In-Plane Ferroelectric Domain Wall Memory with Embedded Electrodes on LiNbO₃ Thin Films

With the formation of mesa-like cells at their surfaces, LiNbO₃ thin films are useful for integrating high-density domain wall memory. However, the material is too hard and inert to etch the cells with inclined side edges that help to diminish polarization retention. Moreover, etching could damage the ferroelectricity of the film. To overcome these drawbacks in forming memory cells directly, we developed a technique to deposit two gapped electrodes in the film surface, without needing to etch the film. While applying an in-plane write voltage above a coercive voltage, the domain within the gap is reversibly switched along with the creation/erasure of conducting domain walls against the peripheral unswitched domain. This technique enables "on"/"off" current read of the written information. Unfortunately, the switched domain within the gap generally has poor retention and a weak wall current arises from the presence of a strong depolarization field. To overcome this problem, we fabricated a type of embedded electrode that diffuses thickness-wise into the LiNbO₃ thin film to form a parallel-plate-like structure to screen the depolarization field. The switched domains now had good retention and carry large wall currents. Alternatively, without the embedded electrodes, the switched domains within the cells can be stabilized with increasing gap distance above a critical length of 320 nm. The two methods foreshadow the possibility in the future to fabricate damage-free LiNbO₃ memory cells without etching.