Fabrication and Characterization of Monolayer MoS2 Grown by CVD

Paulo Sergio Fonseca

Bartels Group

Program of Materials Science & Engineering

Department of Chemistry

MacREU R’side ‘15

Overview

Introduction to MoS2

Goals in MoS2 Research

Accomplishment in: Material Characterization of MoS2

Fabrication of Device

Summary

Acknowledgments

http://research.chem.ucr.edu/groups/bartels

Introduction

What are Transition Metal Dichalcogenides (TMDs)? TMDs are semiconductors with a chemical equation of MX2, where M represents a transition metal such as Mo or W and X is a chalcogen such as S ,Se or Te.

Material under current research is MoS2

WHY study MoS2 ?

- First and foremost: PRICE

- MoS2 is fairly inexpensive

- It is chemically and thermally stable

- MoS2 is desirable due to its ability to grow at the monolayer limit

- Tunable band gap availability

http://www.azonano.com/images/Article_Images/ImageForArticle_3605(1).jpg

Figure 1: Schematic diagram representing three atom layer thickness of monolayer MoS2

Figure 2: Energy band diagram for MoS2

Introduction (cont.) Synthesizing monolayer MoS2 :

- Two of the most popular ways to synthesize MoS2 is by Mechanical Exfoliation and Chemical Vapor Deposition (CVD)

CVD vs Exfoliation?

- CVD is the process by which a volatile precursor is introduced to a substrate which them react to form the desired deposit, the substrate

- Exfoliation is a “scotch tape” type of method in which continuous pealing of the substrate results in the desired product

Which method is preferable when growing monolayer MoS2 ?

In short, it has been proved that CVD grown monolayer MoS2 yields a higher Photoluminescence intensity than exfoliated MoS2

Figure 2: Schematic diagram depicting the process of Chemical Vapor Deposition

Figure 1: Schematic diagram depicting the process of Mechanical Exfoliation

http://m.iopscience.iop.org/1402-4896/2012/T146/014006/article

Introduction (cont.) What is Electron Beam Lithography ?

Electron beam lithography consists of scanning a focused beam of electrons on a surface in which is covered with electron sensitive films coined as “photoresist”. The ultimate goal of EBL is to design and draw custom shapes on your substrate.

Fig 1: Experimental setup of EBL machine

https://www.engr.ucr.edu/cgi-bin/nano/res/index.cgihttp://imagebank.osa.org/getImage.xqy?img=dTcqLmxhcmdlLG9lLTIwLTE5LTIxMjc4LWcwMDU

Fig 2: Step-by-step process of electron beam lithography on a substrate

Brief Introduction to AFMAtomic Force Microscopy is a type of scanning probe microscopy capable of high resolution that works by measuring the roughness of a desired sample

AFM is highly dependent on two types of forces:Van der Waals Forces ( if sample separation from probe is 1nm)

Electrostatic Forces (very small separation between probe and sample ~ 2.5

In AFM, there exists multiple ways to define the topography of a surface including:

Contact Mode

Tapping Mode

Non-Contact Mode (Works by oscillating the cantilever tip at a frequency such that the Van der Waals forces causes a change in frequency which defines the image topography

Figure 1: Schematic diagram of an AFM cantilever tip and sample

https://www.mtholyoke.edu/~menunez/ResearchPage/AFM_mechanism.gif

GoalsGoal: My overall goal is to better understand how monolayer MoS2 behaves as a 2D semi-conductor

Objective: To do so, I propose to:

1) Grow MoS2 monolayer islands via the method of CVD

2) Characterize such islands using Photoluminescence Spectroscopy and AFM

3) Fabricate a pattern on a monolayer island using Electron Beam Lithography

4) Deposit gold and titanium on the fabricated device using Electron Beam Deposition for transport measurements

5) Investigate the electrical properties of such fabricated device by conducting transport measurements

Accomplishments1) CVD Grown Monolayer MoS2

1 m 10 m

Figure 1: 100x (left) and 10x (right) images of monolayer MoS2 film grown by CVD

Figure 2: 100x (left) and 10x (right) images of monolayer MoS2 film grown by CVD

Accomplishments2) Material Characterization of Monolayer MoS2

Photoluminescence Spectroscopy:

1 m

Fig 1: 100x image of monolayer MoS2 film and its corresponding photoluminescence spectrum

Fig 2: 100x image of monolayer MoS2 film and its corresponding photoluminescence spectrum

Accomplishments

2) Material Characterization of Monolayer MoS2

Atomic Force Microscopy:

Using the software “Gwiddyon”, the step height of each triangle was averaged out to 0.7nm

Accomplishments3) Device fabrication on Monolayer MoS2

100x 50x 10x 5x

http://www.iop.org/news/14/apr/page_62966.htmlhttp://nanotubes.rutgers.edu/img_res/1T-FET.pnghttp://nanotubes.rutgers.edu/img_cool/micro/RU_devices_M.png

5 m

10 m 100 m 200 m

Summary

To wrap things up, my project consists of:

- Growing, patterning and characterizing monolayer MoS2

- Investigating the electrical properties of monolayer MoS2 by performing transport measurements on a fabricated device

What Has Been Done? - Grew monolayer MoS2 by CVD

- Characterized monolayer MoS2 islands via AFM and Raman

- Fabricated a device using EBL

Summary (cont.)

Next on the List:

- Investigate the electrical properties of a fabricated monolayer MoS2 device conducting transport measurements

Expectations:

- To gain a better understanding of the electron mobility values in devices based on CVD- grown monolayer MoS2. Through transport

measurements, I will confirm the proposed abilities of the material and improve on device specifications.

Acknowledgements

Special Thanks to my mentors:

Edwin Preciado

Ariana Nguyen

Velveth Klee

MacREU R’side ‘15

ReferencesPlechinger, G., J. Mann, E. Preciado, D. Barroso, A. Nguyen, J. Eroms, C. Schüller, L. Bartels, and T. Korn. "A Direct Comparison of CVD-grown and Exfoliated MoS 2 Using Optical Spectroscopy." Semiconductor Science and Technology Semicond. Sci. Technol. 29.6 (2014): 064008. Web.

Bao, Wenzhong, Xinghan Cai, Dohun Kim, Karthik Sridhara, and Michael S. Fuhrer. "High Mobility Ambipolar MoS2 Field-effect Transistors: Substrate and Dielectric Effects." Appl. Phys. Lett. Applied Physics Letters 102.4 (2013): 042104. Web.

MoS2." MoS2. Rahul Raghvendra, 24 Mar. 2015. Web. 28 July 2015.