Specular X-ray Reflectivity Measurements Research Articles

Effect of gas composition during Pt sputtering on structural and magnetic properties of CoFeB thin films.

Ultrathin Ta/CoFeB/Pt trilayer structures are relevant to a wide range of spintronic applications, from magnetic tunnel junctions to skyrmionics devices. Controlling the perpendicular magnetic anisotropy, Gilbert damping and Dzyaloshinskii-Moriya interaction in the CoFeB layer is key for these applications. We examine the role of sputter gas composition during the Pt overlayer deposition of a Ta/CoFeB/Pt trilayer in Ar, Kr and Xe working gas environments during direct current magnetron sputtering. Decreasing density of the Pt layer (from 21 g/cm3 to 15 g/cm3) was apparent in specular x-ray reflectivity measurements of the trilayer films when increasing the molecular weight of the sputtering gas from Ar to Kr to Xe. Significant effects on the Gilbert damping and the interfacial Dzyaloshinskii-Moriya interaction (DMI) energy were observed, with increases in the damping from 0.037(1) to 0.042(1) to 0.048(1), and reductions in the interfacial DMI from 0.47(4) mJ/m2 to 0.45(5) mJ/m2 to 0.39(4) mJ/m2. The ability to control the perpendicular magnetization and DMI strength of these materials through judicious interfacial control is a means toward magnetic devices with better stability at smaller lateral dimension, key to device scaling for spintronic device arrays.

Anti-reflective structures for photovoltaics: Numerical and experimental design

The effects of different anti-reflective structures on the photovoltaic performance of the silicon solar cell were studied using finite-element modelling and numerical simulations for which experiment alone does not provide a full description. The front surface reflectivity may be mitigated significantly by an anti-reflective coating (ARC) of a suitable thickness. Alternatively, nanoscale surface texturing can effectively trap a greater ratio of incident light to increase optical absorption. The optimized layer thicknesses of the ZnO single layer and SiO2/Si3N4 double layer films were calculated for minimum reflectivity, with the former grown by magnetron sputter deposition and characterized using specular X-ray reflectivity measurements. Based on geometric ray-tracing and solutions to the semiconductor equations, the theoretical photovoltaic performance was simulated and compared for a range of incident angles at an optical intensity of 0.1 Wcm−2, revealing a limit to the angular collection efficiency of the ARC at a grazing incidence angle of 30°. Using ZnO or SiO2/Si3N4 ARCs or surface texturing increases the power conversion efficiency by 20%, 24% and 30% respectively at normal incidence. The insights provided by physical-based modelling on the optimized design parameters of the anti-reflective structures confer a promising pathway for enhancing the external quantum efficiency of photovoltaic devices.

Calcite (1 0 4)–water interface structure, revisited

The structure of the calcite (104)–water interface is reassessed using a new set of high-precision specular X-ray reflectivity measurements. In situ measurements of the specular reflectivity signal to a vertical structural resolution of 0.45Å are used to define the interfacial structure, including vertical displacement patterns of the Ca and CO3 groups as well as the nature of interfacial water. These new data show two organized interfacial water layers, consistent with previous results, and distortion of the interfacial calcite structure to a depth of four to six unit cells, at least two deeper than previously reported. These results are in reasonable agreement with recent computational studies, at least in terms of the locations of the first and second water layers. The difference between the interfacial structure derived from previous X-ray reflectivity results and that presented here emphasizes the need for high-precision measurements to provide a robust understanding of the interfacial structures of reactive minerals in aqueous environments.

A comparative study of Langmuir surfactant films: Grazing incidence x-ray off-specular scattering vs. x-ray specular reflectivity

Surface monolayers assembled on a liquid sub-phase represent a class of systems that is of great interest for studies of phase transitions in quasi-2D systems, chemical self-assembly, surfactant behavior, and biologically relevant monolayers and membranes. X-ray scattering is ideal for studying structural, dynamic, and mechanical properties of these surface monolayers at nanoscale due to the penetrating ability and short wavelength of x-rays. We show here that grazing incidence x-ray off-specular scattering (GIXOS) provides rapid access to in-plane and out-of-plane nanoscale structure, surface fluctuating modes, and potentially bending stiffness. We show that analysis of GIXOS data is highly sensitive to resolution effects. We further present detailed analysis of GIXOS from phospholipid 1,2-dipalmitoyl-phosphatidyl-choline C40H80NO8P (DPPC) and obtain quantitative, angstrom-resolution details of electron density profile normal to the surface that is comparable to those that are obtained from specular x-ray reflectivity measurements. We compare these GIXOS results to x-ray reflectivity measurements performed on the same samples. While electron density and main structural characteristics (such as monolayer thickness) obtained by GIXOS agree with x-ray reflectivity results, the interfaces of GIXOS-derived density profiles are found to be systematically sharper than those obtained with x-ray reflectivity. The possible reasons for these differences are discussed.

X-ray radiation damage of organic semiconductor thin films during grazing incidence diffraction experiments

Since modern synchrotrons with highly intense X-ray beams are in use to investigate organic materials, the stability of soft matter materials during beam exposure is a crucial issue. Grazing incidence X-ray diffraction and specular X-ray reflectivity measurements were performed on thin films of organic semiconducting materials, like poly(3-hexylthiophene) (P3HT), sexithiophene and pentacene. These films were irradiated with an average flux density between 1015 and 1016photons/(smm2) and evidenced a different stability in synchrotron X-ray radiation. The semi-crystalline P3HT showed a clear intensity decrease of the 100 Bragg peak and 020 Bragg peak compared to the rather stable diffraction features of the molecular crystals sexithiophene and pentacene. The difference in synchrotron X-ray radiation stability is explained by the interaction of the X-ray beam with the individual chemical components in the molecules as well as by the different crystallinities of the materials. Furthermore, the semi-crystalline P3HT film exhibited an increase of film thickness after irradiation and the surface roughness slightly decreased. To summarize, this study shows a strong influence of synchrotron X-ray radiation to specific organic thin films like e.g. P3HT, while others like pentacene and sexithiophene are observed as quite stable.

Weak interfacial water ordering on isostructural hematite and corundum (0 0 1) surfaces

Ordering of interfacial water at the hematite and corundum (0 0 1)–water interfaces has been characterized using in situ high resolution specular X-ray reflectivity measurements. The hematite (0 0 1) surface was prepared through an annealing process to produce a surface isostructural with corundum (0 0 1), facilitating direct comparison. Interfacial water was found to display a similar structure on this pair of isostructural surfaces. A single layer of adsorbed water having a large vibrational amplitude was present on each surface and additional ordering of water extended at least 1 nm into the bulk fluid, with the degree of ordering decreasing with increasing distance from the surfaces. Consistent with prior studies of the (0 1 2) and (1 1 0) surfaces of hematite and corundum, the configuration of water above the (0 0 1) surfaces is primarily controlled by the surface structure, specifically the arrangement of surface functional groups. However, interfacial water at the (0 0 1) surfaces displayed significantly larger vibrational amplitudes throughout the interfacial region than at other isostructural sets of hematite and corundum surfaces, indicating weaker ordering. Comparison of the vibrational amplitudes of adsorbed water on a series of oxide, silicate, and phosphate mineral surfaces suggests that the presence or absence of a substantial interfacial electrostatic field is the primary control on water ordering and not the surface structure itself. On surfaces for which charge originates dominantly through protonation–deprotonation reactions the controlling factor appears to be whether conditions exist where most functional groups are uncharged as opposed to the net surface charge. The doubly coordinated functional groups on hematite and corundum (0 0 1) surfaces are largely uncharged under slightly acidic to circumneutral pH conditions, leading to weak ordering, whereas singly coordinated groups on (0 1 2) and (1 1 0) surfaces of these phases are always charged, even when the net surface charge is zero, and induce strong water ordering. Surfaces lacking structural charge can thus be divided into two distinct classes that induce either strong or weak ordering of interfacial water. Surface functional group coordination is the ultimate control on this division as it determines the charge state of such groups under different protonation configurations. Ion adsorption and electron transfer processes may differ between these classes of surfaces because of the effect of water ordering strength on interfacial capacitances and hydrogen bonding.

Relaxations and Interfacial Water Ordering at the Corundum (110) Surface

In situ high resolution specular X-ray reflectivity measurements were used to examine relaxations and interfacial water ordering occurring at the corundum (110)-water interface. Sample preparation affected the resulting surface structure. Annealing in air at 1373 K produced a reconstructed surface formed through an apparently ordered aluminum vacancy. The effect of the reconstruction on in-plane periodicity was not determined. The remaining aluminum sites on the surface maintain full coordination by oxygen and the surface was coated with a layer of physically adsorbed water. Ordering of water further from the surface was not observed. Acid etching of this surface and preparing a surface through annealing at 723 K both produced an unreconstructed surface with identical relaxations and water ordering. Relaxations were confined primarily to the top ∼4 A of the surface and were dominated by an increased distribution width of the fully occupied surface aluminum site and outward relaxation of the oxygen surface...

A simultaneous multiwavelength dispersive X-ray reflectometer for time-resolved reflectometry

Aiming for the realization of time-resolved specular X-ray reflectivity measurements on the sub-second to millisecond timescales, a conceptually new method of measuring specular X-ray reflectivity curves is developed. Using this method the entire profile of the reflectivity curve of interest is measured in place. A horizontally convergent X-ray beam which has a one-to-one correlation between its direction and energy is realized using a curved crystal or laterally graded multilayers on an elliptic substrate. The X-ray beam is then incident on the surface of the specimen placed at the focus such that the glancing angle in the vertical direction is the same for all X-ray components, which are reflected in the vertical direction by the surface and diverge in the horizontal plane. The perpendicular momentum transfer continuously changes as a function of the horizontal ray direction even with fixed glancing angle since the wavelength (energy) changes. The X-ray intensity distribution across the beam direction measured downstream of the specimen using a one- or two-dimensional detector represents the X-ray reflectivity curve. Specular X-ray reflectivity curves are measured with exposure times ranging from 2 ms to 1 s for a gold film of thickness 14.3 nm on a silicon substrate. The potential of this method for time-resolved measurements is demonstrated by recording reflectivity curves with a time resolution of 20 ms from a rotating specimen.

Explanation and correction of false step heights in amplitude modulation atomic force microscopy measurements on alkane films

We use a prototypical alkane film ( n−C 32H 66 or C32) adsorbed on a SiO 2 surface to compare step heights measured by amplitude modulation atomic force microscopy (AM-AFM) with those measured in the contact mode. The C32 film exhibits layers in which the molecules are oriented with their long axis parallel to the SiO 2 surface followed by partial layers of perpendicular molecules. We show that step heights measured in the AM and contact modes agree in all cases except where the step is between a surface formed by a layer of parallel molecules and one of perpendicular molecules. In this case, the AM mode gives a false step height that is as much as 20% lower than that measured in the contact mode and inferred from synchrotron X-ray specular reflectivity measurements. We propose that the weaker van der Waals forces between the AFM tip and a perpendicular layer compared to a parallel layer causes this discrepancy. We show how to correct the false step height by using the approximately linear relationship observed between phase angle (cantilever oscillation relative to the drive signal) and cantilever height measured in an approach curve.

Peptoids that mimic the structure, function, and mechanism of helical antimicrobial peptides

Antimicrobial peptides (AMPs) and their mimics are emerging as promising antibiotic agents. We present a library of "ampetoids" (antimicrobial peptoid oligomers) with helical structures and biomimetic sequences, several members of which have low-micromolar antimicrobial activities, similar to cationic AMPs like pexiganan. Broad-spectrum activity against six clinically relevant BSL2 pathogens is also shown. This comprehensive structure-activity relationship study, including circular dichroism spectroscopy, minimum inhibitory concentration assays, hemolysis and mammalian cell toxicity studies, and specular x-ray reflectivity measurements shows that the in vitro activities of ampetoids are strikingly similar to those of AMPs themselves, suggesting a strong mechanistic analogy. The ampetoids' antibacterial activity, coupled with their low cytotoxicity against mammalian cells, make them a promising class of antimicrobials for biomedical applications. Peptoids are biostable, with a protease-resistant N-substituted glycine backbone, and their sequences are highly tunable, because an extensive diversity of side chains can be incorporated via facile solid-phase synthesis. Our findings add to the growing evidence that nonnatural foldamers will emerge as an important class of therapeutics.

Atomic force microscopy measurements of topography and friction on dotriacontane films adsorbed on a SiO2 surface

We report comprehensive atomic force microscopy (AFM) measurements at room temperature of the nanoscale topography and lateral friction on the surface of thin solid films of an intermediate-length normal alkane, dotriacontane (n-C32H66), adsorbed onto a SiO2 surface. Our topographic and frictional images, recorded simultaneously in the contact mode, reveal a multilayer structure in which one to two layers of molecules adsorb adjacent to the SiO2 surface oriented with their long axis parallel to the interface followed by partial layers of molecules oriented perpendicular to the surface. The thicknesses of the parallel and perpendicular layers that we measured with the AFM agree with those inferred from previous x-ray specular reflectivity measurements on similarly prepared samples. We also observe bulk dotriacontane particles and, in contrast with our previous measurements, are able to determine their location. Above a minimum size, the bulk particles are separated from islands of perpendicularly oriented molecules by regions of exposed parallel layers that most likely extend underneath the particles. We find that the lateral friction is sensitive to the molecular orientation in the underlying crystalline film and can be used effectively with topographic measurements to resolve uncertainties in the film structure. We measure the same lateral friction on top of the bulk particles as on the perpendicular layers, a value that is about 2.5 times smaller than on a parallel layer. Scans on top of parallel layers indicate a constant height but reveal domains having different sublevels of friction. We explain this by the domains having different azimuthal orientations of the molecules.

Structural Studies of the Neural-Cell-Adhesion Molecule by X-ray and Neutron Reflectivity

The structures of adhesion proteins play an important role in the formation of intercellular junctions and the control of intermembrane spacing. This paper describes the combination of neutron and X-ray specular reflectivity measurements to investigate the structure of the ectodomain of the neural-cell-adhesion molecule (NCAM). The measurements with unmodified NCAM suggest the presence of a bend in the extracellular region. Measurements with the polysialic-acid-modified form of NCAM reveal that, at physiological ionic strength, the carbohydrate chains extend beyond the range of the unmodified protein. The excluded volume of the polymer is also ionic-strength-dependent, as expected for a polyelectrolyte. The structural characteristics obtained from these independent analyses of X-ray and neutron reflectivity data agree with each other, with prior reflectivity studies, and with molecular dimensions obtained from direct-force measurements. These results provide structural insights into the configuration of the NCAM ectodomain and the regulation of NCAM adhesion by post-translational modification.

Structural evolution of nanocrystalline/amorphous Fe/Ni75B25 multilayers: temperature dependence of electrical resistivity

X-ray specular-reflectivity measurements have been carried out on nanocrystalline/amorphous Fe/Ni 75 B 25 multilayer films which were sputter-deposited on Si substrates, to investigate the evolution of interface roughness and the correlation between structure and transport properties. A significant interface roughness correlation with increasing Fe/NiB layer repetition was observed. The investigated films indicated a temperature dependent high electrical resistivity—10 4 μΩ-cm at 10 K and 10 3 μΩ-cm at 300 K—with a semiconductor–metal transition like behavior. Selected area electron diffraction revealed the presence of crystalline bcc Fe phase and NiB in amorphous state. The structural and transport properties of the multilayers are discussed.

Structural evolution of nanocrystalline/amorphous Fe/Ni 75B 25 multilayers: temperature dependence of electrical resistivity

X-ray specular-reflectivity measurements have been carried out on nanocrystalline/amorphous Fe/Ni 75B 25 multilayer films which were sputter-deposited on Si substrates, to investigate the evolution of interface roughness and the correlation between structure and transport properties. A significant interface roughness correlation with increasing Fe/NiB layer repetition was observed. The investigated films indicated a temperature dependent high electrical resistivity—10 4 μΩ-cm at 10 K and 10 3 μΩ-cm at 300 K—with a semiconductor–metal transition like behavior. Selected area electron diffraction revealed the presence of crystalline bcc Fe phase and NiB in amorphous state. The structural and transport properties of the multilayers are discussed.

Overpotential deposition of Ag monolayer and bilayer on Au(1 1 1) mediated by Pb adlayer underpotential deposition/stripping cycles

Ultra-thin Ag films on the Au(1 1 1) surface were prepared via overpotential deposition (OPD) in the presence of Pb 2+ ions. By carrying out repetitive Pb adlayer underpotential deposition (UPD) and stripping cycles during Ag bulk deposition, the two-dimensional growth of Ag films was significantly enhanced in high OPD. The Ag monolayer sample was made by comparing the voltammetry curves, in which the signatures for Pb adlayer UPD on Au(1 1 1) changed to that on Ag(1 1 1). As demonstrated by the X-ray specular reflectivity measurements, nearly complete monolayer and bilayer films can be made with optimized deposition procedures. On subatomic scale, however, we found that these films have significant higher root-mean-square displacement amplitudes than those underpotentially deposited Ag monolayer and bilayer on either Au(1 1 1) or Pt(1 1 1).

Depth Profiling of the Lateral Pore Size and Correlation Distance in Thin Porous Silicon Layers by Grazing Incidence Small Angle X-Ray Scattering

Grazing incidence small angle X-ray scattering was used with synchrotron beam to determine lateral pore sizes and distances between pores formed by anodizing thin (∼300 nm) surface layers of p-type silicon substrates with different current densities, electrolyte compositions, and temperatures. The diffuse scattering was recorded by a two-dimensional gas-filled detector and showed an intensity distribution compatible with a pore correlation parallel to the porous silicon layer. This was observed for all the samples irrespective of their anodization conditions. The experimental intensity data were reproduced by assuming a model of spherical pores. The depth profiles of the average pore diameters and interdistances parallel to the surface were obtained and compared with the mean layer porosities determined by previous specular X-ray reflectivity measurements made on the same samples. © 2003 The Electrochemical Society. All rights reserved.

Coverage of the Cd Underpotential Deposited Layer Formed on an Au(111) Substrate: Effect of the Electrolytic Conditions

The surface-normal structure of a Cd layer formed on an Au(111) substrate using underpotential deposition (UPD) in a sulfuric acid solution has been investigated by specular X-ray reflectivity measurements. The measurements were carried out for the Cd UPD layers which were formed at the first and second UPD potential in electrolytic solutions of 1 mM with 10 mM and 100 mM In all cases, a Cd atomic layer is positioned on the Au(111) surface, which is similar to the reconstructed structure, and also a sulfuric acid anion layer is positioned on the Cd atomic layer. Contrary to results of the previous literature, the Cd coverage of the UPD layer depends not on the UPD potential but on the concentration of The coverage of the Cd atoms increases from approximately 0.50 to approximately 0.60 monolayer as the concentration of increases from 10 to 100 mM, and the coverage of the sulfuric acid anions is approximately half the Cd coverage. © 2002 The Electrochemical Society. All rights reserved.

Atomic structure and defect densities in low dielectric constant carbon doped hydrogenated silicon oxide films, deposited by plasma-enhanced chemical vapor deposition

Results of structural characterization by Fourier transform infrared spectroscopy, x-ray diffraction, and specular x-ray reflectivity measurements are employed for the interpretation of electrical measurement data and the deconvoluted distribution of electron states, N(E) of carbon doped hydrogenated silicon oxide (SiOCH) low-k dielectric films. Atomic structure of the films is identified as a mixture of a dominant and totally amorphous SiO2-like phase with a partially polycrystalline SiC phase. The n-type dc conductivity that dominates in this material points to the principal role of the SiC-like phase in the dc transport of the SiOCH material. The deep level transient spectroscopy technique is applied for the N(E) shape studies in the energy range up to 0.7 eV below the conduction band bottom. Typical N(E) values lie in the 1010–1014 eV−1 cm−3 range for films deposited at different ratios of tri-methyl-silane to oxygen flow rate. No correlation between the N(E) shape and the film deposition conditions have been found in this case. The Fermi level position usually lies at 0.18–0.4 eV below conduction band bottom. For the SiOCH films prepared at different levels of rf power densities, the N(E) in the whole studied range increases nearly monotonically with increasing rf power, which is attributed to the SiC-like phase fraction increment. An N(E) peak at 0.25–0.35 eV below conduction band bottom has been found in the films. The possible origin of the peak appearance is discussed.

Grazing incidence X-ray reflectivity of coumarin side-chain polymers used for liquid crystal photoalignment layers

Specular X-ray reflectivity measurements are used to examine photoinduced surface roughness of side-chain coumarin polymers that form liquid crystal photoalignment layers. Thin films were irradiated with polarised or unpolarised ultraviolet light and the X-ray reflectivity at grazing incidence was measured for samples irradiated with different ultraviolet fluences. Kiessig fringes are observed over a large dynamic range and a comparison of the measured and calculated reflectivity gives a value for the surface roughness. The photoinduced roughness increases with ultraviolet fluence, from 5.5 to 20 Å. Negligible photoinduced roughness is detected for a film, which was previously irradiated using conditions that give strong planar liquid crystal alignment. Specular reflectivity data taken with the plane of incidence of the X-ray parallel and perpendicular to the polarisation direction of the ultraviolet light are identical within experimental uncertainty. Hence in-plane anisotropic surface roughness is not observed.

X-Ray Reflectivity and FTIR Measurements of N 2 Plasma Effects on the Density Profile of Hydrogen Silsesquioxane Thin Films

The density depth profile and chemical bond structure of hydrogen silsesquioxane (HSQ) thin films treated with an plasma with varying power and exposure time were measured using specular X-ray reflectivity (SXR) and Fourier transform infrared (FTIR) spectroscopy. The SXR data indicated that the density profile of an untreated HSQ film is not uniform. At least four layers with different electron densities were required to fit the SXR data. For HSQ films treated with either increasing plasma power or plasma exposure time, the film roughness increased and a densified layer was observed at the film/air interface. The thickness of the densified layer increased with both plasma power and plasma exposure time. In the FTIR spectra of the plasma-treated films, the intensities of the Si-O peaks due to the network structure and the Si-OH peak due to water absorption increased and the intensities of the Si-H peaks decreased. The FTIR data show that the plasma converts the HSQ structure into a -like structure and are consistent with the densification observed in the SXR measurements. In general, the HSQ film is more sensitive to increasing plasma power than to increasing plasma exposure time. © 2001 The Electrochemical Society. All rights reserved.