Dispersive charge transport along the surface of an insu - lating layer observed by Electrostatic Force Microscopy

– We report the observation in the direct space of the transport of a few thousand charges submitted to a tunable electric field along the surface of a silicon oxide layer. Charges are both deposited and observed using the same Electrostatic Force Microscope. During the time range accessible to our measurements (i.e. t = 1 ∼ 1000 s), the transport of electrons is mediated by traps in the oxide. We measure the mobility of electrons in the " surface " states of the silicon oxide layer and show the dispersive nature of their motion. It is also demonstrated that the saturation of deep oxide traps strongly enhance the transport of electrons under lateral electric field. Introduction. – Silicon dioxide is nowadays widely used as an insulating material in mi-croelectronic devices: Its homogeneity, and thus its lack of impurities, together with its wide band gap, make it a very good insulating material. However one must notice that electronic devices based on silicon dioxide are very sensitive to static discharges due to extrinsic charges. Therefore one might ask a first question of technological importance : what happens to a few charges on the surface of such a good insulating material ? Will the charges move across the insulating layer towards the conductive underlying substrate or will they move along the surface towards a polarized electrode under the influence of the induced electric field ? We show here, using an Electrostatic Force Microscope, that both kinds of behaviors may be observed and that such a charge distribution is mobile under the influence of an electric field; this latter being low enough to be induced in a normal electronic device such as a MOSFET.