Investigation of Electric Field--Induced Second Harmonic Generation from Fe-Doped SrTiO$_{\mathrm{3}}$ Interfaces

Oxygen vacancy electromigration is a leading contributor to breakdown mechanisms such as resistance degradation in perovskite oxide dielectrics. Greater understanding of oxygen vacancy migration and the correlated formation of structural defects/strain at dielectric interfaces is crucial for improving lifetime and reliability in these materials. We report on structural changes at reduced and oxidized Fe-doped SrTiO$_{\mathrm{3}}$ interfaces detected by electric field-induced second harmonic generation (EFISHG). Under a dc-field, oxygen ions and holes migrate to the anode while oxygen vacancies and electrons migrate to the cathode. Vacancy migration to the cathode leads to electrostrictive distortions, described as Fe:Ti-O bond stretching and bending, in FeTiO$_{\mathrm{6}}$ octahedra. Differences in EFISHG responses are explained by intrinsic electric fields present at the interfaces whose local strength and polarity are influenced by the oxygen vacancy, Fe$^{\mathrm{3+}}$, and Fe$^{\mathrm{4+}}$ concentrations of the crystals. Results show optical SHG is a powerful tool for detecting structural changes at perovskite oxide interfaces due to oxygen vacancy migration.