SCANNING-TUNNELING-MICROSCOPY OBSERVATION OF VARIATIONS OF THE COULOMB STAIRCASE DUE TO CHARGE TRAPPING

We report measurements of Coulomb staircases at low temperatures (1.3-4.2 K) on gold and indium islands which are evaporated on a thin insulating ZrO2 layer on top of a gold sample. In these measurements slow variations of the residual charge Q0 took place. By measuring I-V curves at a repetition rate of about 2-3 Hz, we were able to observe these slow changes. The observed variations are explained by trapping and detrapping of electrons in localized states in the oxide. Since the changes in Q0 do not depend on the temperature in the range between 1.3 and 4.2 K, but strongly depend on the bias voltage, we conclude that the trapping and detrapping process is activated by the large electric field between tip and sample.