Elastic and inelastic electron tunneling with the use of $\mathrm{Si}{\mathrm{O}}_2+\mathrm{Al}{\mathrm{O}}_x$ and $\mathrm{Al}{\mathrm{O}}_x+\mathrm{Si}$ barriers

Elastic and inelastic electron tunneling have been studied in junctions using thermally oxidized $\mathrm{Al}{\mathrm{O}}_x$ barriers doped with Si and ${\mathrm{SiO}}_2$. The effective barrier heights are systematically lowered by the addition of Si and ${\mathrm{SiO}}_2$, and for doping above certain levels an electric-field-induced barrier shift is observed for threshold voltages in the range 0.6 to 1.0 V. Inelastic electron tunneling (IET) spectra have been recorded for the pure $\mathrm{Al}{\mathrm{O}}_x+\mathrm{Si}$ and $\mathrm{Al}{\mathrm{O}}_x+\mathrm{Si}{\mathrm{O}}_2$ barriers as well as for such barriers doped with a range of aromatic ring compounds. Strong modes associated with the formation of $\mathrm{Si}-{\mathrm{H}}_x$ bonds are observed and in the case of molecular doping, junctions fabricated with these mixed barriers show changes in the relative intensities of IET-spectroscopy modes when compared to pure $\mathrm{Al}{\mathrm{O}}_x$ barriers. For very reactive molecules with high $\pi$-resonance energy a small amount of Si (≤ 4 Å) evaporated onto the $\mathrm{Al}{\mathrm{O}}_x$ forms a barrier which allows the observation of higher-energy modes such as C—H stretching modes. These are often quenched due to low-barrier effects in IET-spectroscopy junctions using only pure $\mathrm{Al}{\mathrm{O}}_x$ barriers. At doping levels of Si and ${\mathrm{SiO}}_2$ corresponding to average evaporated thicknesses greater than ~20 Å, the junctions become extremely non-linear, and for bias voltages above 0.6 V they exhibit characteristics of high-voltage Fowler-Nordheim tunneling and internal field emission. The range of perturbations of the $\mathrm{Al}{\mathrm{O}}_x$ barrier produced by doping with Si or ${\mathrm{SiO}}_2$ suggests a strong modification of the surface OH groups. This in turn effects the molecular surface reaction and charge transfer associated with molecular doping.