Strain Engineering 2D MoS$_{2}$ with Thin Film Stress Capping Layers

TL;DR

This paper presents a scalable method to induce and control strain in 2D MoS₂ using thin film stress capping layers, enabling tunable bandgap modifications for device applications.

Contribution

It introduces a novel, scalable technique employing stressed thin films to engineer strain in MoS₂, including monolayers on heterostructures, with minimal defects.

Findings

Strain transfer is effective in multilayer MoS₂ but limited in monolayers on conventional substrates.

Strain of up to 0.85% induces a 75 meV bandgap change in monolayer MoS₂.

Stress transfer depends on film stress, not material, with negligible defects from thermal evaporation.

Abstract

We demonstrate a method to induce tensile and compressive strain into two-dimensional transition metal dichalcogenide (TMDC) MoS$_{2}$ via the deposition of stressed thin films to encapsulate exfoliated flakes. With this technique we can directly engineer MoS$_{2}$ strain magnitude by changing deposited thin film stress, therefore allowing variable strain to be applied on a flake-to-flake level. These thin film stressors are analogous to SiN$_{x}$ based stressors implemented in industrial CMOS processes to enhance Si mobility, suggesting that our concept is highly scalable and may be applied for large-scale integration of strain engineered TMDC devices. We choose optically transparent stressors to allow us to probe MoS$_{2}$ strain through Raman spectroscopy. Combining thickness dependent analyses of Raman peak shifts in MoS$_{2}$ with atomistic simulations, we can explore layer-by-layer strain transfer. MoS$_{2}$ on conventional substrates (SiO$_{2}$, MgO) show strain transfer into the top two layers of multilayer flakes with limited strain transfer to monolayers due to substrate adhesion. To mitigate this limitation, we also explore stressors on van der Waals heterostructures constructed of monolayer (1L) MoS$_{2}$ on hexagonal boron nitride (h-BN). This concept frees the 1L-MoS$_{2}$ allowing for a 0.85$\%$ strain to be applied to the monolayer with a corresponding strain induced bandgap change of 75 meV. By using thin films with higher stress, strain may be engineered to be even higher. Various stressors and deposition methods are considered, showing a stressor material independent transfer of strain that only depends on stressor film force with negligible defects induced into MoS$_{2}$ when thermal evaporation is used.