Local structural changes due to the electric field‐induced migration of oxygen vacancies at Fe‐doped SrTiO₃ interfaces

Abstract

We report on our study of dc voltage-induced structural changes at reduced and oxidized Fe-doped SrTiO 3 (Fe:STO) electrode interfaces using second harmonic generation (SHG) together with photoluminescence (PL) method. We show that oxygen vacancy defects play a critical role in determining the local electrical and structural properties of interfacial depletion regions at Schottky junctions. The SHG results show that the dc electric field causes oxygen ions and vacancies to displace toward the anode and cathode in the low field regime, respectively. This process forms electrostrictive distortions within local interfacial depletion regions which are described by Fe:Ti-O bond stretching and bending. Differences in the EFISHG responses from the oxidized and reduced crystal interfaces are explained according to local oxygen vacancy concentrations and dynamics and their effects on the Schottky barrier heights and depletion region widths at each interface.These results are further supported by our PL measurements. Oxygen ion migration toward the Fe:STO surface leads to enhanced fluorescence intensities from in-gap acceptor states. We demonstrate that SHG and PL measurements are wellsuited for understanding and resolving the underlying causes of dielectric breakdown processes and device failure brought on by dc electric field and ionic defect migrations in perovskite-type electroceramics.

Polar Rectification Effect in Electro-Fatigued SrTiO3-Based Junctions

“…In addition, X-ray diffraction (XRD) results show that the (002) diffraction peak of the fSTO crystal shifts to a lower 2θ angle (the inset of Figure a), which indicates the elongation of the c -axis and therefore the existence of OVs in the whole near-surface region. , Moreover, when the applied electric field is large enough, it can also cause another transient electrostrictive distortion effect (Figure S3). , There are two points worth of special attention. First, the critical electric field for the electro-induced crystal distortion of Ti/fSTO is larger than 12 kV/cm.…”

“…10,11 Moreover, when the applied electric field is large enough, it can also cause another transient electrostrictive distortion effect (Figure S3). 28,29 There are two points worth of special attention. First, the critical electric field for the electro-induced crystal distortion of Ti/fSTO is larger than 12 kV/cm.…”

Assessment of dislocation interaction processes with grain boundaries by nonlinear optical spectroscopy

“…The coherent length, 𝑙𝑙 𝑐𝑐𝑛𝑛ℎ , is the distance over which the SHG light can travel in the material before its intensity drops to zero. The coherent length of the SHG light is defined as [33]:…”

“…As a result, SHG has been employed to study crystal structure, particularly at grain boundaries and interfaces, where inversion symmetry is inherently broken. As a local symmetry probe, optical SHG in the reflection geometry was recently used to investigate structural inhomogeneity at the anode and cathode interfaces of electrically degraded oxidized and reduced Fe-doped SrTiO3 (Fe:STO) single crystals [31][32][33][34][35][36]. Both centrosymmetric Fe 4+ :Ti 4+ -O6 octahedra and non-centrosymmetric Jahn-Teller distortions were identified at the anode interfaces, while oxygen vacancy clusters led to a strong deviation from 4mm group symmetry at the cathode interfaces [37].…”

Assessment of Strain Relaxation and Oxygen Vacancy Migration Near Grain Boundary in SrTiO3 Bicrystals by Second Harmonic Generation

“…Details on the local accumulation of oxygen defects at interfaces indeed are extremely difficult to extract from bulk- or film-based characterization techniques such as the electrical transport measurements, microstructural analysis and high-resolution transmission electron microscopy. − Optical second harmonic generation (SHG) is an effective nonlinear spectroscopic technique for studying crystals that lack inversion symmetry. − As a result, SHG has been employed to study structures at grain boundaries and interfaces where inversion symmetry is inherently broken. As a local symmetry probe, optical reflection SHG was recently used to investigate structural inhomogeneity at the electrode interfaces of electrically degraded, oxidized and reduced Fe-doped SrTiO 3 (Fe:STO) single crystals. − Centrosymmetric Fe 4+ :Ti 4+ -O 6 octahedra and noncentrosymmetric Jahn–Teller distortions were both identified at the anode interfaces, while oxygen vacancy clusters led to a strong deviation from a 4mm group symmetry at the cathode interfaces . These nonlinear optical measurements provided important information on the structural inhomogeneity and defect concentration gradients formed at the degraded Fe:STO anode and cathode interfaces. ,, …”