Temperature-independent resistive oxygen sensors based on SrTi1−xFexO3−δ solid solutions

SrTi0.65Fe0.35O3−δ (STF35) is a potential candidate for oxygen sensors in lean burn engines due to its strong sensitivity to oxygen partial pressure variations and negligible cross-sensitivity to temperature fluctuations. To understand the origin of this unique phenomenon, the temperature coefficient of resistance (TCR) of SrTi1−xFexO3−δ (STF) solid solutions with varying compositions between SrTiO3 and SrFeO3−δ was systematically analyzed. Changes in TCR from negative values at low to positive values at high iron concentrations were found to be correlated to systematically decreasing bandgap energy with increasing Fe/Ti ratio. At an intermediate composition of x = 0.35 (STF35), the bandgap energy is such that the Fermi energy lies just far enough above the valence band to compensate for the temperature-dependence of the mobility, yielding a zero TCR from the product of the free carrier (holes) concentration and mobility terms. We propose that the variations in the bandgap energy of STF solid solutions are due to additional bands derived largely from Fe3+/Fe4+ and Fe2+/Fe3+ states. These bands lie between the O 2p valence and Ti 3d conduction bands of SrTiO3, and due to variations in their position and width, the bandgap energy of STF solid solutions decreases with increasing iron concentration.