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Register a new userCharged Oxygen Vacancy Induced Ferroelectric Structure Transition in Hafnium Oxide
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The discovery of ferroelectric HfO2 in thin films and more recently in bulk is an important breakthrough because of its silicon-compatibility and unexpectedly persistent polarization at low dimensions, but the origin of its ferroelectricity is still under debate. The stabilization of the metastable polar orthorhombic phase was often considered as the cumulative result of various extrinsic factors such as stress, grain boundary, and oxygen vacancies as well as phase transition kinetics during the annealing process. We propose a novel mechanism to stabilize the polar orthorhombic phase over the nonpolar monoclinic phase that is the bulk ground state. Our first-principles calculations demonstrate that the doubly positively charged oxygen vacancy, an overlooked defect but commonly presented in binary oxides, is critical for the stabilization of ferroelectric phase. The charge state of oxygen vacancy serves as a new degree of freedom to control the thermodynamic stability of competing phases of wide-band-gap oxides.
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On highly oxygen deficient thin films of hafnium oxide (hafnia, HfO$_{2-x}$) contaminated with adsorbates of carbon oxides, the formation of hafnium carbide (HfC$_x$) at the surface during vacuum annealing at temperatures as low as 600 {deg}C is reported. Using X-ray photoelectron spectroscopy the evolution of the HfC$_x$ surface layer related to a transformation from insulating into metallic state is monitored in situ. In contrast, for fully stoichiometric HfO$_2$ thin films prepared and measured under identical conditions, the formation of HfC$_x$ was not detectable suggesting that the enhanced adsorption of carbon oxides on oxygen deficient films provides a carbon source for the carbide formation. This shows that a high concentration of oxygen vacancies in carbon contaminated hafnia lowers considerably the formation energy of hafnium carbide. Thus, the presence of a sufficient amount of residual carbon in resistive random access memory devices might lead to a similar carbide formation within the conducting filaments due to Joule heating.
The ferroelectric polarization switching in ferroelectric hafnium zirconium oxide (Hf0.5Zr0.5O2, HZO) in the HZO/Al2O3 ferroelectric/dielectric stack is investigated systematically by capacitance-voltage and polarization-voltage measurements. The thickness of dielectric layer is found to have a determinant impact on the ferroelectric polarization switching of ferroelectric HZO. A suppression of ferroelectricity is observed with thick dielectric layer. In the gate stacks with thin dielectric layers, a full polarization switching of the ferroelectric layer is found possible by the proposed leakage-current-assist mechanism through the ultrathin dielectric layer. Theoretical simulation results agree well with experimental data. This work clarifies some of the critical parts of the long-standing confusions and debating related to negative capacitance field-effect transistors (NC-FETs) concepts and experiments.
The anomalous Hall effect, a hallmark of broken time-reversal symmetry and spin-orbit coupling, is frequently observed in magnetically polarized systems. Its realization in non-magnetic systems, however, remains elusive. Here, we report on the observation of anomalous Hall effect in nominally non-magnetic KTaO3. Anomalous Hall effect emerges in reduced KTaO3 and shows an extrinsic to intrinsic crossover. A paramagnetic behavior is observed in reduced samples using first principles calculations and quantitative magnetometry. The observed anomalous Hall effect follows the oxygen vacancy-induced magnetization response, suggesting that the localized magnetic moments of the oxygen vacancies scatter conduction electrons asymmetrically and give rise to anomalous Hall effect. The anomalous Hall conductivity becomes insensitive to scattering rate in the low temperature limit (T<5 K), implying that the Berry curvature of the electrons on the Fermi surface controls the anomalous Hall effect. Our observations describe a detailed picture of many-body interactions, triggering anomalous Hall effect in a non-magnetic system.
While defects such as oxygen vacancies in correlated materials can modify their electronic properties dramatically, understanding the microscopic origin of electronic correlations in materials with defects has been elusive. Lanthanum nickelate with oxygen vacancies, LaNiO$_{3-x}$, exhibits the metal-to-insulator transition as the oxygen vacancy level $x$ increases from the stoichiometric LaNiO$_3$. In particular, LaNiO$_{2.5}$ exhibits a paramagnetic insulating phase, also stabilizing an antiferromagnetic state below $T_Nsimeq152$K. Here, we study the electronic structure and energetics of LaNiO$_{3-x}$ using first-principles. We find that LaNiO$_{2.5}$ stabilizes a vacancy-ordered structure with an insulating ground state and the nature of the insulating phase is a site-selective paramagnetic Mott state as obtained using density functional theory plus dynamical mean field theory (DFT+DMFT). The Ni octahedron site develops a Mott insulating state with strong correlations as the Ni $e_g$ orbital is half-filled while the Ni square-planar site with apical oxygen vacancies becomes a band insulator. Our oxygen vacancy results can not be explained by the pure change of the Ni oxidation state alone within the rigid band shift approximation. Our DFT+DMFT density of states explains that the peak splitting of unoccupied states in LaNiO$_{3-x}$ measured by the experimental X-ray absorption spectra originates from two nonequivalent Ni ions in the vacancy-ordered structure.
Hafnium oxide (HfO2)-based ferroelectrics offer remarkable promise for memory and logic devices in view of their compatibility with traditional silicon CMOS technology, high switchable polarization, good endurance and thickness scalability. These factors have led to steep rise in research on this class of materials over the past number of years. At the same time, only a few reports on the direct sensing of nanoscale ferroelectric properties exist, with many questions remaining regarding the emergence of ferroelectricity in these materials. While piezoresponse force microscopy (PFM) is ideally suited to probe piezo- and ferro-electricity on the nanoscale, it is known to suffer artifacts which complicate quantitative interpretation of results and can even lead to claims of ferroelectricity in materials which are not ferroelectric. In this paper we explore the possibility of using an improved PFM method based on interferometric displacement sensing (IDS) to study nanoscale ferroelectricity in bare Si doped HfO2. Our results indicate a clear difference in the local remnant state of various HfO2 crystallites with reported values for the piezoelectric coupling in range 0.6-1.5 pm/V. In addition, we report unusual ferroelectric polarization switching including possible contributions from electrostriction and Vegard effect, which may indicate oxygen vacancies or interfacial effects influence the emergence of nanoscale ferroelectricity in HfO2.
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