Dependence of the Structural and Magnetic Properties on the Growth Sequence in Heterostructures Designed by YbFeO3 and BaFe12O19.

The structure and the chemical composition of individual layers as well as of interfaces belonging to the two heterostructures M1 (BaFe12O19/YbFeO3/YSZ) and M2 (YbFeO3/BaFe12O19/YSZ) grown by pulsed laser deposition on yttria-stabilized zirconia (YSZ) substrates are deeply characterized by using a combination of methods such as high-resolution X-ray diffraction, transmission electron microscopy (TEM), and atomic-resolution scanning TEM with energy-dispersive X-ray spectroscopy. The temperature-dependent magnetic properties demonstrate two distinct heterostructures with different coercivity, anisotropy fields, and first anisotropy constants, which are related to the defect concentrations within the individual layers and to the degree of intermixing at the interface. The heterostructure with the stacking order BaFe12O19/YbFeO3, i.e., M1, exhibits a distinctive interface without any chemical intermixture, while an Fe-rich crystalline phase is observed in M2 both in atomic-resolution EDX maps and in mass density profiles. Additionally, M1 shows high c-axis orientation, which induces a higher anisotropy constant K1 as well as a larger coercivity due to a high number of phase boundaries. Despite the existence of a canted antiferromagnetic/ferromagnetic combination (T < 140 K), both heterostructures M1 and M2 do not reveal any detectable exchange bias at T = 50 K. Additionally, compressive residual strain on the BaM layer is found to be suppressing the ferromagnetism, thus reducing the Curie temperature (Tc) in the case of M1. These findings suggest that M1 (BaFe12O19/YbFeO3/YSZ) is suitable for magnetic storage applications.

Structural and Morphological Studies of Pt in the As-Grown and Encapsulated States and Dependency on Film Thickness.

The morphology and crystal structure of Pt films grown by pulsed laser deposition (PLD) on yttria-stabilized zirconia (YSZ)at high temperatures Tg = 900 °C was studied for four different film thicknesses varying between 10 and 70 nm. During the subsequent growth of the capping layer, the thermal stability of the Pt was strongly influenced by the Pt film's thickness. Furthermore, these later affected the film morphology, the crystal structure and hillocks size, and distribution during subsequent growth at Tg = 900 °C for a long duration. The modifications in the morphology as well as in the structure of the Pt film without a capping layer, named also as the as-grown and encapsulated layers in the bilayer system, were examined by a combination of microscopic and scattering methods. The increase in the thickness of the deposited Pt film brought three competitive phenomena into occurrence, such as 3D-2D morphological transition, dewetting, and hillock formation. The degree of coverage, film continuity, and the crystal quality of the Pt film were significantly improved by increasing the deposition time. An optimum Pt film thickness of 70 nm was found to be suitable for obtaining a hillock-free Pt bottom electrode which also withstood the dewetting phenomena revealed during the subsequent growth of capping layers. This achievement is crucial for the deposition of functional bottom electrodes in ferroelectric and multiferroic heterostructure systems.

Thick Does the Trick: Genesis of Ferroelectricity in 2D GeTe-Rich (GeTe)m (Sb2 Te3 )n Lamellae.

The possibility to engineer (GeTe)m (Sb2 Te3 )n phase-change materials to co-host ferroelectricity is extremely attractive. The combination of these functionalities holds great technological impact, potentially enabling the design of novel multifunctional devices. Here an experimental and theoretical study of epitaxial (GeTe)m (Sb2 Te3 )n with GeTe-rich composition is presented. These layered films feature a tunable distribution of (GeTe)m (Sb2 Te3 )1 blocks of different sizes. Breakthrough evidence of ferroelectric displacement in thick (GeTe)m (Sb2 Te3 )1 lamellae is provided. The density functional theory calculations suggest the formation of a tilted (GeTe)m slab sandwiched in GeTe-rich blocks. That is, the net ferroelectric polarization is confined almost in-plane, representing an unprecedented case between 2D and bulk ferroelectric materials. The ferroelectric behavior is confirmed by piezoresponse force microscopy and electroresistive measurements. The resilience of the quasi van der Waals character of the films, regardless of their composition, is also demonstrated. Hence, the material developed hereby gathers in a unique 2D platform the phase-change and ferroelectric switching properties, paving the way for the conception of innovative device architectures.

Formation of Domains within a Lower-to-Higher Symmetry Structural Transition in CrI3.

CrI3 represents one of the most important van der Waals systems on the route to understanding 2D magnetic phenomena. Being arranged in a specific layered structure, it also provides a unique opportunity to investigate structural transformations in dimension-confined systems. CrI3 is dimorphic and possesses a higher symmetry low-temperature phase, which is quite uncommon. It contrasts with vanadium trihalides, which show a higher symmetry high-temperature phase. An explanation of this distinct behavior, together with a large cycle-dependent transition hysteresis, is still an open question. Our low-temperature X-ray diffraction study conducted on CrI3 single crystals complemented by magnetization and specific heat measurements was focused mainly on specific features of the structural transition during cooling. Our results manifest that the structural transition during cooling relates to the formation of structural domains despite the lower symmetry structure transforming to a higher symmetry one. We propose that these domains could control the size of thermal hysteresis.

Effect of Underlayer Quality on Microstructure, Stoichiometry, and Magnetic Properties of Hexaferrite BaFe12O19 Grown on YSZ(111) by Pulsed Laser Deposition.

We have studied the effect of platinum underlayer for two deposited thicknesses on the microstructure, crystalline quality, morphology, chemical composition, and magnetic properties as well as magnetic domain formation of BaFe12O19 (BaM) grown on YSZ(111) by pulsed laser deposition (PLD). We found that PLD platinum deposited with a thickness of 25 nm cannot withstand the dewetting phenomenon occurring during the subsequent BaM layer growth. A smooth and continuous Pt underlayer that possesses a sharp interface and omits the intermixing between the BaM and substrate was successfully achieved for a deposited Pt film thickness of 75 nm. Independent of the thickness of the deposited Pt layer, the c-axis orientation as well as coercivity Hc and the anisotropy HA fields were significantly improved due to a remarkable improvement of lattice mismatch in comparison with the BaM layer grown without a Pt underlayer on YSZ(111). By applying high-resolution X-ray diffraction, scanning and transmission electron microscopy (SEM/TEM), and atomically resolved scanning TEM imaging combined with energy-dispersive X-ray spectroscopy, as well as atomic and magnetic force microscopy, a comprehensive investigation of both structure and chemical composition of the deposited BaM films and their interfacial regions was performed. This study aimed to correlate the enhancement of the overall magnetic properties and of the local spin magnetic domain orientation with the modification of BaM microstructure and chemical composition at the nanometer scale due to the Pt underlayer. Finally, we attempted to understand the mechanisms that control the magnetic properties of these BaM films in order to be able to tailor them.

Mask-Assisted Deposition of Ti on Cyclic Olefin Copolymer Foil by Pulsed Laser Deposition.

Cyclic olefin copolymer (COC) is a novel type of thermoplastic polymer gaining the attention of the scientific community in electronic, optoelectronic, biomedicine and packaging applications. Despite the benefits in the use of COC such as undoubted optical transparency, chemical stability, a good water-vapor barrier and biocompatibility, its original hydrophobicity restricts its wider applicability and optimization of its performances. Presently, we report on the optical and morphological properties of the films of COC covered with Ti in selected areas. The layer of Ti on COC was deposited by pulsed lased deposition processing. The Ti/COC film was characterized by UV-Vis spectroscopy indicating that its transmittance in the visible region decreased by about 20% with respect to the pristine polymer. The quality of the deposited Ti was assessed with the morphology by scanning electron (SEM) and atomic force microscopies (AFM). The modification of the wettability was observed by the sessile drop method indicating a reduction of the native hydrophilicity.

Chiral Light Emission from a Hybrid Magnetic Molecule-Monolayer Transition Metal Dichalcogenide Heterostructure.

Hybrid layered materials assembled from atomically thin crystals and small molecules bring great promises in pushing the current information and quantum technologies beyond the frontiers. We demonstrate here a class of layered valley-spin hybrid (VSH) materials composed of a monolayer two-dimensional (2D) semiconductor and double-decker single molecule magnets (SMMs). We have materialized a VSH prototype by thermal evaporation of terbium bis-phthalocyanine onto a MoS2 monolayer and revealed its composition and stability by both microscopic and spectroscopic probes. The interaction of the VSH components gives rise to the intersystem crossing of the photogenerated carriers and moderate p-doping of the MoS2 monolayer, as corroborated by the density functional theory calculations. We further explored the valley contrast by helicity-resolved photoluminescence (PL) microspectroscopy carried out down to liquid helium temperatures and in the presence of the external magnetic field. The most striking feature of the VSH is the enhanced A exciton-related valley emission observed at the out-of-resonance condition at room temperature, which we elucidated by the proposed nonradiative energy drain transfer mechanism. Our study thus demonstrates the experimental feasibility and great promises of the ultrathin VSH materials with chiral light emission, operable by physical fields for emerging opto-spintronic, valleytronic, and quantum information concepts.

Growth-rate model of epitaxial layer-by-layer growth by pulsed-laser deposition.

We present a numerical model of epitaxial thin-film growth applicable for pulsed-laser deposition on a single crystalline substrate. The model is based on rate equations describing the time development of monolayer coverages and of densities of movable particles on atomically flat terraces. Numerical solution of the equations showed that the time dependence of surface roughness obeys a scaling law, the exponent of which depends on probabilities of various atomistic processes included in the simulation model. From the time dependence of monolayer coverages we calculated x-ray diffracted intensity in a quasiforbidden anti-Bragg reflection and showed that its oscillatory behavior is affected by these probabilities as well. The results show the possibility to study atomistic processes during the deposition from the time dependence of the anti-Bragg intensity measured during deposition.

Compositional and Structural Modifications by Ion Beam in Graphene Oxide for Radiation Detection Studies.

In the present study, graphene oxide foils 10 µm thick have been irradiated in vacuum using same charge state (one charge state) ions, such as protons, helium and oxygen ions, at the same energies (3 MeV) and fluences (from 5 × 1011 ion/cm2 to 5 × 1014 ion/cm2). The structural changes generated by the ion energy deposition and investigated by X-ray diffraction have suggested the generation of new phases, as reduced GO, GO quantum dots and graphitic nanofibers, carbon nanotubes, amorphous carbon and stacked-cup carbon nanofibers. Further analyses, based on Rutherford Backscattering Spectrometry and Elastic Recoil Detection Analysis, have indicated a reduction of GO connected to the atomic number of implanted ions. The morphological changes in the ion irradiated GO foils have been monitored by Transmission Electron, Atomic Force and Scanning Electron microscopies. The present study aims to better structurally, compositionally and morphologically characterize the GO foils irradiated by different ions at the same conditions and at very low ion fluencies to validate the use of GO for radiation detection and propose it as a promising dosimeter. It has been observed that GO quantum dots are produced on the GO foil when it is irradiated by proton, helium and oxygen ions and their number increases with the atomic number of beam gaseous ion.

Crystal structure evolution in the van der Waals vanadium trihalides.

Most transition-metal trihalides are dimorphic. The representative chromium-based triad, CrCl3, CrBr3, CrI3, is characterized by the low-temperature (LT) phase adopting the trigonal BiI3-type while the structure of the high-temperature (HT) phase is monoclinic of AlCl3type (C2/m). The structural transition between the two crystallographic phases is of the first-order type with large thermal hysteresis in CrCl3and CrI3. We studied crystal structures and structural phase transitions of vanadium-based counterparts VCl3, VBr3, and VI3by measuring specific heat, magnetization, and x-ray diffraction as functions of temperature and observed an inverse situation. In these cases, the HT phase has a higher symmetry while the LT structure reveals a lower symmetry. The structural phase transition between them shows no measurable hysteresis in contrast to CrX3. Possible relations of the evolution of the ratioc/aof the unit cell parameters, types of crystal structures, and nature of the structural transitions in V and Cr trihalides are discussed.

Effect of pulse laser frequency on PLD growth of LuFeO3 explained by kinetic simulations of in-situ diffracted intensities.

Atomistic processes during pulsed-laser deposition (PLD) growth influence the physical properties of the resulting films. We investigated the PLD of epitaxial layers of hexagonal LuFeO[Formula: see text] by measuring the X-ray diffraction intensity in the quasiforbidden reflection 0003 in situ during deposition. From measured X-ray diffraction intensities we determined coverages of each layer and studied their time evolution which is described by scaling exponent [Formula: see text] directly connected to the surface roughness. Subsequently we modelled the growth using kinetic Monte Carlo simulations. While the experimentally obtained scaling exponent [Formula: see text] decreases with the laser frequency, the simulations provided the opposite behaviour. We demonstrate that the increase of the surface temperature caused by impinging ablated particles satisfactorily explains the recorded decrease in the scaling exponent with the laser frequency. This phenomena is often overlooked during the PLD growth.

Microstructural modifications induced in Si+-implanted yttria-stabilised zirconia: a combined RBS-C, XRD and Raman investigation.

The structural differences in (100)-, (110)- and (111)-oriented cubic yttria-stabilised zirconia (YSZ) single crystals after implantation with 2 MeV Si+ ions at the fluences of 5 × 1015, 1 × 1016 and 5 × 1016 cm-2 were studied using Rutherford backscattering spectrometry in the channelling mode (RBS-C), X-ray diffraction (XRD) and Raman spectroscopy. The RBS-C results show that the damage accumulation in the 〈110〉 direction exhibits a lower level of disorder (<0.3) than the other orientations (<0.6) and it seems that the (110) crystallographic orientation is the most resistant to radiation damage. The experimental results from the RBS measurement were compared with the results from the XRD measurements. The XRD data were analysed using the standard two-beam dynamical X-ray diffraction theory and the pure isotropic strain was deduced from the fit for the fluence of 5 × 1015 cm-2. It was shown that the maximum value of the isotropic strain does not depend on the surface orientation. The increase in signal intensity at ∼689 cm-1 is probably related to an increase in implantation defects such as oxygen vacancies.

Time-Resolved Morphology and Kinetic Studies of Pulsed Laser Deposition-Grown Pt Layers on Sapphire at Different Growth Temperatures by in Situ Grazing Incidence Small-Angle X-ray Scattering.

Optimizing and monitoring the growth conditions of Pt films, often used as bottom electrodes in multiferroic material systems, represents a highly relevant issue that is of importance for controlling the crystalline quality and performance of ferroelectric oxides such as, e.g. LuFeO3. We performed a time-resolved monitoring of the growth and morphology of Pt films during pulsed laser deposition (PLD) in dependence on the grown film effective thickness and on the growth temperature Tg using in situ grazing incidence small-angle X-ray scattering (GISAXS). Through real-time analysis and modeling of GISAXS patterns, we could fully characterize the influence of Tg on the morphology and on the growth kinetics of the Pt layers. Consequently, critical and characteristic effective thicknesses for the transitions nucleation phase (I)/coalescence phase (II) and coalescence phase (II)/coarsening phase (III) could be determined. In combination with complementary microscopic imaging and chemical mapping via combined SEM/EDXS, we demonstrate the occurrence of a morphological progression in the Pt PLD-grown Pt films, changing from grains at room temperature to a 3D-island morphology at 300 °C, further to a hole-free structure at 500 °C, and finally to a channel structure for 700 and 900 °C. The film topography, as characterized by atomic force microscopy (AFM), favors the PLD growth of Pt layers at temperatures beyond 700 °C where the film is homogeneous, continuous, and hole-free with a flat and smooth surface. The double dependency of the percolation transition on the film effective thickness and on the growth temperature has been established by measuring the electrical conductivity.

X-ray diffraction reveals the amount of strain and homogeneity of extremely bent single nanowires.

Core-shell nanowires (NWs) with asymmetric shells allow for strain engineering of NW properties because of the bending resulting from the lattice mismatch between core and shell material. The bending of NWs can be readily observed by electron microscopy. Using X-ray diffraction analysis with a micro- and nano-focused beam, the bending radii found by the microscopic investigations are confirmed and the strain in the NW core is analyzed. For that purpose, a kinematical diffraction theory for highly bent crystals is developed. The homogeneity of the bending and strain is studied along the growth axis of the NWs, and it is found that the lower parts, i.e. close to the substrate/wire interface, are bent less than the parts further up. Extreme bending radii down to ∼3 µm resulting in strain variation of ∼2.5% in the NW core are found.

Effective algorithm for simulations of layer-by-layer growth during pulsed-laser deposition.

The atomistic simulation of materials growing in the layer-by-layer mode by the pulsed-laser deposition is a significant challenge mainly due to the short timescales in which the fastest processes on the surface occur together with long periods between pulses. We present a kinetic Monte Carlo algorithm which overcomes the scaling problem by approximation of fast diffusion and by neglecting complex chemical processes. The atomic diffusion is modeled as a two-dimensional gas of material units on each layer. The model is based on a few elementary processes-the condensation of units on the surface, their dissolution back to the gas, and interlayer transport, which can be influenced by the Ehrlich-Schwoebel barrier. With these simplifications, the computational time of the algorithm scales only linearly with the size of the substrate while describing physically relevant growth kinetics. We demonstrate that the simplified model is suitable for simulations of layered growth of thin films in the range from quasicontinuous deposition to low-frequency cases. The model is successfully implemented to provide an alternative explanation of the time evolution of layer coverages by interlayer transport after pulses of deposition experimentally observed during perovskite growth [G. Eres et al., Phys. Rev. B 84, 195467 (2011)PRBMDO1098-012110.1103/PhysRevB.84.195467].

Structure Quality of LuFeO3 Epitaxial Layers Grown by Pulsed-Laser Deposition on Sapphire/Pt.

Structural quality of LuFeO 3 epitaxial layers grown by pulsed-laser deposition on sapphire substrates with and without platinum Pt interlayers has been investigated by in situ high-resolution X-ray diffraction (reciprocal-space mapping). The parameters of the structure such as size and misorientation of mosaic blocks have been determined as functions of the thickness of LuFeO 3 during growth and for different thicknesses of platinum interlayers up to 40 nm. By means of fitting of the time-resolved X-ray reflectivity curves and by in situ X-ray diffraction measurement, we demonstrate that the LuFeO 3 growth rate as well as the out-of-plane lattice parameter are almost independent from Pt interlayer thickness, while the in-plane LuFeO 3 lattice parameter decreases. We reveal that, despite the different morphologies of the Pt interlayers with different thickness, LuFeO 3 was growing as a continuous mosaic layer and the misorientation of the mosaic blocks decreases with increasing Pt thickness. The X-ray diffraction results combined with ex situ scanning electron microscopy and high-resolution transmission electron microscopy demonstrate that the Pt interlayer significantly improves the structure of LuFeO 3 by reducing the misfit of the LuFeO 3 lattice with respect to the material underneath.

Anomalous X-ray diffraction from ω nanoparticles in ß-Ti(Mo) single crystals.

Anomalous X-ray diffraction (AXRD) is a technique which makes use of effects occurring near the energy of an absorption edge of an element present in the studied sample. The intensity of the diffracted radiation exhibits an anomalous decrease when the primary beam energy matches the energy needed to excite an electron from an atomic orbital. The characteristics of this step are sensitive to the concentration of the `anomalous' element and its spatial distribution in the sample. In the present investigation, AXRD was employed to study ω particles in a metastable ß titanium alloy Ti-15Mo (in wt%). The experiments were done in an energy range around the Mo K edge at 20.0 keV, allowing investigation of the distribution of Mo in the material, which is rejected from the volume of ω particles during their diffusion-driven growth. This paper deals with diffuse scattering patterns around the (006)ß diffraction maximum. It was observed that different regions of the diffuse scattering exhibited different variations of diffracted intensity with the incident photon energy near the absorption edge. Numerical simulations of diffuse scattering patterns as well as of energy dependences of the scattered intensity were performed. It was found that the observed patterns and their dependence on the primary beam energy can be explained by taking into account (a) elastic deformation of the ß matrix arising from the presence of slightly misfitting ω particles and (b) the presence of a `cloud' of a higher Mo concentration around ω particles.

Indium Incorporation into InGaN Quantum Wells Grown on GaN Narrow Stripes.

InGaN quantum wells were grown using metalorganic chemical vapor phase epitaxy (vertical and horizontal types of reactors) on stripes made on GaN substrate. The stripe width was 5, 10, 20, 50, and 100 µm and their height was 4 and 1 µm. InGaN wells grown on stripes made in the direction perpendicular to the off-cut had a rough morphology and, therefore, this azimuth of stripes was not further explored. InGaN wells grown on the stripes made in the direction parallel to the GaN substrate off-cut had a step-flow-like morphology. For these samples (grown at low temperatures), we found out that the InGaN growth rate was higher for the narrower stripes. The higher growth rate induces a higher indium incorporation and a longer wavelength emission in photoluminescence measurements. This phenomenon is very clear for the 4 µm high stripes and less pronounced for the shallower 1 µm high stripes. The dependence of the emission wavelength on the stripe width paves a way to multicolor emitters.

Self-Seeded Axio-Radial InAs-InAs1-xPx Nanowire Heterostructures beyond "Common" VLS Growth.

Semiconductors are essential for modern electronic and optoelectronic devices. To further advance the functionality of such devices, the ability to fabricate increasingly complex semiconductor nanostructures is of utmost importance. Nanowires offer excellent opportunities for new device concepts; heterostructures have been grown in either the radial or axial direction of the core nanowire but never along both directions at the same time. This is a consequence of the common use of a foreign metal seed particle with fixed size for nanowire heterostructure growth. In this work, we present for the first time a growth method to control heterostructure growth in both the axial and the radial directions simultaneously while maintaining an untapered self-seeded growth. This is demonstrated for the InAs/InAs1-xPx material system. We show how the dimensions and composition of such axio-radial nanowire heterostructures can be designed including the formation of a "pseudo-superlattice" consisting of five separate InAs1-xPx segments with varying length. The growth of axio-radial nanowire heterostructures offers an exciting platform for novel nanowire structures applicable for fundamental studies as well as nanowire devices. The growth concept for axio-radial nanowire heterostructures is expected to be fully compatible with Si substrates.

Characterization of individual stacking faults in a wurtzite GaAs nanowire by nanobeam X-ray diffraction.

Coherent X-ray diffraction was used to measure the type, quantity and the relative distances between stacking faults along the growth direction of two individual wurtzite GaAs nanowires grown by metalorganic vapour epitaxy. The presented approach is based on the general property of the Patterson function, which is the autocorrelation of the electron density as well as the Fourier transformation of the diffracted intensity distribution of an object. Partial Patterson functions were extracted from the diffracted intensity measured along the [000\bar{1}] direction in the vicinity of the wurtzite 00\bar{1}\bar{5} Bragg peak. The maxima of the Patterson function encode both the distances between the fault planes and the type of the fault planes with the sensitivity of a single atomic bilayer. The positions of the fault planes are deduced from the positions and shapes of the maxima of the Patterson function and they are in excellent agreement with the positions found with transmission electron microscopy of the same nanowire.