Barrier heights and electric-field-induced barrier shifts in doped tunnel junctions

$\mathrm{A}\mathrm{l}-\mathrm{Al}{\mathrm{O}}_x-M$ tunnel junctions with Pb and Ag electrodes have been doped with a series of aromatic compounds and with C and Si. An electric-field-induced shift in the $I-\mathrm{v}\mathrm{s}-V$ curves is observed for application of bias voltages above a specific threshold value. This field-induced shift is proportional to the maximum voltage applied and has been analyzed in terms of an effective lowering of the barrier height. The barrier shift is stable indefinitely at 4.2 K but can be completely reversed by a short anneal at 300 K. For the aromatic compounds the magnitude of the barrier shift and the threshold voltage are systematically correlated with the $\pi$-delocalization and resonance energy. Molecules having the largest calculated resonance energy and reactivity indices induce the largest electric field effects. The initial effective barrier height of the doped junction is also correlated with the resonance energy of the molecule. Substitution of Ag for Pb as the cover electrode systematically lowers both the effective barrier height and the threshold voltage while enhancing the electric-field-induced barrier shift. Doping $\mathrm{Al}{\mathrm{O}}_x$ with Si or C produces a strong reduction in effective barrier height, and large electric-field-induced shifts can be observed for both Pb and Ag electrodes. These large changes in electron affinity at the barrier-electrode interface can be associated with hydrogen bonding which involves the charged OH groups on the $\mathrm{Al}{\mathrm{O}}_x$ surface. Analysis of model barriers and the effects on inelastic tunneling spectroscopy are discussed.