Enhanced Photoconductivity at Dislocations in SrTiO₃

Abstract

Dislocations are 1D crystallographic line defects and are usually seen as detrimental to the functional properties of classic semiconductors. It is shown here that this not necessarily accounts for oxide semiconductors in which dislocations are capable of boosting the photoconductivity. Strontium titanate single crystals are controllably deformed to generate a high density of ordered dislocations of two slip systems possessing different mesoscopic arrangements. For both slip systems, nanoscale conductive atomic force microscope investigations reveal a strong enhancement of the photoconductivity around the dislocation cores. Macroscopic in‐plane measurements indicate that the two dislocation systems result in different global photoconductivity behavior despite the similar local enhancement. Depending on the arrangement, the global photoresponse can be increased by orders of magnitude. Additionally, indications for a bulk photovoltaic effect enabled by dislocation‐surrounding strain fields are observed for the first time. This proves that dislocations in oxide semiconductors can be of large interest for tailoring photoelectric functionalities. Direct evidence that electronic transport is confined to the dislocation core points to a new emerging research field.

Mechanical tailoring of dislocations in ceramics at room temperature: A perspective

“…Overview of room-temperature dislocation engineering in SrTiO 3 , displaying the capability of achieving dislocation densities from ∼10 9 /m 2 (in undeformed reference samples) up to ∼10 15 /m 2 , with a continuous plastic zone size from hundreds of nanometers up to mm-or cm-sized regions (in length) without crack formation. The data are extracted from references listed in the figure: Jin et al 57 ; Szot et al 26 ; Wang et al 176 ; Johanning et al 27 ; Kissel et al 13 ; Okafor et al 67 ; Fang et al 44 The author's works are highlighted in green.…”

“…Dislocation‐based simultaneous tuning of the mechanical and functional properties is desirable for application purposes. This has been demonstrated feasible in SrTiO 3 , in which higher dislocation density leads to higher photoconductivity 13 and higher fracture toughness 24 . Hence, it may be practical to ask, what is the upper limit of dislocation density that can be introduced for the selected ceramic materials?…”

“…There were extensive studies on dislocations in semiconductor materials (Si, Ge, GaAs) in the past decades. However, dislocations in semiconductors have long been undesired as they act as scattering and trapping centers, significantly reducing the mobility of charge carriers and degrading the functional properties 13 . Great efforts have been made to avoid dislocations in mainstream semiconductors during the growth of crystals and thin films.…”

“…The charge and local strain field offer abundant opportunities to tune the energy barrier for thermal/electrical scattering and provide sites for enhanced ion/electron transport and chemical reaction rate 15 . This opens an exciting new field for dislocation‐tuned functionality with proofs‐of‐concept established for photoconductivity, 13 electrical conductivity, 16–18 superconductivity, 19 and large‐signal piezoelectric coefficient ( d 33 *) 20 . From the mechanical point of view, once engineered into ceramics, dislocations can improve plasticity and increase fracture toughness (resistance to crack propagation) 21–25 .…”

Engineering dislocation‐rich plastic zones in ceramics via room‐temperature scratching

“…13 In fact, quite some single-crystal ceramic materials can be plastically deformed at room temperature in bulk compression. [14][15][16][17][18][19] Yet, these approaches usually produce plastic zones that contain only narrow slip bands (e.g., several or tens of micrometers in width 20 ) confined to discrete regions in bulk crystals. This not only limits the available dislocation-rich region for further testing but also leads to inefficient use of the entire crystals, which usually are very expensive.…”