Highlights

Ca-Doped Bismuth Ferrite for RRAM Applications

Investigators: C.-H. Yang, S.-Y. Kim, J. Seidel, R. Ramesh

Many interesting phenomena as metal-insulator transition, colossal magnetoresistance, and high-Tc superconductivity can be obtained by adding carriers to insulating parent compounds. The Ca doping effect in a multiferroic BiFeO₃ is investigated. Interestingly we could observe electric field driven phase transition from ferroelectric insulating state to a conducting state. The conductivity changes three orders of magnitude and reversibly returns to the original insulating state by opposite electric field. This phase transition occurs homogeneously with the spatial resolution of 50 nm according to conducting atomic force microscopy (c-AFM). This material can be potentially used for non-volatile resistive random access memory (RRAM).

Electric field driven phase transition between a ferroelectric insulating state and a conducting state in Ca doped BiFeO₃ (a) Using c-AFM technique, the local current under a driving tip voltage of -2 V was measured for Bi_0.8Ca_0.1FeO_3-δ film over the 5 x 5 μm² area after poling with a tip voltage of -12 V on 3 x 3 μm2 area (red square) and sequentially poling with +12 V on 1 x 1 μm² area (green square). (b) The cross-sectional currents along the blue dashed line in (a) are plotted logarithmically. The conductivity change is as large as three orders of magnitude and a reset process (conducting to insulating) was working by the positive poling. (c) In-plane PFM image is presented over the poling area in which an electric field is applied with the same style explained above. High conducting state region has lost the domain contrast showing monotonic brown color which means little piezoresponse along the sensitive axis of lateral tip. On the other hand, as-grown state region outside red square line shows clear stripe domains implying ferroelectric state similar to BiFeO₃. Inside the green box, large piezoresponse of black color and a black/brown contrast are observed proposing ferroelectric state is returned.

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