ON THE DEVELOPMENT OF A NEW METHOD OF TIME-RESOLVED X-RAY REFLECTIVITY AND ITS APPLICATIONS IN MATERIALS SCIENCE
Specular x-ray reflectivity (XRR) is a powerful and non-destructive technique for characterizing surfaces, thin-films, and interfaces. XRR is particularly well suited for in situ studies because it is a highly penetrating and minimally interacting remote probe of the average surface. However, application of XRR to the study of dynamic processes is severely limited by the time resolution of the measurement as traditionally performed. We describe our efforts to develop and demonstrate a novel method for collecting XRR spectra with time resolution of 100 ms using monochromatic radiation that is readily compatible with many synchrotron beamlines and laboratory sources. Our method uses a polycapillary x-ray optic to generate a converging fan of radiation and a position sensitive detector to simultaneously record up to a 5° (in 2θ) portion of the XRR curve in a single snap-shot without mechanical motion. We first demonstrate this method by applying it to characterize an ordering phase transition in a buried epitaxial thin film of La0.7Sr0.3MnO3 – x (LSMO) on SrTiO3 (STO). This transition, from the Perovskite (PV, x=0) to Brownmillerite (BM, x=0.5) phase, is induced by deposition of oxygen-poor STO on top of the LSMO film. Because this transition is kinetically driven, details of the transition could not be studied using traditional techniques. Using our time-resolved measurement, we are able to perform XRR measurements, in situ, during the growth of the STO capping layer by pulsed laser deposition. We are able to observe two distinct transition pathways. Following this effort, we worked to increase the resolution of the measurement. By rotating the sample normal out of the plane of the incident fan, we were able to improve the resolution and quality of the XRR curves, sufficient to extract quantitative information. We used this new method to study diffusion in metal-metal multilayers. We demonstrate we can record XRR spectra while rapidly heating Al/Ni multilayers at a rate of 91 K/s. During this heating, the Al and Ni mix resulting in loss of the periodicity of the multilayer which is readily visible in the XRR spectra. We have demonstrated that we can use the XRR curves to generate Arrhenius plots in order to extract kinetic information about the rapid intermixing reaction.
Applied physics; Materials Science; Brownmillerite; Thin-film Growth; Time-resolved Diffraction; X-ray Reflectivity
Brock, Joel Donald
Van Dover, Robert B.; Schlom, Darrell; Woll, Arthur R.
Materials Science and Engineering
Ph. D., Materials Science and Engineering
Doctor of Philosophy
dissertation or thesis