The elastic component of anisotropic strain dominates the observed shift in the F2g Raman mode of anelastic ceria thin films

Abstract

Raman spectroscopy is applied for non-destructive characterization of strain in crystalline thin films. The analysis makes use of the numerical value of the mode Grüneisen parameter γ, which relates the fractional change in the frequency of a Raman-active vibrational mode and the strain-induced fractional change in the unit cell volume. When in-plane, compressive biaxial strain in aliovalent doped CeO₂-films is relieved by partial substrate removal, the films exhibit values of γ for the F_2g vibrational mode which are ∼30% of the literature values for bulk ceramics under isostatic stress. This discrepancy has been attributed to a negative contribution from the anelastic (time-dependent) mechanical properties of aliovalent-doped ceria. Here we propose a way to “separate” anelastic and elastic contributions to the F_2g mode Grüneisen parameter. Mechanically elastic yttria (Y₂O₃) films on Ti/SiO₂/Si substrate serve as “control”. The values of γ calculated from the change in frequency of the ∼375 cm⁻¹ F_2g Raman-active mode are close to the literature values for bulk yttria under isostatic stress. This work should serve to provide a protocol for characterization of selective sensitivity to different strain components of doped ceria thin films.