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Charge and Spin Transport in Graphene Heterostructures and Cr2Ge2Te6

  • Author(s): Lin, Zhisheng
  • Advisor(s): Shi, Jing
  • et al.

This dissertation summarizes my past work on hydrogenation of graphene, development of Al2O3 tunnel barrier, pressure effects on Cr2Ge2Te6 (CGT), and proximity effects in graphene/WSe2 heterostructures. There are three main parts. The first part is the study of hydrogenation of graphene and the development of atomically smooth Al2O3 tunnel barrier. Graphene device is coated with hydrogen silsesquioxane (HSQ) resist and exposed with electron beam. Graphene is hydrogenated by forming the covalently bonds to the hydrogen atoms and transforms from sp2 to sp3 bonds. By breaking the inversion symmetry perpendicular to the graphene plane, Rashba spin orbit coupling (SOC) is enhanced. We characterized the hydrogenation effects by performing Raman spectroscopy measurements. A clear D and D’ peak grow abruptly with small hydrogenation dosage and keep growing with higher dosage, indicating the increased defects density in graphene. Electrical transport properties are characterized by measuring the gate voltage dependence at different hydrogenation percentages: mobility decreases and graphene becomes more electron-doped upon hydrogenation. The hydrogenation process is reversible, which means the hydrogenation effects are almost gone after annealing. Nonlocal resistivity is 1 to 3 orders of magnitude larger than that of the pristine graphene, which cannot be accounted for by the ohmic contribution assuming uniform graphene channel. The problems of nonlocal measurement method are discussed. The rest of first part is focused on developing the atomically smooth Al2O3 tunnel barrier by sputtering a thin layer of aluminum first and oxidizing it in O2 atmosphere.

The second part is about the pressure effects on the magnetic anisotropy of CGT. Magnetoresistance of CGT bulk crystal is measured under applied hydrostatic pressures up to 2 GPa. Upon the application of hydrostatic pressure, we observe an induced transition of easy axis from c axis to the ab plane of the crystal. Furthermore, we observe a reduction of the band gap of CGT by approximately 0.066 eV once the applied pressure reaches 2 GPa. We verify that the magnetoresistance (MR) change originates from anisotropic magnetoresistance (AMR) by measuring the temperature dependence of MR below and above Curie temperature (TC) under the different applied pressures.

The last part is focused on the proximity effects in bilayer graphene/WSe2 heterostructures and the pressure induced insulating behavior. The enhancement of spin- orbit coupling is verified by observing the weak-anti localization for the graphene region covered with WSe2 but the weak localization (WL) for the uncovered region. The Rashba SOC strength value extracted from the weak-antilocalization (WAL) fitting is about 1 meV while the intrinsic SOC in graphene is about tens of µeV. It increases by more than two orders of magnitude. Graphene covered with WSe2 shows a strong insulating behavior by applying pressure and the insulating behavior is stronger under higher pressure, which is a signature of a band gap opening. Two hypotheses to explain the insulating behavior are discussed. One is the strain induced work function difference in top layer and bottom layer; this difference causes the charge transfer and builds up an electric field, generating the band gap. The other hypothesis is the proximity induced intrinsic electric field built across the bilayer graphene/WSe2 heterostructure, resulting in the breaking of inversion symmetry.

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