Mapping the conducting channels formed along extended defects in SrTiO3 by means of scanning near-field optical microscopy

Abstract

Mixed ionic-electronic-conducting perovskites such as SrTiO3 are promising materials to be employed in efficient energy conversion or information processing. These materials exhibit a self-doping effect related to the formation of oxygen vacancies and electronic charge carriers upon reduction. It has been found that dislocations play a prominent role in this self-doping process, serving as easy reduction sites, which result in the formation of conducting filaments along the dislocations. While this effect has been investigated in detail with theoretical calculations and direct observations using local-conductivity atomic force microscopy, the present work highlights the optical properties of dislocations in SrTiO3 single crystals. Using the change in optical absorption upon reduction as an indicator, two well-defined arrangements of dislocations, namely a bicrystal boundary and a slip band induced by mechanical deformation, are investigated by means of scanning near-field optical microscopy. In both cases, the regions with enhanced dislocation density can be clearly identified as regions with higher optical absorption. Assisted by ab initio calculations, confirming that the agglomeration of oxygen vacancies significantly change the local dielectric constants of the material, the results provide direct evidence that reduced dislocations can be classified as alien matter embedded in the SrTiO3 matrix.

Superconductivity enhancement in polar metal regions of Sr0.95Ba0.05TiO3 and Sr0.985Ca0.015TiO3 revealed by systematic Nb doping

“…One possible explanation that can account for both differences simultaneously is based on consideration of the inhomogeneity of the oxygen vacancies 52 . The thermal reduction procedure used to create the oxygen vacancies for the SrTiO 3−δ single crystal is restricted to dislocation 70 because the formation enthalpy of the oxygen vacancies near the dislocations is significantly lower than that in the stoichiometric SrTiO 3 matrix 71 . Therefore, the inhomogeneous 44,[64][65][66] ).…”

Influence of Defects-Induced Stresses on Birefringence in SrTiO3 Single Crystals

“…In addition, surface properties differ from the bulk because the defect concentration depends on the sample depth [30,31] and impacts crystallographic parameters, such as mosaicity [7,12] and lattice parameter variation. As Hünnefeld et al [30] proved, the occurrence of a long-range strain gradient gives rise to phase transition temperatures in the bulk of the different SrTiO 3 samples between 98.7 K and 105.8 K. In recent years, special attention has been devoted to the presence of dislocations and their role in [6,7,10,12,15,24,29,30,[32][33][34][35][36][37][38][39]. Unsurprisingly, an increase in the transition temperature in a region close to an edge dislocation can be extremely high and reach 255 K and even 340 K, as reported in [9].…”

“…As reported in [12], the ideal distribution of dislocations along the boundary is characterised by a distance of a few nm. It is expected that the extended defects, such as dislocations in bi-crystals reported in [6,7,12], should have a high impact on the optical properties of SrTiO 3 . Indeed, it was reported in [25] that in the SrTiO 3 bi−crystal with a tilt angle of 24 • , there are highly defected regions of 0.1-1 µm in diameter.…”

“…On the other hand, in articles [4,5], several dozen Kelvins above the transition temperature, precursor dynamics connected with the presence of low-temperature clusters were discussed. However, the literature often reports the presence of dislocations in STO single crystals [6][7][8][9], which break local symmetry by inserting SrO and TiO 2 planes into the crystal matrix [7]. Furthermore, according to [10], the band gap shrinks, causing an increase in conductivity due to the dislocations.…”

Is Reduced Strontium Titanate a Semiconductor or a Metal?

“…This effect could also be exploited for resistive switching applications [29]. Using experimental methods with a high spatial resolution, such as optical imaging, in combination with local conductivity atomic force microscopy (LC-AFM), it has been proven that dislocations in SrTiO 3 serve as a template for the formation of metallic filaments upon thermal reduction, thus leading to a preferential reduction in the dislocation-rich surface region [28,30].…”