X-ray Reflectivity Spectroscopy Research Articles

Characteristics of the low electron density surface layer on BaTiO3 thin films

The surfaces of epitaxial BaTiO3 films on SrTiO3 substrates were investigated by x-ray reflectivity (XRR) and angle-resolved x-ray photoelectron spectroscopy (ARXPS). It was shown by XRR analysis that there exists a low electron density surface layer (about 87%–93% of the electron density of the underneath BaTiO3 layer) of 15Å on top of the film. Moreover, ARXPS results revealed a surface core-level shift of Ba in layer of about 11Å, a value which is in agreement with the thickness obtained by XRR, indicating that the surface core-level shift of Ba stems from the low electron density surface layer.

Interdiffusion studies for HfO2/Si by GIXR and XPS

Interdiffusion at the interface between an HfO2thin film and a Si substrate was studied using grazing incidence x-ray reflectivity (GIXR) and x-ray photoelectron spectroscopy (XPS). HfO2thin films were deposited on Si sub strates by the sputtering of a metal-Hf layer, and then they were oxidized/annealed in a N2/O2gas mixture with different O2concentrations. The GIXR and XPS results showed that the out-diffusion of Si atoms from the substrate increased with the oxygen concentration during high temperature annealing, and consequently, it resulted in the formation of a Si-rich Hf-silicate interlayer, i.e., lower-density Hf-silicate with greater thickness. The GIXR results of the interfacial structures were in agreement with the results of XPS and cross-sectional transmission electron microscopy.

Oxidation and Reduction of Ultrathin Nanocrystalline Ru Films on Silicon: Model System for Ru-Capped Extreme Ultraviolet Lithography Optics

Ultrathin ruthenium films are promising capping layers protecting extreme ultraviolet lithography optics against carbon growth and oxidation. The structure and reactivity of 2, 5, and 7 nm thick nanocrystalline Ru films on Si (serving as a model system for Ru capping layers) were studied by multiple techniques including scanning electron microscopy, X-ray diffraction, X-ray reflectivity, and X-ray photoelectron spectroscopy. The structural analysis indicated dense and flat Ru films, consisting of preferentially (0001)-oriented grains. High resolution core-level Ru 3d5/2 and O 1s spectroscopy studies have revealed that these Ru films exposed to O2 ambient are resistant to oxidation up to ∼470 K similar to the oxidation of single-crystalline Ru(0001) surfaces. The reduction of the oxide in the H2 ambient is highly effective and proceeds already below 370 K.

Fabrication and structural analysis of aromatic polyimide ultra-thin films with different fluorine contents

The structural characterizations of aromatic polyimide (PI) ultra-thin films with different fluorine contents were studied. Three types of polyamic acid (PAA) were produced from polymerization of biphenyl dianhydride (BPDA)/4,4′-oxydianiline (ODA), BPDA/2,2′-bis(trifluromethyl)benzidine (TFDB), and 4,4′-(hexafluoro isopropylidene)-diphthalic anhydride (6FDA)/TFDB in N,N-dimethylacetamide (DMAc). Polyamic acid alkyl salts (PAAS) using O,O′,O″-trihexadecanoyl-triethanolamine were transferred onto a Si-wafer substrate to prepare Langmuir–Blodgett (LB) films. The behaviors of PAAS monolayers at the air/water interface were confirmed with π–A isotherms and Brewster Angle Microscopy (BAM) images. The behaviors of PAAS monolayers at the air/water interface depend on the fluorine contents. As there is more fluorine in the main chain of PAAS, the area per molecule of each monolayer was increased. The structure of PAAS LB films with 10, 20, 30, and 40 layers on a substrate were investigated by UV–vis spectroscopy, X-ray diffraction (XRD), X-ray reflectivity, and IR spectroscopy. The obtained PAAS LB films were imidized with a thermal treatment at 300°C. The imidization of PI ultra-thin film was identified by IR spectrum. Atomic force microscopy (AFM) images showed that the particle size of PI ultra-thin films on a Si-wafer were about 7–25nm.

Topotaxial growth of Ti2AlN by solid state reaction in AlN∕Ti(0001) multilayer thin films

The formation of Ti2AlN by solid state reaction between layers of wurtzite-AlN and α-Ti was characterized by in situ x-ray scattering. The sequential deposition of these layers by dual magnetron sputtering onto Al2O3(0001) at 200°C yielded smooth, heteroepitaxial (0001) oriented films, with abrupt AlN∕Ti interfaces as shown by x-ray reflectivity and Rutherford backscattering spectroscopy. Annealing at 400°C led to AlN decomposition and diffusion of released Al and N into the Ti layers, with formation of Ti3AlN. Further annealing at 500°C resulted in a phase transformation into Ti2AlN(0001) after only 5min.

Defining new frontiers in electronic devices with high κ dielectrics and interfacial engineering

Ga 2O 3(Gd 2O 3), a high κ gate dielectric, ultrahigh vacuum (UHV)-deposited on GaAs and InGaAs has unpinned the Fermi level in the high-electron-mobility III–V compound semiconductors for the first time. Systematic heat treatments under various gases and temperatures were studied to achieve low leakage currents of 10 − 8 –10 − 9 A/cm 2 and low interfacial density of states ( D it's) in the range of < 10 11 cm − 2 eV − 1 . By removing moisture from the oxide, thermodynamic stability of the Ga 2O 3(Gd 2O 3)/GaAs heterostructures and the interfaces were achieved with high temperature annealing, the oxide remains amorphous and the interface remains intact with atomic smoothness and sharpness. The Fermi-level unpinning in atomic layer deposition (ALD) Al 2O 3 ex-situ deposited on InGaAs was achieved. Recent work of extremely high-quality nano-thick single crystal oxides of gamma-Al 2O 3 and bixbyite cubic Sc 2O 3 epitaxially grown on Si (111) is discussed. Interfacial manipulation is essential in giving excellent results presented in the paper. X-ray diffraction, reflectivity, and X-ray photoelectron spectroscopy using synchrotron radiation are critical in probing the interfacial properties.

Influence of interface interruption on spin relaxation in GaAs (1 1 0) quantum wells

The influence of interface growth interruption on electron spin relaxation time τs in GaAs/AlGaAs (1 1 0) quantum wells (QWs) grown by solid source molecular-beam epitaxy (SSMBE) has been investigated by grazing incidence X-ray reflectivity and time-resolved Kerr rotation spectroscopy (TRKR). Various inverted interface interruption times were used in growing QWs. Interface roughness of these QW samples was studied by grazing incidence X-ray reflectivity. The simulated results of reflectivity curves by standard software indicated that interface roughness decreased as the interface growth interruption time increased. TRKR measurements at room temperature showed that the appropriate growth interruption could increase spin relaxation time in GaAs/AlGaAs QWs drastically. This dramatic increase was explained by the suppression of the D’yakonov–Perel’ interaction.

X-ray reflectivity and photoelectron spectroscopy study of interdiffusion at the Si∕Fe interface

We have investigated the nature of silicon on iron interface in electron beam deposited Fe∕Si bilayers, with various iron and silicon thicknesses. The Fe and Si layer thicknesses are varied from 30to330Å and 20to86Å, respectively. Grazing incidence x-ray reflectivity and photoelectron spectroscopy measurements were carried out on these samples to determine interface characteristics. Si on Fe (Si̱Fe) interlayer thickness, roughness, and composition do not depend on the thickness of Fe and Si. The thickness of the interlayer is around 13Å. A systematic variation in silicide concentration across this interface is observed by x-ray photoelectron spectroscopy measurement. Change in the density of states in valence band across this interface is also observed by ultraviolet photoelectron measurement.

Study of low energy Ar+ ion irradiated 57Fe/Pt multilayers

This paper reports a study on the effects ofAr+ ion irradiation in57Fe/Pt multilayers havingthe structure Si (substrate)/[57Fe(19 Å)/Pt(25 Å)] × 10. The as-deposited multilayers were irradiated with 90 keVAr+ ions at afluence of 2 × 1016 ions cm−2 at different temperatures. X-ray reflectivity, x-ray diffraction and conversionelectron Mossbauer spectroscopy were used for the characterization of theirradiated samples. Using x-ray reflectivity it was observed that irradiationresults in mixing across the interfaces and deterioration of the multilayerstructure. The mixing results in the formation of fcc FePt phase and noL10 ordering is observed in the samples. The results are discussed in terms of recoildisplacements induced, energy transfers from ions supporting the observations of Bernas et al, using kinetic Monto Carlo simulations.

A comparative study of Co thin film deposited on GaAs (1 0 0) and glass substrates

The structural, magnetic and transport properties of Co/GaAs (1 0 0) and Co/glass thin films have been investigated. The structural measurements reveal the crystalline nature of Co thin film grown on GaAs, while microcrystalline nature in case of glass substrate. The film grown on GaAs shows higher coercivity (49.0 G), lower saturation magnetization (3.65 × 10 −4) and resistivity (8 μΩ cm) values as compared to that on glass substrate (22 G, 4.77 × 10 −4 and 18 μΩ cm). The grazing incidence X-ray reflectivity and photoemission spectroscopy results show the interaction between Co and GaAs at the interface, while the Co layer grown on glass remains unaffected. These observed results are discussed and interpreted in terms of different growth morphologies and structures of as grown Co thin film on both substrates.

Annealing induced structural changes in electron beam evaporated Si/Ge multilayers

The effect of thermal annealing on the structural properties of electron beam evaporated polycrystalline Si/Ge multilayer structures has been studied using Grazing incidence X-ray diffraction (GIXRD), reflectivity (GIXRR) and Raman spectroscopy techniques. The chemical nature of layers at surface and interfaces has been obtained from X-ray photoelectron spectroscopy (XPS) technique, which revealed the presence of impurities only at the top surface in the form of carbides and oxides in the Si layer and in elemental form in the Ge layer. Reflectivity measurements show that the Si/Ge MLS is stable upto 200 °C and roughness of Si on Ge interface is higher as compared to that of Ge on Si interface. Similarly, GIXRD results show that upto 200 °C, MLS is of microcrystalline nature and further annealing at 300 °C and 400 °C, well-defined crystalline peaks of pure Si and Ge are observed, which is in contrast to earlier reported experimental findings. Observed results are interpreted in terms of diffusion induced crystallization and corresponding increase in grain size.

Temperature dependence of the optical properties of ZnSe films deposited on quartz substrate

ZnSe thin films were deposited by pulsed laser ablation on quartz substrate. The films were investigated by different characterization techniques, such as X-ray diffraction, Raman microspectroscopy, absorption, reflectivity, and photoluminescence spectroscopy. The XRD analysis showed the formation of cubic phase polycrystalline films. The Raman spectra confirmed the formation of ZnSe by the presence of TO and LO peaks at 202 cm-1 and 252 cm-1, respectively. The analysis of absorption and reflectivity measurements permits evaluation of the band gap and excitonic energy at low temperature and the temperature dependence of the energy gap. The photoluminescence measurements indicated the possibility of obtaining intrinsic band-band radiative emission up to room temperature.

A comparative study on the structure and properties of nanolayered TiN/NbN and TiAlN/TiN multilayer coatings prepared by reactive direct current magnetron sputtering

Nanolayered multilayer coatings of TiN/NbN and TiAlN/TiN were deposited on Si (100) substrates at various modulation wavelengths (i.e. bilayer thickness, Λ) using a reactive direct current magnetron sputtering system. These coatings were characterized using X-ray diffraction (XRD), X-ray reflectivity, nanoindentation tester and micro-Raman spectroscopy. For TiN/NbN multilayer coatings, at low Λ, the XRD data showed well-resolved first-order negative satellite reflection along (111) principal reflection, whereas, a weak first-order positive satellite reflection was observed for TiAlN/TiN multilayer coatings. Similarly, the X-ray reflectivity data showed well-resolved satellite reflections of second-orders for TiN/NbN multilayer coatings and diffused satellite reflections for TiAlN/TiN multilayer coatings, indicating broad interfaces for TiAlN/TiN multilayers. TiN/NbN and TiAlN/TiN multilayers showed maximum nanoindentation hardness values of 4000 and 3750 kg/mm2, respectively. Single layer coatings of TiN, NbN and TiAlN showed hardness values of 2600, 1800 and 3850 kg/mm2, respectively. The low hardness of TiAlN/TiN multilayers is attributed to diffused interfaces. Raman spectroscopy measurements on the multilayer coatings after heat treatment in air showed that TiN/NbN multilayers were thermally stable (with respect to oxidation) up to 500 °C, whereas TiAlN/TiN multilayers were stable up to 800 °C. The nanoindentation measurements performed on the heat-treated coatings showed that TiN/NbN multilayer hardness reduced to about 2000 kg/mm2 after heating up to 600 °C. On the other hand, TiAlN/TiN multilayers retained hardness as high as 2500 kg/mm2 even after heating up to 700 °C.

Epitaxial ZrC thin films grown by pulsed laser deposition

ZrC thin films were grown on (0 0 1)Si, (1 1 1)Si and (0 0 0 1)sapphire substrates by the pulsed laser deposition (PLD) technique. X-ray diffraction, X-ray reflectivity and Auger electron spectroscopy investigations were used to characterize the structure and composition of the deposited films. It has been found that films grown at temperatures higher than 700 °C under very low water vapor pressures were highly textured. Films deposited on (0 0 1)Si grew with the (0 0 1) axis perpendicular to the substrate, while those deposited on (1 1 1)Si and (0 0 0 1)sapphire grew with the (1 1 1) axis perpendicular to the substrate. Pole figures investigations showed that films were epitaxial, with in-plane axis aligned to those of the substrate.

Structure of reactively sputter deposited tin-nitride thin films: A combined X-ray photoelectron spectroscopy, in situ X-ray reflectivity and X-ray absorption spectroscopy study

Amorphous tin-nitride thin films were prepared by reactive sputter deposition on smooth float glass substrates in a vacuum chamber with an integrated small magnetron source. The films were investigated using in situ reflection mode X-ray absorption spectroscopy and ex situ X-ray photoelectron spectroscopy (XPS). Both the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) were analysed, yielding bond distances, coordination numbers and Debye–Waller factors. XPS yields the chemical composition and the binding state of the constituents of the films, specular X-ray reflectivity allows the determination of the sample density and of the roughness and its changes with film thickness. The results were compared to those of crystalline Sn3N4, indicating that the electronic and atomic structure of the amorphous films determined by EXAFS data analysis are very similar to the stoichiometric reference compound. Two different Sn–N interactions with about 2.09 and 2.19 Å bond distance and 4 and 6 nearest neighbours, respectively, are present. These bond distances are slightly relaxed compared to the crystalline reference material, which is consistent with the sample density, which is reduced by about 8% in comparison to Sn3N4. XPS as well as XANES revealed a Sn valence of about 4+ and the presence of nitric bonds, while XPS also suggests that the nitride is slightly decomposed under X-ray irradiation in ultra-high vacuum.

Studies of the interfacial structure of LaAlO3 thin films on silicon by x-ray reflectivity and angle-resolved x-ray photoelectron spectroscopy

The microstructures of amorphous LaAlO3 thin films deposited on silicon substrates by the laser molecular-beam epitaxy were studied by the x-ray reflectivity and the angle-resolved x-ray photoelectron spectroscopy. It was shown that the film∕substrate interface contains a La-rich LaxAlyOzSi layer and a SiOx layer. It was shown that the electron density of the LaAlO3 layer and the LaxAlyOzSi layer is not homogeneous along the growth direction due to the diffusion of La, Al, and Si. The growth kinetics of the LaAlO3 film was described by three processes: (1) formation of the SiOx layer at the early stage whose thickness saturates rapidly at about 13Å; (2) formation of the LaxAlyOzSi layer by the out diffusion of Si and the inner diffusion of La, Al (mostly La). This stage continues as the film grows (3) In the deposition process of LaAlO3, the distributions of La and Al in the LaAlO3 layer change from inhomogeneous to homogeneous.

Depth Resolved Structural Studies in Multilayers Using X-ray Standing Waves

X-ray based characterization techniques are powerful tools for the study of atomic scale structure of materials. However, high penetrating power of X-rays make them less suitable for depth selective studies, as required in the characterization of multilayer structures. In the present work, it is shown that depth selectivity of the techniques like, X-ray fluorescence, X-ray absorption spectroscopy and nuclear resonance fluorescence can be greatly enhanced by generating X-ray standing waves inside the multilayer structure. The concentration profiles of various elements can be obtained with a depth resolution of the order of 0.1 nm. Depth dependent information about the local structure around a given atom can be obtained from XAFS under standing wave conditions. It is demonstrated that detection of nuclear resonance fluorescence by tuning the energy of the incident X-rays to a Mossbauer transition can yield depth profile of a particular isotope, and can be used for self-diffusion studies. The techniques of X-ray reflectivity and conversion electron Mossbauer spectroscopy are used to provide useful complementary information.

Determination of the electron effective band mass in amorphous carbon from density-functional theory calculations

We generate a series of amorphous carbon materials of densities varying between 2.0 and $3.5\phantom{\rule{0.3em}{0ex}}\mathrm{g}∕{\mathrm{cm}}^{3}$. Their dielectric functions are calculated within the density-functional theory approach, and the low-loss electron energy-loss spectra are evaluated. The dependence of the $\ensuremath{\pi}+\ensuremath{\sigma}$ plasmon energy ${E}_{p}$ on the valence electron density $({n}_{e})$ is determined. We find that the free-electron model in which ${E}_{p}\ensuremath{\propto}{n}_{e}^{0.5}$ is valid. The coefficient of proportionality leads to a ratio of the electron-effective band mass ${m}^{*}$ to the free-electron mass $m$ of ${m}^{*}∕m=0.87$, which is the value obtained using measurements based on the grazing-angle x-ray reflectivity and electron energy-loss spectroscopy [see A. C. Ferrari et al., Phys. Rev. B 62, 11089 (2000)]. This result supports the reliability of amorphous carbon mass densities obtained from EELS data when an electron-effective mass of $0.87\phantom{\rule{0.3em}{0ex}}m$ is used.

Characterization of heterointerfaces in GaAs/MnAs/MnZn-ferrite structures

We have grown GaAs epitaxial films on MnZn-ferrite substrates using MnAs buffer layers and investigated their heterointerfaces with glazing incidence-angle X-ray reflectivity and X-ray photoelectron spectroscopy. It has been found that the heterointerfaces for this structure are quite abrupt and the roughness at the GaAs/MnAs and MnAs/MnZn-ferrite interfaces are 1.1 and 0.2 nm, respectively. We also found that the diffusion of atoms through the GaAs/MnAs interface into the GaAs film is negligible. These results indicate that the MnAs buffer layer for the GaAs/ferrite structure is chemically stable and promising for the application to the future magnetic electronics.

Investigation of the interfacial structure of ultra-thin platinum film deposited by cathodic–arc

Ultra-thin films are of interest in the production of X-ray mirrors that use a multilayer structure. The most commonly used deposition techniques are dc magnetron sputtering and electron beam evaporation; this paper presents results of cathodic–arc deposition. Ultra thin films of platinum with nominal thicknesses in the range 15–65 Å were deposited on silicon substrates and the film structure investigated using X-ray reflectivity and X-ray photoelectron spectroscopy. It has been found that the structure of the deposited films consists of three layers—the platinum film, a silicon oxide layer and a platinum silicide layer. In contrast to dc magnetron and electron beam deposited films, the silicide layer of cathodic–arc deposited films have a higher density and greater thickness, which is attributed to the higher energy process of this deposition technique. These attributes of the cathodic–arc deposited films suggest that the deposition technique is not suitable for production of mirrors of materials that react with each other, but for materials that do not the deposition technique is potentially more favourable than that of e-beam and magnetron sputtering.