Transition metal dichalcogenide (TMD) monolayers, such as WSe2, WS2, MoSe2, and MoS2, possess distinct physical properties due to the strong coupling between spin and valley degrees of freedom.(1, 2) As monolayer TMDs have a direct bandgap lying in visible range, they have been studied extensively by optical methods.(2, 3) Heterostructures of monolayer TMDs with other functional materials are currently attracting significant attention due to the opportunities to access and utilize their spin-valley degrees of freedom through electrical means.(4) For instance, TMD-ferromagnet heterostructures have been employed recently to study spin current generation in TMDs. (4, 5) The quality of atomically thin TMDs, however, is strongly affected by deposition techniques of metallic layers and have not been fully investigated.(6) In this work, we report the fabrication of Pt/Co multilayer using pulsed laser deposition (PLD) on monolayer WSe2 grown bymetalorganic chemical vapor deposition (MOCVD) on single crystalline (0001)-oriented Al2O3 substrates. PLD is a plasma based deposition technique capable of tuning of kinetic and thermodynamic conditions over an expanse range to elucidate and control fundamental structure-property relationships across a wide variety of material classes. (7)Using Raman Spectroscopy, we monitor deposition induced damage on monolayer WSe2. The pressure of Argon process gas is found to suppress deposition induced defects in WSe2, which indicates that the primary source of defect generation comes from ion bombardment. Further, we report on magnetometry and spin torque measurements of our WSe2-ferromagnet heterostructures and demonstrate the generation of spin current from TMD layer. We anticipate that our results will advance the electrical investigation of spin-valley and spin generation phenomena in 2D hybrid heterostructures for spintronics.
New Publication! S. Chae, , J. Lee, K. A. Mengle, J. T. Heron, and E. Kioupakis Appl. Phys. Lett. 114, 102104 (2019)
Abstract: Ultra-wide-band-gap (UWBG) semiconductors have tremendous potential to advance electronic devices as device performance improves superlinearly with the increasing gap. Ambipolar doping, however, has been a major challenge for UWBG materials as dopant ionization energy and charge compensation generally increase with the increasing bandgap and significantly limit the semiconductor devices that can currently be realized. Using hybrid density functional theory, we demonstrate rutile germanium oxide (r-GeO2) to be an alternative UWBG (4.68 eV) material that can be ambipolarly doped. We identify SbGe, AsGe, and FO as possible donors with low ionization energies and propose growth conditions to avoid charge compensation by deep acceptors such as VGe and NO. On the other hand, acceptors such as AlGe have relatively large ionization energies (0.45 eV) due to the formation of localized hole polarons and are likely to be passivated by VO, Gei, and self-interstitials. Yet, we find that the co-incorporation of AlGe with interstitial H can increase the solubility limit of Al and enable hole conduction in the impurity band. Our results show that r-GeO2 is a promising UWBG semiconductor that can overcome current doping challenges and enable the next generation of power electronics devices.
Full text available from Applied Physics Letters
The award comes from the Ceramics program within the NSF Division of Materials Research. The project is to focus on the understanding the roles of defects and disorder on the dielectric properties of entropy-stabilized thin film materials.
Established in 2017 to honor 100 years of one of the University’s oldest and most prestigious scholarships, the Barbour Centennial Award helps graduate student research via financial support for travel, resources, and living expenses. Barbour Scholar alumnae from across the globe came together to contribute to this gift.
This past week, both Sieun and Peter gave talks at the annual American Ceramic Society Electronic materials and applications (EMA) conference on defect formation and magnetic disorder in entropy stabilized oxides.
Defect and disorder driven dielectric properties of entropy-stabilized oxides.
Abstract: Entropy-stabilized oxides (ESO) are a solid solution of five or more binary oxides in a single lattice, stabilized by the large configurational entropy from cationic disorder. Due to their tunable chemical heterogeneity and intrinsic disorder, ESO are expected to demonstrate novel functional behavior. Point defects in oxides, however, can have a strong influence on functional properties, yet an understanding of point defects in ESO is unknown. Here we present on a theoretical and experimental investigation of point defects and disorder in (MgCoNiCuZn)O-based ESO using density functional theory (DFT) and dielectric measurements. We theoretically predicted that the thermodynamic stability of vacancies in ESO strongly depends on their nearest-neighbor configuration, indicating that the types and concentrations of defects can be tuned by the composition of cations, particularly Cu. Our calculated dielectric constant varies depending on vacancy and cation composition. To experimentally characterize these materials, we have integrated single crystalline entropy-stabilized oxide thin films into vertical capacitor devices by using MgO/SrTiO3 buffered conductive Si substrates and performed dielectric testing over a wide range of frequencies. We varied the composition of the films and observed the effect of local lattice distortion that arises from the composition of Cu on the dielectric behavior of ESO.
Magnetic Frustration Engineering Through Stereochemical Disorder in Single Crystalline Entropy-Stabilized Oxides
Abstract: A unique benefit to entropic stabilization is the increased solubility of elements, which opens a broad compositional space with subsequent local chemical and structural disorder resulting from different atomic sizes and preferred coordinations of the constituents. It is unknown, however, to what degree this structural disorder contributes to material functionalities. In the antiferromagnetic entropy-stabilized oxides studied here, we see that by tuning the concentration of local structural frustrations caused by Jahn-Teller active cations, we induce or reclaim a large degree of disorder in the magnetic lattice of the material. This effect can be utilized to tune the anisotropy and magnetic structure of the oxide to approach that of an isotropic spin glass, yet still in a single crystalline material. Our results reveal that the unique characteristics of entropy stabilized materials can be utilized to realize novel magnetism in oxide thin films.