Charge stability on thin insulators studied by atomic force microscopy

Charge diffusion in thin Al2O3 layers has been investigated by Atomic Force Microscopy (AFM). The layers were made by anodic oxidation of Al plates, in order to obtain plane and homogeneous amorphous oxides of known thicknesses. Under dry-nitrogen atmosphere, the charges are deposited by contact electrification: a deposit voltage is applied between the Al substrate of the layer and the metallized AFM tip brought to contact with the oxide. This process is perfectly controllable and reproducible, the quantity of charges deposited being proportional to the deposit voltage. Afterwards the tip is lifted up and scans the surface of the oxide in order to observe the diffusion of the deposited charges. Two behaviors were observed for the diffusion process depending on the thickness and on the deposit voltage. These results are interpreted by introducing an inhomogeneous trap distribution in the layer, the diffusion process being considered mainly as diffusion by hopping transport in the bulk.