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 100-1500 eV 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 300 mm in length and 5 kg 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.

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, 10 W; O 2 flow, 30 sccm; exposure time, 10 min 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.

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 Šthickness deposited with an adhesive layer of 20 ŠCr 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.

Influence of spin orbit splitting and satellite transitions on nickel soft X-ray optical properties near its L2,3 absorption edge region.

Transition elements exhibit strong correlations and configuration interactions between core and valence excited states, which give rise to different excitations inside materials. Nickel exhibits satellite features in its emission and absorption spectra. Effects of such transitions on the optical constants of nickel have not been reported earlier and the available database of Henke et al. does not represent such fine features. In this study, the optical behaviour of ion beam sputter deposited Ni thin film near the L2,3-edge region is investigated using reflection spectroscopy techniques, and distinct signatures of various transitions are observed. The soft X-ray reflectivity measurements in the 500-1500 eV photon energy region are performed using the soft X-ray reflectivity beamline at the Indus-2 synchrotron radiation source. Kramers-Kronig analysis of the measured reflectivity data exhibit features corresponding to spin orbital splitting and satellite transitions in the real and imaginary part of the refractive index (refraction and absorption spectra). Details of fine features observed in the optical spectra are discussed. To the best of our knowledge, this is the first study reporting fine features in the measured optical spectra of Ni near its L2,3-edge region.

Investigation of the buried planar interfaces in multi-layered inverted organic solar cells using x-ray reflectivity and impedance spectroscopy.

The hole and electron extracting interlayers in the organic solar cells (OSCs) play an important role in high performing devices. The present work focuses on an investigation of Zinc oxide/bulk heterojunction (ZnO/BHJ) and BHJ/MoO x (Molybdenum oxide) buried planar interfaces in inverted OSC devices using the optical contrast in various layers along with the electrical measurements. The x-ray reflectivity (XRR) analysis demonstrates the formation of additional intermixing layers at the interfaces of ZnO/BHJ and BHJ/MoO x . Our results indicate infusion of PC71BM into ZnO layer up to ~4 nm which smoothen the ZnO/BHJ interface. In contrast, thermally evaporated MoO x molecules diffuse into PTB7-Th dominant upper layers of BHJ active layer resulting in an intermixed layer at the interface of MoO x /BHJ. The high recombination resistance (~5 kΩ cm2) and electron lifetime (~70 µs), obtained from the impedance spectroscopy (IS), support such vertical segregation of PTB7-Th and PC71BM in the active layer. The OSC devices, processed in ambient condition, exhibit high power conversion efficiency of 6.4%. We consider our results have great significance to understand the structure of buried planar interfaces at interlayers and their correlation with the electrical parameters representing various interfacial mechanisms of OSCs.

Influence of the core-hole effect on optical properties of magnesium oxide (MgO) near the Mg L-edge region.

The influence of the core-hole effect on optical properties of magnesium oxide (MgO) is established through experimental determination of optical constants and first-principles density functional theory studies. Optical constants (δ and ß) of MgO thin film are measured in the spectral region 40-300 eV using reflectance spectroscopy techniques at the Indus-1 synchrotron radiation source. The obtained optical constants show strong core exciton features near the Mg L-edge region, causing significant mismatch with Henke's tabulated values. On comparing the experimentally obtained optical constants with Henke's tabulated values, an edge shift of ∼3.0 eV is also observed. Distinct evidence of effects of core exciton on optical constants (δ and ß) in the near Mg L-edge absorption spectra are confirmed through first-principles simulations.

Optical constants of e-beam-deposited zirconium dioxide measured in the 55-150 Å wavelength region using the reflectivity technique.

In the present study, optical constants of e-beam-deposited zirconium dioxide (ZrO2) thin film are determined in the 55-150 Å soft x-ray wavelength region using the angle-dependent reflectivity technique. Soft x-ray reflectivity measurements are carried out using the reflectivity beamline at the Indus-1 synchrotron radiation source. Derived optical constants (δ and ß) are compared with the tabulated values of Henke et al. [http://henke.lbl.gov/optical_constants/asf.html]. It is found that the measured δ values are consistently lower than the tabulated bulk values in the 70-150 Å wavelength region. In this region, the delta values are lower by 19%-24% from the tabulated data. Below the Zr M4 edge (66.3 Å), a deviation in delta values is found as ∼2%-21%. These changes are attributed to growth-related defects (oxygen and voids) and variation in film stoichiometry. To the best of our knowledge, the present study gives the first reported experimental values of optical constants for ZrO2 in the 55-150 Å wavelength region.

Optical properties of zirconium carbide in 60-200 Å wavelength region using x-ray reflectivity technique.

Optical constants of zirconium carbide (ZrC) have been determined in the soft x-ray region of 60-200 Å wavelength using angle-dependent x-ray reflectivity measurements. Reflectivity measurements are carried out at the reflectivity beamline of the Indus-1 synchrotron radiation source. Derived optical constants (δ and ß) are compared with the tabulated values [At. Data Nucl. Data Tables 54, 181 (1993)]. The optical constants are 15%-35% lower than bulk values in the 60-200 Å wavelength region. Near the Zr M4 edge 187 eV (66.3 Å), the δ values were close to bulk values with a deviation of 5%-10%. A large deviation of ∼20% was found in beta values especially near the Zr M4 edge region, whereas it was in close agreement in the wavelength range away from the edge. To the best of our knowledge, this paper gives the first reported experimental values of optical constants for zirconium carbide in the 60-200 Å wavelength region.

Analysis of higher harmonic contamination with a modified approach using a grating analyser.

Soft x-ray spectra of the toroidal grating monochromator (TGM) at the reflectivity beamline of Indus-1 synchrotron source are analyzed for higher harmonic contribution. A diffraction grating of central line spacing 1200 l/mm is used to disperse the monochromatic beam received from TGM to quantify the harmonic contents in the 50-360 Å wavelength range. In order to calculate the harmonic contamination, conventionally the intensity of higher order peak is divided by first order peak intensity of the desired wavelength. This approach is found to give wrong estimate as first order peak itself is overlapped by higher order peaks. In the present study, a modified approach has been proposed to calculate harmonic contamination where the intensity contributions of overlapping orders have been removed from the first order diffraction peak of the desired wavelength. It is found that the order contamination in the TGM spectra is less than 15% in the wavelength range of 90-180 Å. The total harmonic contribution increases from 6%-60% in the wavelength range of 150-260 Å. The critical wavelength of Indus-1 is 61 Å hence the harmonic contamination below 90 Å is significantly low. The results obtained with modified approach match well with those obtained by quantitative analysis of multilayer reflectivity data. The obtained higher harmonics data are used to fit the transmission of aluminum edge filter in the 120-360 Å wavelength range.

Fine structures in refractive index of sapphire at the L(II,III) absorption edge of aluminum determined by soft x-ray resonant reflectivity.

The optical constants of sapphire crystal (α-Al(2)O(3)) and amorphous Al(2)O(3) in the soft x-ray region (67-85 eV) around the aluminum LII,III absorption edge (73.1 eV) are determined by angle-dependent x-ray reflectivity. The differences between the optical constant values of both the samples are discussed. The fine structures obtained in the absorption of crystalline sapphire are explained. An absorption feature at 70.2 eV is observed for the first time for crystalline alumina. Both datasets are compared to the tabulated values of Henke et al. [At. Data Nucl. Data Tables 54, 181 (1993)], Weaver et al. [Physik Daten, Physics Data: Optical Properties of Metals (Fach-information zentrum, 1981), Vols. 18-1 and 18-2], and [Handbook of Optical Constants of Solids II (Academic, 1991)].

NbC/Si multilayer mirror for next generation EUV light sources.

In the present study we report a new multilayer combination comprised of refracting layers of niobium carbide and spacer layers of silicon as a more stable and high reflecting combination for the 10 - 20 nm wavelength region. The reflectivity of the new combination is comparable to Mo/Si conventional mirrors. Annealing experiments carried out with NbC/Si multilayer at 600°C temperature showed a ~2.5% drop in the soft x-ray reflectivity along with a marginal contraction in the multilayer period length. The multilayer structure is found stable after the heat treatment. Crystallization of the niobium carbide and silicon layers is responsible for the compaction in the period length as revealed by the grazing incidence x-ray diffraction measurements. No signature of silicide formation or any other chemical species could be detected. The multilayer structures were grown by ion beam sputtering technique using a compound target of niobium carbide. Soft x-ray reflectivity measurements performed at the Indus-1 and BESSY-II synchrotron radiation sources are found in good agreement with the simulations.

Quantitative determination of higher harmonic content in the soft x-ray spectra of toroidal grating monochromator using a reflection multilayer.

Use of a grating monochromator causes a problem of higher harmonic contaminations in a synchrotron beamline operating in the soft x ray/vacuum ultraviolet region. Generally gratings are used to experimentally determine the higher harmonic contaminations. In this method, the relative contribution of contaminant wavelengths is measured with respect to the first harmonic wavelength (desired wavelength). The quantitative fit of grating spectra is rather complex, and therefore qualitative analysis is carried out. Analysis of multilayer reflectivity data has become rather simple with recent advances in the theoretical modeling. Therefore we propose to use a multilayer mirror and analyze its reflectivity data for quantitative determination of harmonic contamination in a soft x ray beamline. In the present study we used a Mo/Si multilayer of d=97 Å to quantify the spectral purity of 600 lines/mm toroidal grating at the reflectivity beamline of Indus-1 450 MeV synchrotron source. The measured reflectivity spectra at each wavelength is analyzed and the actual contribution of higher harmonics in the incident beam is obtained. Details of methodology and results are discussed.

X-ray optics simulation using Gaussian superposition technique.

We present an efficient method to perform x-ray optics simulation with high or partially coherent x-ray sources using Gaussian superposition technique. In a previous paper, we have demonstrated that full characterization of optical systems, diffractive and geometric, is possible by using the Fresnel Gaussian Shape Invariant (FGSI) previously reported in the literature. The complex amplitude distribution in the object plane is represented by a linear superposition of complex Gaussians wavelets and then propagated through the optical system by means of the referred Gaussian invariant. This allows ray tracing through the optical system and at the same time allows calculating with high precision the complex wave-amplitude distribution at any plane of observation. This technique can be applied in a wide spectral range where the Fresnel diffraction integral applies including visible, x-rays, acoustic waves, etc. We describe the technique and include some computer simulations as illustrative examples for x-ray optical component. We show also that this method can be used to study partial or total coherence illumination problem.

Optical properties of indium phosphide in the 50-200 Å wavelength region using a reflectivity technique.

The optical constants of indium phosphide (InP) in the soft x-ray region of 50-200 Å are determined from angle-dependent reflectivity measurements. The measurements are carried out using the reflectivity beam line at the Indus-1 synchrotron source. The derived optical constants are compared with tabulated values of Henke et al. [At. Data Nucl. Data Tables 54, 181 (1993)]. Experimental values of δ and ß are in close agreement with the tabulated values in the lower wavelength region of 50-120 Å. The experimental value indicates an edge shift of 0.4 Å toward the lower wavelength side from the phosphorous L-edge value of 92 Å. However, above the 120 Å region, where the indium N(2) edge falls at 160.7 Å, there is a huge difference between experimental and tabulated values. Both delta and beta values are significantly higher. In contrast to tabulated values of the ß/δ ratio, which is more than 1 above the 140 Å region, the experimental measured ratio is found to be less than 1. This study presents the first reported experimental values of optical constants for InP in this wavelength range, to the best of our knowledge.

Effect of tin diffusion on the optical behavior of float glass in the soft-x-ray region.

The optical responses of two sides of float glass in the soft-x-ray region were studied at the Indus-1 synchrotron facility. To the best of our knowledge these are the first experimentally obtained optical data for both sides of float glass in the soft-x-ray region. Optical constants delta and beta were determined by use of angle-dependent reflectance techniques in the wavelength range 80-200 A. On the side of the glass that was tin indiffused, a significant difference in delta value from that of the non-tin-side surface was observed. The measured data were compared with Henke's tabulated value of SiO2. The surface roughness of float glass was separately determined by hard-x-ray reflectivity to minimize the number of fitting variables. The effect of a contamination layer on the determination of optical constants was avoided by an appropriate sample-cleaning method.