Ferroelectricity in Si-Doped Hafnia: Probing Challenges in Absence of Screening Charges.

The ability to develop ferroelectric materials using binary oxides is critical to enable novel low-power, high-density non-volatile memory and fast switching logic. The discovery of ferroelectricity in hafnia-based thin films, has focused the hopes of the community on this class of materials to overcome the existing problems of perovskite-based integrated ferroelectrics. However, both the control of ferroelectricity in doped-HfO2 and the direct characterization at the nanoscale of ferroelectric phenomena, are increasingly difficult to achieve. The main limitations are imposed by the inherent intertwining of ferroelectric and dielectric properties, the role of strain, interfaces and electric field-mediated phase, and polarization changes. In this work, using Si-doped HfO2 as a material system, we performed a correlative study with four scanning probe techniques for the local sensing of intrinsic ferroelectricity on the oxide surface. Putting each technique in perspective, we demonstrated that different origins of spatially resolved contrast can be obtained, thus highlighting possible crosstalk not originated by a genuine ferroelectric response. By leveraging the strength of each method, we showed how intrinsic processes in ultrathin dielectrics, i.e., electronic leakage, existence and generation of energy states, charge trapping (de-trapping) phenomena, and electrochemical effects, can influence the sensed response. We then proceeded to initiate hysteresis loops by means of tip-induced spectroscopic cycling (i.e., "wake-up"), thus observing the onset of oxide degradation processes associated with this step. Finally, direct piezoelectric effects were studied using the high pressure resulting from the probe's confinement, noticing the absence of a net time-invariant piezo-generated charge. Our results are critical in providing a general framework of interpretation for multiple nanoscale processes impacting ferroelectricity in doped-hafnia and strategies for sensing it.

The flexoelectric effect in Al-doped hafnium oxide.

After the successful introduction as a replacement for the SiO2 gate dielectric in metal-oxide-semiconductor field-effect transistors, HfO2 is currently one of the most studied binary oxide systems with ubiquitous applications in nanoelectronics. For years, the interest of microelectronic downscaling has focused on tuning the dielectric constant of HfO2, particularly for monoclinic and tetragonal phases. Recently, Müller et al. showed the occurrence of ferroelectricity in orthorhombic HfO2 obtained by doping with Si, Y or Al which can alter the centrosymmetric atomic structure of the elemental binary oxide. Ferroelectric HfO2 is characterized by a permanent electric dipole that can be reversed through the application of an external voltage. As all ferroelectrics, a strong coupling between the polarization and the deformation exists, a property which has allowed the development of piezoelectric sensors and actuators. However, ferroelectrics also show a coupling between the electrical polarization and the deformation gradient, defined as flexoelectricity. In essence, the free charge inside the material redistributes in response to strain gradients, inducing a net non-zero dipole moment, eventually reaching polarization reversal by the sole application of a mechanical stress. Here we show the flexoelectric effect in Al-doped hafnium oxide, using the tip of an atomic force microscope (AFM) to maximize the strain gradient at the nanometre scale. Our analysis indicates that pure mechanical force can be used for the local polarization control of sub-100 nm domains. Due to the full compatibility of HfO2 in the modern CMOS process, the discovery of flexoelectricity in hafnia paves the way for (1) nanoscopic memory bits that can be written mechanically and read electrically, (2) tip-induced reprogrammable ferroelectric-based logic and (3) electromechanical transducers.

Phase equilibria and transition mechanisms in high-pressure AgCl by ab initio methods.

The theoretical study of pressure-driven phase transformations by means of ab initio quantum mechanical methods, in the frame of the extended Landau approach, is considered. A specific application to AgCl is presented: the system shows, on increasing pressure, four polymorphs with rock salt- (Fmm), KOH- (P2(1)/m), TlI- (Cmcm), and CsCl- (Pmm) type structures. The method of constant-pressure enthalpy minimization was used for all phases, by fully relaxing the corresponding crystal structures. Periodic ab initio energy calculations were performed by the CRYSTAL03 code, employing a DFT-GGA-PBE functional with a localized basis set of Gaussian-type functions. The three phase transitions were predicted to occur at 3.5, 6.0, and 17.7 GPa, respectively, against pressures of 6.6, 10.8, and 17 GPa from literature experimental results. The rock salt- to KOH-type and KOH- to TlI-type displacive transformations show a weak first-order character. The TlI- to CsCl-type reconstructive transition is sharply first-order, and its kinetic mechanism was studied in detail on the basis of a P2(1)/m pathway, similar to that previously found for the rock salt- to CsCl-type transformation of NaCl. An activation enthalpy of 0.011 eV was found at the equilibrium pressure of 17.7 GPa.