Flexible HfO2_2-based ferroelectric memristor

The development of the next generation of flexible electronics for biomedical applications requires the implementation of flexible active elements, potentially microcontrollers. The further step in this direction includes the development of devices for data processing directly on-chip, in particular, devices for neuromorphic computing. One of the key elements put forward within this paradigm is the memristor—the device emulating the plasticity of biological synapses. Due to the internal temporal dynamics of conductance, second-order memristors exhibit the most natural emulation of a biological synapse. Among different types of second-order memristors, ferroelectric memristors show the best cell-to-cell and cycle-to-cycle reproducibility. Here, we demonstrate a flexible ferroelectric second-order memristor on a mica substrate based on the 5-nm-thick polycrystalline Hf$_{0.5}$Zr$_{0.5}$O$_2$ film. The conductance (synaptic weight) modulation with $_{OFF}/_{ON}$ ratio ∼20 is achieved via the gradual switching of the ferroelectric domains affecting the potential barrier in the structure. The devices demonstrate high reproducibility and various synaptic functionalities, including paired-pulse potentiation and paired-pulse depression. Functional properties persist both during static bending and after more than 100 bending cycles with a radius down to 1 cm

Electronic band alignment and electron transport in Cr/BaTiO3/PtCr/BaTiO_{3}/Pt tunnel junctions

Electroresistance in ferroelectric tunnel junctions is controlled by changes in the electrostaticpotential profile across the junction upon polarization reversal of the ultrathin ferroelectricbarrier layer. Here, hard X-ray photoemission spectroscopy is used to reconstruct the electricpotential barrier profile in as-grown Cr/BaTiO$_3$(001)/Pt(001) heterostructures. Transportproperties of Cr/BaTiO$_3$/Pt junctions with a sub-lm Cr top electrode are interpreted in terms oftunneling electroresistance with resistance changes of a factor of 30 upon polarization reversal.By fitting the I-V characteristics with the model employing an experimentally determined electricpotential barrier we derive the step height changes at the BaTiO$_3$/Pt (Cr/BaTiO$_3$) interfaceþ0.42(0.03) eV following downward to upward polarization reversal

Resistive switching effect in HfxAl1−xOyHf_xAl_{1−x}O_y with a graded Al depth profile studied by hard X-ray photoelectron spectroscopy

The multilevel resistive switching effect in $Pt/Hf_xAl_{1-x}O_y$/$TiN$ stacks has been studied by hard X-ray photoelectron spectroscopy (HAXPES). Atomic layer deposition was used to grow $Hf_xAl_{1-x}O_y$ films with graded Al depth profile, where the engineered Al concentration across the film facilitates the desired oxygen vacancies' profile. The method to derive the electrical potential profile across dielectric from the HAXPES spectra is proposed. By combining the information on the chemical state at both interfaces with the extracted potential distribution, a qualitative model for the resistive switching effect in the $Pt/Hf_xAl_{1-x}O_y$/$TiN$ metal-insulator-metal stack is proposed. According to this model, the conductive filaments controlling the resistivity are formed by the electric field driven condensation of the charged oxygen vacancies in the oxide