Electroforming and switching in oxides of transition metals: The role of metal-insulator transition in the switching mechanism

Electroforming and switching effects in sandwich structures based on anodic films of transition metal oxides (V, Nb, Ti, Fe, Ta, W, Zr, Hf, Mo) have been studied. After being electroformed, some materials exhibited current-controlled negative resistance with S-shaped V-I characteristics. For V, Fe, Ti, and Nb oxides, the temperature dependences of the threshold voltage have been measured. As the temperature increased, V-th decreased to zero at a critical temperature T-0, which depended on the film material. Comparison of the T-0 values with the temperatures of metal-insulator phase transition for some compounds (T-t = 120 K for Fe3O4, 340 K for VO2, similar to 500 K for Ti2O3, and 1070 K for NbO2) showed that switching was related to the transition in the applied electric field. Channels consisting of the above-mentioned lower oxides were formed in the initial anodic films during the electroforming. The possibility of formation of these oxides with a metal-insulator transition was confirmed by thermodynamic calculations. (C) 1996 Academic Press, Inc.