Electric Modulation of Conduction and Optical Characteristics in Ca-doped BiFeO3 Films for Memory Device Applications
This dissertation details the synthesis and properties of Ca-doped BiFeO3 thin films. The phase diagram was established as a function of the Ca doping concentration and temperature through structural analysis to study the effect of divalent-ion-calcium doping on BiFeO3 films. A ferroelectric-paraelectric boundary exists around at 10% Ca doping ratio. For this doping concentration, the largest transition of conduction states was observed. The mechanisms for the observed effects are discussed on the basis of the interplay of ionic and electronic conduction through the redistribution of oxygen vacancies by application of electric fields. Application of electric fields enables to control and manipulate this electronic transition to the extent that a p-n junction can be created, erased and inverted in this material. Beside this conduction transition, the visible shading effect was observed with application of electric fields. Based on these reversible transitions of the electrical and optical characteristics of Ca-doped BiFeO3, electrical and optical memory devices are proposed in this dissertation. The memory device built with a hetero-junction presented a nonvolatile conduction modulation with a rectified current behavior, which is essential for realizing random access memory devices. The switching mechanism is elucidated by junction property modification ascribed to the change of local oxygen vacancy concentration. In addition, the optical characteristics of Ca-doped BiFeO3 films for potential electro-optic memory devices were measured and evaluated. Detailed discussion about the origin of the shading effect is given based on the change of a polar order parameter and an oxygen vacancy ordered state.