Low temperature electronic transport in homogeneous and semicontinuous thin bismuth films.

Abstract

Low temperature electronic transport properties have been studied in thin bismuth films with an emphasis on investigating departures from weak disorder behavior due to a semicontinuous and inhomogeneous film structure. Homogeneous films are examined in detail to characterize the material properties. Films of thickness below $\\sim$80A are semicontinuous with percolation characteristics. By controllably oxidizing individual films in air or oxygen we have observed a crossover in the transport behavior that is associated with the formation of oxide tunnel barriers between bismuth particles. The temperature dependence of resistance crosses over from the lnT behavior typical of weakly disordered two-dimensional systems to a quasi-1D dependence, 1/$\\sqrt{T}$. The thermoelectric power, which is T-linear for low oxidation, diverges dramatically with decreasing temperature at the same level of oxidation at which the change in resistance behavior is observed. We interpret these results in the context of the physics of ultrasmall-capacitance tunnel junctions where a Coulomb blockade, associated with the single-electron charging energy $E\\sb{C}$ = $e\\sp2$/2C, suppresses electron tunneling at low voltage bias. The thermopower for a small junction including these charging energy effects is calculated in an Appendix. For metals at low temperatures the thermopower is predicted to be activated with the characteristic energy $E\\sb{C}$. We use this result to extract $E\\sb{C}$ as a function of the effective oxide barrier resistance in the films. Our data indicate that the onset of a Coulomb blockade occurs abruptly when the average barrier resistance exceeds the fundamental value $R\\sb{Q}$ $\\simeq$ $\\hbar$/$e\\sp2$.