Direct X-Ray Measurements of Strain in Monolayer MoS$_{2}$ from Capping Layers and Geometrical Features

TL;DR

This study uses direct x-ray diffraction to measure how capping layers induce significant tensile strain in monolayer MoS$_{2}$, revealing the influence of interfacial adhesion and patterning on strain levels.

Contribution

It provides the first direct x-ray measurements of strain in monolayer MoS$_{2}$ caused by various capping layers and geometrical features, quantifying strain up to 2%.

Findings

Strain is mainly controlled by interfacial adhesion.

Patterned structures exhibit enhanced strain.

Tensile strain up to 2% observed in MoS$_{2}$.

Abstract

Strain induced through fabrication, both by patterning and capping, can be used to change the properties of two-dimensional (2D) materials or other thin films. Here, we explore how capping layers impart strain to monolayer MoS$_{2}$ using direct x-ray diffraction measurements of the lattice. We first observe the impact of naturally-oxidized metal layers ($\sim$1.5 nm Al) and subsequently-deposited Al$_{2}$O$_{3}$ (15 nm to 25 nm thick) on the 2D material, and find that the strain imparted to MoS$_{2}$ is mainly controlled by the interfacial adhesion of the seed layer in addition to the substrate adhesion. Then, using test structures which mimic transistor contacts, we measure enhanced strain from such patterns compared to blanket films. Furthermore, we observe significant tensile strain - up to 2% in monolayer MoS$_{2}$, one of the largest experimental values to date on a rigid substrate - due to highly-stressed blanket metal capping layers. These results provide direct evidence supporting previous reports of strain effects in 2D material devices.