Spectroscopic Ellipsometry for Simultaneous Thickness and Defect Concentration Changes in Oxide Thin Films
Authors: Anderson Lawrence O , Perry Nicola H. 
Published: May 2020

Measurements of strain and stoichiometry changes needed to determine coefficients of chemical expansion (CCEs) can be very time-intensive for bulk materials due to the long equilibration times needed at the multiple instruments and conditions. Additionally, bulk polycrystalline materials give macroscopic responses that include both lattice and microstructural contributions, and the responses are often therefore averaged over the randomized orientation of grains. Thin films provide an avenue to rapidly assess steady-state properties of such chemically expanding materials after short dwells due to their low volume and high surface area. Additionally, the controlled growth of thin films can be used to fabricate polycrystalline samples or single crystals with tailored crystallographic orientation, grain dimensions, and induced strain for controlled experiments. Although techniques exist for in situ analysis of thickness changes or defect concentration changes in thin films, there are few (if any) techniques that can measure both simultaneously. Further, the limited existing techniques to probe defect concentrations of thin films, e.g., in-plane electrical conductivity, cross-plane chemical capacitance, or in situ X-ray diffraction, require additional knowledge such as the dominant carrier mobility, the interface capacitance, or the coefficient of chemical expansion respectively (which is what we wish to determine).

Spectroscopic ellipsometry has been widely used to analyze film thicknesses and optical properties, but we show that the optical properties obtained from the measurement can also be used to quantify defect concentrations, specifically oxygen non-stoichiometry. This work builds on our prior in situ optical absorption studies of oxygen concentrations and dynamics in thin films [1-4]. When we induce an oxygen stoichiometry change in a film, ellipsometry is, in principle, able to provide a measurement of the corresponding thickness change and oxygen stoichiometry change by a single technique.

To evaluate the effectiveness of this technique initially, a SrTi0.65Fe0.35O3 thin film has been scanned after anneals in O2 and Ar. The modelled expansion and optical constants, derived from the data, were compared with previous work, and thicknesses were verified by X-ray reflectometry (XRR). For materials whose extinction coefficient corresponding to oxygen concentration is already known, the oxygen stoichiometry change can be estimated from the measured absorption coefficient change. Additionally, a variation of the Clausius-Mossotti equation is proposed to estimate the defect concentration in materials that lack prior optical extinction coefficient data corresponding to the defect of interest. These preliminary results suggest that spectroscopic ellipsometry may be used for the rapid acquisition of in situ data to quantify thin film CCEs in various environments.

[1] Perry, N. H., Kim, J. J., & Tuller, H. L. (2018). Oxygen surface exchange kinetics measurement by simultaneous optical transmission relaxation and impedance spectroscopy: Sr (Ti, Fe) O3-x thin film case study. Science and Technology of advanced MaTerialS, 19(1), 130-141.

[2] Perry, N. H., Pergolesi, D., Bishop, S. R., & Tuller, H. L. (2015). Defect chemistry and surface oxygen exchange kinetics of La-doped Sr (Ti, Fe) O3− α in oxygen-rich atmospheres. Solid State Ionics, 273, 18-24.

[3] Chen, T., Harrington, G. F., Sasaki, K., & Perry, N. H. (2017). Impact of microstructure and crystallinity on surface exchange kinetics of strontium titanium iron oxide perovskite by in situ optical transmission relaxation approach. Journal of Materials Chemistry A, 5(44), 23006-23019.

[4] Perry, N. H., Kim, N., Ertekin, E., & Tuller, H. L. (2019). Origins and Control of Optical Absorption in a Nondilute Oxide Solid Solution: Sr (Ti, Fe) O3–x Perovskite Case Study. Chemistry of Materials, 31(3), 1030-1041.