X-ray Reflectivity Measurements Research Articles

Faster and lower-dose X-ray reflectivity measurements enabled by physics-informed modeling and artificial intelligence co-refinement

An approach is presented for analysis of real-time X-ray reflectivity (XRR) process data not just as a function of the magnitude of the reciprocal-space vector q, as is commonly done, but as a function of both q and time. The real-space structures extracted from the XRR curves are restricted to be solutions of a physics-informed growth model and use state-of-the-art convolutional neural networks (CNNs) and differential evolution fitting to co-refine multiple time-dependent XRR curves R(q, t) of a thin film growth experiment. Thereby it becomes possible to correctly analyze XRR data with a fidelity corresponding to standard fits of individual XRR curves, even if they are sparsely sampled, with a sevenfold reduction of XRR data points, or if the data are noisy due to a 200-fold reduction in counting times. The approach of using a CNN analysis and of including prior information through a kinetic model is not limited to growth studies but can be easily extended to other kinetic X-ray or neutron reflectivity data to enable faster measurements with less beam damage.

Faster and lower-dose X-ray reflectivity measurements enabled by physics-informed modeling and artificial intelligence co-refinement.

An approach is presented for analysis of real-time X-ray reflectivity (XRR) process data not just as a function of the magnitude of the reciprocal-space vector q, as is commonly done, but as a function of both q and time. The real-space structures extracted from the XRR curves are restricted to be solutions of a physics-informed growth model and use state-of-the-art convolutional neural networks (CNNs) and differential evolution fitting to co-refine multiple time-dependent XRR curves R(q, t) of a thin film growth experiment. Thereby it becomes possible to correctly analyze XRR data with a fidelity corresponding to standard fits of individual XRR curves, even if they are sparsely sampled, with a sevenfold reduction of XRR data points, or if the data are noisy due to a 200-fold reduction in counting times. The approach of using a CNN analysis and of including prior information through a kinetic model is not limited to growth studies but can be easily extended to other kinetic X-ray or neutron reflectivity data to enable faster measurements with less beam damage.

Narrowband aperiodic multilayers with flat spectral response for plasma diagnostics

Aperiodic W/Si multilayer mirrors with the narrow bandwidth and the flat spectral response are presented as X-ray reflectors operated at 8.05 keV for plasma diagnostics. In this manuscript, aperiodic W/Si samples with different periods and thickness ranges were prepared on ultra-smooth silicon (100) substrates by direct-current magnetron sputtering. Grazing incidence X-ray reflection measurements were conducted to characterize the thickness distribution and the interfacial width of multilayer samples. Depth profiles of major elements in a periodic multilayer were investigated using time-of-flight secondary ion mass spectrometry to qualitatively estimate the effect of impurities from the residual gas. Cross-sectional transmission electron microscope images of the aperiodic multilayer indicate the asymmetric interfaces with the increasing interfacial width. The cumulative roughness model and the diffusion layer model were employed to fit the thickness sequence, interfacial roughness and diffusion zone width from GIXRR curves for further optimization with the given interfacial effect. The reflection spectrum of the optimal W/Si sample was measured at the beamline BL09B of Shanghai Synchrotron Radiation Facility. The measured results of the aperiodic W/Si multilayer exhibit the (56.2 ± 1.3)% reflectivity, 936eV bandwidth and 581eV flat-band region.

In situ X-ray reflectivity and GISAXS study of mesoporous silica films grown from sodium silicate solution precursors

An environmentally friendly and inexpensive silica source, sodium silicate solution, was applied to synthesize a free-standing mesoporous silica film at the air/liquid interface, exploiting the co-assembly of cetyltrimethylammonium bromide and polyethylenimine. The effect of the composition of the solution used for the film formation on the mesostructure of the as-synthesized silica films, characterized by small angle X-ray scattering (SAXS), was investigated. The initial film formation time is estimated by the change in surface pressure with time. Additionally, a possible formation process of the mesostructured silica film is proposed using data from in situ grazing incidence small angle X-ray scattering (GISAXS) and X-ray reflectivity (XRR) measurements. A free-standing film with a wormlike structure was formed at the interface and reorganized into a 2D hexagonal ordered structure while drying at room temperature, after removal from the air/solution interface. The ordered 2D hexagonal structure, however, could only be retained to some extent during calcination, in samples where nitrate ions are present in the film formation solution. • Free standing mesostructured silica films were prepared from alkaline solutions • Cheap, more environmentally friendly sodium silicate precursors were used • Polyelectrolyte-surfactant templates allow film growth under alkaline conditions • In situ study shows initial wormlike mesostructures become more ordered upon drying

Determination of the interdiffusion coefficient at low temperature in Ni/NiCr nanometer-scale multilayers using X-ray reflectivity

The interdiffusion coefficient in the Ni/Cr system was determined at low temperature using Ni/Ni0.78Cr0.22 nanometer-scale multilayers with periods of 3.65 and 4.50 nm. X-ray reflectivity measurements were carried out to monitor the 1st Bragg peak intensity with annealing time, at both 400 and 450 °C temperatures. The obtained kinetics of the composition modulation decay allowed determining the interdiffusion coefficients of the uniform final solid solution, Ni0.89Cr0.11. Values in the order of 10−21 to 10-20 cm2.s−1 at 400 °C and 450 °C, respectively, were found, in line with extrapolations from data of Cr diffusion obtained at higher temperature.

Differential Surface Adsorption Phenomena for Conventional and Novel Surfactants Correlates with Changes in Interfacial mAb Stabilization.

Protein adsorption on surfaces can result in loss of drug product stability and efficacy during the production, storage, and administration of protein-based therapeutics. Surface-active agents (excipients) are typically added in protein formulations to prevent undesired interactions of proteins on surfaces and protein particle formation/aggregation in solution. The objective of this work is to understand the molecular-level competitive adsorption mechanism between the monoclonal antibody (mAb) and a commercially used excipient, polysorbate 80 (PS80), and a novel excipient, N-myristoyl phenylalanine-N-polyetheramine diamide (FM1000). The relative rate of adsorption of PS80 and FM1000 was studied by pendant bubble tensiometry. We find that FM1000 saturates the interface faster than PS80. Additionally, the surface-adsorbed amounts from X-ray reflectivity (XRR) measurements show that FM1000 blocks a larger percentage of interfacial area than PS80, indicating that a lower bulk FM1000 surface concentration is sufficient to prevent protein adsorption onto the air/water interface. XRR models reveal that with an increase in mAb concentration (0.5-2.5 mg/mL: IV based formulations), an increased amount of PS80 concentration (below critical micelle concentration, CMC) is required, whereas a fixed value of FM1000 concentration (above its relatively lower CMC) is sufficient to inhibit mAb adsorption, preventing mAb from co-existing with surfactants on the surface layer. With this observation, we show that the CMC of the surfactant is not the critical factor to indicate its ability to inhibit protein adsorption, especially for chemically different surfactants, PS80 and FM1000. Additionally, interface-induced aggregation studies indicate that at minimum surfactant concentration levels in protein formulations, fewer protein particles form in the presence of FM1000. Our results provide a mechanistic link between the adsorption of mAbs at the air/water interface and the aggregation induced by agitation in the presence of surfactants.

The growth of high-crystalline quality Mn2Au (110) thin films

M n 2 Au is a bimetallic antiferromagnetic with an extraordinarily high Néel temperature of 1600 K. Mn2Au has also been found to demonstrate Néel-order spin–orbit torque, allowing for electrical switching of the antiferromagnetic Néel vector. This has led to much interest in this material for spintronic applications. Here, we detail the growth of high quality Mn2Au (110) thin films. The films were grown on a high quality Pt (111) buffer layer, which was grown on an Al2O3 (0001) substrate. The Mn2Au films have excellent crystalline quality as demonstrated by x-ray diffraction, x-ray reflection, and reflection high energy electron diffraction measurements. The Mn2Au (110) films are found to grow with three equivalent in-plane rotation domains, caused by the Pt/Al2O3 substrate.

Nanoscale Structure of Langmuir-Blodgett Film of Bent-Core Molecules.

Bent-core mesogens (BCMs) are a class of thermotropic liquid crystals featuring several unconventional properties. However, the interpretation and technological exploitation of their unique behavior have been hampered by the difficulty of controlling their anchoring at surfaces. To tackle this issue, we report the nanoscale structural characterization of BCM films prepared using the Langmuir–Blodgett technique. Even though BCMs are quite different from typical amphiphilic molecules, we demonstrate that stable molecular films form over water, which can then be transferred onto silicon substrates. The combination of Brewster angle microscopy, atomic force microscopy, and X-ray reflectivity measurements shows that the molecules, once transferred onto a solid substrate, form a bilayer structure with a bottom layer of flat molecules and an upper layer of upright molecules. These results suggest that Langmuir–Blodgett films of BCMs can provide a useful means to control the alignment of this class of liquid crystals.

Cr/C multilayer growth on a heavy metal layer for upgrading of high efficiency tender x-ray gratings.

To increase efficiency of single layer gratings used in the tender x-ray range, a high reflectance multilayer can be directly grown on single layer gratings. Multilayer growth quality was studied by depositing the Cr/C multilayer on a Pt single layer using flat substrates. Their structure quality and adhesion were characterized by atomic force microscopy (AFM), grazing incidence x-ray reflectivity (GIXRR), x-ray scattering (XRS), x-ray diffraction (XRD), and layer adhesion measurement. AFM results showed that the surface roughness was 0.218 nm for the multilayer without the Pt layer and 0.272 nm for the multilayer with the Pt layer. As GIXRR results showed, the average interface widths were 0.39 nm for the multilayer without the Pt layer and 0.42 nm for the multilayer with the Pt layer. XRS results indicated that the existence of a Pt layer enlarged slightly the roughness of the multilayer. Simulation results exhibited that these slight changes caused by the Pt layer had an insignificant effect on reflectivity. As XRD results displayed, the crystallization of the Pt layer had negligible effects on the crystallization of Cr in films. The layer adhesion measurement revealed that the critical loads to peel off the layer from the substrate were 84.64 mN for the multilayer without the Pt layer and 33.99 mN for the multilayer with the Pt layer. After 6 months, the latter layer structure is undamaged, demonstrating that the coating is not easily peeled off. This study proves the feasibility to upgrade a low efficiency single Pt layer grating to a highly efficient multilayer grating.

Study of interface reaction in a B4C/Cr mirror at elevated temperature using soft X-ray reflectivity.

Boron carbide is a prominent material for high-brilliance synchrotron optics as it remains stable up to very high temperatures. The present study shows a significant change taking place at 550°C in the buried interface region formed between the Cr adhesive layer and the native oxide layer present on the silicon substrate. An in situ annealing study is carried out at the Indus-1 Reflectivity beamline from room temperature to 550°C (100°C steps). The studied sample is a mirror-like boron carbide thin film of 400 Å thickness deposited with an adhesive layer of 20 Å Cr on a silicon substrate. The corresponding changes in the film structure are recorded using angle-dependent soft X-ray reflectivity measurements carried out in the region of the boron K-edge after each annealing temperature. Analyses performed using the Parratt recursive formalism reveal that the top boron carbide layer remains intact but interface reactions take place in the buried Cr-SiO2 region. After 300°C the Cr layer diffuses towards the substrate. At higher temperatures of 500°C and 550°C the Cr reacts with the native oxide layer and tends to form a low-density compound of chromium oxysilicide (CrSiOx). Depth profiling of Si and Cr distributions obtained from secondary ion mass spectroscopy measurements corroborate the layer model obtained from the soft X-ray reflectivity analyses. Details of the interface reaction taking place near the substrate region of boron carbide/Cr sample are discussed.

Investigation of Exchange Bias in Irmn/Nife Bilayer Thin Films Using Magnetoresistance Technique

A bottom pinned Si/SiO2/Ta(20)/IrMn(15)/NiFe(10)/Ta(5) film has been fabricated using an ion beam sputtering system at room temperature. Post-deposition magnetic annealing was performed on sputtered multilayer thin film in a vacuum of 2´10-6 mbar at 250˚C for one hour in an in-plane magnetic field of 1.8 kG. The X-ray diffraction and X-ray reflectivity measurements have been performed for the structural and thickness analysis, respectively. The roughness of individual layers was determined using XRR measurements. The longitudinal MOKE measurements have been performed to measure the exchange bias, coercivity, and magnetic anisotropy at room temperature. The magnetoresistance measurement technique is employed to demonstrate the thermal evaluation of the exchange bias effect, training effect, and magnetization reversal asymmetry at room temperature and low temperature. The magnetoresistance measurement revealed a large exchange bias, pronounced magnetization reversal asymmetry, and enhancement in the training effects at 20 K. The experimental result exhibits the occurrence of temperature-dependent change from uniaxial to biaxial exchange anisotropy.

Annealing effect on SiNx/SiO2 superlattice with ultrathin sublayer fabricated using plasma-enhanced atomic layer deposition

Because of the precise control of film thickness, atomic layer deposition (ALD) offers significant advantages in fabricating high-quality superlattice films. For the first time, a SiNx/SiO2 superlattice with an ultrathin SiNx sublayer layer (1.5 nm) and a SiO2 barrier layer (3 nm) was successfully fabricated using plasma-enhanced ALD. Transmission electron microscopy (TEM) measurements demonstrate the maintenance of multilayer structures with smooth interfaces. Next, the annealing effect on the microstructural, optical, and chemical properties of these SiNx/SiO2 superlattice structures was thoroughly investigated. With increasing annealing temperature, X-ray reflection (XRR) and TEM measurements revealed consistent changes in interface, morphology, roughness, density, and thickness. After high-temperature annealing, Fourier-transform infrared spectroscopy (FTIR) measurements combined with X-ray photoelectron spectroscopy (XPS) measurements revealed significant reductions in Si–H-related defects and absorption bands. Photoluminescence (PL) was used to characterize the light emission property, and it was discovered that increasing the annealing temperature decreased the PL peak intensity at ∼550 and 900 nm. The silicon nanocrystals (Si–NCs)-related PL peak (∼900 nm) became dominant as the annealing temperature increased. The findings in this paper shed light on the annealing effect on the new superlattice material prepared via ALD. Improved PL properties of this type of superlattice are expected and can be obtained in future research by optimizing the ALD deposition process and annealing parameters.

Nondestructive x-ray reflectivity analysis of Al distributions of ultraviolet-cured spin-coated resist films hybridized with trimethylaluminum

Ultraviolet (UV)-cured spin-coated resist films of 100 nm and thinner thicknesses made with bisphenol A-based dimethacrylate monomers on silicon substrates were hybridized by multiple-pulsed vapor infiltration of sequential trimethylaluminum (TMA) doses and a final H2O dose. Nondestructive x-ray reflectivity measurements without film shrinkage enabled characterization of the internal layer structures of the UV-cured films unmodified before hybridization and hybridized by TMA infiltration and chemical fixing. It was possible to determine the thickness, density, and interface roughness of each layer in the unmodified and hybridized UV-cured films. The TMA infiltration and chemical fixing resulted in the formation of the highest-density Al-rich layer near the film surface. The presence of the highest-density layer arising from the most abundant component, Al, was confirmed through energy-dispersive x-ray spectroscopy and time-of-flight secondary ion mass spectrometry. The increase in the cycle number of TMA dose from 100 to 150 and 200 cycles had little effect on organic–inorganic hybridization of the 100 nm-thick UV-cured films. The thickness of the outermost Al-rich layer near the film surface was approximately 30 nm, which was unchanged when the film thicknesses of 100, 75, and 50 nm were changed. The outermost skin layer suppressed the infiltration of TMA into the UV-cured films. The infiltration of TMA into the UV-cured films progressed as the film thicknesses decreased.

X-ray reflectivity from curved surfaces as illustrated by a graphene layer on molten copper

The X-ray reflectivity technique can provide out-of-plane electron-density profiles of surfaces, interfaces, and thin films, with atomic resolution accuracy. While current methodologies require high surface flatness, this becomes challenging for naturally curved surfaces, particularly for liquid metals, due to the very high surface tension. Here, the development of X-ray reflectivity measurements with beam sizes of a few tens of micrometres on highly curved liquid surfaces using a synchrotron diffractometer equipped with a double crystal beam deflector is presented. The proposed and developed method, which uses a standard reflectivity θ-2θ scan, is successfully applied to study insitu the bare surface of molten copper and molten copper covered by a graphene layer grown in situ by chemical vapor deposition. It was found that the roughness of the bare liquid surface of copper at 1400 K is 1.25 ± 0.10 Å, while the graphene layer is separated from the liquid surface by a distance of 1.55 ± 0.08 Å and has a roughness of 1.26 ± 0.09 Å.

Background Pressure Induced Structural and Chemical Change in NiV/B4C Multilayers Prepared by Magnetron Sputtering

NiV/B4C multilayers with a small d-spacing are suitable for multilayer monochromator working at a photon energy region from 5 to 8 keV, or photon energy region from 10 to 100 keV. To investigate the influence of background pressure during fabrication, NiV/B4C multilayers with a d-spacing of 3.0 nm were fabricated by magnetron sputtering with different background pressures. The grazing incidence x-ray reflectivity (GIXR) and transmission electron microscopy (TEM) measurement illustrated the structural change that happened in NiV/B4C multilayers when background pressure is high. The electron dispersive x-ray spectroscopy (EDX) of NiV/B4C multilayer deposited with a high background pressure suggests a gradient distribution of oxygen, which corresponds to the gradient thickness change. The detailed x-ray absorption near edge spectroscopy (XANES) comparison of NiV/B4C multilayers, NiV coating, and B4C coating showed the chemical state change induced by background pressure. We concluded that during the deposition, vanadium oxide promoted the oxidation of boron. In order to fabricate a good performance of NiV/B4C multilayers, the background pressure needs lower than 1 × 10−4 Pa.

Maxwell-Wagner Relaxation-Driven High Dielectric Constant in Al2O3/TiO2 Nanolaminates Grown by Pulsed Laser Deposition.

Multilayer nanolaminates (NLs) of alternate ultrathin sublayers of Al2O3 and TiO2 (ATA) with the thickness ranging ∼2 to 0.5 nm were fabricated by optimized pulsed laser deposition (PLD). Maxwell-Wagner (M-W) relaxation-induced interfacial polarization was realized and engineered by precisely controlling the sublayer thicknesses and the number of interfaces. X-ray reflectivity and cross-sectional transmission electron microscopy measurements of ATA NLs revealed an artificial periodicity with well-defined uniformly thick amorphous sublayers with chemically and physically distinct interfaces down to a sublayer thickness of ∼0.8 nm. The dielectric constants and loss of ATA NLs were found to increase from ∼60 to 670 and decrease from ∼0.9 to 0.16, respectively, as sublayer thicknesses reduced from ∼2 to 0.8 nm. However, for a sublayer thickness below 0.8 nm, the trend was reversed. Furthermore, temperature-dependent impedance spectroscopy studies revealed two distinct thermally activated relaxation processes, corresponding to TiO2 and Al2O3 sublayers, corroborating the M-W relaxation. The conductivity contrast between the sublayers of ATA NLs enhanced with reducing sublayer thickness and plateaued at a sublayer thickness of ∼0.8 nm, resulting in dominant M-W interfacial polarization and a high cut-off frequency of ∼50 kHz. These results demonstrate that ATA NLs grown by PLD may find application as potential high-k materials for next-generation nanoelectronic devices.

Magnetic-field and temperature dependence of anomalous Hall effect in Pt/Cr2O3/Pt trilayer

The Pt/Cr2O3 interface exhibits a variety of spin-related phenomena. In this study, we investigated the anomalous Hall effect (AHE) of a Pt/40-nm-thick Cr2O3/Pt trilayer grown on Al2O3(0001), where Cr2O3 is an antiferromagnetic (AFM) insulator. All layers were grown epitaxially on the substrate, and X-ray reflectivity measurement showed an interfacial roughness of approximately 0.2 nm at each interface. The AHE resistance showed a nonlinear magnetic-field dependence at 300 K. Below 250 K, a clear hysteresis with coercivity was observed. The coercivity was approximately 7 T above 150 K and approximately 6 T below 100 K. The remanent AHE resistance shows a finite value below 285 K. The temperature dependence forms a broad peak with a maximum at approximately 200 K and gradually decreases with decreasing temperature. This temperature dependence is similar to that reported for the interfacial magnetic moment on Cr2O3(0001). Based on these results, the remanent AHE coupled with the AFM order parameter or the Néel vector was successfully detected in the 40-nm-thick Cr2O3 layer grown on the conductive layer.

Anomalous Critical Field Dependence of Cr-Nb-Cr Trilayers on Niobium Thickness

Cr-Nb-Cr trilayer thin films were deposited using DC/RF sputtering with Cr layer thickness maintained at 7.5 nm and Nb thickness varying from 30 nm to 229 nm, in order to study the interplay of antiferromagnetism and superconductivity. Various viewpoints have been expressed in the literature with respect to analyzing the anomalous properties that have been identified in superconductor/magnetic thin films. The study of Cr-Nb-Cr thin films were taken up to identify the existence of similar anomalous superconducting properties on account of antiferromagnetic-superconductor interfaces. X-ray reflectivity measurements showed high quality of thin film structure with minimum surface and interface roughness. Transport measurements down to 2K were carried out in the presence of magnetic fields up to 2 T applied perpendicular to the trilayers to deduce Tc. The suppression of Tc due to Cr layer is larger than proximity effect of similar metal films. It could be explained by proximity effect using antiferromagnetic Cr layer. Upper critical field measurements show a distinct nonmonotonic dependence of upper critical magnetic field and the slope on Nb layer thickness. The analysis due to WHH theory to deduce upper critical fields for the trilayer thin films studied did not match with experimental values. Although studies performed on Cr-Nb-Cr trilayers did not show any anomaly in Tc, it clearly showed a depression of Tc much largerthan proximity effect, non monotonic behavior in Bc2(0) and dBc2/dTc behavior with Nb layer thickness.

Electronic and magnetic properties of epitaxial thin film of Nd0.5Ba0.5MnO3

Contradictory reports about the electronic and magnetic behavior of bulk Nd0.5Ba0.5MnO3 with uniform disorder exist in the literature. In this work, we investigated single crystalline, A-site disordered thin films of Nd0.5Ba0.5MnO3, grown on the SrTiO3 substrate by pulsed laser deposition. The epitaxial growth of these films in the layer-by-layer fashion has been confirmed by in situ reflection high energy electron diffraction, atomic force microscopy, x-ray reflectivity, and x-ray diffraction measurements. Interestingly, these films are found to be electrically insulating and exhibit spin-glass behavior at low temperatures. This offers a distinct approach to study Ba based half-doped manganites and an opportunity to further manipulate competing charge/orbital ordering and ferromagnetism in such systems, through heterostructure engineering.

Molecular beam epitaxy of TmTe thin films on SrF2 (111)

The odd parity nature of 4f states characterized by strong spin–orbit coupling and electronic correlations has led to a search for novel topological phases among rare earth compounds, such as Kondo systems, heavy Fermions, and homogeneous mixed-valent materials. Our target system is thulium telluride thin films whose bandgap is expected to be tuned as a function of lattice parameter. We systematically investigate the growth conditions of TmxTey thin films on SrF2 (111) substrates by molecular beam epitaxy. The ratio between Te and Tm supply was precisely tuned, resulting in two different crystalline phases, which were confirmed by x-ray diffraction and x-ray photoemission spectroscopy. By investigating the crystalline quality as a function of the substrate temperature, the optimal growth conditions were identified for the desired Tm1Te1 phase. Additional low energy electron diffraction and reflective high energy electron diffraction measurements confirm the epitaxial growth of TmTe layers. X-ray reflectivity measurements demonstrate that homogeneous samples with sharp interfaces can be obtained for varied thicknesses. Our results provide a reliable guidance to prepare homogeneous high-quality TmTe thin films and thus serve as a basis for further electronic investigations.