Network for Computational Nanotechnology NCN UC Berkeley Univ
Network for Computational Nanotechnology (NCN) UC Berkeley, Univ. of Illinois, Norfolk State, Northwestern, Purdue, UTEP First time user guide for RTD-NEGF Samarth Agarwal, Mathieu Luisier, Zhengping Jiang, Michael Mc. Lennan, Gerhard Klimeck Samarth Agarwal
What are RTDs? Quantum transmission for a single barrier is less than one. Quantum transmission for a double barrier (or more) is equal to one at some energies. This is due to the resonant states present in the well region. Samarth Agarwal
What are RTD’s? . . . cont’d Current This fact enables RTDs to exhibit Negative Differential Resistance. Peak in current: Emitter fermi level(Green) is aligned with a resonance(Red). NDR: Current is a decreasing function of voltage. Trough in current: Emitter fermi level not aligned with a resonance. Samarth Agarwal Voltage
Purpose of the tool RTD-NEGF: Resonant Tunneling Diode Simulator using Non-equilibrium Green’s Fn. Schrodinger Equation: Using Green’s Functions for open systems Self-consistently Open Systems: Charge can be exchanged at the boundary Samarth Agarwal Poisson equation for Open Systems
If you just hit simulate… The listed plots are generated. Samarth Agarwal
Default Outputs Conduction band: Bulk conduction band profile + electrostatic potential Normalized Current: Current as a function of energy Transmission Coefficient: Quantum transmission as a function of energy Samarth Agarwal
Geometry Two barrier transmission Triple barrier transmission Resonances in adjoining wells couple and split. (3 nm barriers and 5 nm long wells) Samarth Agarwal
Barrier Thickness Thicker barriers I-V curve: Barrier thickness reduced to 4. 8 nm I-V curve for default structure: Barrier thickness 5 nm Greater Confinement in the well. Longer lifetime of resonances Lower Current Samarth Agarwal
Potential Models Linear Drop Thomas-Fermi No self-consistency. Quantum calculation on linearly varying potentials Quantum calculation on potential determined self-consistently using semi-classical charge. Samarth Agarwal Hartree Potential determined self-consistently using quantum mechanical charge.
Potential Models with asymmetric structures Linear Drop Thomas-Fermi Hartree Default structure: Symmetric barriers More charge accumulation in asymmetric structures. Hartree gives the most accurate description. Asymmetric barriers: Width of second barrier increased to 8 nm. Samarth Agarwal
Reservoir relaxation model Exponential Decay Energy Independent Reservoir relaxation model: Treatment of optical potential below the conduction band edge. Samarth Agarwal Optical potential: Necessary to include broadening of states.
Reservoir Relaxation Models : Impact on I-Vs Valley current dominated by scattering. Optical potential determined by relaxation models, represents scattering. Energy independent model predicts higher valley current, because of higher scattering. Samarth Agarwal I-V curve: Default structure, Energy Independent relaxation model. Higher valley current. I-V curve: Default structure, Exponentially damped relaxation model. Lower valley current.
References • For the Non-equilibrium Green’s function formalism: https: //nanohub. org/topics/negf • Simulation using tight-binding and NEGF: Quantum device simulation with a generalized tunneling formula, Gerhard Klimeck, Roger Lake, R. Chris Bowen, William R. Frensley, and Ted S. Moise, Appl. Phys. Lett. 67, 2539 (1995), DOI: 10. 1063/1. 114451. • Online courses: Quantum transport: https: //nanohub. org/resources/6172, Fundamentals of Nanoelectronics: https: //nanohub. org/resources/5346 • Comparison of tight-binding and transfer-matrices: https: //nanohub. org/resources/pcpbt Samarth Agarwal
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