Atomistic simulations of 2D materials and van der Waal’s heterostructures for beyond-Si-CMOS devices




Valsaraj, Amithraj

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The unique electrical and optical properties of two-dimensional (2D) materials has spurred intense research interest towards development of nanoelectronic devices utilizing these novel materials. The atomically thin form of 2D materials translates to excellent electrostatic gate control even at nanoscale channel length dimensions, near-ideal two-dimensional carrier behavior, and perhaps conventional and novel devices applications. Monolayer transition metal dichalcogenides (TMDs) are novel, gapped 2D materials. Toward device applications, I consider MoS₂ layers on dielectrics, in particular in this work, the effect of vacancies on the electronic structure. In density-functional-theory (DFT) simulations, the effects of near-interface oxygen vacancies in the oxide slab, and Mo or S vacancies in the MoS₂ layer are considered. Band structures and atom-projected densities of states for each system and with differing oxide terminations were calculated, as well as those for the defect-free MoS₂-dielectrics system and for isolated dielectric layers for reference. Among the results, I find that with O-vacancies, both the HfO₂-MoS₂ and the Al₂O₃-MoS₂ systems appear metallic due to doping of the oxide slab followed by electron transfer into the MoS₂, in manner analogous to modulation doping. The n-type doping of monolayer MoS₂ by high-k oxides with O-vacancies is confirmed through collaborative experimental work in which back-gated monolayer MoS₂ FETs encapsulated by oxygen deficient high-k oxides have been characterized. Van der Waal’s heterostructures allow for novel devices such as two-dimensional-to-two-dimensional tunnel devices, exemplified by interlayer tunnel FETs. These devices employ channel/tunnel-barrier/channel geometries. However, during layer-by-layer exfoliation of these multi-layer materials, rotational misalignment is the norm and may substantially affect device characteristics. In this work, by using density functional theory methods, I consider a reduction in tunneling due to weakened coupling across the rotationally misaligned interface between the channel layers and the tunnel barrier. As a prototypical system, I simulate the effects of rotational misalignment of the tunnel barrier layer between aligned channel layers in a graphene/hBN/graphene system. Rotational misalignment between the channel layers and the tunnel barrier in this van der Waal’s heterostructure can significantly reduce coupling between the channels by reducing, specifically, coupling across the interface between the channels and the tunnel barrier. This weakened coupling in graphene/hBN/graphene with hBN misalignment may be relevant to all such van der Waal’s heterostructures. TMDs are viable alternatives to graphene and hBN as channel and tunnel barrier layers, respectively, for improved performance in interlayer tunnel FET device structures. In particular, I used DFT simulations to study the bilayer-graphene/WSe₂/bilayer-graphene heterostructure as well as single and multilayer ReS₂-layer systems. Significant roadblocks to the widespread use of TMDs for nanoelectronic devices are the large contact resistance and absence of reliable doping techniques. Hence, I studied substitutional doping of, and evaluated various metal contacts to MoS₂ by computing the density of states for the systems. Metal contacts that pin the Fermi level within the desired band are optimal for device applications. My simulation results suggest that monolayer (ML) MoS₂ can be doped n-type or p-type by substituting for an S atom in the supercell with a group-17 Cl atom or a group-15 P atom, respectively. My simulations also suggest that Sc and Ti would serve as excellent contacts to n-type ML MoS₂ due to the strong bonding and large number of states near the Fermi level. But the theoretical expectations are tempered by the material characteristics, i.e., the extremely reactive nature of Sc and the oxidation prone nature of Ti atoms. I also studied commonly used Ag and Au metal contacts to ML MoS₂, which exhibited medium strength bonding to MoS₂ and an apparent pinning of the Fermi level nearer to the nominal MoS₂ conduction band edge


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