Strain engineering 2D MoS2 with thin film stress capping layers
T Pena and SA Chowdhury and A Azizimanesh and A Sewaket and H Askari and SM Wu, 2D MATERIALS, 8, 045001 (2021).
We demonstrate a method to induce tensile and compressive strain into two-dimensional transition metal dichalcogenide (TMDC) MoS2 via the deposition of stressed thin films to encapsulate exfoliated flakes. With this technique we can directly engineer MoS2 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 complementary metal-oxide-semiconductor (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 MoS2 strain through Raman spectroscopy. Combining thickness dependent analyses of Raman peak shifts in MoS2 with atomistic simulations, we can explore layer-by-layer strain transfer. MoS2 on conventional substrates (SiO2, 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) MoS2 on hexagonal boron nitride (h-BN). This concept frees the 1L-MoS2 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 MoS2 when thermal evaporation is used.
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