50 research outputs found
Electrical Properties of Epitaxial Ferroelectric Heterostructures
In the context of miniaturization of devices, ferroelectric materials are used as multifunctional materials for their well-known intrinsic properties, especially for the switching of polarization in an applied electric field. The high-quality epitaxial thin film structures are used for the possibility to study different effects as low dimensions, interface, strain and strain gradients on ferroelectric materials and other electric characteristics, also representing a possibility to obtain new phenomena and properties that can be used for development of new devices with different functionalities. This chapter is a summary of the ferroelectric and dielectric behaviour of epitaxial thin films of Pb(Zr,Ti)O3 (PZT) and BaTiO3 (BTO) obtained by pulsed laser deposition and the correlation with structural quality of the layers and with different electrostatic conditions induced either by electrodes or by the different interlayers. For this purpose in the first part, studies regarding the influence of the substrates and of different top electrodes are performed for Pb(Zr,Ti)O3 (PZT) 52/48. In the second part, we focused on artificial multiferroic structures from alternating layers of PZT 20/80 or BaTiO3 (BTO) as ferroelectric phase and CoFe2O4 (CFO) as magnetic material. We found that interface configuration and strain engineering could control ferroelectric hysteresis, the capacitance or the leakage current magnitude
Atomistic Simulations of Methylammonium Lead Halide Layers on PbTiO<sub>3</sub> (001) Surfaces
The substantial increase
in the power conversion efficiency of
hybrid perovskite solar cells, to date reaching more than 20% in the
laboratory, has strongly motivated research on this class of organicâinorganic
materials and related devices, particularly based on CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3â<i>x</i></sub>X<sub><i>x</i></sub>/TiO<sub>2</sub> heterostructures (X = Cl,Br). Taking
under consideration that a ferroelectric substrate may act as an efficient
electron transporter, positively influencing charge collection across
the interface and allowing the tuning of the halide perovskite (HP)
- ferroelectric junction, we performed extensive density functional
theory calculations on CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3â<i>x</i></sub>Cl<sub><i>x</i></sub> layers deposited
on tetragonal PbTiO<sub>3</sub> (PTO) (001) surfaces, to study their
structural and electronic properties. The main findings of this study
are as follows. (i) A ferroelectric polarization pointing from the
PTO/HP interface to the PTO is favorable for the photogenerated electrons
transfer across the interface and their transport to the collecting
electrode. (ii) The PTO internal electric field leads to a position
dependent energy levels diagram. (iii) The HP gap may be tuned by
chlorine concentration at the interface, as well as the by the surface
terminations of PbTiO<sub>3</sub> and hybrid perovskite layers. (iv)
The presence of the PTO ferroelectric surface is likely to have just
a slight orientational effect on the (CH<sub>3</sub>NH<sub>3</sub>)<sup>+</sup> dipoles
Normal and inverted hysteresis in perovskite solar cells
Hysteretic effects are investigated in perovskite solar cells in the standard
FTO/TiO/CHNHPbICl/spiro-OMeTAD/Au configuration. We
report normal (NH) and inverted hysteresis (IH) in the J-V characteristics
occurring for the same device structure, the behavior strictly depending on the
pre-poling bias. NH typically appears at pre-poling biases larger than the open
circuit bias, while pronounced IH occurs for negative bias pre-poling. The
transition from NH to IH is marked by a intermediate mixed hysteresis behavior
characterized by a crossing point in the J-V characteristics. The measured J-V
characteristics are explained quantitatively by the dynamic electrical model
(DEM). Furthermore, the influence of the bias scan rate on the NH/IH hysteresis
is discussed based on the time evolution of the non-linear polarization.
Introducing a three step measurement protocol, which includes stabilization,
pre-poling and measurement, we put forward the difficulties and possible
solutions for a correct PCE evaluation.Comment: 11 pages, 10 figure
Competition between ferroelectric and semiconductor properties in Pb(Zr0.65Ti0.35)O-3 thin films deposited by sol-gel
Asymmetric metalâferroelectricâmetal (MFM) structures were manufactured by solâgel deposition of a lead zirconate-titanate (PZT with Zr/Ti ratio 65/35) film on Pt-coated Si, with a Au top electrode. The average remnant polarization of 9 ”C/cm2 and the coercive field of 39 kV/cm were obtained from the hysteresis loop measurements. A detailed analysis of the polarizationâelectric field (PâE), capacitanceâvoltage (CâV), and currentâvoltage (IâV) measurement results allowed us to estimate the near-electrode space-charge region thickness (roughly half of the film thickness at zero voltage), net doping concentration (around 1018 cmâ3), built-in potential (in the 0.4â0.8 V range, depending on the injecting electrode), and dynamic dielectric constant (5.2). The current logarithmâvoltage dependence for the field-enhanced Schottky emission obeys a "1/4" law. The spectral distribution of the short circuit current measured under continuous light illumination in the 290â800 nm range exhibits a cutoff wavelength at 370 nm and a maximum sensitivity at about 340 nm. The estimated band-gap energy of the PZT material is 3.35 eV. The MFM structure is discussed in terms of two back-to-back Schottky diodes with a ferroelectric material in between. It is concluded that the semiconductor properties of the films are not negligible and, in certain conditions, are dominating over the ferroelectric one
Investigation of the Boron removal effect induced by 5.5 MeV electrons on highly doped EPI- and Cz-silicon
This study focuses on the properties of the BO
(interstitial Boron~-~interstitial Oxygen) and CO
(interstitial Carbon~-~interstitial Oxygen) defect complexes by
\SI{5.5}{\mega\electronvolt} electrons in low resistivity silicon. Two
different types of diodes manufactured on p-type epitaxial and Czochralski
silicon with a resistivity of about 10~cm were irradiated with
fluence values between \SI{1e15}{\per\square\centi\meter} and
\SI{6e15}{\per\square\centi\meter}. Such diodes cannot be fully depleted and
thus the accurate evaluation of defect concentrations and properties
(activation energy, capture cross-section, concentration) from Thermally
Stimulated Currents (TSC) experiments alone is not possible. In this study we
demonstrate that by performing Thermally Stimulated Capacitance (TS-Cap)
experiments in similar conditions to TSC measurements and developing
theoretical models for simulating both types of BO
signals generated in TSC and TS-Cap measurements, accurate evaluations can be
performed. The changes of the position-dependent electric field, the effective
space charge density profile as well as the occupation of the
BO defect during the electric field dependent electron
emission, are simulated as a function of temperature. The macroscopic
properties (leakage current and ) extracted from current-voltage
and capacitance-voltage measurements at \SI{20}{\celsius} are also presented
and discusse
Influence of orbital contributions to the valence band alignment of Bi2O3, Fe2O3, BiFeO3, and Bi0.5Na0.5TiO3
The formation of an interface between Bi2O3, Fe2O3, BiFeO3, Bi0.5Na0.5TiO3, and the high work function metallic RuO2 is studied using photoelectron spectroscopy with in situ RuO2 deposition. Schottky barrier heights are derived and the valence band maximum energies of the studied materials are aligned with respect to each other as well as to other functional oxides like SrTiO3 and PbTiO3. The energy band alignment follows systematic trends compared to a large number of oxides, and can be understood in terms of the contribution of Fe 3d and Bi 6s/6p (lone pair) orbitals to electronic states near the valence band maximum. The results indicate that the valence band maxima are largely determined by the local environment of the cations, which allows to estimate valence band maximum energies of oxides with multiple cations from those of their parent binary compounds. The high valence band maximum of BiFeO3 is consistent with reported p-type conduction of acceptor doped material, while the high conduction band minimum makes n-type conduction unlikely
Effects of a high-dose 24-h infusion of tranexamic acid on death and thromboembolic events in patients with acute gastrointestinal bleeding (HALT-IT): an international randomised, double-blind, placebo-controlled trial
Background: Tranexamic acid reduces surgical bleeding and reduces death due to bleeding in patients with trauma.
Meta-analyses of small trials show that tranexamic acid might decrease deaths from gastrointestinal bleeding. We
aimed to assess the effects of tranexamic acid in patients with gastrointestinal bleeding.
Methods: We did an international, multicentre, randomised, placebo-controlled trial in 164 hospitals in 15 countries.
Patients were enrolled if the responsible clinician was uncertain whether to use tranexamic acid, were aged above the
minimum age considered an adult in their country (either aged 16 years and older or aged 18 years and older), and
had significant (defined as at risk of bleeding to death) upper or lower gastrointestinal bleeding. Patients were
randomly assigned by selection of a numbered treatment pack from a box containing eight packs that were identical
apart from the pack number. Patients received either a loading dose of 1 g tranexamic acid, which was added to
100 mL infusion bag of 0·9% sodium chloride and infused by slow intravenous injection over 10 min, followed by a
maintenance dose of 3 g tranexamic acid added to 1 L of any isotonic intravenous solution and infused at 125 mg/h
for 24 h, or placebo (sodium chloride 0·9%). Patients, caregivers, and those assessing outcomes were masked to
allocation. The primary outcome was death due to bleeding within 5 days of randomisation; analysis excluded patients
who received neither dose of the allocated treatment and those for whom outcome data on death were unavailable.
This trial was registered with Current Controlled Trials, ISRCTN11225767, and ClinicalTrials.gov, NCT01658124.
Findings: Between July 4, 2013, and June 21, 2019, we randomly allocated 12 009 patients to receive tranexamic acid
(5994, 49·9%) or matching placebo (6015, 50·1%), of whom 11 952 (99·5%) received the first dose of the allocated
treatment. Death due to bleeding within 5 days of randomisation occurred in 222 (4%) of 5956 patients in the
tranexamic acid group and in 226 (4%) of 5981 patients in the placebo group (risk ratio [RR] 0·99, 95% CI 0·82â1·18).
Arterial thromboembolic events (myocardial infarction or stroke) were similar in the tranexamic acid group and
placebo group (42 [0·7%] of 5952 vs 46 [0·8%] of 5977; 0·92; 0·60 to 1·39). Venous thromboembolic events (deep vein
thrombosis or pulmonary embolism) were higher in tranexamic acid group than in the placebo group (48 [0·8%] of
5952 vs 26 [0·4%] of 5977; RR 1·85; 95% CI 1·15 to 2·98).
Interpretation: We found that tranexamic acid did not reduce death from gastrointestinal bleeding. On the basis of our
results, tranexamic acid should not be used for the treatment of gastrointestinal bleeding outside the context of a
randomised trial