INTERPRETATION OF ATOMIC-FORCE MICROSCOPE (AFM) SIGNALS FROM SURFACE-CHARGE ON INSULATORS

Charge stored on the surface of an insulator at the sub-micron length scale can be observed with an atomic force microscope (AFM). The dynam ics of the charge spreading due to three mechanisms are calculated: (a ) surface conduction; (b) surface charge spreading due to self-repulsi on; and (c) bulk space charge motion, which is necessarily coupled to a surface charge. Self-similar motion occurs in all three cases. The A FM signals decay with time, and the shapes of the decay curves are sim ilar. We show that the surface-charge-perturbed cases can be identifie d because the time constant depends on the magnitude of the initial ch arge, but that separating the pure surface spreading from the bulk pro cess may prove difficult.