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Synapses play a vital role in information processing, learning, and memory formation in the brain. By emulating the behavior of biological synapses, electronic synaptic devices hold the promise of enabling high‐performance, energy‐efficient, and scalable neuromorphic computing. Ferroelectric memristive devices integrate the characteristics of both ferroelectric and memristive materials and present a far‐reaching potential as artificial synapses. Here, it is reported on a new ferroelectric device on silicon, a field‐effect memristor, consisting of an epitaxial ultrathin ferroelectric Hf0.5Zr0.5O2 film sandwiched between an epitaxial highly doped oxide semiconductor SrTiO3‐δ and a top metal. Upon a low voltage of less than 2 V, the field‐effect modulation in the semiconductor enables to access multiple states. The device works in a large time domain ranging from milliseconds down to tens of nanoseconds. By gradually switching the polarization by identical pulses, the ferroelectric diode devices can dynamically adjust the synaptic strength to mimic short‐ and long‐term memory plasticity. Ionic contributions due to redox processes in the oxide semiconductor beneficially influence the device operation and retention. [ABSTRACT FROM AUTHOR]

Pb(Zr,Ti)O3 (PZT) is the most common ferroelectric (FE) material widely used in solid‐state technology. Despite intense studies of PZT over decades, its intrinsic band structure, electron energy depending on 3D momentum k, is still unknown. Here, Pb(Zr0.2Ti0.8)O3 using soft‐X‐ray angle‐resolved photoelectron spectroscopy (ARPES) is explored. The enhanced photoelectron escape depth in this photon energy range allows sharp intrinsic definition of the out‐of‐plane momentum k and thereby of the full 3D band structure. Furthermore, the problem of sample charging due to the inherently insulating nature of PZT is solved by using thin‐film PZT samples, where a thickness‐induced self‐doping results in their heavy doping. For the first time, the soft‐X‐ray ARPES experiments deliver the intrinsic 3D band structure of PZT as well as the FE‐polarization dependent electrostatic potential profile across the PZT film deposited on SrTiO3 and LaxSrMn1−xO3 substrates. The negative charges near the surface, required to stabilize the FE state pointing away from the sample (P+), are identified as oxygen vacancies creating localized in‐gap states below the Fermi energy. For the opposite polarization state (P−), the positive charges near the surface are identified as cation vacancies resulting from non‐ideal stoichiometry of the PZT film as deduced from quantitative XPS measurements. [ABSTRACT FROM AUTHOR]

Thin film metal–ferroelectric–metal capacitors with an equal mixture of hafnium oxide and zirconium oxide as the ferroelectric material are fabricated using iridium oxide as the electrode material. The influence of the oxygen concentration in the electrodes during crystallization anneal on the ferroelectric properties is characterized by electrical, chemical, and structural methods. Forming gas, O2, and N2 annealing atmospheres significantly change the ferroelectric performance. The use of oxygen‐deficient electrodes improves the stabilization of the ferroelectric orthorhombic phase and reduces the wake‐up effect. It is found that oxygen‐rich electrodes supply oxygen during anneal and reduce the amount of oxygen vacancies, but the nonferroelectric monoclinic phase is stabilized with a negative impact on the ferroelectric properties. [ABSTRACT FROM AUTHOR]

The discovery of ferroelectricity in doped HfO2 represents an excellent opportunity to overcome the obstacles in manufacturing reliable ferroelectric field effect transistors (FeFET) for nonvolatile memory applications, considering that HfO2 is compatible with Si and Ge and it is already used in semiconductor industry. The presence of interface defects may have detrimental effects on the operation of FeFETs, so their role is systematically investigated in this study in correlation with the substrate doping. Metal–ferroelectric–semiconductor (MFS) structures are fabricated by depositing Hf0.5Zr0.5O2 (HZO) layers on n‐type Ge substrate. Their electric properties are compared with those of MFS structures obtained by depositing HZO on p‐type Ge, to study the influence of the doping. It is found that, although the ferroelectric properties of HZO are similar, the capacitance and impedance of the MFS structures behave differently. For n‐Ge, the occupation probability of a large number of low‐lying interface defect acceptor states, charges the interface negatively which adversely affects the C–V response of the MFS, albeit without harming the ferroelectric (P–V) hysteresis. Although the interface defects do not harm ferroelectricity, they could inhibit inversion in p‐type Ge or accumulation in n‐type Ge so they should be taken into account when designing Ge FeFET devices. [ABSTRACT FROM AUTHOR]

Development of reproducible, low‐cost fabrication technologies that are readily adaptable to large‐scale production, is one of the main challenges in the field of perovskite solar cells (PSCs). So far, for all the other layers in a solar cell, large‐area deposition methods have been adapted, except for mesoporous fabrication. Herein, the fabrication of mesoporous TiO2 scaffolds using a large‐area deposition technique, such as spray coating, is shown. Moreover, this technique induces the formation of a very specific reticulated structure with well‐delimited, oval‐shaped cavities. The cavities have irregular dimensions, with diameters in the range of 3–7 μm, ≈350 nm height, resulting in an overall increase in roughness of one order of magnitude, compared with the spin‐coated mesoporous scaffold. Using this rough structured mesoporous TiO2 in PSCs not only does not affect the efficiency of solar cells but actually improves it from an average of 10% to 12% in comparison with the devices containing a spin‐coated mesoporous scaffold. [ABSTRACT FROM AUTHOR]

The competition between interface barrier in the Schottky–Mott limit and polarization driven mechanism is established during gradual formation of metal (Au) – ferroelectric (BaTiO3) interface. X‐ray photoelectron spectroscopy provides core level energies and valence band positions in the contact region, to monitor the band alignment from the very first stages of metal deposition on BaTiO3. The band bending at metal/ferroelectric (FE) interface is extracted from the shift of core levels (Ba 3d, Ti 2p) as a function of the metal thickness. It is shown that the interface band alignment mechanism involves a well‐defined polarization orientation washing out the bending expected from the work function difference. The sudden modification of the binding energies within ferroelectric at the first 2 Å Au indicates that the ferroelectric compensation mechanism triggered by the metal overlayer initiates already at ultrathin top layer, while subsequent growth contributes only at a gradual correction of the potential in the FE. The emerging picture is confirmed in first‐principle calculation indicating the preferences of Au to grow preferentially to different terminated regions and to stabilize distinct ferroelectric polarization. [ABSTRACT FROM AUTHOR]

The competition between interface barrier in the Schottky–Mott limit and polarization driven mechanism is established during gradual formation of metal (Au) – ferroelectric (BaTiO3) interface. X‐ray photoelectron spectroscopy provides core level energies and valence band positions in the contact region, to monitor the band alignment from the very first stages of metal deposition on BaTiO3. The band bending at metal/ferroelectric (FE) interface is extracted from the shift of core levels (Ba 3d, Ti 2p) as a function of the metal thickness. It is shown that the interface band alignment mechanism involves a well‐defined polarization orientation washing out the bending expected from the work function difference. The sudden modification of the binding energies within ferroelectric at the first 2 Å Au indicates that the ferroelectric compensation mechanism triggered by the metal overlayer initiates already at ultrathin top layer, while subsequent growth contributes only at a gradual correction of the potential in the FE. The emerging picture is confirmed in first‐principle calculation indicating the preferences of Au to grow preferentially to different terminated regions and to stabilize distinct ferroelectric polarization. [ABSTRACT FROM AUTHOR]