First-Principles Studies of Two-dimensional Semiconductor Heterostructures and Magnetic Materials
2020
Online
Hochschulschrift
Zugriff:
Semiconductor heterostructures provide an exciting setting for novel electronic and optoelectronic devices. Energy band alignment in semiconductor heterostructures is one of the most important parameters of design since it controls the electrical and optical properties. Extensive library of two-dimensional (2D) semiconductors allow us to create a wide range of heterostructures made by stacking different 2D semiconductors on top of each other, held by van der Waals forces.In this thesis we employ state-of-the-art first-principles calculations based on density functional theory (DFT) to investigate energy band alignments in two-dimensional (2D) semiconductor heterostructures. The Anderson and midgap models are often used in the study of semiconductor heterojunctions, but for van der Waals (vdW) vertical heterostructures they have shown only very limited success. Using the group-IV monochalcogenides as a prototypical system, we propose a linear response model and compare the effectiveness of these models in predicting DFT band alignments, band types and bandgaps. We show that the true band alignment can be obtained by the linear response model, which falls in between the Anderson and midgap models. Our proposed model can be characterized by an interface dipole $eV_{h} = \alpha\times(E_{m2}-E_{m1})$, where the linear response coefficient $\alpha$ = 0 and 1 corresponds to the Anderson and midgap model respectively, and $E_{m}$ is the midgap energy of the monolayer which can be viewed as an effective electronegativity. For group-IV monochalcogenides, we show that $\alpha$ = 0.34 best captures the DFT band alignment of the vdW heterostructure, and we discuss the viability of the linear response model considering other effects such as strains and band hybridization. Topological insulator has been touted as a very promising class of material for spintronics applications. A particular property of interest is its intrinsic spin-momentum locking protected by time reversal symmetry. However, in current state-of-the-art molecular beam epitaxial grown materials, various defects has been observed. Here we investigate effects of rotational defects and basal twins on electronic properties of topological insulators (TI) such as Bi$_{2}$Te$_{3}$, Bi$_{2}$Se$_{3}$, and Sb$_{2}$Te$_{3}$. We demonstrate that basal twins and defects affect the local band structure of the TI thin films. However, it’s most important feature remains relatively robust, i.e. the important 90$^{0}$ spin-momentum locking of the surface states. Finally, we study MgV$_2$O$_4$ as a promising and chemical material for spintronics applications, due to its excellent lattice matching with MgO. We show that the ferromagnetic (FM) phase of this material is half-metallic, and its half-metallicity is preserved even after interfacing with MgO, as we validate through their heterostructure calculations. Interestingly, we also show that the FM phase of spinel compound MgV$_2$O$_4$ host three-dimensional flat band (FB) right near the Fermi level, consequently yielding a large anomalous Hall effect (AHE, $\sigma \approx 670\,\Omega^{-1}\cdot cm^{-1}$). We explore experimental feasibility of stabilizing this FM phase through strain and doping engineering, and the results suggest that experimentally accessible amount of hole doping might induce a AFM-FM phase transition.
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First-Principles Studies of Two-dimensional Semiconductor Heterostructures and Magnetic Materials
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Autor/in / Beteiligte Person: | Ghasemi Azadani, Javad |
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Veröffentlichung: | 2020 |
Medientyp: | Hochschulschrift |
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