Accessible Switching of Electronic Defect Type in SrTiO₃ via Biaxial Strain

Abstract

Elastic strain is used widely to alter the mobility of free electronic carriers in semiconductors, but a predictive relationship between elastic lattice strain and the extent of charge localization of electronic defects is still underdeveloped. Here we considered SrTiO₃, a prototypical perovskite as a model functional oxide for thin film electronic devices and nonvolatile memories. We assessed the effects of biaxial strain on the stability of electronic defects at finite temperature by combining density functional theory (DFT) and quasiharmonic approximation (QHA) calculations. We constructed a predominance diagram for free electrons and small electron polarons in this material, as a function of biaxial strain and temperature. We found that biaxial tensile strain in SrTiO₃ can stabilize the small polaron, leading to a thermally activated and slower electronic transport, consistent with prior experimental observations on SrTiO₃ and distinct from our prior theoretical assessment of the response of SrTiO₃ to hydrostatic stress. These findings also resolved apparent conflicts between prior atomistic simulations and conductivity experiments for biaxially strained SrTiO₃ thin films. Our computational approach can be extended to other functional oxides, and for the case of SrTiO₃ our findings provide concrete guidance for conditions under which strain engineering can shift the electronic defect type and concentration to modulate electronic transport in thin films.