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.