The Pennsylvania State University the Graduate School THE

The Pennsylvania State University The Graduate School THE EFFECTS OF INTERFACE AND SURFACE CHARGE ON TWO DIMENSIONAL TRANSITORS FOR NEUROMORPHIC, RADIATION, AND DOPING APPLICATIONS A Dissertation in Electrical Engineering by Andrew J. Arnold © 2020 Andrew J. Arnold Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2020 The dissertation of Andrew J. Arnold was reviewed and approved by the following: Thomas Jackson Professor of Electrical Engineering Co-Chair of Committee Saptarshi Das Assistant Professor of Engineering Science and Mechanics Dissertation Advisor Co-Chair of Committee Swaroop Ghosh Assistant Professor of Electrical Engineering Rongming Chu Associate Professor of Electrical Engineering Sukwon Choi Assistant Professor of Mechanical Engineering Kultegin Aydin Professor of Electrical Engineering Head of the Department of Electrical Engineering ii Abstract The scaling of silicon field effect transistors (FETs) has progressed exponentially following Moore’s law, and is nearing fundamental limitations related to the materials and physics of the devices. Alternative materials are required to overcome these limitations leading to increasing interest in two dimensional (2D) materials, and transition metal dichalcogenides (TMDs) in particular, due to their atomically thin nature which provides an advantage in scalability. Numerous investigations within the literature have explored various applications of these materials and assessed their viability as a replacement for silicon FETs. This dissertation focuses on several applications of 2D FETs as well as an exploration into one of the most promising methods to improve their performance. Neuromorphic computing is an alternative method to standard computing architectures that operates similarly to a biological nervous system. These systems are composed of neurons and operate based on pulses called action potentials. The neurons communicate with each other through connections called synapses which release neurotransmitters in response to incoming action potentials. By exploiting hysteresis effects in MoS2 transistors, it is found that applied gate pulses can be used to directly model several key behaviors governing biological neurotransmitter release. This enables the FET to function as a synaptic device which mimics the biological behavior more completely than in typical neuromorphic devices. In particular, the gate pulse polarity, the number of pulses, and the pulse magnitude are used to mimic the bipolar, quantal, and stochastic nature of neurotransmitter release. Additionally, it is found that the long trap state decay time can be used as an analog to long-term potentiation which is a process responsible for biological learning and memory. Radiation resistance is an important factor for electronics used in certain space and nuclear applications. Numerous studies have investigated the effects of high energy radiation on 2D electronics. However, these studies typically do not account for the electrical effects of radiation damage to the gate dielectric which also contributes to the total change in device characteristics. A novel experimental setup which uses four samples and takes advantage of the unique properties of 2D materials can eliminate this iii factor which may obscure the radiation effects on the 2D material itself. The four samples are an unirradiated control sample, a sample where both the flakes and substrate are irradiated, a sample where only the substrate is irradiated, and a sample where only the flakes are irradiated. Using this experimental configuration, it is found that upon exposure to He+ ion radiation at a fluence of 1015 ions/cm2, damage to the electrical characteristics of MoS2 FETs with on a 50 nm Al2O3 gate dielectric is mostly induced by damage to the flakes themselves and that oxide damage has a minor but nonzero effect. Similar results of lesser magnitude are found for devices irradiated with protons at a fluence of 1.26 × 1016 ions/cm2. However, in both cases the devices are able to maintain high ON currents and ON/OFF ratios. The main limiting factor preventing the fabrication of high-quality contacts to 2D materials is Schottky barrier formation due to Fermi level pinning. In conventional silicon FETs, heavily doped regions under the contacts are used to reduce the contact resistance. However, substitutional doping in 2D FETs is impractical due to high-temperature requirements. A promising alternative is surface charge transfer doping (SCTD). A comprehensive experimental study supported with Sentaurus TCAD simulations is used to isolate the thickness dependence of surface doping performed through oxygen plasma exposure. It is found that the plasma produced damage for thin flakes, and that as the flake thickness is increased, the desired threshold voltage shift decreases and the minimum OFF state current increases. Sentaurus simulations show that this is caused by a failure of the doping charge to influence the 2D semiconductor near the back-gate due to the increased distance between them. It is also shown that leaving an undoped region in the channel produces the desired improved contact performance while preventing the excessive threshold voltage shifts produced by high levels of uniform doping. Additionally, Sentaurus simulations are used to extract the thickness trends of this extension doping. Finally, the combined experimental and simulation results are used to determine the material and doping charge requirements for effective use of SCTD for improved device performance. iv TABLE OF CONTENTS List of Figures ............................................................................................................................................ vii List of Tables ............................................................................................................................................ xiv Acknowledgements ................................................................................................................................... xv Chapter 1: Introduction ............................................................................................................................. 1 Introduction ........................................................................................................................................ 1 2D Materials....................................................................................................................................... 1 2D Field Effect Transistors ................................................................................................................ 2 Fermi Level Pinning .......................................................................................................................... 4 Electrical Characteristics of Schottky Barrier FETs .......................................................................... 9 Two Paths of Carrier Injection ......................................................................................................... 12 Improving the Contact Resistance of SB FETs Through Fermi Level Depinning .......................... 16 Improving the Contact Resistance of 2D FETs Through Hybridization and Phase Engineering .... 20 Dissertation Organization ................................................................................................................ 23 References ...................................................................................................................................... 24 Chapter 2: MoS2 Synaptic Device ............................................................................................................ 29 Introduction ...................................................................................................................................... 29 MoS2 FET ........................................................................................................................................ 30 Neurons and Synapses ..................................................................................................................... 33 MoS2 FET Pulse Response .............................................................................................................. 36 MoS2 FET Postsynaptic Current ...................................................................................................... 40 Long-Term Potentiation ................................................................................................................... 43 OFF State Biasing ............................................................................................................................ 44 Conclusion ....................................................................................................................................... 46 References ........................................................................................................................................ 48 Chapter 3: Radiation Resistance of MoS2 ............................................................................................... 51 Introduction ...................................................................................................................................... 51 Experimental Setup .......................................................................................................................... 53 + Dependence of Electrical Effects on He Fluence ..........................................................................

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