Resistive donor-doped SrTiO3 sensors: I, basic model for a fast sensor response

In contrast to dense donor-doped SrTiO3 (STO) ceramics and single crystals, porous fine grained thick films reveal a surprisingly fast resistivity response in reply to a change of the oxygen partial pressure (pO2) within some 10 ms even at temperatures below 900 °C. Although this unexpected behavior was attributed to a grain boundary effect, the resistivity versus pO2 plot shows a similar characteristic as found for dense ceramics or single crystals, respectively. This observation suggests that cation vacancies, which significantly influence the electrical behavior of the bulk, but which are known to equilibrate only at highest temperatures, may also play a key role in the formation of resistive grain boundaries in the moderate temperature range. For mobility reasons, a change in the cation vacancy concentration might then be limited to a few monolayers on each side of the interface and a very high interface density of defect states is needed to explain the drastic resistivity change of the sensor observed in the experiment. We propose a generalized point defect model that involves the formation of a near-interface space charge region. The latter is found to affect the local defect equilibria significantly. As a consequence, the concentration of each defect type differs from the bulk value. The fast sensor response might then originate from a space charge induced increase of the cation vacancy concentration situated only near the interface that exceeds the value predicted from electroneutral defect considerations by 2 orders of magnitude. The local formation of cation vacancies causes a strong depletion of electrons. The space charge itself is formed due to the difference in mobilities of ionic and electronic species.