DC Resistance Degradation of SrTiO$_3$: The Role of Virtual-Cathode Needles and Oxygen Bubbles.

This study of highly accelerated lifetime tests of SrTiO$_3$, a model semiconducting oxide, is motivated by the interest in reliable multilayer ceramic capacitors and resistance-switching thin-film devices. Our analytical solution to oxygen-vacancy migration under a DC voltage -- the cause of resistance degradation in SrTiO$_3$ -- agrees with previous numerical solutions. However, all solutions fail to explain why degradation kinetics feature a very strong voltage dependence, which we attribute to the nucleation and growth of cathode-initiated fast-conducting needles. While they have no color contrast in SrTiO$_3$ single crystals and are nominally invisible, needles presence in DC-degraded samples -- in silicone oil and in air -- was unambiguously revealed by in-situ hot-stage photography. Observations in silicone oil and thermodynamic considerations of voltage boundary conditions further revealed a cooccurrence of copious oxygen bubbling and the onset of final accelerating degradation, suggesting sudden oxygen loss is a precursor of final failure. Remarkably, both undoped and Fe-doped SrTiO$_3$ can emit electroluminescence at higher current densities, thus providing a vivid indicator of resistance degradation and a metal-to-insulator resistance transition during cooling. The implications of these findings to thin ceramic and thin film SrTiO$_3$ devices are discussed, along with connections to similar findings in likewise degraded fast-ion yttria-stabilized zirconia.