Current-induced energy barrier suppression for electromigration from first principles

Abstract

We present an efficient method for evaluating current-induced forces in nanoscale junctions, which naturally integrates into the non-equilibrium Green's function formalism implemented within density functional theory. This allows us to perform dynamical atomic relaxation in the presence of an electric current while also evaluating the current-voltage characteristics. The central idea consists in expressing the system energy density matrix in terms of Green's functions. In order to validate our implementation we perform a series of benchmark calculations, both at zero and finite bias. Firstly we evaluate the current-induced forces acting over an Al nanowire and compare them with previously published results for fixed geometries. Then we perform structural relaxation of the same wires under bias and determine the critical voltage at which they break. We find that, while a perfectly straight wire does not break at any of the voltages considered, a zigzag wire is more fragile and snaps at 1.4 V, with the Al atoms moving against the electron flow. The critical current density for the rupture is estimated to be 9.6×10 10 A/cm 2 , in good agreement with the experimentally measured value of 5×10 10 A/cm 2 . Finally we demonstrate the capability of our scheme to tackle the electromigration problem by studying the current-induced motion of a single Si atom covalently attached to the sidewall of a (4,4) armchair single-walled carbon nanotube. Our calculations indicate that if Si is attached along the current path, then current-induced forces can induce migration. In contrast, if the bonding site is away from the current path, then the adatom will remain stable regardless of the voltage. An analysis based on decomposing the total force into a wind and an electrostatic component, as well as on a detailed evaluation of the bond currents, shows that this remarkable electromigration phenomenon is due solely to the position-dependent wind force.

Nonconservative dynamics in long atomic wires

“…Fluctuating forces are due to the corpuscular nature of electrons and are responsible for processes such as Joule heating [1][2][3]. The average force on the other hand contains, among other contributions, the familiar electron wind force [4][5][6][7][8][9]. In recent years the wind force has become the focus of renewed attention due to the realization that it is nonconservative [3,5,[10][11][12][13][14][15][16].…”

Effect of molecular conformations on the electronic transport in oxygen-substituted alkanethiol molecular junctions

“…7). [48][49][50] Different from the Au-trans-S(CH 2 CH 2 O) n CH 3 -Au junctions with n = 3, 4, and 5 and the Au-helical-S(CH 2 CH 2 O) 3 CH 3 -Au junction where the terminal methyl group dominates the electronic coupling at the right molecule-electrode interface, the oxygen atom in the CH 2 CH 2 O group adjacent to the terminal methyl group not only couples more strongly with the right gold electrode but also shortens the pathway of electrons in the Au-helical-S(CH 2 CH 2 O) n CH 3 -Au junctions with n = 4 and 5. Therefore, the conductance decay constant of 0.39/atom fitted with all of the non-hydrogen atoms in the molecular backbone is somewhat underestimated, but it only increases to 0.44/atom when the terminal methyl groups in the Au-helical-S(CH 2 CH 2 O) n CH 3 -Au junctions with n = 4 and 5 are not considered.…”

Proportion effect in diblock co-oligomer molecular diodes

“…It should be pointed out that for such polar molecules, the bias-dependent structural relaxation may be meaningful, since the molecular dipole may interact with the strong local electric field generated from the bias and modify the molecular structure asymmetrically, which can also contribute to the rectification [25,26]. The bias-dependent structural relaxation in the first-principles frame can be achieved by calculating the current-induced force [27]. Here, due to the huge calculation cost we only carried out the structural optimization at zero bias.…”