Electronic properties and device applications of rotationally controlled van der Waals heterostructures

Kim, Kyounghwan
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Van der Waals heterostructure is a highly versatile platform to unveil two-dimensional (2D) electron physics and to explore new device functionalities. In this dissertation, we present comprehensive experimental studies of van der Waals heterostructures considering the rotational control between different 2D layers. A hemispherical handle polymer stamp is introduced to improve 2D layer transfer, which allows rotational control when stacking 2D layers. To verify the accuracy of rotational control, we demonstrate a rotationally aligned double monolayer graphene using a sequential pick-up of two graphene layers, which originate from a single crystal domain, and compare it with Bernal stacked bilayer graphene using Raman spectroscopy, scanning probe microscopy (SPM), and electrical transport measurements under an applied transverse electric-field. We also demonstrate resonant tunneling in double bilayer graphene heterostructures using hexagonal boron nitride (hBN) as an interlayer dielectric, in which rotational alignment between the two bilayer graphene is the key ingredient for the device functionality. To further highlight the significance of highly accurate rotational alignment between different layers, we demonstrate tunable moiré crystals in small-twist-angle bilayer graphene. A comprehensive electron transport study is conducted, and the data are compared with SPM results. We observe transport gaps at ±8 electrons per moiré unit cell, along with a conductivity minimum at charge neutrality with twist angle less than 1°. In magnetic fields, we also observe the emergence of a Hofstadter butterfly in the energy spectrum, with four-fold degenerate Landau levels (LLs), and broken symmetry quantum Hall states at filling factors, ν = ±1, ±2, ±3. We demonstrate dual-gated tungsten diselenide (WSe₂) interlayer tunneling devices, consisting of two WSe₂ monolayers with controlled rotational alignment, and separated by a thin hBN tunnel barrier. In samples where the two WSe₂ have a 0° relative twist the tunneling current-voltage characteristics reveal resonant tunneling, manifested by negative differential resistance, which stem from energy and momentum conservation. Because WSe₂ possesses strong spin-orbit coupling which leads to coupled spin-valley degrees of freedom, controlling the angle between the two WSe₂ monolayers allows us to probe the conservation of spin-valley degree of freedom in tunneling.