Modeling Atomic Layer Deposition of Alumina as an Ultra-thin Tunnel Barrier using Reactive Molecular Dynamics*

In this study, we have utilized the reactive molecular dynamics (MD) simulations to model the Atomic Layer Deposition (ALD) process that forms an ultra-thin film of a tunnel barrier made of amorphous alumina. The reactive MD simulation is advantageous in comparison to the ab-initio MD simulation since it offers lower computational cost and the capability to model over a relatively longer simulation period and for a larger scale. To further refine and parameterize the existing ReaxFF potentials, we used the Density Functional based Tight Binding (DFTB) -based in conjunction with the Python toolkit as implemented in the Amsterdam Modeling Suite (AMS) program package. The additional training sets were made from evaluations of the potential energy surface (PES) scans of various ALD-relevant species including H₂O, OH, and the ALD precursors trimethylaluminum (TMA) and Bis(20ethyl-1,3-cyclopentadien-1-yl) magnesium (C₁₄H₁₈Mg), as well as the bond-dissociation energies during the reactions that occur when they interact. We systematically evaluated the role of the various ALD precursors including the TMA and C₁₄H₁₈Mg and the water pulse toward the chemical reactions that take place on the surface during ALD. We additionally evaluated the role of experimentally- observable parameters including the operating temperature and precursor concentrations for the internal structure of the amorphous alumina/magnesium as the final deposition products. Lastly, we systematically assessed the role of wetting layers as the means to improve the quality and performance of the tunnel barrier.