Resistive switching mechanism was investigated on a model nonvolatile memory device structure composed of an amorphous InGaZnO (a-IGZO) insulator layer and TiN and indium tin oxide (ITO) electrodes by directly observing the microstructural and compositional changes in the device during the switching operation. In the pristine state, microstructure of the device was characterized by well-defined smooth interface between the a-IGZO switching layer and electrodes. Once the device is in a set state by applying positive bias to the TiN electrode, the TiN/a-IGZO interface became markedly rough with irregularly localized structures protruded into the a-IGZO layer from the TiN electrode. Transmission electron microscopy revealed that these protruded structures are polycrystalline TiN which have been formed by the migration of Ti species during the voltage sweep due to the electric field between positively biased TiN and grounded ITO electrodes. The device structures showed bistable unipolar switching behavior while they were transparent to visible light with reasonable transmittance of about 77%. (C) 2012 Elsevier B.V. All rights reserved.