Inelastic Tunneling in Pb–Zn or ZnS–Oxide–Pb Junctions

A series of tunneling measurements using Pb–Zn or ZnS–oxide–metal thin film junctions in the normal state is reported. The samples are prepared by depositing a film of Zn or ZnS on top of the first Pb film prior to oxidation. For both dopants a metallic Zn layer of monolayer thickness is present between the Pb and the insulator. A fraction of a monolayer of Zn is sufficient to substantially enhance the structure usually attributed to electrode phonons and to eliminate that usually attributed to PbOx barrier phonons. The latter is replaced by structure which may be related to ZnO in Zn-doped junctions and ZnS in one type of ZnS-doped junctions. No barrier phonon structure whatever appears in a second type of ZnS-doped junction. The smooth background conductance is also modified by the doping. These results are discussed in terms of recent theoretical work. They suggest that the coupling to barrier and electrode phonons is at least as important in determining the tunneling characteristic as the phonon spectra themselves, and that the metal-insulator interface is critical in determining the coupling.