Quench-Condensed Metallic Clusters Studied by Single Electron Tunneling

Doktorsavhandling, 2002

Using angle deposition through conventional electron-beam lithographic masks as the sample resistance was measured in situ, we defined constrictions with widths and lengths of about 5 nm in thin (1-3) nm granular films. The nominal film thickness in the constriction could be varied stepwise by in situ film depositions, changing the grain size and the configuration of the tunneling paths. As the film thickness was increased, the multigrain structure in constriction was replaced by a Single Electron Transistor (SET) with a single island dominating in the current transport. Despite the random nature of cluster formation, samples with periodic gate dependency and well-formed Coulomb diamonds were routinely obtained in every experimental run. The first implementation of this method resulted in a room-temperature operating SET. At helium temperatures this method allows to isolate a single cluster in a Quench-Condensed (QC) film and to fabricate, in a controllable way, SET transistors with charging energies up to 100 meV. The main result of this thesis is that the QC clusters are not frozen; their shape is self-adjusted to minimize the electronic energy. As a result, the cluster's shape is sensitive to the electrostatic and thermal perturbations. We observed a reversible temperature-driven distortion of QC clusters at temperatures below ~ 7 K. This effect is attributed to a phase transition in bismuth clusters of a new type, with the cluster shape as the order parameter. At zero temperature the cluster is deformed due to spontaneous distortion of the ground state, and above the transition temperature the thermal perturbation recovers the spherical symmetry. The electron energy dependence on the cluster distortion implies that not all quantum states in QC cluster are chaotic. This conclusion is further supported by the observation of resonances in tunneling spectra taken on a single bismuth cluster placed in a well-defined SET geometry.