93 research outputs found
Strain engineering of ferroelectric domains in KxNa1âxNbO3 epitaxial layers
The application of lattice strain through epitaxial growth of oxide films on lattice mismatched
perovskite-like substrates strongly influences the structural properties of ferroelectric
domains and their corresponding piezoelectric behavior. The formation of different
ferroelectric phases can be understood by a strain-phase diagram, which is calculated
within the framework of the LandauâGinzburgâDevonshire theory. In this paper, we illustrate
the opportunity of ferroelectric domain engineering in the KxNa1âxNbO3 lead-free
material system. In particular, the following examples are discussed in detail: (i) Different
substrates (NdGaO3, SrTiO3, DyScO3, TbScO3, and GdScO3) are used to systematically
tune the incorporated epitaxial strain from compressive to tensile. This can be exploited
to adjust the NaNbO3 thin film surface orientation and, concomitantly, the vector of
electrical polarization, which rotates from mainly vertical to exclusive in-plane orientation.
(ii) In ferroelectric NaNbO3, thin films grown on rare-earth scandate substrates, highly
regular stripe domain patterns are observed. By using different film thicknesses, these
can be tailored with regard to domain periodicity and vertical polarization component.
(iii) A featured potassium concentration of x = 0.9 of KxNa1âxNbO3 thin films grown on
(110) NdScO3 substrates favors the coexistence of two equivalent, monoclinic, but
differently oriented ferroelectric phases. A complicated herringbone domain pattern is
experimentally observed which consists of alternating MC and a1a2 domains. The coexistence
of different types of ferroelectric domains leads to polarization discontinuities
at the domain walls, potentially enabling high piezoelectric responses. In each of these
examples, the experimental results are in excellent agreement with predictions based on
the linear elasticity theory
Approaching the High Intrinsic Electrical Resistivity of NbO2 in Epitaxially Grown Films
NbO2 is a promising candidate for resistive switching devices due to an insulator-metal transition above room temperature, which is related to a phase transition from a distorted rutile structure to an undistorted one. However, the electrical resistivity of the NbO2 thin films produced so far has been too low to achieve high on-off switching ratios. Here, we report on the structural, electrical, and optical characterization of single-crystalline NbO2 (001) thin films grown by pulsed laser deposition on MgF2 (001) substrates. An annealing step reduced the full width at half maximum of the NbO2 (004) x-ray Bragg reflection by one order of magnitude, while the electrical resistivity of the films increased by two orders of magnitude to about 1k Omega cm at room temperature. Temperature-dependent resistivity measurements of an annealed sample revealed that below 650K, two deep-level defects with activation energies of 0.25eV and 0.37eV dominate the conduction, while above 650K, intrinsic conduction prevails. Optical characterization by spectroscopic ellipsometry and by absorption measurements with the electric field vector of the incident light perpendicular to the c-axis of the distorted rutile structure indicates the onset of fundamental absorption at about 0.76eV at room temperature, while at 4K, the onset shifts to 0.85eV. These optical transitions are interpreted to take place across the theoretically predicted indirect bandgap of distorted rutile NbO2
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Stability of ZnSe-Passivated Laser Facets Cleaved in Air and in Ultra-High Vacuum
Catastrophic optical mirror damage (COMD) is one of the main failure mechanisms limiting the reliability of GaAs based laser diodes. Here, we compare the facet stability of ZnSe-passivated ridge-waveguide lasers (RWLs) that are cleaved in air and subsequently cleaned using atomic hydrogen with RWLs that are cleaved in ultra-high vacuum. RWLs cleaved in ultra-high vacuum show a superior performance and reach power densities up to 58 MW/cm 2 under extended continuous wave operation at 1064 nm. This is attributed to the reduction of defects at the interface between ZnSe and the cleaved facet as evidenced by transmission electron microscopy and X-ray diffraction
Structural and optical investigation of non-polar (1-100) GaN grown by the ammonothermal method
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Journal of Applied Physics 113, 203513 (2013) and may be found at https://doi.org/10.1063/1.4807581.We studied the structural and optical properties of state-of-the-art non-polar bulk GaN grown by the ammonothermal method. The investigated samples have an extremely low dislocation density (DD) of less than 5 Ă 104âcmâ2, which results in very narrow high-resolution x-ray rocking curves. The a and c lattice parameters of these stress-free GaN samples were precisely determined by using an x-ray diffraction technique based on the modified Bond method. The obtained values are compared to the lattice parameters of free-standing GaN from different methods and sources. The observed differences are discussed in terms of free-electron concentrations, point defects, and DD. Micro Raman spectroscopy revealed a very narrow phonon linewidth and negligible built-in strain in accordance with the high-resolution x-ray diffraction data. The optical transitions were investigated by cathodoluminescence measurements. The analysis of the experimental data clearly demonstrates the excellent crystalline perfection of ammonothermal GaN material and its potential for fabrication of non-polar substrates for homoepitaxial growth of GaN based device structures
Self-stabilization of the equilibrium state in ferroelectric thin films
(K,Na)NbO3 is a lead-free and sustainable ferroelectric material with electromechanical parameters comparable to Pb(Zr,Ti)O3 (PZT) and other lead-based solid solutions. It is therefore a promising candidate for caloric cooling and energy harvesting applications. Specifically, the structural transition from the low-temperature Mc- to the high-temperature c-phase displays a rich hierarchical order of domains and superdomains, that forms at specific strain conditions. The relevant length scales are few tens of nanometers for the domain and few micrometers for the superdomain size, respectively. Phase-field calculations show that this hierarchical order adds to the total free energy of the solid. Thus, domains and their formation has a strong impact on the functional properties relevant for electrocaloric cooling or energy harvesting applications. However, monitoring the formation of domains and superdomains is difficult and requires both, high spatial and high temporal resolution of the experiment. Synchrotron-based time-resolved X-ray diffraction methods in combination with scanning imaging X-ray microscopy is applied to resolve the local dynamics of the domain morphology with sub-micrometer spatial and nanosecond temporal resolution. In this regime, the material displays a novel self-stabilization mechanism of the domain morphology, which may be a general property of first-order phase transitions
Scanning X-ray nanodiffraction from ferroelectric domains in strained K0.75Na0.25NbO3 epitaxial films grown on (110) TbScO3
Scanning X-ray nanodiffraction on a highly periodic ferroelectric domain pattern of a strained K0.75Na0.25NbO3 epitaxial layer has been performed by using a focused X-ray beam of about 100 14;nm probe size. A 90°-rotated domain variant which is aligned along [1 2]TSO has been found in addition to the predominant domain variant where the domains are aligned along the [12]TSO direction of the underlying (110) TbScO3 (TSO) orthorhombic substrate. Owing to the larger elastic strain energy density, the 90°-rotated domains appear with significantly reduced probability. Furthermore, the 90°-rotated variant shows a larger vertical lattice spacing than the 0°-rotated domain variant. Calculations based on linear elasticity theory substantiate that this difference is caused by the elastic anisotropy of the K0.75Na0.25NbO3 epitaxial layer
Effect of thermal annealing on the optical and structural properties of (311)B and (001) GaAsBi/GaAs single quantum wells grown by MBE
The effect of Furnace Annealing (FA) and Rapid Thermal annealing (RTA) on the structural and optical properties of GaAs1âââxBix/GaAs single quantum wells grown on (001) and (311)B substrates by molecular beam epitaxy was investigated. The structural properties were investigated by high-resolution x-ray diffraction (HR-XRD) and Transmission Electron Microscopy. The Bi concentration profiles were determined by simulating the HR-XRD 2ΞâÏ scans using dynamical scattering theory to estimate the Bi content, lattice coherence, and quality of the interfaces. The Bi composition was found to be similar for both samples grown on (001) and (311)B GaAs substrates. However, the simulations indicate that the Bi composition is not only limited in the GaAsBi quantum well (QW) layer but also extends out of the GaAsBi QW toward the GaAs barrier. Photoluminescence (PL) measurements were performed as a function of temperature and laser power for samples with a nominal Bi composition of 3%. PL spectra showed that (001) and (311)B samples have different peak energies at 1.23âeV and 1.26âeV, respectively, at 10âK. After RTA at 300â°C for 2âmin, the PL intensity of (311)B and (001) samples was enhanced by factors of âŒ2.5 and 1.75, respectively. However, for the (001) and (311)B FA samples, an enhancement of the PL intensity by a factor of only 1.5 times could be achieved. The enhancement of PL intensity in annealed samples was interpreted in terms of PL activation energies, with a reduction in the alloy disorder and an increase in the Bi cluster
Non-adiabatic small polaron hopping in the n=3 Ruddlesden-Popper compound Ca4Mn3O10
Magnetotransport properties of the compound Ca4Mn3O10 are interpreted in
terms of activated hopping of small magnetic polarons in the non-adiabatic
regime. Polarons are most likely formed around Mn3+ sites created by oxygen
substoichiometry. The application of an external field reduces the size of the
magnetic contribution to the hopping barrier and thus produces an increase in
the conductivity .We argue that the change in the effective activation energy
around TN is due to the crossover to VRH conduction as antiferromagnetic order
sets in.Comment: 29 pages, 7 figure
ACTRIS non-methane hydrocarbon intercomparison experiment in Europe to support WMO GAW and EMEP observation networks
The performance of 18 European institutions involved in long-term non-methane hydrocarbon (NMHC) measurements in ambient air within the framework of the Global Atmosphere Watch (GAW) and the European Monitoring and Evaluation Programme (EMEP) was assessed with respect to data quality objectives (DQOs) of ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure Network) and GAW. Compared to previous intercomparison studies the DQOs define a novel approach to assess and ensure a high quality of the measurements. Having already been adopted by GAW, the ACTRIS DQOs are demanding with deviations to a reference value of less than 5% and a repeatability of better than 2% for NMHC mole fractions above 0.1 nmol mol(-1). The participants of the intercomparison analysed two dry gas mixtures in pressurised cylinders, a 30-component NMHC mixture in nitrogen (NMHC_N-2 /at approximately 1 nmol mol(-1) and a whole air sample (NMHC_air), following a standardised operation procedure including zero-and calibration gas measurements. Furthermore, participants had to report details on their instruments and assess their measurement uncertainties. The NMHCs were analysed either by gas chromatography-flame ionisation detection (GC-FID) or by gas chromatography-mass spectrometry (GC-MS). For the NMHC_N-2 measurements, 62% of the reported values were within the 5% deviation class corresponding to the ACTRIS DQOs. For NMHC_air, generally more frequent and larger deviations to the assigned values were observed, with 50% of the reported values within the 5% deviation class. Important contributors to the poorer performance in NMHC_air compared to NMHC_N-2 were a more complex matrix and a larger span of NMHC mole fractions (0.03-2.5 nmol mol(-1)). The performance of the participating laboratories were affected by the different measurement procedures such as the usage of a two-step vs. a one-step calibration, breakthroughs of C-2-C-3 hydrocarbons in the focussing trap, blank values in zero-gas measurements (especially for those systems using a Nafion (R) Dryer), adsorptive losses of aromatic compounds, and insufficient chromatographic separation.Peer reviewe
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