Effects of barrier preparation on inelastic electron tunneling

Al-$\mathrm{Al}{\mathrm{O}}_x$-Pb junctions prepared for inelastic-electron-tunneling spectroscopy (IETS) have been studied as a function of OH structure associated with the $\mathrm{Al}{\mathrm{O}}_x$ barrier. The charged OH structure has been modified by cooling and roughening the substrate and by varying the exposure to ${\mathrm{H}}_2$O during fabrication. A model barrier calculation using a WKB approximation for the tunneling current and a computer fitting procedure for obtaining a fit to the experimental $I-V$ curves has allowed quantitative comparison of the barrier parameters resulting from the different preparation procedures. In all cases the $\mathrm{Al}{\mathrm{O}}_x$ barrier asymmetry is dominated by the charged OH structure and the polarizable dipole layer associated with this interface structure. Cooling the substrate or roughening the substrate during preparation enhances the OH mode structure and reduces the barrier asymmetry by several electron volts from that observed for standard room-temperature preparation of smooth junctions. Exposure of rough substrates to water vapor can completely reverse the barrier asymmetry. The contribution of the OH groups can be modeled as a dipole layer contributing a high, thin barrier near the second electrode. A two-barrier model can be used to fit the elastic-tunneling data quite well. For doped junctions used for molecular-vibrational-mode analysis an enhanced OH structure contributed by the barrier generally produces a strong reduction of vibrational-mode intensity. A sharp, well-defined dipole layer appears to enhance the IET spectrum, possibly through the presence of a high, thin barrier as well as through the detailed polarization properties of the barrier. Additional OH adsorption reduces and disorders this barrier, leading to less electron-phonon coupling in the excitation of IETS modes.