Transport Properties of 2D Heterostructures Determined via First Principles Calculations
Type of DegreePhD Dissertation
Restriction TypeAuburn University Users
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Atomically thin, two-dimensional materials have demonstrated their great potentials in nanoelectronics due to their mechanical and optoelectronic properties. With atomic layers free of dangling bonds, these materials possess the unique ability to be exfoliated and reassembled forming heterostructures while preserving sharp crystalline interfaces. As each material has different electronic structure and properties, heterostructures open new paths for enhancing or creating new, tailored features that are unseen in their bulk counterparts by inducing the property of one material on to another, so as to realize emergent phases. Theoretical descriptions capturing the quantum mechanical details offer valuable information to understand the properties of these materials and control the interlayer coupling in the heterostructures they can form. For instance, the interaction of materials with their surroundings, whether by stacking order, layer orientation, charge induction, or magnetization, may induce a change in their physical properties. The enhancement, or impedance, of these proximity effects on the quantum mechanical features requires a detailed, atomistic viewpoint. In this work, we characterize the charge transport in two-dimensional materials and the heterostructures they can form using first principles calculations within the density functional theory. We elucidate the origins of carrier injection through different mechanisms for lateral junctions composed of metallic and semiconducting transition metal dichalcogenides. We rationalize the contact resistance in these systems in terms of the phase, composition and length of the channel. We find that transmission in metal-metal junctions is nearly ideal as electrode Bloch states remain delocalized through the channel, departing only due to momentum mismatch between states in the lead and channel. We find contact resistance in metal-semiconducting systems to degrade as a result of large Schottky barriers. However, near band edges, contact resistance values are about an order of magnitude lower than those obtained experimentally suggesting that doping and phase-engineering could be employed to overcome this issue. Beyond lateral heterostructures, we characterize the modulation of charge transport in vertically stacked heterostructures composed of a variety of two-dimensional materials. Through the application of external, out-of-plane strain, we estimate the piezoresistive properties for junctions based on transition metal dichalcogenides. To circumvent the computational burden of full-scale heterostructures, we develop a compact model based solely on bulk properties to predict the gauge factor in these systems. Specifically, tunneling decay rates and Schottky barrier heights play a vital role in maximizing the piezoresistive response. Finally, we characterize magnetic tunnel junctions based on two-dimensional materials. We propose a model that depends on the physical properties of bulk constituents, avoiding the computational burden associated with full quantum transport simulations. Notably, our model can handle systems with arbitrary epitaxies, which are typically challenging for first principles calculations. We investigate various configurations of heterostructures formed by different magnetic channels, with particular focus on Fe-dihalides, which we predict to exhibit tunneling magnetoresistance values of up to 10,000%. We present methods for tuning the magnetoresistance including adjusting the channel thickness and band alignments. The excellent agreement observed between our model and full quantum transport calculations in heterostructures demonstrates the promising potential for accelerated data-driven screening of two-dimensional material candidates suitable for spintronic applications and devices.