Conductance Changes Produced by the Controlled Addition of Foreign Atoms to the Barrier of Al-Insulator-Metal Tunnel Junctions

The conditions necessary for the occurrence of the two types of zero-bias conductance anomalies observed in certain tunnel junctions, (i) the large conductance dip ("giant resistance anomaly") and (ii) the small narrow conductance peak, were studied by introducing controlled amounts of impurities between the ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ barrier and the top electrode $M$ of $\mathrm{A}\mathrm{l}-{\mathrm{Al}}_2{O}_3-M$ tunnel junctions. The elements Ti, Cr, Co, Cu, and their oxides, as well as CaO and Ge, were used in layer thicknesses up to 30 Å. The effect of the dopant layer is dependent on the choice of Al or Ag for the metal $M$. This result is attributed to the different chemical reactivities of these metals. The small conductance peak treated by the theory of Appel-baum was found to occur only when unoxidized magnetic impurity was adjacent to the top electrode. Oxidized magnetic impurities, as well as CaO and Ge, produce a large conductance reduction. However, it was found that the reduction is very sharp for Ge and for partially reduced layers of the transition-metal oxides, but quite broad for CaO and well-oxidized transition metals. Although the conductance peak is associated with magnetic scattering, Appelbaum's theory does not account for the observed saturation of the peak at $T\sim 1$ °K as $T$ is lowered. Reduction of the conductance peak by magnetic fields up to 150 kG yielded gyromagnetic ratios $g=1.50\mathrm{}\pm 0.15$ for Cr and $g=1.25\mathrm{}\pm 0.13$ for Ti. These values imply very strong exchange coupling of the magnetic dopants to the conduction electrons, in disagreement with the values predicted from the size and the saturation temperature of the conductance peak. The sharp conductance dip may be due to the fact that the partially reduced dopant layer forms an amorphous semiconductor and that tunneling occurs to a distribution of states which rapidly increases in density away from the Fermi level. The broad conductance reduction produced by well-oxidized dopants is attributed to the addition of a potential barrier > 0.5 eV high to the ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ barrier.