characterizations show very good magnetic properties, allowing the perpendicular and parallel magnetocrystalline anisotropy constants to be measured down to 100 K, using the Chappert model to fit ferromagnetic resonance measurements. The film magnetic properties match the properties of bulk stoichiometric MnCoGe in the hexagonal structure, with a Curie temperature of similar to 269 K and a negligible coercive field at room temperature. The only difference between the magnetic properties of bulk and thin film specimens appears to be the film shape anisotropy, forcing the internal magnetic field to be contained in the film plane. (C) 2017 Elsevier B.V. All rights reserved.

The direct observation at room temperature (RT) of the noncentrosymmetric orthorhombic topological Weyl semimetal phase in epitaxial thin films of MoTe2 grown on InAs(111)/Si(111) substrates by molecular beam epitaxy (MBE) is reported. The orthorhombic phase is typically found at lower temperatures but its observation at RT in this work is attributed to the enlarged lattice parameters, influenced by the substrate, which stabilize an interlayer antibonding state compatible with the orthorhombic stacking. First-principles calculations predict eight type II Weyl nodes which are located below (but near) the Fermi energy making them accessible to charge transport and creating the prospect for practical applications exploiting the nontrivial topological properties. The orthorhombic phase coexists with an unconventional triclinic layer stacking which is different than the monoclinic or orthorhombic structures but it is centrosymmetric and topologically trivial.

In this work, we study growth and migration of atomic defects in MoSe2 on graphene using multiple advanced transmission electron microscopy techniques to explore defect behavior in vdW heterostructures. A MoSe2/graphene vdW heterostructure is prepared by a direct growth of both monolayers, thereby attaining an ideal vdW interface between the monolayers. We investigate the intrinsic defects (inversion domains and grain boundaries) in synthesized MoSe2, their evolution amid growth processing steps, and their influence on the formation and movement of extrinsic defects. Electron diffraction identifies a preferential interlayer orientation of 2° between MoSe2 and graphene, which is caused by the presence of intrinsic IBD defects. Extrinsic defects (point and line defects) are generated by in situ electron irradiation in the MoSe2 layer. Our results shed light on how to independently modify the MoSe2 atomic structure in vdW heterostructures for potential utilization in device processing.

Single and few layers of the two-dimensional (2D) semimetal ZrTe2 are grown by molecular beam epitaxy on InAs(111)/Si(111) substrates. Excellent rotational commensurability, van der Waals gap at the interface and moiré pattern are observed indicating good registry between the ZrTe2 epilayer and the substrate through weak van der Waals forces. The electronic band structure imaged by angle resolved photoelectron spectroscopy shows that valence and conduction bands cross at the Fermi level exhibiting abrupt linear dispersions. The latter indicates massless Dirac Fermions which are maintained down to the 2D limit suggesting that single-layer ZrTe2 could be considered as the electronic analogue of graphene.

Recently, tuning the electronic properties of atomically thin semi-conducting materials has become one of the exciting challenges. The modification of atomic structures by introducing ordered atomic defects has been demonstrated as a powerful strategy to control their band structures [1]. Inversion domain boundary (IDB) in transition metal selenides, such as MoSe2, is one of the interesting ordered defect structures, which locally introduces 1D metallic channels in 2D atomic layers [2]. Understanding the mechanism of formation, elimination and migration of IDBs will be thus a key to control the electronic property of 2D layers.

M. T. Dau1,2, C. Vergnaud1,2, A. Marty1,2, F. Rortais1,2, C. Beigné1,2, H. Boukari1,3, E. BelletAmalric1,4, O. Renault1,5, C. Alvarez1,6, H. Okuno1,6, P. Pochet1,6 and M. Jamet1,2 1 Université Grenoble Alpes, F-38000 Grenoble, France 2 INAC-SPINTEC, CEA/CNRS, F-38000 Grenoble, France 3 CNRS, Insitut NEEL, F-38000 Grenoble, France 4 INAC-PHELIQS, CEA F-38000 Grenoble, France 5 CEA, LETI, Minatec campus, F-38054 Grenoble, France 6 INAC-MEM, CEA, F-38000 Grenoble, France Contact: minhtuan.dau@cea.fr

Glass-ceramic scintillators are a field rich in research opportunities and potential applications. The chapter leads the reader in the direction of potential exploration routes to improve such materials so that they may be used commercially. The work described here encompasses novel X-ray, gamma-ray, alpha-particle, and neutron scintillator materials based on glass ceramics. Synthesis methods, characterization, and applications, including medical and homeland security, are discussed. These materials have enormous potential owing to their ability to accept a broad range of elemental compositions, protect normally hygroscopic crystalline materials, and be cast in almost any shape or size. Consequently, the materials have a great future in radiation devices and are also attractive for fundamental science studies.

Molecular beam epitaxy technique has been used to deposit a single layer and a bilayer of MoSe2 on sapphire. Extensive characterizations including in-situ and ex-situ measurements show that the layered MoSe2 grows in a scalable manner on the substrate and reveals characteristics of a stoichiometric 2H-phase. The layered MoSe2 exhibits polycrystalline features with domains separated by defects and boundaries. Temperature and magnetic field dependent resistivity measurements unveil a carrier hopping character described within two-dimensional variable range hopping mechanism. Moreover, a negative magnetoresistance was observed, stressing a fascinating feature of the charge transport under the application of a magnetic field in the layered MoSe2 system. This negative magnetoresistance observed at millimeter-scale is similar to that observed recently at room temperature in WS2 flakes at a micrometer scale [Zhang et al., Appl. Phys. Lett. 108, 153114 (2016)]. This scalability highlights the fact that the underlying physical mechanism is intrinsic to these two-dimensional materials and occurs at very short scale.

The potential use of germanide thin films as self‐aligned metallization in Ge‐based Complementary Metal‐Oxide Semiconductor (CMOS) technology has drawn interest in the solid‐state reactions of Ge and metal films [1‐3]. In order to integrate these germanide thin films as a contact material, a complete analysis of the solid‐state reactions of amorphous Ge (a‐Ge) and metals is required. The solid‐state reactions in the Co‐Ge thin films systems previously examined by in situ x‐ray diffracting annealing experiments only probed the crystal structures present in the film and did not identify the morphological variations of the process [1]. Furthermore, to the authors knowledge previous works Co‐Ge thin film systems have only analyzed systems with Ge in excess of Co. In this study, the phase formation and crystallization behavior of a‐Ge and Co thin film layers are investigated by ex situ transmission electron microscopy (TEM) coupled with in situ x‐ray diffraction (XRD) annealing experiments. Four different specimens were prepared, alternating the stacking order and film thickness (200 nm and 30 nm), in order to explore the influence of the free surface on the crystallization and the phase reactions with Co in excess of a‐Ge. Thin film layers of a‐Ge and Co were deposited via electron beam evaporation onto (100) silicon wafers with the native oxide film present on the surface (~1–2 nm). The thin film specimens were characterized by TEM before and after annealing during the phase formation process. Bright field TEM images of the 200 nm Co on top of 30 nm a‐Ge stack are shown in Figure 1: (a) as‐deposited, (b) annealed at 280°C and (c) 400°C. Upon heating to 280°C, the a‐Ge layer reacts with the Co layer to form CoGe, which then undergoes an additional phase transformation to CoGe 2 with continued heating to 400°C. The influence of the free surface ( fs ) on the crystallization process was examined on the 200 nm a‐Ge and 30 nm Co film stack. The scanning electron microscopy images of the specimens after annealing to 400°C are shown in Figure 2. (a) The specimen with a‐Ge at the free surface (a‐Ge fs ) shows two distinct contrast indicating the presence of two different crystalline phases near the surface, while (b) the specimen with Co at the free surface (Co fs ) shows a more homogeneous contrast throughout the surface. Both specimens have voids visible at the surface, which formed during annealing. By viewing the specimens in cross‐section, the presence of large voids is visible in (c) the specimen a‐Ge fs , but the voids are not present in (d) the specimen with Co fs . The resulting formation of voids in only one of two specimens indicates the free surface has a direct influence of the phase formation process.