Soft X-ray Reflectivity Research Articles

Using magneto-optical effects in soft X-ray reflectivity to study current driven magnetization reversal

The soft X-ray reflectivity technique is frequently utilized for studying magnetization reversal in thin films due to its elemental and depth sensitivity. The characteristic hysteresis loops measured with this technique are dependent on both the magnetization direction in magnetic materials and the incident soft X-ray polarization. In this note, we have discussed these magneto-optical effects in soft X-ray reflectivity measurements. These effects can be exploited to probe magnetization reversal mechanisms driven by stimuli beyond conventional means of magnetic field. To demonstrate this, we have presented our investigations on current-induced magnetization switching in ferromagnet (FM)/heavy metal(HM) heterostructures.

Applicability of energy-theta mapping in resonant soft X-ray reflectometry to probe depth dependence of oxidation state and crystallographic environment in iron oxide multilayers

In the present paper we discuss applicability of the synchrotron method of resonant x-ray reflectometry to non-destructive depth profiling of multilayer heterostructures with low optical contrast between the sublayers. The two-dimensional mapping approach comprising evaluation and measurement of reflectance as a function of photon energy and grazing angle is shown to provide a convenient way to effectively utilize the enhancement of optical contrast between the sublayers that exists at the absorption edges of chemical elements due to particular spectral shape peculiarities related to oxidation state, crystallographic environment and magnetization. In the present study the evaluation of the resonant X-ray reflectivity maps is performed using a specially developed modeling and fitting software. Estimation of the photon energy / grazing angle combinations at which the reflectance is most sensitive to the minor changes in the physical properties of the sublayers is illustrated by the example of ferroic-on-semiconductor nanoscale heterostructures composed of nanoscale film of metallic iron oxidized from either top or bottom side. The subtle differences in resonant soft x-ray reflectance caused by variation of the oxide film thickness, oxidation state and crystallographic environment are discussed. The presented approach is applicable to a large class of heterostructures composed of sublayers that can be distinguished only by the differences in their near edge X-ray absorption fine structure.

A versatile beamline for soft x-ray reflectivity, absorption, and fluorescence measurements at Indus-2 synchrotron source.

A versatile beamline for performing reflectivity, fluorescence, and absorption experiments in the soft x-ray region of 100-1500eV is commissioned on a bending magnet port of the Indus-2 synchrotron source. A high vacuum 2-axis reflectometer with x, y, and z sample scanning stages is installed. This reflectometer is used to measure the reflectivity of large samples up to 300mm in length and 5kg in weight. This feature is useful for characterizing x-ray optical elements, such as mirrors, gratings, and multilayers. A flange mounted silicon drift detector is installed in the downstream of the reflectometer for soft x-ray fluorescence measurements. The soft x-ray absorption measurements are carried out in the total electron yield and partial fluorescence yield modes. Integration of three different experimental techniques in the experimental station makes the beamline versatile for materials science applications as it provides structural, chemical, and electronic state information by performing the required experiments in an identical environment. The beamline uses a varied line spacing plane grating monochromator and gives a high flux (∼109 to 1011 photon/s) with a moderate resolution (λ/Δλ ~1000-5000). A three-mirror-based higher harmonic setup is incorporated to get rid of harmonics and to get a high spectral purity monochromatic beam with less than 0.1% harmonic content. In the present article, the beamline optical scheme, mechanical configuration, and details of the experimental setups are presented, along with a few representative results of each experimental mode.

Composition Dictates Molecular Orientation at the Heterointerfaces of Vapor-Deposited Glasses.

Physical vapor deposition (PVD) can prepare organic glasses with a preferred molecular orientation. The relationships between deposition conditions and orientation have been extensively investigated in the film bulk. The role of interfaces on the structure is less well understood and remains a key knowledge gap, as the interfacial region can govern glass stability and optoelectronic properties. Robust experimental characterization has remained elusive due to complexities in interrogating molecular organization in amorphous, organic materials. Polarized soft X-rays are sensitive to both the composition and the orientation of transition dipole moments in the film, making them uniquely suited to probe molecular orientation in amorphous soft matter. Here, we utilize polarized resonant soft X-ray reflectivity (P-RSoXR) to simultaneously depth profile the composition and molecular orientation of a bilayer prepared through the physical vapor deposition of 1,4-di-[4-(N,N-diphenyl)amino]styryl-benzene (DSA-Ph) on a film of aluminum-tris(8-hydroxyquinoline) (Alq3). The bulk orientation of the DSA-Ph layer is controlled by varying deposition conditions. Utilizing P-RSoXR to depth profile the films enables determination of both the bulk orientation of DSA-Ph and the orientation near the Alq3 interface. At the Alq3 surface, DSA-Ph always lies with its long axis parallel to the interface, before transitioning into the bulk orientation. This is likely due to the lower mobility and higher glass transition of Alq3, as the first several monolayers of DSA-Ph deposited on Alq3 appear to behave as a blend. We further show how orientation at the interface correlates with the bulk behavior of a codeposited glass of similar blend composition, demonstrating a straightforward approach to predicting molecular orientation at heterointerfaces. This work provides key insights into how molecules orient during vapor deposition and offers methods to predict this property, a critical step toward controlling interfacial behavior in soft matter.

Implementing 0.1 nm B4C barriers in ultrashort period 1.0 nm W/Si multilayers for increased soft x-ray reflectance

Ultrashort period 1.0 nm W/Si multilayers have potential as dispersive Bragg reflectors in high-resolution x-ray fluorescence. However, formation of WSix leads to poor optical performance. To address this, we introduce ultrathin 0.1 nm B4C diffusion barriers in sputter-deposited 1.0 nm W/Si, inhibiting W–Si interaction. We demonstrate that the peak reflectance at a wavelength of 0.834 nm increased with a factor of 3.4 compared to W/Si. Diffuse scattering measurements reveal no change in interfacial roughness when applying B4C barriers compared to W/Si. X-ray reflectivity analysis shows a substantial increase in optical contrast between Si and W as well as sharper transitions between the layers. Chemical analysis suggests that the B4C barrier reduces formation of WSix through partial substitution of W-silicide bonds with W-carbide/boride bonds, leading to an increase in optical contrast. The resulting structure of W/Si with B4C barriers offers a compelling alternative to the more established W/B4C multilayer at the ultrashort scale due to its superior soft- and hard x-ray reflectance.

Effect of electronic transitions on near edge optical properties of off-stoichiometric boron carbide thin films

In the present study, soft x-ray optical properties of off-stoichiometric boron carbide thin films are investigated, and the structure and chemical composition of the film is analyzed using angle dependent x-ray reflectivity and x-ray photoelectron spectroscopy techniques. Energy dependent soft x-ray reflectivity measured at a fixed grazing angle of 1.5° is used to determine the optical constants in the boron K edge region by applying the Kramers–Kronig technique. The measured optical constants show near edge fine features corresponding to σ* and π* resonances. The electronic transitions corresponding to σ* resonance cause a 40%–75% increase in the delta value in the above boron K edge region. The π* transitions corresponding to off-stoichiometric nature of the boron carbide are observed in the absorption spectra near ∼192.7 eV. Details of the measured soft x-ray optical properties of the off-stoichiometric boron carbide thin film are discussed.

Boron carbide thin film surface characterization after graphitic carbon removal using low-pressure oxygen gas RF plasma.

B 4 C-coated thin film mirrors are used in high brilliance synchrotron and x-ray free electron laser beamlines due to their low absorption coefficient and high thermal stability. As in the case of gold, platinum, and other thin film mirrors, B 4 C-coated mirrors also are affected due to synchrotron radiation-induced carbon contaminations in beamlines. In the present study, a graphitic carbon (C) layer deposited on top of boron carbide (B x C) thin film surface is removed by five successive oxygen radio frequency (RF) plasma exposures (RF power, 10W; O 2 flow, 30sccm; exposure time, 10min each). Before and after the carbon layer removal, structural and compositional properties of the B x C/C bilayer are characterized by soft x-ray reflectivity, x-ray photoelectron spectroscopy, grazing angle x-ray diffraction, and Raman spectroscopy techniques. Characterization results reveal that in the first four exposures the carbon layer thickness decreases continuously without affecting the B x C layer properties; however, in the fifth exposure, the carbon layer is completely removed along with a partial etching of the B x C layer too.

Determining Structure and Thermodynamics of A-b-(B-r-C) Copolymers.

The self-assembly of block copolymers (BCPs) is dictated by their segregation strength, χN, and while there are well-developed methods for determining χ in the weak and strong segregation regimes, it is challenging to accurately measure χ of copolymers with intermediate segregation strengths, especially when copolymers have inaccessible order-disorder transition temperatures. χeff is often approximated by using strong segregation theory (SST), but utilizing these values to estimate the interface width (wm) of BCPs in the intermediate segregation regime often results in predictions that deviate significantly from measured values. Therefore, we propose using the extent of mixing, quantified as the normalized interface width wm/L0, where L0 is the block copolymer pitch, as a thermodynamic parameter. We experimentally measure wm and L0 for a series of lamellar A-b-(B-r-C) copolymers via resonant soft X-ray reflectivity and extract values of χeffN based on previous data collected for A-b-B copolymers. The composition profiles measured via reflectivity match the extracted χeffN values, while those calculated with SST predict much more mixed composition profiles. The extracted χeff values agreed quantitatively between copolymers of different molecular weights. We believe that this methodology will be well-suited for block copolymers used in lithographic applications due to their inaccessible order-disorder transition temperatures, intermediate values of χN, and the importance of wm for line edge roughness metrics.

Boron migration during amorphous to crystalline transformation in CoFeB/MgO multilayers: A reflectivity study

Migration of boron in CoFeB/MgO multilayer during amorphous to crystalline transformation in CoFeB has been studied using Polarized Neutron Reflectivity (PNR) and anomalous Soft X-ray Reflectivity (SXR). PNR gives information about the possible redistribution of B in the multilayer during thermal annealing, as well as about the variation in the average magnetic moment. Sensitivity of the soft X-ray reflectivity to the movement of boron is enhanced by doing a series of measurements with X-ray energy spanning across the K absorption edge of B. Both the PNR and anomalous SXR results clearly suggest that upon thermal annealing, boron prefers to move away from the MgO/CoFeB interfaces, and forms a layer of boride in the middle of the magnetic layer. These results are important in view of the existing ambiguity regarding the role of boron in CoFeB/MgO based magnetic tunnel junctions.

Design of a soft x-ray emission spectrometer setup for soft x-ray reflectivity beamline of Indus-2

A soft x-ray reflectivity beamline is operational on Indus-2 synchrotron source since 2016 for reflectivity, transmission and absorption measurements in 100-1500 eV photon energy range. To perform spectroscopic experiments in 60-1000 eV spectral range a soft x-ray emission spectrometer (SXES) is designed which will serve as an additional experimental station on the beamline. The SXES setup is designed using aberration corrected varied line spacing flat filed gratings which will disperse soft x-ray emission photons on a CCD screen. Gratings of nominal line density of 1200 l/mm and 2400 l/mm are used to cover 60-250 eV and 120-1000 eV energy range respectively.

Interface analysis of Mg/Sc and Sc/Mg bilayers using X-ray reflectivity

• Interfaces at the Mg/Sc and Sc/Mg bilayers are investigated. • Bilayers are studied by secondary ion mass spectrometry and x-ray reflectivity. • Mg-on-Sc and Sc-on-Mg interfaces in the bilayers are found to be asymmetric. Mg/Sc and Sc/Mg bilayers were studied to investigate their interfaces. Time-of-flight secondary ion mass spectroscopy and grazing incidence x-ray reflectivity are combined to determine the realistic structure of the bilayer stack: thickness, refractive index, and roughness of layers and interlayers respectively. In addition, soft x-ray reflectivity measurements provide sensitive information about the chemical environment of scandium, when the incident photon energy is tuned in the vicinity of the Sc L-absorption edge (∼ 400 eV). Both Mg-on-Sc and Sc-on-Mg interfaces have high roughness (∼1 nm) and a MgSc compound interlayer but are shown to be asymmetric.

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.

High reflectivity Co/Mg multilayer working in the broad soft x-ray range of 350–770 eV

A Co/Mg multilayer was proposed and optimized to work in the broad soft x-ray range of 350–770 eV. The multilayers with a d-spacing of 6.0 nm showed large interface widths of 1.95–1.07 nm which were significantly improved to 0.65–0.62 nm by using a higher Co sputtering power. Grazing incidence x-ray reflectometry, atomic force microscopy, transmission electron microscopy and x-ray diffraction measurements were used to study the multilayer structure and a more ordered polycrystalline structure was found in the multilayers deposited with higher Co power, which can explain the great improvement. The effect of Ar sputtering pressure on the layer structure was also studied. The measured soft x-ray reflectivity results indicated a high reflectivity of 18%–27.5% at 400–700 eV can be reached if a saturated number of bilayers of 50 were deposited.

Characterization of the Interfacial Orientation and Molecular Conformation in a Glass-Forming Organic Semiconductor.

The ability to control structure in molecular glasses has enabled them to play a key role in modern technology; in particular, they are ubiquitous in organic light-emitting diodes. While the interplay between bulk structure and optoelectronic properties has been extensively investigated, few studies have examined molecular orientation near buried interfaces despite its critical role in emergent functionality. Direct, quantitative measurements of buried molecular orientation are inherently challenging, and many methods are insensitive to orientation in amorphous soft matter or lack the necessary spatial resolution. To overcome these challenges, we use polarized resonant soft X-ray reflectivity (p-RSoXR) to measure nanometer-resolved, molecular orientation depth profiles of vapor-deposited thin films of an organic semiconductor Tris(4-carbazoyl-9-ylphenyl)amine (TCTA). Our depth profiling approach characterizes the vertical distribution of molecular orientation and reveals that molecules near the inorganic substrate and free surface have a different, nearly isotropic orientation compared to those of the anisotropic bulk. Comparison of p-RSoXR results with near-edge X-ray absorption fine structure spectroscopy and optical spectroscopies reveals that TCTA molecules away from the interfaces are predominantly planar, which may contribute to their attractive charge transport qualities. Buried interfaces are further investigated in a TCTA bilayer (each layer deposited under separate conditions resulting in different orientations) in which we find a narrow interface between orientationally distinct layers extending across ≈1 nm. Coupling this result with molecular dynamics simulations provides additional insight into the formation of interfacial structure. This study characterizes the local molecular orientation at various types of buried interfaces in vapor-deposited glasses and provides a foundation for future studies to develop critical structure-function relationships.

Three-dimensional chemical-state imaging with reflection-mode soft x-ray absorption spectroscopy.

In this study, a method for reflection-mode soft x-ray absorption spectroscopy was developed to realize three-dimensional chemical-state imaging. Soft x rays from a pinhole were reflected by the sample, and the magnified image was observed with a two-dimensional detector. This technique was applied to a Co film with an Au-island-covered surface to obtain the surface chemical state images with a spatial resolution of several tens of micrometers. Furthermore, the soft x-ray reflection spectra within and outside the Au layer were extracted from the images by changing the photon energy. Distinct differences were observed at the Co absorption edge. By considering anomalous x-ray scattering around the Co L-edges in the simulation, the reflection spectrum near the absorption edge in the nm depth resolution was reproduced. In the region without the Au layer, the results were well reproduced, assuming that 4nm CoO was formed at the surface. These results demonstrate the feasibility of three-dimensional imaging of the chemical states in multilayer films.

Effect of surface oxidation on soft x-ray optical properties of ion beam sputter deposited amorphous AlN thin film

In the present study, structural and compositional analyses of reactive ion beam sputter deposited aluminum nitride (AlN) thin film of thickness 100 Å are carried out using x-ray reflectivity and x-ray photoelectron spectroscopy techniques. Soft x-ray optical response of the film is derived from energy dependent soft x-ray reflectivity measurements performed in photon energy region of 380–1700 eV. Optical constants (δ and β) obtained from the reflectivity spectra show features corresponding to absorption edges of the constituent elements. Observed fine features in the β profile are further confirmed from x-ray absorption (XAS) measurements carried out in the total electron yield mode. The measured XAS spectra are correlated with electronic and compositional properties of the AlN film. The effects of surface oxidation on soft x-ray optical properties of the AlN thin film are discussed.

Dependence of exchange bias on structure of antiferromagnet in Fe/IrMn[formula omitted

Single-crystal exchange biased multilayer thin-films consisting of an Fe layer on single-crystal chemically ordered L12-IrMn3 and disordered γ-IrMn3 were investigated using circularly and linearly polarised soft X-ray reflectivity to determine the interfacial magnetic structure between Fe and IrMn3. In ordered L12-IrMn3, we found that the Mn uncompensated moments at the interface are strongly pinned and only Fe rotational moments are observed. In disordered γ-IrMn3 the uncompensated moments are partially pinned, where the Mn moments are rotatable but the rotations are restricted. These findings are a crucial development in exchange bias theory to understand the force that drives the magnetic reversal process in ordered and disordered IrMn3.

Resonant Soft X-ray Reflectivity in the Study of Magnetic Properties of Low-Dimensional Systems

In this review, the technique of resonant soft X-ray reflectivity in the study of magnetic low-dimensional systems is discussed. This technique is particularly appealing in the study of magnetization at buried interfaces and to discriminate single elemental contributions to magnetism, even when this is ascribed to few atoms. The major fields of application are described, including magnetic proximity effects, thin films of transition metals and related oxides, and exchange-bias systems. The fundamental theoretical background leading to dichroism effects in reflectivity is also briefly outlined.

Evolution of the interface microstructure of short-period Cr/Ti multilayers with increase in number of bi-layers

Evolution of interface microstructure with addition of more number of layers has been investigated on Cr/Ti multilayer which is important for application as a mirror or monochromator in the “water window” soft X-ray spectral regime. Two sets of Cr/Ti multilayers at moderate and low bi-layer thickness regime have been prepared for this purpose using Ion Beam Sputtering technique. In one set, Cr/Ti multilayer with 25, 50, 60 and 100 numbers of bilayers with moderate bi-layer thickness of 3.8 nm have been deposited. In another set, four samples each having low bilayer thickness of 2.4 nm with 25, 50, 75 and 100 number of bi-layers have been prepared and the multilayers have subsequently been characterized with Specular X-ray reflectivity and diffuse X-ray scattering measurements at grazing incidence with hard X-rays. It is observed that cumulative effect of interface imperfections became more pronounced with increasing number of bilayers resulting in continuous increase in the interface width in both the thickness regimes; however, its evolution is not exactly same for the two sets of samples. Diffuse X-ray scattering result reveals that for the first set of samples, interface imperfections are dominated by interface roughness with no change in the interface diffusion with increase in number of layers, while for the second set, both interface roughness as well as interface diffusion contribute towards the cumulative accumulation of interface imperfections which is also confirmed by cross-sectional Transmission Electron Microscopy. Finally, performance of the multilayers in water window regime has been checked by soft X-ray reflectivity measurements using synchrotron radiation. The above results have direct implication in utilisation of these multilayers over a wide bi-layer thickness regime required to cover the full ‘water-window’ spectrum.

Buried Structure in Block Copolymer Films Revealed by Soft X-ray Reflectivity.

Interactions between polymers and surfaces can be used to influence properties including mechanical performance in nanocomposites, the glass transition temperature, and the orientation of thin film block copolymers (BCPs). In this work we investigate how specific interactions between the substrate and BCPs with varying substrate affinity impact the interfacial width between polymer components. The interface width is generally assumed to be a function of the BCP properties and independent of the surface affinity or substrate proximity. Using resonant soft X-ray reflectivity the optical constants of the film can be controlled by changing the incident energy, thereby varying the depth sensitivity of the measurement. Resonant soft X-ray reflectivity measurements were conducted on films of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) and PS-b-poly(methyl methacrylate) (PS-b-PMMA), where the thickness of the film was varied from half the periodicity (L0) of the BCP to 5.5 L0. The results of this measurement on the PS-b-P2VP films show a significant expansion of the interface width immediately adjacent to the surface. This is likely caused by the strong adsorption of P2VP to the substrate, which constrains the mobility of the junction points, preventing them from reaching their equilibrium distribution and expanding the observed interface width. The interface width decays toward equilibrium moving away from the substrate, with the decay rate being a function of film thickness below a critical limit. The PMMA block appears to be more mobile, and the BCP interfaces near the substrate match their equilibrium value.