Strain Effects in Transition Metal Dichalcogenide Field-Effect Transistors

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Title: Strain Effects in Monolayer Transition Metal Dichalcogenide Field-Effect TransistorsStrain is a powerful variable in the design and performance of electron devices. Not only has it played a central role in boosting Si technology, but it continues to enable entirely new types of devices like straintronics and electronic skins. In comparison to bulk semiconductors, monolayer transition metal dichalcogenides (TMDs) have considerably richer relationships with strain due to their atomic thickness. This not only allows these materials to access deformations not possible with micromachined Si, but also results in strain being an unintentional artifact of many common device processing steps. This project will investigate the effects of strain on field-effect transistors (FETs) with monolayer TMD channels, which can advance the performance limits of these devices and solve major integration and reliability challenges associated with deleterious strain. Knowledge distilled from this project will support K-12 science, technology, engineering, and mathematics (STEM) programs in the region through the Caruth Institute for Engineering Education (CIEE), an educational center located at the investigator's university. This includes the development of hands-on summer campus hosted at CIEE, and the development of STEM-related lesson plans for school districts around the region that promote diversity in engineering.The goal of this project is to provide a rigorous investigation of the effect of strain on the low-field and high-field transport in monolayer TMDs and at their contacts. The proposed research has potential to be transformative in the field of electronic devices as strain can dramatically modulate the band structure, driving record piezoresistive effects in the low-field transport and tuning saturation velocity in the high-field transport. Furthermore, new quantitate knowledge on the effects of strain at contacts can advance technologies such as flexible devices, where contact failure is often described empirically. In addition to the uniform strain, the effects of short-range strain fluctuations on the transport will be characterized and fit to newly developed transport models. Devices will be fabricated on flexible substrates with varying channel lengths, which allows for a decoupling of the channel and contacts. Strain will be imparted using mechanical deformations, and characterized using Raman spectroscopy and photoluminescence. Electrical characterizations will be fit to low-field (Boltzmann Transport Equation) and high-field (multivalley Monte Carlo) transport models that account for several scattering mechanisms. This includes phonons, neutral and charge defects, and the development of some of the first models to account for short-range strain variation. All knowledge generated by this project will be disseminated through publications and inclusions in the investigator's courses to reach the broadest possible audience.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.