Crossbar Operation of BiFeO₃/Ce-CaMnO₃ Ferroelectric Tunnel Junctions: From Materials to Integration

Abstract

Ferroelectric Tunnel Junctions (FTJs) are a candidate for the hardware realization of synapses in artificial neural networks. The fabrication process for a 784x100 crossbar array of 500 nm large FTJs, exhibiting effective On/Off currents ratio in the range 50~100, is presented. First, the epitaxial 4 nm-$BiFeO_3 /$ $Ca_{0.96}Ce_{0.04}$ $MnO_3 // YAlO_3$ is combined with Ni electrodes. The oxidation of Ni during the processing affects the polarity of the FTJ and the On/Off ratio, which becomes comparable to that of CMOS-compatible $HfZrO_4$ junctions. The latter have a wider coercive field distribution: consequently, in test crossbar arrays, $BiFeO_3$ exhibits a smaller cross-talk than $HfZrO_4$. Furthermore, the relatively larger threshold for ferroelectric switching in $BiFeO_3$ allows the use application of half-programming schemes for supervised and unsupervised learning. Second, the heterostructure is combined with W and Pt electrodes. The design is optimized for the controlled collapse chip connection to neuromorphic circuits.