NEMO5 and 2D Materials: Tuning Bandstructures, Wave Functions and Electrostatic Screening
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The successful isolation of graphene in 2004 opened up the exciting new research field of 2D materials. These materials host a long list of unique mechanical, electrical and chemical features that promise important device applications. Reliable performance predictions of 2D nanodevices must embrace coherent quantum mechanical effects (tunneling, confinement and interferences) atomistic effects (corrugation, subatomic confinement) and incoherent effects (phonon scattering and device imperfections). Subatomic resolution is needed, but techniques must be efficient enough to model real-size devices. Recently, the multipurpose simulation tool NEMO5 was augmented with the maximally localized Wannier function (MLWF) representation. This representation offers a good balance between numerical efficiency and subatomic resolution. MLWF parameterizations are highly transferable and free from ambiguities that have plagued empirical tight binding models.
In this talk, I will briefly discuss the MLWF approach and compare it to DFT and atomistic tight binding. Initial results using the MLWF approach for 2D material based devices will be discussed and compared to experiments. These results unveil systematic band structure changes as functions of the layer thickness and the applied gate potential. The electrostatic response depends on the location of the band edges in the Brillouin zone, their degeneracy and associated wavefunctions. All these properties turn out to be tunable. Scattering rates, mobilities and density of states are tightly bound to such band structure details as well. Even the bandgap is a function of the layer thickness and the applied electric field. Fitting NEMO5’s gate control of bandgaps to experimental data allows us to deduce the layer thickness dependence of the dielectric constant in the 2D materials. The enhancements discussed in this talk provide NEMO5 with the new capabilities needed to play an important role in the exploration of novel 2D devices.
Tillmann Kubis graduated to PhD at the Technical University Munich (Germany) in theoretical semiconductor physics in 2009. He is currently a Research Assistant Professor in the Network for Computational Nanotechnology at Purdue University. His work includes development and implementation of new algorithms for general quantum transport within the nonequilibrium Green’s function method. His algorithms are published in the academic open source semiconductor nanodevice modeling tool NEMO5. This code is used among many academic and industrial groups including Intel, Samsung, Lumileds, and TSMC. His research currently addresses electron and phonon transport, transport-ready Hamiltonian extraction from density functional theory methods, spin transport with topological insulators and design optimizations of terahertz quantum cascade lasers and nitride based light emitting diodes.
Nano-Engineered Electronic Device Simulation Node (NEEDS), Network for Computational Nanotechnology, Birck Nanotechnology Center
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Room 2001, Birck Nanotechnology Center, Purdue University, West Lafayette, IN