ItemEngineering The Palladium-WSe₂ Interface Chemistry for Field Effect Transistors with High-Performance Hole Contacts(Amer Chemical Soc, 2018-12-07) Smyth, Christopher M.; Walsh, Lee A.; Bolshakov, Pavel; Catalano, Massimo; Addou, Rafik; Wang, Luhua; Kim, Jiyoung; Kim, Moon J.; Young, Chadwin D.; Hinkle, Christopher L.; Wallace, Robert M.; 0000-0001-5566-4806 (Wallace, RM); 0000-0003-0690-7423 (Young, CD); 0000-0003-2781-5149 (Kim, J); 0000-0002-6688-8626 (Walsh, LA); 0000-0002-5485-6600 (Hinkle, CD); 0000-0002-5454-0315 (Addou, R); 70133685 (Kim, J); Smyth, Christopher M.; Walsh, Lee A.; Bolshakov, Pavel; Catalano, Massimo; Addou, Rafik; Wang, Luhua; Kim, Jiyoung; Kim, Moon J.; Young, Chadwin D.; Hinkle, Christopher L.; Wallace, Robert M.Palladium has been widely employed as a hole contact to WSe₂ and has enabled, at times, the highest WSe₂ transistor performance. However, there are orders of magnitude variation across the literature in Pd-WSe₂ contact resistance and I-ON/I-OFF ratios with no true understanding of how to consistently achieve high-performance contacts. In this work, WSe₂ transistors with impressive I-ON/I-OFF ratios of 10(6) and Pd-WSe₂ Schottky diodes with near-zero variability are demonstrated utilizing Ohmic-like Pd contacts through deliberate control of the interface chemistry. The increased concentration of a PdSeₓ intermetallic is correlated with an Ohmic band alignment and concomitant defect passivation, which further reduces the contact resistance, variability, and barrier height inhomogeneity. The lowest contact resistance occurs when a 60 min post-metallization anneal at 400 degrees C in forming gas (FG) is performed. X-ray photoelectron spectroscopy indicates this FG anneal produces 3x the concentration of PdSeₓ and an Ohmic band alignment, in contrast to that detected after annealing in ultrahigh vacuum, during which a 0.2 eV hole Schottky barrier forms. Raman spectroscopy and scanning transmission electron microscopy highlight the necessity of the fabrication step to achieve high-performance contacts as no PdSeₓ forms, and WSe₂ is unperturbed by room temperature Pd deposition. However, at least one WSe₂ layer is consumed by the necessary interface reactions that form PdSeₓ requiring strategic exploitation of a sacrificial WSe₂ layer during device fabrication. The interface chemistry and structural properties are correlated with Pd-WSe₂ diode and transistor performance, and the recommended processing steps are provided to enable reliable high-performance contact formation. ItemControlled Crack Propagation for Atomic Precision Handling of Wafer-Scale Two-Dimensional Materials(American Association for the Advancement of Science, 2018-10-11) Shim, J.; Bae, S. -H; Kong, W.; Lee, D.; Qiao, K.; Nezich, D.; Park, Y. J.; Zhao, R.; Yue, Ruoyu; Zhou, Guanyu; Hinkle, Christopher L.; Yue, Ruoyu; Zhou, Guanyu; Hinkle, Christopher L.Although flakes of two-dimensional (2D) heterostructures at the micrometer scale can be formed with adhesive-tape exfoliation methods, isolation of 2D flakes into monolayers is extremely time consuming because it is a trial-and-error process. Controlling the number of 2D layers through direct growth also presents difficulty because of the high nucleation barrier on 2D materials. We demonstrate a layer-resolved 2D material splitting technique that permits high-throughput production of multiple monolayers of wafer-scale (5-centimeter diameter) 2D materials by splitting single stacks of thick 2D materials grown on a single wafer. Wafer-scale uniformity of hexagonal boron nitride, tungsten disulfide, tungsten diselenide, molybdenum disulfide, and molybdenum diselenide monolayers was verified by photoluminescence response and by substantial retention of electronic conductivity. We fabricated wafer-scale van der Waals heterostructures, including field-effect transistors, with single-atom thickness resolution. © 2018 American Association for the Advancement of Science. All rights reserved. ItemNucleation and Growth of WSe₂: Enabling Large Grain Transition Metal Dichalcogenides(IOP Publishing Ltd, 2017-09-22) Yue, Ruoyu; Nie, Yifan; Walsh, Lee A.; Addou, Rafik; Liang, Chaoping; Lu, Ning; Barton, Adam T.; Zhu, Hui; Che, Zifan; Barrera, Diego; Cheng, Lanxia; Cha, Pil-Ryung; Chabal, Yves J.; Hsu, Julia W. P.; Kim, Jiyoung; Kim, Moon J.; Colombo, Luigi; Wallace, Robert M.; Cho, Kyeongjae; Hinkle, Christopher L.; 0000-0002-2910-2938 (Liang, C); Yue, Ruoyu; Nie, Yifan; Walsh, Lee A.; Addou, Rafik; Liang, Chaoping; Lu, Ning; Barton, Adam T.; Zhu, Hui; Che, Zifan; Barrera, Diego; Cheng, Lanxia; Chabal, Yves J.; Hsu, Julia W. P.; Kim, Jiyoung; Kim, Moon J.; Wallace, Robert M.; Cho, Kyeongjae; Hinkle, Christopher L.The limited grain size (< 200 nm) for transition metal dichalcogenides (TMDs) grown by molecular beam epitaxy (MBE) reported in the literature thus far is unsuitable for high-performance device applications. In this work, the fundamental nucleation and growth behavior of WSe₂ is investigated through a detailed experimental design combined with on-lattice, diffusion-based first principles kinetic modeling to enable large area TMD growth. A three-stage adsorption-diffusion-attachment mechanism is identified and the adatom stage is revealed to play a significant role in the nucleation behavior. To limit the nucleation density and promote 2D layered growth, it is necessary to have a low metal flux in conjunction with an elevated substrate temperature. At the same time, providing a Se-rich environment further limits the formation of W-rich nuclei which suppresses vertical growth and promotes 2D growth. The fundamental understanding gained through this investigation has enabled an increase of over one order of magnitude in grain size for WSe₂ thus far, and provides valuable insight into improving the growth of other TMD compounds by MBE and other growth techniques such as chemical vapor deposition (CVD). ItemChemical Bonding and Defect States of LPCVD Grown Silicon-Rich Si₃N₄ for Quantum Dot Applications(A V S: Science & Technology of Materials, Interfaces, and Processing, 2014-03) Mohammed, Shakil; Nimmo, Michael T.; Malko, Anton V.; Hinkle, Christopher L.; 0000 0001 2678 9765 (Malko, AV); Mohammed, Shakil; Nimmo, Michael T.; Malko, Anton V.; Hinkle, Christopher L.Si-rich Si₃N₄ (SRN) thin films were investigated to understand the various defect states present within the SRN that can lead to reduced performance in quantum dot based devices made of these materials. The SRN films, deposited by low pressure chemical vapor deposition followed by furnace anneals over a range of temperatures, were determined to be comprised of two distinct phase separated SRN regions with different compositions (precipitates within a host matrix). Photoluminescence (PL) spectra showed multiple peaks convoluted together within the visible and near-visible range. Depending on deposition and annealing conditions, the films displayed changes in PL peak intensities which were correlated with chemical bonding utilizing x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, spectroscopic ellipsometry, and capacitance-voltage measurements. It is found that the PL originates from defect-state to defect-state and band edge to defect-state electronic transitions. ItemIn Situ Study of the Role of Substrate Temperature during Atomic Layer Deposition of HfO2 on InP(2013-10-16) Dong, Hong; Santosh, KC; Qin, Xiaoye; Brennan, Barry; McDonnell, Steven; Zhernokletov, Dmitry; Hinkle, Christopher L.; Kim, Jiyoung; Cho, Kyeongjie; Wallace, Robert M.; 70133685 (Kim, J)The dependence of the "self cleaning" effect of the substrate oxides on substrate temperature during atomic layer deposition (ALD) of HfO₂ on various chemically treated and native oxide InP (100) substrates is investigated using in situ X-ray photoelectron spectroscopy. The removal of In-oxide is found to be more efficient at higher ALD temperatures. The P oxidation states on native oxide and acid etched samples are seen to change, with the total P-oxide concentration remaining constant, after 10 cycles of ALD HfO₂ at different temperatures. An (NH₄)₂ S treatment is seen to effectively remove native oxides and passivate the InP surfaces independent of substrate temperature studied (200°C, 250°C and 300°C) before and after the ALD process. Density functional theory modeling provides insight into the mechanism of the changes in the P-oxide chemical states.