ECE Spring Seminar Series

This is a past event

NEP Speaker, Stephen McDonnell, University of Virginia


Title: "In-Vacuo Studies of Transition Metal Dichalcogenide Synthesis and Integration" 



The last decade has seen fevered interest in 2D materials for nanoelectronics. Previous studies focused primarily on tribological and energy storage applications due to the weak van der Waals bonding that exists between the atomic layers. These materials, which include graphene, hexagonal boron nitride, and the plethora of transition metal dichalcogenide (TMD) combinations, have electronic structures exhibiting metallic, semiconducting, and insulating properties. This promises devices with scalability to the atomic limit combined with defect-free interfaces. Realizing this promise has not proved trivial. Defects in the naturally occurring material can dominate their properties, and even synthesized materials can suffer from high impurity concentration. Process residues such as photo-resists can impact device performance. Metal depositions can result in the formation of unexpected interface compounds that can dominate the contact behavior.


We study both the synthesis and integration of 2D materials for nano- and optoelectronic applications. Using an in-vacuo MBE-ARPES cluster tool, we grow TMDs from elemental sources using van der Waals epitaxy. The growth can be carried out on a range of substrates include insulating, semiconducting, and metallic materials. The layer-by-layer nature of these growths can be verified in-situ using reflection, high-energy electron diffraction. We use in-vacuo angle-resolved photoemission spectroscopy to study the electronic structure of these materials as a function of composition, substrate, and processing conditions. The in-vacuo set-up is vital to determining the intrinsic properties of these materials, many of which readily oxidize in air.


In addition to studying the synthesis of these materials, we also investigate how these materials interface with metals that are commonly used as contacts in electronic devices. It is well known that low work-function metals can react with TMDs to form metal-chalcogen compounds at the metal-semiconductor interface. Recently it has been shown that the process conditions during the electron beam deposition of metals can control the interface reaction. In particular, the reactor base pressure and deposition rate can be used to tune the interface chemistry. For example, the partial pressure of oxidizing species, that are typically present in a high-vacuum reactor, is sufficiently high to completely oxidize titanium as it is deposited by e-beam. This results in a sharp interface between TiO2 and the semiconductor interfaces; however, it is the properties of TiO2 rather than Ti that are dominating electrical and thermal conduction across the interface.


I will present a summary of our work on TMD synthesis, integration on novel substrates, and the interfaces formed between 2D materials and typical device contact metals.



Stephen McDonnell graduated from Dublin City University with a B.Sc. in Applied Physics in 2004 and a Ph.D. in Physical Sciences in 2009 before joining the Materials Science Department at UT-Dallas as a postdoctoral associate. In 2015 he joined the Materials Science and Engineering Department at the University of Virginia as an Assistant Professor. His current research interests are centered on the integration of 2D materials into device architectures involving interfaces with 3D materials. Device applications for these materials include photovoltaics, logic, low powered transistors, flexible electronics, and photoelectrochemistry. The McDonnell group seeks to synthesis 2D materials by Molecular Beam Epitaxy and study their electronic structure by in-vacuo photoelectron spectroscopy techniques including XPS, APRES, and IPES. The group also investigates the interface chemistry between these materials and device relevant insulators and metals so as to correlate interface chemistry with device characteristic and ultimately allow for interface engineering that will allow these materials to be utilized to their full potential.

Friday, April 26, 2019 at 11:15am to 12:05pm

Evans Hall, Evans 204
Evans Hall, University of Delaware, Newark, DE 19716, USA

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Students, Academics, Faculty & Staff, Lectures & Programs, College of Engineering, Community, Lectures and Programs



Electrical and Computer Engineering



ENGR - Electrical & Computer Engineering
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Amber Spivey

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