First principles non-equilibrium Green's function modeling of vacuum and oxide barrier tunneling

Kirk H Bevan, Purdue University

Abstract

Vacuum and oxide barrier electron tunneling phenomena have been studied at length for several decades. Yet with electron device barrier widths now commonly measured in atomic units, complex quantum mechanical phenomena such as wavefunction coupling, surface states, and interface bonds have begun to play a pivotal role. In this context we theoretically examine scanning tunneling microscopy (STM) and magnetic tunnel junction atomic scale devices. Our electron transport methodology couples the non-equilibrium Green’s function formalism (NEGF) self-consistently with density functional theory (DFT) to capture high and low bias electron tunneling in the absence of significant inelastic scattering. A quantitative physical model is put forward for analyzing STM measurements. The model is shown to produce quantitative agreement with STM height measurements of styrene chains adsorbed on hydrogen passivated Si(100), significantly better than the often used Tersoff-Hamann approximation. Current-voltage derived contact distance between the STM tip and substrate is shown to be a function of the applied potential profile and semiconductor band gap. Secondly, we examine the STM contact properties of a molecular resonant tunneling diode on heavily p-type doped Si(100) and show that negative differential resistance is severely limited by electron charging, quantum capacitance and contact coupling. This has important implications for prototyping nanoscale devices with STM current-voltage measurements, where the operation regime can be limited by the contact quality between the STM tip and device. Subsequently, we address the problem of transferring bulk derived atomic parameters to thin barriers and interfaces. A straight forward method for extracting semi-empirical pseudopotentials (SEPs) from real space DFT calculations is detailed. The transferability of bulk derived Fe and MgO SEPs to Fe/MgO/Fe tunnel junctions is examined in detail. It is found that though bulk parameters can be optimized to induce only small interface errors, these errors nonetheless induce unphysical interface potential scattering which adversely affects the tunneling current. However, we show that SEPs extracted directly from NEGF-DFT calculated Fe/MgO/Fe sandwich electronic structure calculations are able to accurately capture interface properties. The results underscore the need for separate device parameter sets for tunnel junction interface surface states and bulk material regions.

Degree

Ph.D.

Advisors

Datta, Purdue University.

Subject Area

Electrical engineering|Condensed matter physics

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