UCLA

The operation of ferroelectric memory (FeRAM) is mediated by microscopic ferroelectric domains. Nonidealities such as material inhomogeneities, charge injection, and oxygen vacancy motion affect the switching characteristics of these microscopic domains in ways that are not well understood. Better imaging techniques would reveal the underlying physics and aid the development of future FeRAM. We apply scanning transmission electron microscope(STEM) electron beam-induced current (EBIC) imaging to hafnium zirconium oxide(Hf0.5Zr0.5O2, HZO) ferroelectric capacitors. We measure the capacitors’ switching characteristics in situ using the nano-positive-up negative-down (nano-PUND) method. STEM EBIC maps the depolarization field in the capacitor with strong contrast that is easily calibrated in MV/cm. Coercive-field mapping reveals that individual domains prefer either the P↑ or the P↓ state. This preference results from local variations in the remanent background electric fields and manifests as a spread in the polarization switching current peaks. When the sample is heated beyond the Curie temperature and cooled back to room temperature,the contrast-generating depolarization field in the domains vanishes, suggesting the disappearance of the ferroelectric orthorhombic phase. Subsequent poling on the capacitor yields a significantly reduced polarization compared to the state before heating. However, performing the same operation with simultaneous heating and biasing induces an overall remanent background electric field that recovers a larger fraction of the polarization, bringing it closer to its state before heating. With its ability to map internal fields and coercive fields at high spatial resolution, STEM EBIC imaging is a revolutionary new tool for characterizing ferroelectric materials and devices.