DC resistance degradation of SrTiO3: The role of virtual‐cathode needles and oxygen bubbles

Abstract

AbstractThis study of highly accelerated lifetime tests of SrTiO3, 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 SrTiO3—agrees with previous numerical solutions. Yet 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 SrTiO3 single crystals, hence nominally “invisible,” needle’s 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 and kinetic considerations further revealed that copious oxygen bubbling and general reduction mark the onset of final accelerating degradation toward failure. Conversely, if oxygen vacancies cannot be sufficiently depleted from the near‐anode region to render it sufficiently conductive, then final failure is postponed, which is often the case at lower temperatures and voltages when the lifetime tests are incomplete. Remarkably, both undoped and Fe‐doped SrTiO3 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 SrTiO3 devices are discussed, along with connections to similarly DC‐degraded fast‐ion yttria‐stabilized zirconia.