Soft X-ray Reflection Spectroscopy for Nano-Scaled Layered Structure Materials

www.nature.com/scientificreports OPEN Received: 25 January 2018 Accepted: 11 September 2018 Published: xx xx xxxx Soft X-ray Reflection Spectroscopy for Nano-Scaled Layered Structure Materials A. Majhi1,2, Maheswar Nayak1,2, P. C. Pradhan1,2, E. O. Filatova3, A. Sokolov4 & F. Schäfers4 We introduce a novel approach that addresses the probing of interfacial structural phenomena in layered nano-structured films. The approach combines resonant soft x-ray reflection spectroscopy at grazing incidence near the “critical angle” with angular dependent reflection at energies around the respective absorption edges. Dynamic scattering is considered to determine the effective electron density and hence chemically resolved atomic profile across the structure based on simultaneous data analysis. We demonstrate application of the developed technique on the layered model structure C (20 Å)/B (40 Å)/Si (300 Å)/W (10 Å)/substrate. We precisely quantify atomic migration across the interfaces, a few percent of chemical changes of materials and the presence of impurities from top to the buried interfaces. The results obtained reveal the sensitivity of the approach towards resolving the compositional differences up to a few atomic percent. The developed approach enables the reconstruction of a highly spatio-chemically resolved interfacial map of complex nano-scaled interfaces with technical relevance to many emerging applied research fields. Today thin films and nano-structured layer systems find a wide range of applications in materials science1–4 due to their tunable optical, structural, electronic, magnetic and superconducting properties. Often the quality of films is a governing factor that determines the critical parameters of these devices5–8. Any deviation of their physical, chemical and geometrical parameters from desirable ones causes fluctuations in their properties; for example, the complete disappearance of quantum effects in nano-electronic devices or the catastrophic drop of reflectance of ultra short-period x-ray multilayer mirrors. The problem becomes more complicated due to the formation of interlayers, owing to atomic migration, chemical reactions or implantations in metal-oxide-semiconductor gate stacks, which may impact the functionality of the devices by for examples, affecting the effective work function of electrodes9, the optical contrast10 or their magnetization11. All of these, in turn, stipulate the higher requirements on the technology of thin film synthesis and quality control. One of the key issues is the precise determination of atomic and chemical composition profiles at various interfaces in layered structures with an in-depth resolution approaching the scale of interatomic distances (~1 Å), which pushes the development of novel approaches. Commonly used transmission electron microscopy (TEM) imaging allows one to attain the desired resolution, provided the interface roughness is small. However, the analysis of quantitative atomic composition appears to be problematic. Additionally, it is a destructive method and the detection of light (low-Z) elements in the presence of high-Z ones becomes difficult12. Similarly, the routinely used photoemission spectroscopy or the analysis of the fluorescent radiation in combination with argon ion-sputtering technique are also destructive techniques and may introduce artefacts during sample preparation13. Therefore, there is a need for development of alternative analytical techniques (less invasive than TEM), which would allow one to quantify chemically and spatially resolved atomic concentration profiles across the hetero-structure with a depth resolution better than 10 Å. Among the non-destructive techniques, the combined x-ray standing wave and x-ray reflectivity14 are limited in their success owing to lack of sensitivity to structures with low contrast interfaces15, and/or low-Z materials. Also neutron reflectivity (NR) is complementary to x-ray reflectivity16,17, but NR has some known limitations18. Finally, hard X-ray photoelectron spectroscopy (HAXPES)19 and depth-resolved soft X-ray emission spectroscopy20 are 1 Synchrotrons Utilization Section, Raja Ramanna Centre for Advanced Technology, Indore, 452013, India. 2Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai, 400094, India. 3St Petersburg State University, Ulyanovskaya 3, Peterhof, St Petersburg, 198504, Russian Federation. 4Helmholtz-Zentrum Berlin, Institute for Nanometre Optics and Technology, Berlin, Germany. Correspondence and requests for materials should be addressed to M.N. (email: ) Scientific Reports | (2018) 8:15724 | DOI:10.1038/s41598-018-34076-5 1 www.nature.com/scientificreports/ the most suitable non-destructive spectroscopic methods. Nevertheless, in all these methods various physical models are used during processing of the original data, which often introduce its own limitations. In the present work, using resonance phenomena in the soft x-ray region by exploring both spectral and angular dependencies of the reflection coefficient, we develop a novel approach to probe the interfacial structural phenomena in layered nano-structured films. Owing to its excellent chemical sensitivity, high contrast variation and high resolution such a technique could clearly overcome the previously mentioned limitations. Nowadays spectral dependent reflection spectroscopy emerges as a potential tool for atomic and electronic structures of materials and is utilized in different contexts at relatively larger incidence angles21–27. Similarly, reflection spectroscopy at constant momentum transfer qz, is utilized in different contexts for epitaxially grown transition-metal oxides4,28–30. For example, the sensitivity of constant qz-reflection to a marker layer (LaxSr1-xTiO3) with concentration x = 0.006 was demonstrated for a structurally nearly perfect SrTiO3 film (ref.30). Subsequently, the layered structure was determined by fitting reflection spectra with a-priori information on the film. Similarly, based on the atomic slices approach, reflection spectra of constant qz and angular dependence were modeled for an epitaxial LaSrMnO4 film to extract information on layer termination and the stacking sequence of the atomic planes30. However, the idea of creating a depth of formation of reflected beam within a nano-scale range near the “critical angle” is not clearly understood. Such an attempt could potentially probe the physico-chemical characteristics of the nano-scaled layer structure by varying the angle of incidence. Additionally, shallow incidence angles near the critical angle provide much more sensitivity. This facilitates the discrimination of the chemical state of the overlaying surfaces from that of the underlying layer. Relying on the above mentioned method, we combine the high sensitivity of energy- and angle-dependent near-edge reflection spectra to obtain a quantitative spectroscopic profile of complex nano-layered s (...truncated)