The Pennsylvania State University The Graduate School SYNTHESIS AND CHARACTERIZATION OF TWO-DIMENSIONAL MATERIALS FOR BEYOND COMPLEMENTARY METAL OXIDE SEMICONDUCTOR TECHNOLOGY A Dissertation in Materials Science and Engineering by Rui Zhao © 2019 Rui Zhao Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2019 ii The dissertation of Rui Zhao was reviewed and approved* by the following: Joshua A. Robinson Associate Professor of Materials Science and Engineering Dissertation Advisor Chair of Committee Mauricio Terrones Distinguished Professor of Physics, Chemistry, and Materials Science and Engineering Roman Engel-Herbert Associate Professor of Materials Science and Engineering, Chemistry and Physics Saptarshi Das Assistant Professor of Engineering, Science & Mechanics John Mauro Professor of Materials Science and Engineering Chair, Intercollege Graduate Degree Program Associate Head for Graduate Education, Materials Science and Engineering *Signatures are on file in the Graduate School iii ABSTRACT Rapid development in Semiconductor research has brought excitement and challenges. New materials have pushed both academic and industry progress forward at a fast pace. Since the discovery of one-atomic layer of graphene in 2004, tremendous efforts have been devoted to pushing its properties to meet the technology demands, establish metrics and help accomplish milestones. The family of two-dimensional materials continue to expand. Transition metal dichalcogenides (TMDs) help compensate for the zero-bandgap of graphene and greatly increase the opportunities for their applications in electronics, optoelectronics, bioelectronics and medical- related applications. To enable electronic chips with higher density and faster processing/switching speeds and address technology bottlenecks, new correlated or functionalized two-dimensional materials are being positioned under the research spotlight. As one example, 1T phase tantalum disulfide (1T- TaS2) has presented a Mott state at around 180K, at which an abrupt change in its resistivity occurs. This unique property could help achieve two-dimensional Landau switch and realize ultra-low power, collective-state devices. However, large-scale synthesis of single crystalline monolayer of two-dimensional materials is still at its infancy. Fundamental questions as to the structural-property relationships of new materials or heterostructures await to be addressed. This dissertation focuses on the synthesis and characterizations of functional two-dimensional nanomaterials for beyond complementary metal oxide semiconductor industry. This dissertation starts with introducing the state of art of semiconductor industries and the progress of two-dimensional transition metal dichalcogenides (Chapter 1). It then addresses the two-dimensional materials’ synthesis methods and related characterization techniques (Chapter 2). Precursor selections and precursor-substrate interactions are discussed in Chapter 3. Then two specific nanomaterials are specially addressed in the next three chapters. Chapter 4 includes iv dissertation research work on synthesizing and characterization of pristine 1T-TaS2 while chapter 5 focus its functionalization and address the potential application and challenges. Chapter 6 is concentrated on 2H-MoS2 and evaluate its potentials in copper diffusion barriers. Chapter 7 provides a summary for all my research work and concludes this dissertation. v TABLE OF CONTENTS LIST OF FIGURES ................................................................................................................. vii LIST OF TABLES ................................................................................................................... xviii ACKNOWLEDGEMENTS ..................................................................................................... xix Chapter 1 Introduction ............................................................................................................ 1 1-1 Overview of semiconductor industry ......................................................................... 1 1-2 Overview of two-dimensional transition metal dichalcogenides (TMDs) ................. 4 1.2.1 Structures of TMDs ......................................................................................... 5 1.2.2 Band structures of TMDs ................................................................................ 7 1.2.3 Charge density waves (CDWs) ....................................................................... 8 1-3 Overview of chemical functionalization of TMDs .................................................... 9 1.3.1 Doping of TMDs ............................................................................................. 10 1.3.2 Intercalation of TMDs ..................................................................................... 11 1.3.3 Alloying of TMDs ........................................................................................... 14 1-4 Overview of heterostructures based on TMDs .......................................................... 15 Chapter 2 Thin film characterizatons and device measurements ............................................ 18 2-1 The synthesis “Atlas” ................................................................................................ 18 2-2 Epitaxial thin films .................................................................................................... 22 2-3 The synthesis “Atlas” for two-dimensional (2D) materials ....................................... 24 2-4 Thin film characterizations ........................................................................................ 27 2-4-1 Raman spectroscopy and Photoluminescence ................................................ 27 2-4-2 Scanning electron microscopy and energy-dispersive X-ray spectroscopy .... 30 2-4-3 Scanning force microscopy ............................................................................ 32 2-4-4 X-ray photoelectron spectroscopy .................................................................. 35 2-4-5 Transmission electron microscopy ................................................................. 37 2-5 Basics of field effect transistors (FET) ...................................................................... 39 2-5-1 Field effect transistors .................................................................................... 39 2-5-2 Device fabrication ........................................................................................... 41 2-5-3 Device measurements ..................................................................................... 46 Chapter 3 Precursor and substrate interactions ....................................................................... 49 3-1 Introduction to precursor selections ........................................................................... 49 3-2 Growth assistance agents ........................................................................................... 51 3-3 The impact of chalcogenide precursors on growth substrate: a study of Si-S-Te ternary phase diagram .............................................................................................. 57 3-4 Synthesis and characterization of ultra-thin-silica (UTS) .......................................... 60 Chapter 4 1T-Tantalum Disulfide (1T-TaS2) .......................................................................... 62 vi 4-1 State of the art of 1T-TaS2 ......................................................................................... 63 4-2 Synthesis of 1T-TaS2 ................................................................................................. 68 4-3 Raman spectroscopy of 1T-TaS2 CDW structures ..................................................... 72 4-4 Substrate impacts on 1T-TaS2 phase transitions ........................................................ 74 Chapter 5 Functionalization of 1T-TaS2 ................................................................................. 81 5-1 Doping of 1T-TaS2..................................................................................................... 81 5-2 Intercalation of 1T-TaS2 ............................................................................................ 85 5-3 Heterostructure construction of 1T-TaS2 and MoS2 .................................................. 92 Chapter 6 2H-Molybdenum Disulfide .................................................................................... 99 6-1 Synthesis and characterization of 2H-MoS2 .............................................................. 99 6-2 Nb-doping of 2H-MoS2 ............................................................................................. 103 6-3 Nb-doping of MoS2 as copper diffusion barrier ........................................................ 107 Chapter 7 Dissertation summary ............................................................................................. 111 Reference ................................................................................................................................. 114 vii LIST OF FIGURES Figure 1-1: (a) The supply and demand in the semiconductor device. (b) The market continues to drive integrated system towards more diversified (More than Moore) and more minimized (More Moore). Both figures are retrieved from reference 22. ............... 2 Figure 1-2: (a) A diagram aims to address the domains and subdomains of Internet of Things (IoT).