Formation of nanoconstriction in thin insulating films by applying electric field.

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Metal (Al)/insulator (SiO_2 and/or MgO)/metal (Au) stacking structure with thin (5-20nm) insulating films have been fabricated. The electrically insulating state changes into conductive state by applying d.c. voltage.The threshold voltage from insulating state to conductive state (V_t) has been measured as a function of the thickness for SiO_2 (t_<SiO2>) and MgO (t_<MgO>) films, respectively. Results are as follows : for SiO2 films, V_t [V] =0.7 [V/nm] ・t_<SiO2> [nm], and for MgO films, V_t [V] =0.44 [V/nm]・t_<MgO> [nm].The Au anodic surfaces before and after applying field have been observed using scanning tunneling microscope (STM). The morphology on the Au anodic surface changed for the each process to form nanoconstrictions in the insulating films. Present experiment revealed the fact that the electrical change in the MIM structure form insulating to conducting related the formation of nanoconstriction due to the deformation of the surface of anodic material.