Abstract
We have studied the oxygen sensitivity of Pt-Pd/p-type CaFe2O4 and Pt-Pd/Rh/p-type CaFe2O4 diodes, where the CaFe2O4 shows Fermi-level pinning. Reversible oxygen responses were observed for both diodes at 100 °C, where current under bias increased with increasing oxygen concentration. Response time was decreased by Rh doping. Thus, these diodes act as an oxygen sensor at a relatively low temperature. The Schottky barrier height φ, determined from the measured photocurrent-bias curve, decreased with an increase in oxygen concentration. The experimental value of dφ/d log PO2 was 0.11 eV/decade, and was much larger than the theoretical value of 0.018 eV/decade based on the surface redox reaction mechanism in the surface state. The difference in this value indicates that the bias is applied to the bulk as well as to the space-charge layer at the surface.
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