Electron mobility variations in surface-charged indium tin oxide thin films

A large variation in electrical resistance induced by an electrochemical surface charge is observed for the ultrathin films of indium tin oxide (ITO). A decrease and increase in resistance is noticed when the negative and positive surface charge is applied to the ITO thin films, respectively. An increase in total effect-size is obtained with a decrease in film thickness. Two contributions are considered to account for the measured effect: the variation in the carrier density and the modification of the charge carrier mobility. The first contribution, which is estimated from the Hall-effect measurements and applied surface charge density, can explain only a small fraction of the observed variation in electronic transport. The correlation of the other contribution (i.e., a variation in the electron mobility) with the film morphology and the local electronic states is examined by scanning tunneling microscopy and spectroscopy studies. Scanning tunneling spectra suggest that a local variation in charge carrier density exists on the grain surfaces and at the grain boundaries. Upon electrochemical surface charging, this local variation in density of states should result in an increase in the electronic roughness of the surface and a deeper penetration of the applied electric field at the grain boundaries. Thus, it is considered that a pronounced surface and grain boundary scattering of the conducting electrons is responsible for the large (electric) field effect observed in highly conducting oxides such as ITO.