Browsing by Author "Walsh, Lee A."
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Item Engineering 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.Item Nucleation 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).