Convergence Issues in Ab-Initio Transport Calculation through Oxide Barriers and Molecules

摘要

We use ab-initio simulations to calculate tunneling currents through atomic scale structures. Two issues affecting the outcome of the calculations have been investigated: (1) the description of the wavefunction tails set by the choice of basis set and (2) the interaction between the structure and the electrodes. We selected two systems for investigation: Si/SiO_2Si MOS device and Au/benzene-l,4-dithiol/Au. Transport was calculated using non-perturbative scattering theory operating on a Hamiltonian generated by the local orbital SIESTA code On issue (1) we show that the exponential leakage current decay with barrier thickness of ～1 decade/2 A measured on Si/SiO_2/Si can only be reproduced in SIESTA using at least a single-ζ plus polarization (SZP) basis set. On issue (2) we show that the number of electrode atomic layers affected by the molecular structure is large and that underestimating that number may result in a considerable overestimation of the tunneling current.