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2021 Highlights

User 2DCC Research Highlights 2021

Project Summary: The electronic properties of transition metal dichalcogenides (TMDs) vary dramatically depending on their phase (2H vs. 1T) and ans the energy to convert between the two phases is predicted to be relatively low. This has sparked interest in the use of the phase change properties of TMDs for active components in electronic devices. A combination of mechanical strain and charge transfer from a metal substrate has previously been suggested to induce the phase transition from 2H to 1T.  In this study, x-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) were used to identify charge transfer processes and changes in the structural phase for MoS2, MoSe2 and MoTe2 monolayers on Au surfaces annealed up to 500oC. Although charge transfer does appear to occur, we do not find spectroscopic evidence for the 2H to 1T phase change in exfoliated TMDs. However, both helium-ion irradiated exfoliated TMDs and layers grown by metalorganic chemical vapor deposition (MOCVD) show shifts in their XPS spectra that could be interpreted as a phase change but instead are the result of TMD/Au hybridization, grain boundaries and defects interacting with the Au surface.

Published in: J. Phys. Chem. 2020, 124, 25361-25368.

2DCC Role: The wafer-scale MoS2 monolayer samples used for this study were grown by MOCVD in the 2DCC facility. The user PI (J.T. Robinson) and his postdoctoral scholars have been active participants in 2DCC activities including participating in annual User Committee meetings and presenting posters at Graphene and Beyond.

Project Summary: The topological Hall effect (THE) is a phenomenon that is a consequence of a Berry phase created by spin textures in real space. Interfacing a topological insulator with a magnetic insulator provides a model platform for studying this phenomenon in a well-controlled manner. This papers reports the first clear evidence for the THE in heterostructures that combine a model topological insulator (Bi2Se3) with a ferromagnetic insulator (BaFe12O19). The key signature of the THE is an “excess” Hall resistance when the perpendicular to the plane magnetization of the ferromagnet is reversed by an external magnetic field. These samples show a conventional anomalous Hall effect (AHE) at high temperature (T > 80 K) but a pure THE develops in the temperature range of T = 2–3 K. Over T = 3–80 K, the two effects coexist but show opposite temperature dependence. Control measurements, calculations, and simulations together suggest that the observed THE originates from skyrmions (spin textured “bubbles”) that arise due to a Dzyaloshinskii–Moriya interaction at the interface. The estimated strength of this interaction is substantially higher than that in more conventional heavy metal-based systems widely studied in the spintronics community. Publication: Nano Lett. 2021, 21, 1, 84–90

2DCC Role: The Bi2Se3 thin films were grown by MBE in the multi-module MBE system in the 2DCC facility on BaFe12O19 substrates provided by the user (Wu). The user and his graduate students have been active participants in 2DCC User Committee meetings and also participated in the reverse site visit for the 2DCC renewal.

Project Summary: To assess the potential of transition metal dichalcogenides (TMDs) for future circuits, it is important to study the variation in key device parameters across a large number of devices. Here we benchmark device-to-device variation in field-effect transistors (FETs) based on wafer-scale monolayer MoS2 and WS2. Our study involves 230 MoS2 FETs and 160 WS2 FETs with channel lengths ranging from 5 µm down to 100 nm. We use statistical measures to evaluate key FET performance indicators for benchmarking these TMD monolayers against existing literature as well as ultra-thin body Si FETs. Our results show consistent performance of the 2D FETs across 1×1 cm2 chips owing to high quality uniform layers and clean transfer onto device substrates. We demonstrate record high carrier mobility of 33 cm2V-1s-1 in WS2 FETs, which is a 1.5X improvement compared to the best literature report. Our results confirm the technological viability of 2D FETs in future integrated circuits. Published in Nature Communications 2021, 12, 1-12.

2DCC Role: The wafer-scale MoS2 and WS2 monolayer samples used for this study were grown by metalorganic chemical vapor deposition (MOCVD) in the 2DCC facility. The device results provide a benchmark for academic and industry users who are working with similar 2DCC material.

In-House 2DCC Research Highlights 2021

Project Summary: Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS2 requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which form when oppositely oriented triangular domains coalesce. Here we demonstrate fully coalesced unidirectional WS2 monolayers on 2 in. diameter c-plane sapphire by metalorganic chemical vapor deposition (MOCVD) using a multistep growth process to achieve epitaxial WS2 monolayers. Transmission electron microscopy analysis reveals that the WS2 monolayers are largely free of IDBs but instead have translational boundaries that arise when WS2 domains with slightly offset lattices merge together. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that preserve the aligned domain structure. The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with optical and transport properties approaching those of exfoliated flakes.

Published in: ACS Nano 15,2, 2532-2541 (2021).

2DCC Role: The MOCVD growth studies were carried out in the 2DCC Thin Films facility (Redwing).  Structural and optical characterization of the layers were carried out by 2DCC-affiliated faculty (Alem, Terrones) and transport measurements were performed by an internal user (Das).

Understanding the superconductivity at the interface of FeSe/SrTiO3 is a problem of contemporary interest in condensed matter physics because of the significant increase in the critical temperature (Tc ~ 50 K) for the onset of superconductivity compared to that of bulk FeSe crystals (Tc ~ 9 K). Additional interest in this problem arises from the possibility of an unconventional pairing mechanism. We used the 2DCC multimodule molecular beam epitaxy (MBE) and surface characterization facility to study the influence of various capping layers on the Tc of ultrathin films of FeSe grown on SrTiO3. The multimodule facility’s in vacuo four-probe electrical resistance measurement capability provided critical information about the Tc of MBE-grown FeSe films in their pristine state, while ex situ magneto-transport measurements elucidated the key role of distinct charge transfer from different capping layers (compound FeTe, non-metallic Te, and metallic Zr). Our results show that FeTe provides an optimal cap that barely influences the inherent Tc found in pristine FeSe/SrTiO3, while the transfer of holes from a non-metallic Te cap completely suppresses superconductivity and leads to insulating behavior. We also used ex situ magneto-resistance measurements in FeTe-capped FeSe films to extract the angular dependence of the in-plane upper critical magnetic field. Our observations reveal an almost isotropic in-plane upper critical field. Although this does not show any obvious signature of exotic physics, our study provides insight into the symmetry and pairing mechanism of high temperature superconductivity in FeSe.

 

Published in: Phys. Rev. Materials 5, 034802 (2021).

Project Summary: The sticking coefficients of thermally evaporated chalcogen elements selenium and tellurium were experimentally determined as a function of temperature. Their direct and quantitative determination provides important insights to comprehend and realistically model the growth kinetics of chalcogenide-based film growth.

A direct way to determine the ratio of chalcogenide elements sticking to a film surface is to measure the rate of mass accumulated, which was achieved by depositing selenium and tellurium on a quartz crystal microbalance held at different temperatures. Pronounced reduction of the sticking coefficients by a factor of 4 in a very narrow temperature range of 20°C and 30°C around temperatures of 35°C and 115°C were found for selenium and tellurium, pointing towards the critical need of precise temperature control during chalcogenide film growth using molecular beam epitaxy. The results reveals that unlike tellurium, selenium is supplied in different chemical forms with different desorption characteristics.

J. Vac. Sci. Technol. A 2021, 39, 023413. DOI:10.1116/6.0000736

Project Summary: Dilute magnetic semiconductors, achieved through substitutional doping of magnetic atoms into semiconducting systems, enable experimental modulation of spin dynamics for novel magneto-electric or magneto-optical devices, especially in 2D transition metal dichalcogenides that accentuate interactions and activate valley degrees of freedom.

Project Summary: The first ReaxFF force field developed for 2D-WSe2 provides the community with a highly efficient means that describe material growth, phase transitions, defect formation and migration and thus can provide valuable atomistic insights into experimental efforts on growth, phase, and defect engineering as a function of the local chemical environment. This potential can elucidate further the morphological evolution of monolayers in different environments in terms of loading conditions and defect concentrations/distributions. Interactions between vacancies and ripples in a 2D layers (“ripplocations”) suggest that vacancies could stabilize buckled structures by modulating the strain energy and possibly open a venue for sweeping out undesirable defects such as vacancies from 2D WSe2.

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