Nature of Single-Localized-Electron States Derived from Tunneling Measurements

We have used the resistance fluctuations of very small-area ($0.03 \mu {\mathrm{m}}^2<A<\mathrm{}2\mathrm{}\mu {\mathrm{m}}^2$) tunnel junctions to study the population kinetics of individual localized electron traps in the tunnel-barrier material. We present evidence that the rate-limiting kinetic step for charge fluctuations above ∼ 15 K is thermal activation between two low-energy ionic configurations. Below 15 K the transitions between these potential wells appear to be dominated by direct configurational tunneling of the ions forming the trap.