62 research outputs found
Rhombohedral Hf0.5Zr0.5O2 thin films:Ferroelectricity and devices
This thesis reports a new polar r-phase, with large polarization and robustness. According to the crystal studies on different substrates, specific guidelines are given on how to achieve this new ferroelectric r-phase in HZO thin films. Taking advantage of the great properties of HZO thin films with this new phase, the films were integrated as tunnel barriers into multiferroic tunnel junction. The properties of this device are studied, which give some insights for the future possible application. An intermediate stage has been achieved which unites 4-resistance states, 10^6% TER and TMR reversal upon electric field switching. Success in stabilizing such a stage for a large enough number of cycles will offer a new type of multifunctional device with important advantages with respect to current solutions
Direct Epitaxial Growth of Polar (1-x)HfO2-(x)ZrO2 Ultrathin Films. on Silicon
Ultra-thin Hf1-xZrxO2 films have attracted tremendous interest owing to their
Si-compatible ferroelectricity arising from polar polymorphs. While these
phases have been grown on Si as polycrystalline films, epitaxial growth was
only achieved on non-Si substrates. Here we report direct epitaxy of polar
phases on Si using pulsed laser deposition enabled via in situ scavenging of
the native a-SiOx under ballistic conditions. On Si (111), polar rhombohedral
(r)-phase and bulk monoclinic (m-) phase coexist, with the volume of the former
increasing with increasing Zr concentration. R-phase is stabilized in the
regions with a direct connection between the substrate and the film through the
compressive strain provided by an interfacial crystalline c-SiO2 layer., The
film relaxes to a bulk m-phase in regions where a-SiOx regrows. On Si (100), we
observe polar orthorhombic o-phase coexisting with m-phase, stabilized by
inhomogeneous strains at the intersection of monoclinic domains. This work
provides fundamental insight into the conditions that lead to the preferential
stabilization of r-, o- and m-phases.Comment: 18 pages of manuscript, 7 figure
Guidelines for the stabilization of a polar rhombohedral phase in epitaxial Hf0.5Zr0.5O2 thin films
The unconventional Si-compatible ferroelectricity in hafnia-based systems,
which becomes robust only at nanoscopic sizes, has attracted a lot of interest.
While a metastable polar orthorhombic (o-) phase (Pca21) is widely regarded as
the responsible phase for ferroelectricity, a higher energy polar rhombohedral
(r-) phase is recently reported on epitaxial HfZrO4 (HZO) films grown on (001)
SrTiO3 (R3m or R3), (0001) GaN (R3), and Si (111). Armed with results on these
systems, here we report a systematic study leading towards identifying
comprehensive global trends for stabilizing r-phase polymorphs in epitaxially
grown HZO thin films (6 nm) on various substrates (perovskites, hexagonal and
Si).Comment: Final version as it appears in the 50th Anniversary issue of
Ferroelectric
Magneto-ionic control of spin polarization in multiferroic tunnel junctions
Magnetic tunnel junctions (MTJs) with Hf0.5Zr0.5O2 barriers are reported to
show both tunneling magnetoresistance effect (TMR) and tunneling
electroresistance effect (TER), displaying four resistance states by magnetic
and electric field switching. Here we show that, under electric field cycling
of large enough magnitude, the TER can reach values as large as 10^6%.
Moreover, concomitant with this TER enhancement, the devices develop electrical
control of spin polarization, with sign reversal of the TMR effect. Currently,
this intermediate state exists for a limited number of cycles and understanding
the origin of these phenomena is key to improve its stability. The experiments
presented here point to the magneto-ionic effect as the origin of the large TER
and strong magneto-electric coupling, showing that ferroelectric polarization
switching of the tunnel barrier is not the main contribution
Direct observation of reversible oxygen migration and phase transitions in ferroelectric Hf0.5Zr0.5O2 thin-film devices
Circulating micronutrient levels and their association with sepsis susceptibility and severity: a Mendelian randomization study
Background: Sepsis, a global health challenge, necessitates a nuanced understanding of modifiable factors for effective prevention and intervention. The role of trace micronutrients in sepsis pathogenesis remains unclear, and their potential connection, especially with genetic influences, warrants exploration.Methods: We employed Mendelian randomization (MR) analyses to assess the causal relationship between genetically predicted blood levels of nine micronutrients (calcium, β-carotene, iron, magnesium, phosphorus, vitamin C, vitamin B6, vitamin D, and zinc) and sepsis susceptibility, severity, and subtypes. The instrumental variables for circulating micronutrients were derived from nine published genome-wide association studies (GWAS). In the primary MR analysis, we utilized summary statistics for sepsis from two independent databases (UK Biobank and FinnGen consortium), for initial and replication analyses. Subsequently, a meta-analysis was conducted to merge the results. In secondary MR analyses, we assessed the causal effects of micronutrients on five sepsis-related outcomes (severe sepsis, sepsis-related death within 28 days, severe sepsis-related death within 28 days, streptococcal septicaemia, and puerperal sepsis), incorporating multiple sensitivity analyses and multivariable MR to address potential heterogeneity and pleiotropy.Results: The study revealed a significant causal link between genetically forecasted zinc levels and reduced risk of severe sepsis-related death within 28 days (odds ratio [OR] = 0.450; 95% confidence interval [CI]: 0.263, 0.770; p = 3.58 × 10−3). Additionally, suggestive associations were found for iron (increased risk of sepsis), β-carotene (reduced risk of sepsis death) and vitamin C (decreased risk of puerperal sepsis). No significant connections were observed for other micronutrients.Conclusion: Our study highlighted that zinc may emerges as a potential protective factor against severe sepsis-related death within 28 days, providing theoretical support for supplementing zinc in high-risk critically ill sepsis patients. In the future, larger-scale data are needed to validate our findings
Operando observation of reversible oxygen migration and phase transitions in ferroelectric devices
Operando observation of reversible oxygen migration and phase transitions in ferroelectric devices
Operando observation of reversible oxygen migration and phase transitions in ferroelectric devices
Unconventional ferroelectricity, robust at reduced nanoscale sizes, exhibited by hafnia-based thin-films presents tremendous opportunities in nanoelectronics. However, the exact nature of polarization switching remains controversial. Here, we investigate epitaxial Hf0.5Zr0.5O2 (HZO) capacitors, interfaced with oxygen conducting metals (La0.67Sr0.33MnO3, LSMO) as electrodes, using atomic resolution electron microscopy while in situ electrical biasing. By direct oxygen imaging, we observe reversible oxygen vacancy migration from the bottom to the top electrode through HZO and reveal associated reversible structural phase transitions in the epitaxial LSMO and HZO layers. We follow the phase transition pathways at the atomic scale and identify that these mechanisms are at play both in tunnel junctions and ferroelectric capacitors switched with sub-millisecond pulses. Our results unmistakably demonstrate that oxygen voltammetry and polarization switching are intertwined in these materials
Operando observation of reversible oxygen migration and phase transitions in ferroelectric devices
- …