Applied Science and Convergence Technology 2022; 31(6): 128-132
Published online November 30, 2022
Copyright © The Korean Vacuum Society.
Department of Organic Material Science and Engineering, School of Chemical Engineering, Pusan National University, Busan 46241, Republic of Korea
X-ray is an essential probe for observing, from the nano to atomic scale, the physical and chemical properties of thin films, such as film thickness, electron densities, and features related to crystal structures. In particular, bright and collimated synchrotron (SR) X-rays have enabled various in situ experiments combined with multiple measurements of X-rays and other probes. In this report, we provide basic information on SR-related X-ray experiments, such as X-ray reflectivity-Total reflection X-ray fluorescence, Grazing incidence wide-angle X-ray scattering, and 3-dimensional reciprocal space mapping, for thin film research.
Keywords: Thin film, Synchrotron X-rays, X-ray scattering, X-ray fluorescence
Understanding structures at the nano and atomic scales is essential for managing the properties of thin films. The optoelectronic and mechanical properties of thin films are strongly affected by their structural characteristics, including the film thickness and shape, and the size of the embedded nanostructures. Crystalline properties, such as crystallinity, domain size, and domain orientations, as well as chemical compositions, are fundamental information for controlling the physical and chemical behaviors of thin-film systems. In a thin-film system, the role of the surface and interface is important for understanding the changes in material properties. Recently, it has become necessary to understand the changes in materials under in situ operando conditions according to external environmental stimuli such as heat treatment, electric bias, pressure, and electromagnetic wave irradiation. Because changes in the crystalline and material properties of thin films must be closely monitored, X-rays have become a powerful tool to non-invasively measure these changes at the atomic scale.
X-rays,which are electromagnetic waves of several keV, interact with the scattering or absorption of electrons in an atom. The energy of X-rays is as high as a few tens of keV, corresponding to the binding energies of the
The wavelength of X-rays is a few Å, which is the order of magnitude of atom-atom distances in a unit cell of the crystal structure. Therefore, we can observe a strong Bragg X-ray scattering signal for a crystalline film composed of periodically ordered atoms. That is, the collective elastic scattering intensities (Bragg signals) of X-rays are a function of the correlation lengths of the electron densities. Figure 1(a) shows a schematic diagram of X-ray scattering divided into two regimes, small and wide-angle, by the scattering angle 2θ. We can investigate the momentum transfer
The X-ray intensity from a lab source is not sufficient for obtaining scattering signals from thin films because of the relatively small scattering volume. In addition, the X-ray energies should be varied for elemental-sensitive scattering or fluorescence signals of thin films. Various in situ and operando experiments are crucial for understanding the device’s performance in real situations. Synchrotron (SR) Xrays are very bright and collimated within a few tens of microns, enabling numerous types of in situ experiments with high resolutions in reciprocal space and time. For elemental discrimination, we could easily tune the SR X-ray energies for anomalous X-ray scattering and X-ray absorption experiments.
In this study, we focus on the basic information for SR X-ray experiments of thin-film systems: X-ray reflectivity-Total reflection X-ray fluorescence (XRR-TXRF), grazing incidence wide-angle X-ray scattering (GIWAXS), and 3-dimensional reciprocal space mapping (3D RSM). X-ray signals near the critical angle are important because we can obtain depth-dependent information on thin-film systems. Therefore, we introduce TXRF combined with X-ray reflectivity, which is a powerful method for thin-film characterization [1–4]. GIWAXS is also a key analysis technique for obtaining crystalline features within thin films [5–7]. In addition, 3D RSM of a Bragg signal is briefly presented, which can show phenomena related to defects such as grain boundaries in thin-film systems [8–10].
X-ray reflectivity does not measure atoms individually but rather measures the overall electron density distribution of a film in the vertical direction. X-rays are reflected from the interfaces of different electron densities, and the reflection follows Snell’s law. Therefore, X-ray reflectivity is a function of electron density along the surface normal direction expressed in Eq. (1), and it provides information on the film thickness, density, and roughness of the surface and interfaces of a film . Because the X-ray wavelength is of a few Å, the reflectivity can vary with changes in thickness and roughness on the Å scale, allowing high spatial resolution analysis of atomic structures. In Eq. (1),
Figure 2 shows the schematic and simulated reflectivity patterns when X-rays are incident on a thick substrate at angles as low as θ ~ 0.1 − 5°. As shown in Fig. 2(a), reflection, refraction, and absorption occur at the interface between two different media, similar to other electromagnetic waves. These phenomena are described by the refractive index (
From Snell’s law [Eq. (6)] and the boundary conditions, the Fresnel equation for X-ray reflectivity (
Figure 2(b) shows the calculated reflectivity of a thick LaAlO3 substrate and the SLD with a roughness of σ = 0 Å (black line) and σ = 3 Å (red) . As previously mentioned, the refractive index
In the caseof a single slab, X-rays were reflected at the surface and interface, and two reflected X-rays interfered with each other, as shown in Fig. 2(c). As a result of the interference, the Kiessig fringe, which is the oscillation of the intensity with respect to the scattering angle, appeared in the X-ray reflectivity data, and the oscillation period was determined by the slab thickness. The reflectance of a single slab is expressed by Eq. (8), where
Figure 2(d) shows the calculated reflectivity patterns and corresponding SLD profiles of the SrTiO3(10 nm)/LaAlO3 systems under three different conditions. The first two cases show different surface roughness values, and the last case shows different X-ray energies. The film thickness can be obtained in the kinematic approximation region, that is,
To understand the effect of surface roughness, we assumed two films with different surface roughness of 3 and 7 Å and assumed an X-ray reflectivity measurement of 10 keV. The data show that the intensity decreased significantly as
All layered film systems can be modeled as homogeneous
Figure 3(b) shows the calculated reflectivity and corresponding SLD of a Pt(50 Å)/Ni(50 Å) bilayer on a c-cut sapphire substrate. There are complicated oscillations originating from bilayer structures, and it is easy to recognize that the shortest period of oscillation corresponds to the total film thickness of the bilayer, which is approximately 100 Å. Before a reflectivity fit, a robust estimation of the layer thicknesses is possible by obtaining a Fourier transform of the normalized reflectivity.
X-ray evanescent waves were formed under the condition of total external reflection. X-rays traveled along the surface and penetrated the medium, and the X-ray penetration depth ξ was determined by the angles of incidence and total external reflection, as described in Eq. (9). Figure 4(a) shows the evanescent wave at θ ≤ θc, and the penetration depth is defined as the position at which the evanescent wave intensity became 1/3 of the intensity at the surface. The penetration depth increases from a few to thousands of nanometers with respect to the incident angles, as shown in Fig. 4(b), then we can investigate depth-dependent crystalline properties and compositional information. Figure 4(c) shows a schematic diagram of the TXRF setup. At incident X-rays slightly higher than the absorption edge of the element to be probed, the fluorescence signals of the element were enhanced, and the signal was proportional to the intensity of the penetrated evanescent wave.
Surface segregation of cations has been an important issue for solid oxide fuel cell cathode materials at operating temperatures. In 2016, Yu
Investigating GIWAXS is valuable for observing the crystalline properties of polycrystalline thin films. In GIWAXS, the X-ray incident angle is small and close to the critical angle, and widely scattered Xrays are measured using a two-dimensional (2D) detector. It is a powerful method, especially for organic materials or very thin films, because the scattering volume increases owing to the long optical path of X-rays at a small grazing incident angle. Using the same analogy as TXRF, we can investigate the depth-dependent crystal structure by changing the X-ray incident angle. Figure 5 depicts a general scheme of the GIWAXS setup: the incident angles of the X-ray are set within 0.5θc ≤ θ ≤ 3θc and the sample-to-detector distance (SDD) is close to approximately 300 mm to measure the scattering signal over wide angles. It is worth noting that high collimation with a few hundreds of micron-sized beams is necessary because the illumination area increases significantly at the grazing angle, and fine tuning of the incident angles is important to determine the penetration depth of X-rays. As shown in Fig. 5, the detector pixels assigned to
The preferred orientations with respect to the azimuthal angles can be observed using GIWAXS, as shown in Fig. 6. The left two-column images show the schematic view of GIWAXS for a Debye ring of a Bragg peak and the corresponding crystal domain orientations. For randomly oriented crystal domains, the color of the Debye ring was uniform, indicating that the Bragg peak intensities did not change along the circle. If the preferred orientations of the domains existed with a random distribution, there were bright spots in the Debye ring, indicating the preferential ordering of crystals. Finally, only one spot was shown for the crystal domains perfectly aligned along one azimuthal angle, as shown at the bottom of Fig. 6(a). As shown in the schematic, the Debye ring had some width along
A material system is defined by the atomic density distribution of its elements. The atomic density distribution can be simply assumed to be the electron density distribution when considering the interaction between the matter and X-rays. The X-ray scattering intensity is the Fourier transform of the autocorrelation function of electron density in real space, which is a function of 2π/
2D RSM is a decisive method for evaluating the strained status of the film by the substrate. 2D RSM is generally measured using a point detector, but it is very time consuming, especially if it is very thin or has low crystallinity. Recently, 2D X-ray detectors with high spatial and time resolutions, as well as a wide dynamic range in intensity, have become possible, opening the door to 3D visualization of reciprocal spaces. Figure 8(b) shows the schematic of the 3D RSM setup. A 3D RSM can be obtained by obtaining 2D scattering images rocking the incident angle around the Bragg angle with a fixed 2D detector at the scattering angle to be measured.
By analyzing the 3D RSM of a Bragg peak, it is possible to investigate crystal domains along specific directions such as in-plane axes compared to those of substrates, anisotropic diffuse signals due to thermal vibrations, and 3D defect distributions. Ha et el. investigated the role of defects in the metal-insulator transition (MIT) of VO2 thin films grown on c-cut sapphire substrates using 3D RSM at various temperatures . They confirmed diffuse peaks originating from rutile-like defects separating the monoclinic ordered domains and their behavior in reciprocal space during heating near MIT. The analysis of 3D scattering signals provided two pre-transition states in the structural phase transition near MIT, where two different signals from monoclinic ordering and planar rutile-like defects were treated separately in a 3D reciprocal space.
In this study, we introduced SR-based X-ray experiments with brief explanations of thin-film characterization. Highly bright, collimated, and energy-tunable SR X-rays provided various X-ray techniques, such as XRR, TXRF, GIWAXS, and 3D RSM, with high spatial and time resolution. In addition, in situ real-time changes in films were observed using SR X-rays under external stimuli, such as heat, pressure, mechanical stress, electrical bias, and electromagnetic irradiation. Owing to these advantages, the application field of SR X-rays is increasing, allowing the development of new materials and elucidating the mechanisms of physical and chemical changes in materials.
The authors declare no conflicts of interest.