2D Semiconductor Materials and Devices

Dongzhi Chi, K.E.Johnson Goh, Andrew T.S Wee

• Publisher: Elsevier
• Year: 2019
• Print ISBN: 9780128161876
• eBook ISBN: 9780128165898
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

2D Semiconductor Materials and Devices reviews the basic science and state-of-art technology of 2D semiconductor materials and devices. Chapters discuss the basic structure and properties of 2D semiconductor materials, including both elemental (silicene, phosphorene) and compound semiconductors (transition metal dichalcogenide), the current growth and characterization methods of these 2D materials, state-of-the-art devices, and current and potential applications.- Reviews a broad range of emerging 2D electronic materials beyond graphene, including silicene, phosphorene and compound semiconductors- Provides an in-depth review of material properties, growth and characterization aspects—topics that could enable applications- Features contributions from the leading experts in the field

Chapter 1

Two-dimensional semiconductor transition metal dichalcogenides: basic properties

Aleksandr Rodin^1,2, ¹Centre for Advanced 2D Materials, National University of Singapore, Singapore, ²Yale-NUS College, Singapore

Abstract

The purpose of this chapter is to introduce transition metal dichalcogenides (TMDCs). Starting with the lattice structure, we explore the consequences of the material symmetries by drawing on similarities between these materials and graphene. We then proceed to the band structure and its dependence on the material thickness. Again, using graphene as a familiar example, we highlight the differences between TMDC and graphene bands. On the basis of this, we provide a generic Hamiltonian that describes TMDCs. From this, we move to the spin–orbit coupling and discuss its consequences for these materials (e.g., valley selection, spin lifetime). Finally, we address excitonic and plasmonic effects. Where applicable, we provide a connection between theory and experiments to give a more complete picture.

Keywords

TMDC properties; TMDC physics; introduction to TMDCs; basic TMDCs; TMDC semiconductors

1.1 Introduction

When describing the concept of two-dimensional (2D) materials to a layperson, the phrase “one atom thick” is frequently used. A particularly inquisitive interlocutor then learns about the Nobel-winning work on graphene [1] and how her pencil is, in fact, a mine of this precious 2D substance. This is, typically, as far as the conversations with dilettante material scientists go, with them believing that graphene is the 2D material.

While graphene is, without a doubt, a revolutionary member of the 2D family, a recent theoretical work [2] has identified over 1800 compounds that are “easily or potentially exfoliable.” Even though these materials vary dramatically in their properties, they collectively reveal that the mono-species singe-atom-thick graphene is an exception rather than the rule. Instead, most of the predicted and observed 2D substances are composed of multiple elements and, at their thinnest, form layers that are several atoms thick.

The concept of a multiatom-thick “layer” might appear artificial. After all, it is possible to make very thin films using a variety of materials. When doing so, however, one will invariably have to cut some interatomic bonds at the surface of the sample. This gives rise to dangling bonds that can react with the environment. For layered materials, this is not so. Within individual layers the constituent atoms are covalently bonded. Stacking these laminae give rise to the bulk form of the material which is held together by the relatively weak interlayer van der Waals forces. Because the bonds involving electron sharing are contained within the layers, cutting the material parallel to their plane does not result in dangling bonds. In addition, the disparity between the inter- and intralayer bonding strengths makes possible the removal of individual layers from the bulk, while retaining their structural integrity.

The fact that most of the predicted and observed layers possess a well-defined multiatomic thickness does not preclude them from being 2D. These finite-thickness crystal structures exhibit periodicity only in their plane, meaning that the crystal momentum for the Bloch waves can only be defined in two dimensions. Thus the dimensionality of the system is determined not by their physical dimensions but also by the dimensionality of the crystal momentum. In a similar fashion, nanotubes are 1D despite being “tubes,” and atoms are 0D even though electrons move around the nucleus in all three dimensions.

The transition between 2D monolayers and 3D bulk is not abrupt. To appreciate this, consider a chain of single-state quantum dots with nearest neighbor hopping. A short two-dot dimer with two split states is closer in nature to a single dot than to an infinitely long chain with a well-formed Bloch band. In the same way, stacking multiple layers on top of each other does not immediately invalidate the 2D view of the system.

Despite the fact that out of the hundreds of predicted 2D compounds only a small fraction have been realized experimentally so far, these observed materials exhibit a wide variety of behaviors and contain metals, semimetals, semiconductors, and insulators. One group of the layered structures that occupies an important position in the library of observed 2D systems is the transition metal dichalcogenides (TMDCs). TMDCs have been known for decades, and a paper in 1969 [3] discussed the structural and electronic properties of their bulk forms. At the same time, this family of materials was largely ignored until after the isolation of graphene when the layered structure of TMDCs became of interest to the scientific community.

As can be deduced from their name, the general chemical formula for TMDCs is MX₂, where M is a transition metal and X is a chalcogen (sulfur, selenium, or tellurium). Due to a large number of ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of contributors
• Preface
• Introduction
• Chapter 1. Two-dimensional semiconductor transition metal dichalcogenides: basic properties
• Chapter 2. Novel phenomena in two-dimensional semiconductors
• Chapter 3. Chemical vapor deposition growth of 2D semiconductors
• Chapter 4. Molecular beam epitaxy fabrication of two-dimensional materials
• Chapter 5. Optical characterization of two-dimensional semiconductors
• Chapter 6. Atomic structure of defects in transitional metal dichalcogenides using transmission electron microscopy
• Chapter 7. STM/STS and ARPES characterization—structure and electronic properties
• Chapter 8. Reducing the dimensionality of novel materials: one-dimensional silicon nanoribbons
• Chapter 9. Recent advances in black phosphorus and transition metal dichalcogenide–based electronic and optoelectronics devices
• Index