In Situ Optical Absorption Studies of Point Defect Kinetics and Thermodynamics in Oxide Thin Films

Abstract

The authors review the use of in situ optical absorption to probe point defect equilibria and kinetics in oxides, with particular attention to thin films and high temperature measurements evaluating changes in oxygen content. The introduction notes the impact of point defects on oxide behavior, including differences in thin films vs. bulk materials, and describes methods to evaluate point defect equilibria and kinetics; advantages of optical absorption among these methods are highlighted. Section 2 provides an overview of how optical absorption and point defect concentrations are related, including various possible structural and electronic origins of defect-mediated absorption changes corresponding to oxygen content changes. Section 3 traces the development of historical understanding of optical absorption in nonstoichiometric oxides and emergence of thin film applications and measurement approaches that resulted. Section 4 details the procedure for determining surface exchange coefficients from optical transmission relaxation measurements and provides specific examples of how this approach uniquely has enabled new insights into thin film surface exchange behavior with and without current collectors. Section 5 provides examples of optically-derived diffusivity measurements and comparisons of bulk and thin This article is protected by copyright. All rights reserved. 2 film defect equilibria. Section 6 outlines potential opportunities and future developments in this area, and Section 7 concludes the manuscript.

Simultaneous Electrical, Electrochemical, and Optical Relaxation Measurements of Oxygen Surface Exchange Coefficients: Sr(Ti,Fe)O3−d Film Crystallization Case Study

“…The OTR technique has recently been applied to measure kinetics and point defect concentrations in films of the fluorite-structured Ce 0.9 Pr 0. 18,21,48,49,52 Here, we will present a brief overview of the nature and origins of optical absorption and its relationship to oxygen nonstoichiometry in nondilute STF; however, for a more detailed treatment, see ref 51 or the review by Buckner et al 40 Early work focused on a dilute case: the model system SrTiO 3 with dilute Fe on Ti sites. Mossbauer spectroscopy measurements suggested that, in that system, under oxidizing conditions, Fe changes valence from Fe 3+ to Fe 4+ as oxygen is added.…”

“…Some aspects of these considerations have been raised in our earlier work. 24,40,48 4.2.1. Driving Forces.…”

Pushing the Study of Point Defects in Thin Film Ferrites to Low Temperatures Using In Situ Ellipsometry

“…A very promising family of methods for tracking point defects in oxides thin films are UV-visible optical characterization techniques. [4,25] As recently reviewed by Buckner et al, [26] point defects strongly modify the electronic band structure of many oxides, giving rise to unique changes of optical properties that can be tracked by optical methods. Although UV-visible techniques cannot directly probe the concentration of point defects, by measuring the evolution of the dielectric properties of the thin film by in situ Brower analysis, it is possible to indirectly achieve a quantification of the defect chemistry of the material, similar to the measurement of the electronic conductivity but without the limitation of a non-constant charge carrier mobility.…”

Direct Measurement of Oxygen Mass Transport at the Nanoscale

“…However, the deployment of strategies for increasing mass transport at the interface level is often hindered by a lack of understanding of the fundamental controlling mechanisms, which remain largely undisclosed to date. [11,16] Several techniques based on electrical conductivity relaxation, [17] weight changes, [18] electrochemical impedance spectroscopy, [19] and color changes, [20][21][22] are widely employed in order to retrieve global information on ion transport in oxides, but are insufficient for the study of local kinetics at the nanoscale. The most common technique for the investigation of ion transport properties (oxygen diffusivity-D-and surface exchange coefficient-k) is isotopic exchange depth profiling (IEDP) in combination with secondary-ion mass spectrometry (SIMS).…”