20 research outputs found
Ferroelectric domain nucleation and switching pathways in hafnium oxide
Nanoscale ferroelectrics that can be integrated into microelectronic
fabrication processes are highly desirable for low-power computing and
non-volatile memory devices. However, scalable novel ferroelectric materials,
such as hafnium oxide (HfO2), remain in a state of development, and a clear
understanding of the effects of relevant compositional and processing
parameters to control their ferroelectric properties and the actual
polarization switching mechanisms are still under investigation. One key
fundamental knowledge gap is the polarization switching pathway in
ferroelectric hafnia. To further our fundamental understanding of domain
nucleation and switching, we have studied polarization switching pathways in
HfO2-x thin films in real-time at the atomic scale using transmission electron
microscopy. We employed differential phase contrast imaging that allows for the
acquisition of both hafnium and oxygen atomic column signals and facilitates
the observation of relative movement of atomic columns between both
sublattices. Our results demonstrate that the switching pathway involves a
transient tetragonal-like local structure, as oxygen ions shift in locations
and remain within their parent hafnium polyhedra
Recommended from our members
Thermal conductivity of nano-grained SrTiO3 thin films
We measure the thermal conductivities of nano-grained strontium titanate (ng-SrTiOâ) films deposited on sapphire substrates via time-domain thermoreflectance. The 170ânm thick oxide films of varying grain-size were prepared from a chemical solution deposition process. We find that the thermal conductivity of ng-SrTiOâ decreases with decreasing average grain size and attribute this to increased phonon scattering at grain boundaries. Our data are well described by a model that accounts for the spectral nature of anharmonic Umklapp scattering along with grain boundary scattering and scattering due to the film thickness.Keywords: thin films,
time-domain reflectometry,
grain boundaries,
grain size,
phonon-phonon interactions,
strontium compounds,
thermoreflectance,
anharmonic lattice modes
nanostructured materials
electron-phonon interactions
liquid phase deposition
nanofabrication
thermal conductivit
Asymmetric Electrode Work Function Customization via Top Electrode Replacement in Ferroelectric and FieldâInduced Ferroelectric Hafnium Zirconium Oxide Thin Films
Abstract Nonâvolatile memory device structures such as ferroelectric randomâaccess memory and ferroelectric tunnel junctions employ switchable spontaneous polarization to hold binary states. These devices can potentially benefit from the imposition of spontaneous internal biases and their resulting effect on the polarization properties of the ferroelectric (or fieldâinduced ferroelectric/antiferroelectric) layer. While HfO2âbased thin films are ideal candidates for implementation into these devices due to their scalability and silicon compatibility, the phase purity of these oxides is sensitive to the selection of electrode material, preventing incorporation of asymmetric electrode layers into such structures. Within this work, electrode replacement following postâmetallization anneal processing is introduced as a route to achieve ferroelectric and fieldâinduced ferroelectric HfxZr1âxO2 (HZO) thin films with electrodeâindependent phase constitutions. The effects of this process and the corresponding internal biases imposed across the HZO layers due to asymmetric work functions are investigated. It is shown that internal biases vary in magnitude in accordance with prediction based on the work functions of the replaced electrode layers and affect remanent polarization magnitudes. Accordingly, electrode replacement presents a processing route that can readily produce HZO films with spontaneous internal biases and electrodeâindependent phase constitutions, facilitating implementation of these ferroelectrics into the next generation device structures
Step-free GaN surfaces grown by confined-area metal-organic vapor phase epitaxy
A two-step homoepitaxial growth process producing step-free surfaces on low dislocation density, Ga-polar ammonothermal GaN single crystals is described. Growth is conducted under very low supersaturation conditions where adatom incorporation occurs predominantly at step edges, and lateral growth is strongly preferred. The achievable step-free area is limited by the substrate dislocation density. For ammonothermal crystals with an average dislocation density of âŒ1 Ă 104 cmâ2, step-free mesas up to 200 Ă 200 ÎŒm2 in size are achieved. These remarkable surfaces create a unique opportunity to study the effect of steps on the properties and performance of semiconductor heterostructures
Neutron Irradiation Effects on Domain Wall Mobility and Reversibility in Lead Zirconate Titanate Thin Films
The effects of neutron-induced damage on the ferroelectric properties of thin film lead zirconate titanate (PZT) were investigated. Two sets of PbZr0.52Ti0.48O3 films of varying initial quality were irradiated in a research nuclear reactor up to a maximum 1 MeV equivalent neutron fluence of (5.16± 0.03) x 1015 cm-2. Changes in domain wall mobility and reversibility were characterized by polarization-electric field measurements, Rayleigh analysis, and analysis of first order reversal curves (FORC). With increasing fluence, extrinsic contributions to the small-signal permittivity diminished. Additionally, redistribution of irreversible hysterons towards higher coercive fields was observed accompanied by the formation of a secondary hysteron peak following exposure to high fluence levels. The changes are attributed to the radiation-induced formation of defect dipoles and other charged defects, which serve as effective domain wall pinning sites. Differences in damage accumulation rates with initial film quality were observed between the film sets suggesting a dominance of pre-irradiation microstructure on changes in macroscopic switching behavior
Recommended from our members
Chemically Homogeneous Complex Oxide Thin Films Via Improved Substrate Metallization
A long-standing challenge to the widespread application of complex oxide thin films is the stable and robust integration of noble metal electrodes, such as platinum, which remains the optimal choice for numerous applications. By considering both work of adhesion and stability against chemical diffusion, it is demonstrated that the use of an improved adhesion layer (namely, ZnO) between the silicon substrate and platinum bottom electrode enables dramatic improvements in the properties of the overlying functional oxide films. Using BaTiOâ and Pb(Zr,Ti)Oâ films as test cases, it is shown that the use of ZnO as the adhesion layer leads directly to increased process temperature capabilities and dramatic improvements in chemical homogeneity of the films. These result in significant property enhancements (e.g., 300% improvement to bulk-like permittivity for the BaTiOâ films) of oxide films prepared on Pt/ZnO as compared to the conventional Pt/Ti and Pt/TiO[subfield x] stacks. A comparison of electrical, structural, and chemical properties that demonstrate the impact of adhesion layer chemistry on the chemical homogeneity of the overlying complex oxide is presented. Collectively, this analysis shows that in addition to the simple need for adhesion, metal-oxide layers between noble metals and silicon can have tremendous chemical impact on the terminal complex oxide layers.Keywords: electrodes, ferroics, thin films, electronic structures, functional coating
Thickness-Independent Vibrational Thermal Conductance across Confined Solid-Solution Thin Films
We experimentally show that the thermal conductance across confined solid-solution crystalline thin films between parent materials does not necessarily lead to an increase in thermal resistances across the thin-film geometries with increasing film thicknesses, which is counterintuitive to the notion that adding a material serves to increase the total thermal resistance. Confined thin epitaxial Ca0.5Sr0.5TiO3 solid-solution films with systematically varying thicknesses in between two parent perovskite materials of calcium titanate and (001)-oriented strontium titanate are grown, and thermoreflectance techniques are used to accurately measure the thermal boundary conductance across the confined solid-solution films, showing that the thermal resistance does not substantially increase with the addition of solid-solution films with increasing thicknesses from Ό1 to Ό10 nm. Contrary to the macroscopic understanding of thermal transport where adding more material along the heat propagation direction leads to larger thermal resistances, our results potentially offer experimental support to the computationally predicted concept of vibrational matching across interfaces. This concept is based on the fact that a better match in the available heat-carrying vibrations due to an interfacial layer can lead to lower thermal boundary resistances, thus leading to an enhancement in thermal boundary conductance across interfaces driven by the addition of a thin vibrational bridge layer between two solids
Crystallographic changes in lead zirconate titanate due to neutron irradiation
Piezoelectric and ferroelectric materials are useful as the active element in non-destructive monitoring devices for high-radiation areas. Here, crystallographic structural refinement (i.e., the Rietveld method) is used to quantify the type and extent of structural changes in PbZr0.5Ti0.5O3 after exposure to a 1 MeV equivalent neutron fluence of 1.7 Ă 1015 neutrons/cm2. The results show a measurable decrease in the occupancy of Pb and O due to irradiation, with O vacancies in the tetragonal phase being created preferentially on one of the two O sites. The results demonstrate a method by which the effects of radiation on crystallographic structure may be investigated