Two-Dimensional van der Waals Bilayer Heterostructure Towards Band-to-Band Tunneling and Photocatalytic Water Splitting Applications
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Authors
Ferdous, Naim
Issue Date
2024
Type
Dissertation
Language
en_US
Keywords
Alternative Title
Abstract
The potential of van der Waals (vdW) heterostructure to incorporate the outstanding features of stacked materials to meet various application requirements has drawn considerable attention. According to the band alignments, vdW heterojunctions can be divided into three types: type I (straddling), II (staggered), and III (broken-gap). In this dissertation, three novel vdW bilayer heterostructures with staggered and broken-gap band alignment are proposed based on layered gallium oxide (Ga2O3), silicon carbide (SiC), germanium carbide (GeC) and aluminum nitride (AlN) by means of first principles density functional theory. The exceptional quantum tunneling mechanisms of type-III heterostructures make them a promising design strategy for tunneling field-effect transistors. A unique Ga2O3/SiC vdW bilayer heterostructure with inherent type-III broken gap band alignment has been revealed through first-principles calculation. Due to the overlapping band structures, a tunneling window of 0.609 eV has been created, which enables the charges to tunnel from the VBM of the SiC layer to the CBM of the Ga2O3 layer and fulfills the required condition for band-to-band tunneling. External electric field and strain can be applied to tailor the electronic behavior of the bilayer heterostructure. Positive external electric field and compressive vertical strain enlarge the tunneling window and enhance the band-to-band tunneling (BTBT) scheme. Under external electric field as well as vertical and biaxial strain, the Ga2O3/SiC vdW hetero-bilayer maintains the type-III band alignment, revealing its capability to tolerate the external electric field and strain with resilience. All these results provide a compelling platform for the Ga2O3/SiC vdW bilayer to design high-performance tunneling field effect transistor.
Besides, the structural, electronic and optical properties of a novel SiC/AlN vdW bilayer heterostructure have been systematically investigated through first-principles calculations. Notably, the heterostructure presents an inherent type-II band orientation wherein the photogenic holes and electrons are spatially separated in the SiC layer and the AlN layer, respectively. Our results indicate that the SiC/AlN heterostructure occupies a suitable band-gap of 2.97 eV, straddling the kinetic overpotentials of the hydrogen and oxygen production reactions. The heterostructure has an ample absorption profile ranging from the ultraviolet to the near-infrared regime, while the absorption intensity reaches up to 2.16×105 cm-1. In addition, external strain modulates the optical absorption of the heterostructure effectively.
Moreover, this dissertation reports the intriguing potential of a novel 2D van der Walls hetero-bilayer consisting of GeC and AlN in the photocatalytic water splitting method to generate hydrogen. The GeC/AlN heterostructure has an appropriate band gap of 2.05 eV, wherein the band edges are in proper energetic positions to provoke the water redox reaction to generate hydrogen and oxygen. The type-II band alignment of the bilayer facilitates the real-space spontaneous separation of the photogenerated electrons and holes in the different layers, improving the photocatalytic activity significantly. Analysis of the electrostatic potential and the charge density difference unravels the build-up of an inherent electric field at the interface, preventing electron-hole recombination. The ample absorption spectrum of the bilayer from the ultra-violet to the near-infrared region, reaching up to 8.71×105 /cm, combined with the resiliency to the biaxial strain, points out the excellent photocatalytic performance of the bilayer heterostructure. On top of rendering useful information on the key features of the GeC/AlN hetero-bilayer, the dissertation offers informative details on the experimental design of the van der Walls bilayer heterostructure for solar-to-hydrogen conversion applications.