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Optical and Electrical Properties of Two-Dimensional Materials

Abstract : Two-dimensional (2D) semiconductor materials exhibit overwhelming electrical, optical, magnetic, thermal and other advantages, which enables their great potential applications in ultra-thin, transparent and highly integrated optoelectronic devices. Searching new two-dimensional materials and exploring their optimal performance, as well as expanding the practical application of two-dimensional materials have been the cores of the researches of two-dimensional materials. This thesis focuses on the vertical magnetic control of the CoFeB film on a large-area single-layer MoS₂ film, which could expand the potential of two-dimensional materials in spin optical detectors, the Polarized Photodetection (anisotropy) based on noval two-dimensional semiconductor GeAs, and the optical characterizations of group IV-VI compounds like SnS and ZnSnS alloys. This paper introduces them in detail through the following three parts: 1. We research the fabrication of the Ta/CoFeB/MgO structures with large perpendicular magnetic anisotropies (PMA) on the full coverage MoS₂ monolayers. By optimizing the thickness of the CoFeB layer and the annealing temperature, a large perpendicular interface anisotropy energy of 0.975 mJ/m² has been obtained at the CoFeB/MgO interface. By analyzing the structural and the chemical properties of the heterostructure, it is found that the insertion of MgO between the ferromagnetic metal (FM) and the 2D material can effectively block the diffusion of the FM atoms into the 2D material, and that the Ta layer plays a critical role to efficiently absorb B atoms from the CoFeB layer to establish the PMA. From the results of ab initio calculations, the MgO thickness can be tuned to modify the MoS₂ band structure, from an indirect bandgap with 7 MLs MgO layers to a direct bandgap with 3 MLs MgO layers. The proximity effect induced by Fe results in a splitting of 10 meV in the valence band at the Γ point of the 3MLs MgO structure while it is negligible for the 7MLs MgO structure. 2. we research the anisotropic optical characterization of a group IV-V compound, Germanium Arsenic (GeAs), with anisotropic monoclinic structure. The in-plane anisotropic optical nature of GeAs crystal is further investigated by the polarization-resolved absorption spectroscopy (400-2000 nm) and the polarization-sensitive photodetectors. In the visible-to-near-infrared range, the 2D GeAs nanoflakes demonstrate the distinct perpendicular optical reversal with an angle of 75~80 degrees on both of the linear dichroism and the polarization-sensitive photodetection. Obvious anisotropic features and the high dichroic ratio of Ipmax/Ipmin ~ 1.49 at 520 nm and Ipmax/Ipmin ~ 4.4 at 830 nm are measured by the polarization-sensitive photodetection. The polarization-dependent photocurrent mapping implied that the polarized photocurrent mainly occurred at the Schottky photodiodes at the electrode/GeAs interface. 3. We research optical characterizations of group-IV-VI compounds like SnS and ZnSnS alloys. SnS nanosheets exhibit carrier mobility of 37.75 cm²·V⁻¹·s⁻¹, photoresponsivity of 310.5 A/W and external quantum efficiency of 8.56×104% at 450 nm. Optical absorption around the absorption edge presents obvious polarization sensitivity with the highest optical absorption dichroic ratio of 3.06 at 862 nm. Due to the anisotropic optical absorption, the polarized photocurrent appears upon the periodic change affected by the polarized direction of the incident light at 808 nm. The ZnSnS alloys combine the advantageous optical parameters of SnS and ZnS₂, which belong to the direct band structure of n-type 2D semiconductors. The carrier mobility of the alloy is 65 cm² V⁻¹ S⁻¹ and the on/off ratio under white-LED illumination is as high as 51.
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Submitted on : Friday, November 5, 2021 - 4:08:02 PM
Last modification on : Saturday, November 6, 2021 - 4:21:19 AM


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Ziqi Zhou. Optical and Electrical Properties of Two-Dimensional Materials. Physics [physics]. Université de Lorraine; University of Chinese academy of sciences, 2021. English. ⟨NNT : 2021LORR0141⟩. ⟨tel-03417249⟩



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