Selectivity and composition dependence of response of gas-sensitive resistors. Part 3.—Properties of the solid solution series (CrNbO4)x(Sn1 –y,SbyO2)1 –x(0 ⩽x⩽ 1, y= 0,0.01, 0.05)
The response of gas-sensitive resistors fabricated from solid solution compounds of the form (CrNbO4)x(Sn1 –ySbyO2)1 –x(0 ⩽ x⩽ 1;y = 0,0.01, 0.05) to carbon monoxide, propane and water have been studied. An n-p transition occurred at a CrNbO4 stoichiometry that increased with the level of Sb5+ doping. XPS demonstrated the surface enrichment of Nb5+ in the CrNbO4 substituted compounds and a Fermi level shift of 0.6 eV between the n-type and p-type materials. The n-type compounds exhibited higher sensitivity to CO and C3H8 that the p-type compounds, which was rationalised using a compensated semiconductor conduction model in which both the energy of the surface acceptor state relative to the conduction band, and the surface acceptor state density determined the sensitivity. Both the n-type and p-type compounds exhibited a resistance decrease upon exposure to water, and this was explained in terms of the relative energies of the trap states Oads– and OHads: OHads lies higher in energy than Oads– in the n-type but lower in the p-type compounds.