ELECTRICAL-TRANSPORT PROPERTIES OF THIN-FILM METAL-OXIDE-METAL NB2O5 OXYGEN SENSORS

The electrical properties of thin-film metal-oxide-metal Nb2O5 oxygen sensors have been investigated in the temperature range 400-600 degree s C. They are shown to be strongly dependent on the electrode material . With Nb electrodes, the cathode is a good electron injector. At low bias, the oxygen pressure dependence of the conductivity follows P(O-2 )(-1/n) with n equal 1.0 +/- 0.2 over a pressure range extending at le ast from 10(5) to 10 Pa. A model is proposed explaining this behaviour by the presence of surface electron traps associated with chemisorbed oxygen. The films present a very large concentration of chemisorption sites as atmospheric oxygen is able to diffuse into the bulk through channels running between the Nb2O5 chains of the crystal structure. Ab ove a certain threshold voltage, a non-linear regime where the current density is proportional to the square of the applied voltage is obser ved. This regime is explained by the anodic oxidation of the Nb anode. Cr and Pt electrodes behave like valve contacts: they are blocking at low voltage and injecting at high applied voltage. Structures shorter than 50 mu m are shown to be depleted of their oxygen vacancies for e lectric fields in the range 10(3)-10(4) V cm(-1). The electrical admit tance presents a steep non-linear double-injection regime above a cert ain threshold voltage. In this bias range the current density varies l ike V-n with n greater than or equal to 7. The threshold voltage is se nsitive to the ambient and decreases when the sensor is exposed to an atmosphere with low oxygen concentration. It is shown that the migrati on of ionized oxygen vacancies induced by the electric field actually controls the electronic transport through the metaloxide-metal structu re. These results provide new insights into the capabilities and limit ations of Nb2O5 sensors.