|Author:||Chan, Hung Lit|
|Title:||Vertical graphene tunnelling heterostructures with ultrathin ferroelectric film as a tunnel barrier|
|Advisors:||Hao, Jianhua (AP)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Applied Physics|
|Pages:||xvi, 102 pages : color illustrations|
|Abstract:||Two-dimensional (2D) layered materials have attracted enormous interests in both fundamental research and industrial applications. When one dimension is restricted in size, 2D materials exhibit distinct properties, which are different from their bulk materials. With its unique 2D structure, graphene, a single layer of carbon atoms in a hexagonal lattice, reveals remarkable electronic, thermal, optical and mechanical properties. Recently, graphene can be coupled with ferroelectric materials to form functional graphene/ferroelectric hybrid structure. Ferroelectric materials show a spontaneous ferroelectric polarization, which can be switched via an external electric field. The novel graphene/ferroelectric heterostructure can yield fascinating behaviors and reveal great potential for various functional devices. In this thesis, firstly, a ferroelectric tunnel junction (FTJ) based on graphene/BaTiO₃/Nb:SrTiO₃ heterostructure can be fabricated. The crystal quality and layer number of the graphene nanosheets are studied by Raman spectroscopy. The crystal structure of the BaTiO₃/Nb:SrTiO₃ is revealed by X-ray diffraction (XRD) characterization. The chemical composition of the BaTiO₃ films is investigated by X-ray photoelectron spectroscopy (XPS). The morphology and ferroelectricity of the BaTiO₃ thin films are determined with piezoresponse force microscopy (PFM). The electrical resistance switching is studied in the graphene/BaTiO₃/Nb:SrTiO₃ heterostructure. The ON/OFF conductance ratio is found to increase with decreasing Nb concentration from 1.0 wt% to 0.1 wt% on the Nb:SrTiO₃ semiconductor substrates, due to ferroelectric modulation of barrier height and width. A remarkable ON/OFF ratio up to 10³ is obtained in the devices when introducing Nb concentration of 0.1 wt% at room temperature. Furthermore, good retention property and switching reproducibility can be achieved in the devices, which are suitable for non-volatile memory applications. Secondly, vertical graphene heterostructure FET (VGHFET) employing ultrathin ferroelectric film as a tunnel barrier can be fabricated. The ferroelectric switching may add tunability to the VGHFET. The multilayer devices are based on Au/Sm:BiFeO₃/graphene/SiO₂/Si, Au/Al₂O₃/graphene/BaTiO₃/Nb:SrTiO₃ as well as Au/Al₂O₃/graphene/BaTiO₃/La₀.₇Sr₀.₃3MnO₃/SrTiO₃ heterostructure. The output and transfer electrical characteristics of the devices can be observed, which are beneficial for logic electronic applications. In conclusion, vertical graphene tunneling heterostructure with ultrathin ferroelectric barrier has been studied. The electronic properties and device demonstrations of FTJs and VGHFETs have been investigated. These fundamental studies provide a platform for further research of 2D/ferroelectric hybrid structure and show promise for future applications on the nanoscale.|
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