Grazing Incidence X-ray Reflectivity Measurements Research Articles

Early recrystallization of Ni/Ti multilayer due to disorder in the Ni layer

Disorders present at the interfaces drive solid state amorphization (SSA) and recrystallization in Ni/Ti multilayers. In this communication, the effect of disorders present in the bulk Ni layers is studied on SSA and recrystallization. The disorder in the Ni layers is created by sputtering Ni layers of the Ni/Ti multilayers under the mixed ambience of argon and nitrogen. It has been observed recently that the neutron optical performances of Ni/Ti graded multilayers or supermirrors can be improved by sputtering of Ni layers under the mixed ambience of argon and nitrogen. In this work, Ni/Ti periodic multilayers deposited under pure argon ambience and under mixed ambience of argon and nitrogen have been annealed under vacuum at 200 °C, 400 °C, and 600 °C for 2 h. All the annealed multilayer samples along with as-deposited multilayers are characterized by x-ray diffraction and grazing incidence x-ray reflectivity measurements in specular and non-specular modes. Atomic diffusion within the layers and across interfaces has been studied as a function of annealing temperature for both sets of multilayers. It is found that the interstitial nitrogen atoms present in the Ni layers play a crucial role in the diffusivity at room temperature as well as high temperature and thus influence the SSA and recrystallization phenomena.

Stability of Cr/C multilayer during synchrotron radiation exposure and thermal annealing.

The stability of Cr/C multilayer during irradiation or thermal annealing was investigated using grazing incidence X-ray reflectivity measurement, X-ray photoelectron spectroscopy, X-ray diffraction analysis, small-angle X-ray scattering analysis, and soft X-ray reflectivity measurement. One sample was irradiated with a white beam of synchrotron radiation and five other samples were annealed at various temperatures. The 18-h irradiation treatment caused local surface contaminants but did not affect the buried stacks. The annealing treatment resulted in increased reflectivity at approximately 1.2 keV, and the multilayer remained stable for temperature up to 700 °C. Thus, the Cr/C multilayers exhibited excellent stability during irradiation and thermal treatments and can be used for the mirrors and multilayer gratings of third-generation synchrotron radiation systems.

Interface evolution of Co/Ti multilayers with ultra-short period

Behaviour of interface properties and propagation of interfacial atomic diffusion have been investigated systematically in Co/Ti multilayers with ultra-short (∼18 Å) bi-layer thickness. Co/Ti multilayers with 10,20,40,60 and 80 bi-layers have been grown using an indigenously built in-line dc magnetron sputtering system having application as reflecting optics in the water window soft X-ray regime of 23–44 Å. Specular and diffused Grazing Incidence X-ray reflectivity measurements with hard X-rays have been used to evaluate detailed interfacial structure of the multilayers. It is observed that interfacial atomic diffusion of Co into Ti layers is significant in such ultra-short period multilayer structures and it increases as more number of layers are added in the multilayer stack. As a result of Co diffusion into Ti the physical density of Ti layer increases and also effective thickness of Co layer decreases. Since the thickness of the Ti layers are in sub-nm range, it is found that this diffusion is not limited only to the interface region of Co-on-Ti, rather it spreads throughout the bulk of Ti layer. Finally, soft X-ray reflectivity measurement on the multilayers with synchrotron radiation at 28 Å wavelength shows that the peak reflectivity is limited by the significant diffusion at the Co/Ti interfaces.

Interface studies of Mo/Si multilayers with carbon diffusion barrier by grazing incidence extended X-ray absorption fine structure

Single layer Mo and Si thin films deposited for different deposition times on c-Si substrates by Ion beam sputtering technique have been characterized by Grazing Incidence X-ray Reflectivity (GIXRR) measurements. Subsequently, few Mo/Si multilayers having 5 bi-layers with different Mo to bi-layer thickness ratio (Γ) at a fixed bi-layer thickness have been deposited and characterized by GIXRR measurements. Investigation has been carried out on the different behaviors of the interfaces and also variation of the interface width with Γ has been studied. Finally, 25 bi-layer Mo/Si multilayer samples having the lowest Γ value and with and without carbon diffusion barrier layer at both the Mo/Si interfaces are deposited and the interfaces have been characterized by hard X-ray and soft X-ray reflectivity measurements. The study shows improvement in reflectivities of the multilayer with carbon diffusion barrier layer from 38% to 54% at 130 Å wavelength. To have further insight into the modification of the interface due to introduction of the carbon barrier layer, depth selective study of both the multilayers have been carried out by Grazing Incidence Extended X-ray Absorption Fine Structure measurements using synchrotron radiation and thereby investigating the local structures around Mo atoms in Si bulk region and Si-on-Mo interface. This study confirms the reduction of diffusion of Mo and Si layer into each other when the carbon buffer layers are present at the interfaces, thus reducing the chance of formation of MoSi2.

Temperature dependent optical characterization of Ni-TiO2 thin films as potential photocatalytic material

Along with other transition metal doped titanium dioxide materials, Ni-TiO2 is considered to be one of the most efficient materials for catalytic applications due to its suitable energy band positions in the electronic structure. The present manuscript explores the possibility of improving the photocatalytic activity of RF magnetron sputtered Ni-TiO2 films upon heat treatment. Optical, structural and morphological and photocatalytic properties of the films have been investigated in detail for as deposited and heat treated samples. Evolution of refractive index (RI) and total film thickness as estimated from spectroscopic ellipsometry characterization are found to be in agreement with the trend in density and total film thickness estimated from grazing incidence X-ray reflectivity measurement. Interestingly, the evolution of these macroscopic properties were found to be correlated with the corresponding microstructural modifications realized in terms of anatase to rutile phase transformation and appearance of a secondary phase namely NiTiO3 at high temperature. Corresponding morphological properties of the films were also found to be temperature dependent which leads to modifications in the grain structure. An appreciable reduction of optical band gap from 2.9 to 2.5 eV of Ni-TiO2 thin films was also observed as a result of post deposition heat treatment. Testing of photocatalytic activity of the films performed under UV illumination demonstrates heat treatment under atmospheric ambience to be an effective means to enhance the photocatalytic efficiency of transition metal doped titania samples.

Study of interface correlation in W/C multilayer structure by specular and non-specular grazing incidence X-ray reflectivity measurements

W/C/W tri-layer thin film samples have been deposited on c-Si substrates in a home-built Ion Beam Sputtering system at 1.5 × 10−3 Torr Ar working pressure and 10 mA grid current. The tri-layer samples have been deposited at different Ar+ ion energies between 0.6 and 1.2 keV for W layer deposition and the samples have been characterized by specular and non-specular grazing incidence X-ray reflectivity (GIXR) measurements. By analyzing the GIXR spectra, various interface parameters have been obtained for both W-on-C and C-on-W interfaces and optimum Ar+ ion energy for obtaining interfaces with low imperfections has been found. Subsequently, multilayer W/C samples with 5-layer, 7-layer, 9-layer, and 13-layer have been deposited at this optimum Ar+ ion energy. By fitting the specular and diffused GIXR data of the multilayer samples with the parameters of each interface as fitting variables, different interface parameters, viz., interface width, in-plane correlation length, interface roughness, and interface diffusion have been estimated for each interface and their variation across the depth of the multilayers have been obtained. The information would be useful in realizing W/C multilayers for soft X-ray mirror application in the <100 Å wavelength regime. The applicability of the “restart of the growth at the interface” model in the case of these ion beam sputter deposited W/C multilayers has also been investigated in the course of this study.

Effect of substrate bias and oxygen partial pressure on properties of RF magnetron sputtered HfO2 thin films

Among all dielectric materials, hafnium oxide qualifies to be one of the most significant candidates as high index optical coating material due to its excellent chemical and thermal stability with SiO2, the low refractive index material, apart from its excellent laser radiation resistance. In this article, microstructural, physical, and optical properties of two sets of hafnium oxide thin films deposited by radio frequency (RF) magnetron reactive sputtering under a mixed ambient of argon and oxygen have been investigated: one set at various oxygen partial pressure and with substrate biasing by 50 W pulse direct current (DC) and another set at similar gaseous ambient as above but without any substrate bias. Structure of all the HfO2 thin films have been found to be monoclinic through grazing incidence x-ray diffraction measurements. Mass density of the samples has been estimated by grazing incidence x-ray reflectivity measurements and compared with the atom density of the samples estimated through Rutherford back scattering study. The evolutions of optical properties of the films with respect to the variation of oxygen partial pressure and substrate biasing have been investigated by transmission spectrophotometry and spectroscopic ellipsometry. It has been observed that RF sputtering with pulse DC substrate bias and with 15–30% oxygen partial pressure in the ambient helps in achieving better quality HfO2 films with low void fraction and high refractive index.

Structure and properties of gallium nitride thin films deposited on Si (111) by using radio-frequency magnetron sputtering

The structure and the properties of gallium nitride (GaN) thin films prepared by using a radiofrequency (RF) magnetron sputtering method are reported. The GaN thin films were deposited on p-type (111)-oriented silicon substrates at room temperature in a pure nitrogen atmosphere. X-ray photoelectron spectra showed that the films were mainly composed of gallium and nitrogen. An oxygen impurity was found to be incorporated into the films during the deposition. The refractive index of the GaN films was measured to be 2.36 at a wavelength of 633 nm. The optical bandgap of the GaN films was determined to be approximately 3.31 eV. From the grazing incidence X-ray reflectivity measurements, the density and the surface roughness of the GaN films were estimated to be 5.827 g/cm3 and 2.95 nm, respectively. The X-ray diffraction analysis revealed that the crystalline phase of the GaN films changed from wurtzite to zinc-blende as the working gas pressure was reduced. The observed phase change was found to be independent of the orientation of the Si substrate, but to be strongly correlated with the biaxial compressive stress.

Substrate surface effect on the structure of cubic BN thin films from synchrotron-based X-ray diffraction and reflection

Cubic BN (cBN) thin films prepared by mass-selected ion beam deposition technique (MSIBD) on Si substrates with different surface roughness were studied by synchrotron-based grazing incidence X-ray diffraction (GI-XRD) and X-ray reflectivity (XRR) measurements. The BN films are mostly composed of two phases. One is cBN phase, the other is hexagonal BN (hBN) phase. The cubic phase content of the thin films is dependent on the roughness of their corresponding substrates. The smooth substrate surface is helpful for the nucleation of the cBN phase. cBN phase is mostly grown in the near surface region of the films and there is a hBN interlayer at the film–substrate interface. GIXRD and XRR are proved to be powerful tools for analyzing the structure of the cBN thin films.

Correlation of interface roughness for ion beam sputter deposited W/Si multilayers

W/Si multilayers having 5, 7, 9, 13, 17, and 25 layers have been deposited on c-Si substrates by Ion Beam Sputtering technique and have been characterized by specular and diffused grazing incidence X-ray reflectivity measurements. Information regarding the density, thickness and interface widths of individual layers of the multilayer stacks have been obtained from the theoretical fitting of the specular reflectivity spectra while fitting of the diffused X-ray reflectivity have yielded information regarding roughness and diffusivity at the individual interfaces along with the in-plane correlation lengths of roughness of the individual layers and the vertical correlation length of the whole multilayer structure. Investigations have been carried out on the different behavior of W-on-Si and Si-on-W interfaces and on the variation of the above parameters with the increase in number of layers in the multilayer structures.

Characteristics of m-Plane InN Films Grown on ZnO Substrates at Room Temperature by Pulsed Laser Deposition

We have grown single-crystal m-plane (1100) InN films on m-plane (1100) ZnO substrates at low substrate temperatures by the use of pulsed laser deposition. X-ray diffraction and electron back-scattered diffraction measurements have revealed that pure m-plane InN grows at substrate temperatures below 300 °C. The full width at half maximum values of the X-ray rocking curves for m-plane InN grown at room temperature (RT) are as low as 0.12 and 0.12° with X-ray incidence directions normal to [0001] and [1120], respectively. Grazing-incidence X-ray reflectivity measurements have revealed that interfacial layers of greater than 5 nm in thickness are formed in the samples grown at temperatures above 300 °C, while no interfacial layers are formed between InN and ZnO at RT. The suppression of interfacial reactions by reducing the growth temperature down to RT is probably the reason for the improvements in crystalline quality that are observed.

Effects of oxygen flow rate on microstructure and optical properties of aluminum oxide films deposited by electron beam evaporation technique

Reactive evaporation technique has been used to deposit thin films of alumina (Al 2O 3) on crystalline Si substrates at ambient temperatures in an electron beam (e-beam) evaporation system using alumina granules as evaporant material. The loss of oxygen due to dissociation of alumina has been compensated by bleeding high purity oxygen gas into the system during evaporation. A set of samples were prepared at different flow rates of oxygen and the films have been characterized by Spectroscopic Ellipsometry (SE), Atomic Force Microscopy (AFM), Grazing Incidence X-ray Reflectivity (GIXR) and X-ray Photoelectron Spectroscopy (XPS) measurements. The density and optical properties of the films showed interesting variation with oxygen flow rates.

Reactively Sputtered Nanocrystalline ZrN Film as Extremely Thin Diffusion Barrier between Cu and SiO2

The results of analyses by X-ray diffraction, transmission electron microscopy, and grazing incidence X-ray reflectivity measurement indicate that a 5-nm-thick ZrN film interposed between Cu and SiO2 shows excellent barrier properties, tolerating annealing up to at least 500 °C for 30 min. The X-ray diffraction pattern reveals a decrease in the intensity of the Cu(111) reflection upon annealing at 800 °C, suggesting a failure of the thin barrier due to Cu diffusion through the barrier. We are confident that the formation of a continuous nanocrystalline ZrN film in a uniform fashion in a stable phase with a slightly nitrogen-rich composition is a cause of the excellent features obtained. The formation process of the ZrN film is discussed in terms of the nucleation process of reactive sputtering at a low deposition temperature.

Alkyl and Alkoxyl Monolayers Directly Attached to Silicon: Chemical Durability in Aqueous Solutions

For practical application of self-assembled monolayers (SAMs), knowledge of their chemical durability in acidic or basic solutions is important. In the present work, a series of SAMs directly immobilized on a silicon (111) surface through Si-C or Si-O-C covalent bonds without a native oxide layer were prepared by thermally activated chemical reactions of a hydrogen-terminated Si(111) substrate with linear molecules, i.e., 1-hexadecene, 1-hexadecanol, 1-dodecanol, and n-dodecanal, to investigate the durability of the SAMs to HF and Na2CO3 solutions. While grazing incidence X-ray reflectivity measurements showed that all the as-prepared SAMs had almost the same film density and molecular coverage, keeping the original step and terrace structure of Si(111) as is observed by atomic force microscopy, they gave different degradation behaviors, i.e., pitting and concomitant surface roughening in both solutions. 1-hexadecene SAM was stable against immersion in both solutions, while the other SAMs were damaged within 60 min, most likely due to the difference in chemical bonding modes at the SAM/Si interface, i.e., Si-C and Si-O-C.

Study of interfacial structure of HfO2thin film on Si by grazing incidence x-ray reflectivity

The interfacial structures of an HfO2thin film on a Si substrate were studied by grazing incidence x-ray reflectivity (GIXR) in this work. The HfO2thin films were fabricated on Si substrates by the sputtering and subsequent oxidization/annealing with different interfacial layers. The GIXR results revealed that the density and thickness of the Hf-silicate interlayer was different for different samples. The composition of the Hf-silicate interlayer was related to the oxygen atoms; more oxygen atoms led to the formation of Si-rich Hf-silicate. The interfacial structures were appropriately obtained by GIXR measurements; these results were in agreement with the results of x-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy.

Synchrotron x-ray scattering study of thin epitaxial Pr2O3 films on Si(001)

We investigate the structural and interfacial properties of thin Pr2O3 films on Si(001) substrates grown by molecular beam epitaxy using synchrotron grazing incidence x-ray diffraction and reflectivity measurements in ultrahigh vacuum. The epitaxial films consist of two orthogonal [101]-oriented cubic domains, with equal proportion. The average in-plane domain sizes are larger than the film thickness. Scans along crystal truncation rods confirm the cubic Mn2O3 structure of the epitaxial layer. A small amount of hexagonal Pr2O3 is found in thin films. Its fraction increases with increasing layer thickness indicating that it is not confined to the interface. Reflectivity measurements reveal an additional layer at the Pr2O3∕Si(001) interface, which is extended by in situ annealing. Transmission electron microscopy of the samples confirms the structural properties of the films found by the x-ray measurements and shows that the interfacial layers are nonuniform with a coexistence of crystalline and amorphous regions.

Thickness dependent structural and magnetic properties of ultra-thin Fe/Al structures

The structural and magnetic properties of electron beam evaporated ultra-thin Fe/Al structures are studied as a function of Fe layer thickness, while keeping the Al layer thickness constant. The grazing incidence X-ray reflectivity measurements carried out on the structures having Fe layer thickness ≤20 A show substantial intermixing between the layers during deposition, indicated by a loss of periodicity. These structures resemble a composite single layer film consisting of Fe and Al clusters. However, for thicker Fe layers (≥30 A), the appearance of a first order Bragg peak in the reflectivity patterns indicates the formation of a better-multilayered structure. These results are also supported by AFM and resistivity measurements. The X-ray diffraction measurements show that in all the multilayer films, deposited Fe layers are textured mainly along (110) direction. The corresponding magnetic measurements show a soft magnetic behaviour of the films with an in-plane easy direction of the magnetization. The observed soft magnetic behaviour in these samples is explained in terms of (i) weak crystalline magnetic anisotropy due to small crystal grains and magnetostriction and (ii) the morphological and structural changes occurring due to the variation in the Fe layer thickness below the critical value in the deposited structures.

Correlation of structural, chemical, and magnetic properties in annealed Ti∕Ni multilayers

Ti ∕ Ni multilayer samples have been synthesized on float glass substrates using an electron-beam evaporation technique under ultrahigh vacuum conditions at room temperature. Grazing incidence x-ray diffraction (GIXRD) and grazing incidence x-ray reflectivity (GIXRR) techniques were used, respectively, to study structural modifications and to determine corresponding changes in microstructural parameters, such as individual layer thickness, interface roughness, and electron density due to annealing treatment. In addition to this, the chemical nature of the surface and interfaces of these samples were also studied using a depth profile x-ray photoelectron spectroscopy (XPS) technique. The GIXRD measurement show clear amorphization of the as-deposited multilayer sample annealed in the temperature range of 300–400 °C. The corresponding GIXRR measurement indicates the formation of a sufficiently thick layer of Ti–Ni at interfaces converting the Ti∕Ni bilayer into a Ti∕Ti–Ni∕Ni trilayer multilayer structure. The precipitation of the Ti–Ni alloy phase at the interface in the case of samples annealed at 400 °C has been confirmed by XPS measurements. The magnetization behavior investigated using the magneto-optical Kerr effect technique clearly shows well the saturation magnetization behavior for all samples annealed up to 300 °C, while the sample annealed at 400 °C does not show saturation magnetization. The corresponding coercivity value (Hc) is also found to be changed drastically from 15.5 to 0.6 Oe. This observed magnetization behavior is discussed and correlated with structural and chemical changes in the multilayer structure.

Molecular volume and electronic and vibrational polarizibilities for amorphous LaAlO3

Grazing incidence x-ray reflectivity measurements are used to determine the density of sputter-deposited LaAlO3 and anodized LaAl films. Together with refractive index and dielectric constant measurements, it is demonstrated that a coherent picture emerges explaining the low dielectric constant of the amorphous films (∼13) as compared to the single-crystal value (∼26). The importance of molecular volume dependence of the electronic and vibrational molecular polarizabilities is underlined.

Pulsed laser deposition of metallic multilayers: the influence of laser power on microstructure

Multilayers of Co and Cu on Si(111) substrates have been produced by pulsed laser deposition at the second harmonic of a Nd:YAG laser (532 nm). The effect of varying the laser power on the film microstructure has been investigated using grazing incidence X-ray reflectivity measurements. Quantitative analysis of the reflectivity curves indicates that higher laser powers are associated with greater intermixing at the Co/Cu interfaces. Offset scans indicate that there is conformal roughness. The deposition process introduces some droplets into the layers, principally of Cu.