Contact Engineering in 2D Material Heterostructures
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Date
2018
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Doctoral Thesis
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yes
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Abstract
Two-dimensional (2D) materials like graphene, MoS2 and hexagonal boron nitride (hBN) are building blocks for novel electronic and photonic device applications beyond the operating limits of traditional bulk semiconductor devices. Stacking multiple 2D materials into vertical heterostructures opens up further degrees of freedom for engineering device functionality and facilitates the emergence of new physical phenomena. However, for building high-quality heterostructures, preserving the surface cleanliness of 2D materials poses a major challenge. In this thesis, we show that substantial residues and compressive strain can be present in 2D materials transferred from PDMS, a widely used polymer for heterostructure assembly. We demonstrate that these residues can be minimized by cleaning the PDMS surface in UV-ozone prior to exfoliation. Accumulated strain, interfacial bubbles and wrinkles can be efficiently removed by vacuum annealing after transfer, thereby restoring the surface morphology of transferred flakes to their native state. Furthermore, integration of Ohmic contacts into heterostructures is essential for obtaining high performance devices. However, no scalable methodology for gaining electrical access to monolayer MoS2 buried within heterostructures currently exists. We overcome this challenge by fabricating reliable one-dimensional edge contacts to hBN encapsulated monolayer MoS2, and for the first time, achieve performance metrics similar to conventional top contacts. Quantum transport simulations reveal that edge contacts exhibit a higher carrier transmission probability in comparison with top contacts, independent of contact length. Thus, edge contacts open the door to further shrinkage of the device footprint attainable with 2D semiconductors and mark an important step towards practical realization of encapsulated devices with macroscopically homogeneous electrical and optical characteristics.
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published
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ETH Zurich
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Subject
2D materials; van der Waals heterostructures; MoS2; Electrical contacts; Field effect transistors
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03944 - Novotny, Lukas / Novotny, Lukas