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Two-Dimensional Molybdenum Disulfide Field-Effect Transistors and its Related Heterostructures

  • Author(s): Chai, Yu
  • Advisor(s): Ozkan, Cengiz S
  • et al.

From the standpoint of mainstream IC manufacturing, newly introduced two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), represented by molybdenum disulfide (MoS2), may turn out to be the game changer in the relay of MOSFET downscaling process. However, Schottky barrier formed at metal-semiconductor interface makes MoS2 transistors operate way below their capability. In the first part of this study, by depositing two kinds of contact metal combinations on the same MoS2 sample, the effect of Schottky and Ohmic contacts on the extrinsic field-effect mobility has been studied. The result indicates that non-optimal contacts can become the “bottleneck’ that hinders carrier transport, making transistors operate way below their intrinsic performance limit. A highly transparent M-S interface should be regarded as a prerequisite for stabilizing transistors in deep triode region and further mobility extraction. In the second part, a “passivation first, metallization second” technique is developed for fabricating edge contacts to MoS2 in two heterostructures - Al2O3/MoS2/SiO2 and h-BN/MoS2/h-BN. Electrostatic gating effect has been characterized through the configuration of back-gated FETs. A plasma etching step with volatile product, and subsequent smooth side wall profiles are found related to more efficient Ohmic-like channel conduction. This technique is applicable to both exfoliated and synthesized TMDs, and it presents a useful route for preserving the pristine quality of 2D semiconductor from material preparation to device characterization. In the end, strain is exerted to MoS2 channel by depositing a silicon nitride stress capping layer that covers the entire transistor. Current on/off ratio and other transistor performance metrics are measured as the transistor evolves from back-gate, to top-gate and finally, strain-gate configurations. A 58% increase in electron mobility and 46 % increase in on-current magnitude are observed in strain-gated, compared with top-gated transistors. This is the first study that directly links the strain effect to device performance of MoS2 top-gated transistors.

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