Transition metal dichalcogenides heterostructure-based electronic and optoelectronic devices




Mohammed, Omar Badr

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Advances in the techniques used to combine various two-dimensional (2D) materials into a single heterostructure with almost defect-free interfaces, along with the unique properties of these 2D materials, make these heterostructures appealing, both as a platform to investigate basic physics and for the potential applications they offer. In the first part of this work, we investigate the electrical properties of the rhenium disulfide (ReS₂) 2D material, which is a member of the transition metal dichacogenides (TMDs) family. ReS₂ is a direct band gap material, regardless of its thickness, with its conduction band minimum located at the Γ-point. The observation of a p-type branch in the transfer characteristics allows the fabrication of a p-n junction, which is covered in chapter two. WSe₂/hBN/ReS₂ vertical heterostructure is another p-n junction that has been investigated, where we make a diode with a rectification ratio of up to 10⁶. In chapter three, we report the fabrication and characterization of a vertical resonant interlayer tunneling field-effect transistor (ITFET) created using exfoliated, few-layer ReS₂ flakes as the electrodes, and hexagonal boron nitride (hBN) as the tunnel barrier. Due to the Γ-point conduction band minimum, ReS₂-based system offers the possibility of resonant interlayer tunneling, and associated low-voltage negative differential resistance (NDR) without a complicated rotational alignment of the electrode crystal orientations. Substantial NDR is observed, which appears consistent with in-plane crystal momentum conserving tunneling, although considerably broadened by scattering consistent with low mobility ReS₂ flakes. And in chapter four of this dissertation, we present an experimental comparison between two devices; the first is an ITFET with its two ReS₂ electrodes are misaligned, while the other has its two electrodes having a zero-degree rotational alignment. in both cases we have been able to achieve an NDR although, in the second device the NDR peak-to-valley ratio is lower due to the added top gate, and the less-perfect interfaces as a result of the etching step during fabrication, where both increase scattering. We also, investigated the effect of changing tunnel barrier thickness on the peak current, an increase of about two orders of magnitude in the peak current associated with the use of the two monolayer thinner hBN tunnel barrier in this study, i.e., about one order of magnitude per hBN monolayer, consistent with a Gr/hBN/Gr system. Finally, we used one of our NDR devices to realize an SRAM memory cell


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