Top electrode material related bipolar memory and unipolar threshold resistance switching in amorphous Ta2O5 films

Tantalum oxide (Ta₂O₅) is one of the most studied materials for its stable resistance switching and potential application in nonvolatile memory devices. Top electrode and essential switching material are two critical points dominating its switching characteristics. Here, Ta₂O₅ films of amorphous nature (a-Ta₂O₅) with tunable thicknesses were made by changing the applied voltage during anodic oxidation of Ta-metal foils. The resistance-switching behavior of an a-Ta₂O₅ film in a metal/a-Ta₂O₅/Ta configuration was investigated by using a sputtered W or Ag metal film as the top electrode. The unipolar threshold switching phenomenon was observed using W as top electrode (W_TE), while bipolar switching behaviors were achieved using active Ag metal as top electrode (Ag_TE). The thickness of the a-Ta₂O₅ film shows an obvious effect on the SET voltage in a W_TE/a-Ta₂O₅/Ta device. The interfacial redox reaction induced formation of more conductive Ta-rich suboxide and the Joule heating effect are proposed to contribute to the unipolar threshold switching behavior. It is also suggested that the bipolar switching could have resulted from the electrochemical reaction-induced dissolution and growth of Ag conducting channels inside the Ta₂O₅ films.