AVS 68 Session EM-WeA: Compound Semiconductors

Commonly studied transition metal dichalcogenide (TMD) crystals exhibit polymorphism, where the electronic structure of the material changes significantly with a change in the crystal structure. MoTe₂ has gained particular interest because the potential energy difference between the semiconducting 2H and semi-metallic 1T' phase is the lowest (40 meV) among TMD polymorphs, making it promising for low voltage phase change memory and transistors. Although the 2H phase is the most stable form, it can be transformed to 1T' by 0.3 - 3% by tensile strain thereby causing a large change in the electronic conductivity. Recent studies have experimentally shown this phase transition by mechanically stretching the entire substrate or applying local mechanical strain using atomic force microscopy (AFM) tip, but both strain methods would be difficult to implement in CMOS. What is needed is a straining mechanism driven by field-effect, where a single device can be controlled electrically by a nearby gate.

Here, we employ an ‘ionomer’ or single-ion conductor to impart strain. An ionomer contains mobile cations but has anions that are covalently bonded to a polymer backbone. Under an applied electric field, the cations accumulate at the MoTe₂ surface, effectively controlling electron transport in the material, while anions maintain their position in the polymer backbone creating a charge imbalance. The imbalance causes the ionomer to bend, which then induces strain in the MoTe₂.

In this work, the electrical and structural properties of a suspended MoTe₂ FET are measured simultaneously using a home-built set-up combining electrical measurements with Raman spectroscopy. With no gate voltage (V_G) applied, the insulating 2H phase is confirmed. For V_G > 2.5 V, the 1T’ phase is detected by a significant decrease in the electrical resistance accompanied by a characteristic 1T’ peak in the Raman spectra. Mapping the 2H and 1T’ peaks across the entire flake reveals that the phase transition is reversible (i.e., the flake reverts to the semiconducting 2H phase when the voltage is removed), which is an essential feature of memory. The output characteristic of the FET shows a large change in the I_D-V_G slope for V_G >1.5 V. Further, metallic conduction is confirmed by the positive temperature coefficient of resistance for V_G = +2 and +3 V. Lastly, time-dependent Raman spectroscopy is performed to study the phase change dynamics. The demonstrated gate-controlled reversible straining method can be easily extended to strain other types of two-dimensional materials to explore fundamental properties as well as discover new device mechanisms.