Strain-Resilient Field-Effect Transistors Based on Wrinkled Graphene/MoS₂ Heterostructures

Realizing stretchable field-effect transistors (FETs) and integrated circuitry requires new materials that are simultaneously strain resilient yet have high electronic mobility. Two-dimensional (2D) materials are one of the most promising materials for flexible and stretchable FETs because of their extraordinary mechanical, electrical, and optical properties. For example, transistors from 2D heterostructures or with ALD grown gates on flexible substrates have been shown to accommodate strains of up to 1.5% and have device mobilities of 233 cm²/Vs^1,2. Meanwhile, leveraging principles of wrinkling, crumpling or kirigami have all been used to impart stretchability of well above 100% by leveraging the ultra-low bending modulus of 2D materials to minimize strain through 3D deformation³. These strategies have been used to demonstrate stretchable devices like electrodes, photodetectors, or strain sensors. The next step with enormous potential for high mobility yet stretchable integrated circuitry is to bring the 3D deformation with 2D heterostructures together to demonstrate stretchable FETs which leverage all the strengths of 2D materials.<br/>In this work, we fabricated stretchable FETs on a soft substrate where 2D materials formed all the active electronic layers. Patterned graphene forms the electrodes and local gate, while MoS2 serves as the transistor channel. ALD HfO₂ is the dielectric layer and also serves as the mechanical support. The entire structure is less than 15 nm thick, so it has a very low bending modulus, enabling easy 3D deformations that minimize strain. To achieve stretchability, we leveraged the principles of self-assembled wrinkling and kirigami. Transferring the device onto the pre-strained elastomer and releasing the strain induces wrinkling of the devices with wavelengths of 7.7 microns, smaller than the transistor channel. We patterned kirigami cuts onto the electrodes and between devices, which serve to help relieve strain and guide the wrinkling patterns. We examined the electrical properties of the FETs under compression and stretching. The initial experimental data shows that before applying strain, the FET on current was 0.14 μA, while the on/off current ratio was ~10³. The current device performance is limited by high contact resistance, which has been shown can be much lower in graphene-contacted MoS2 devices on hard substrates, and can likely be improved. As a very promising sign, the electrical performance did not change for up to 21% compression.<br/>This work provides a new strategy of strain-resilient 2D heterostructure FETs, which opens up new applications like strain-resilient integrated circuits, as well as fundamental investigations of how heterogeneous strain will affect electron transport.<br/><br/>References<br/>1. Lee, G. H. <i>et al.</i> Flexible and transparent MoS2 field-effect transistors on hexagonal boron nitride-graphene heterostructures. <i>ACS Nano</i> <b>7</b>, 7931–7936 (2013).<br/>2. Zhu, W. <i>et al.</i> Black Phosphorus Flexible Thin Film Transistors at Gighertz Frequencies. <i>Nano Lett.</i> <b>16</b>, 2301–2306 (2016).<br/>3. Zhang, Z., Tian, Z., Mei, Y. & Di, Z. Shaping and structuring 2D materials via kirigami and origami. <i>Mater. Sci. Eng. R Reports</i> <b>145</b>, 100621 (2021).