Electrical transport properties of thin-film metal-oxide-metal Nb 2O 5 oxygen sensors

Abstract

The electrical properties of thin-film metal-oxide-metal Nb 2O 5 oxygen sensors have been investigated in the temperature range 400–600°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 least 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 Nb 2O 5 chains of the crystal structure. Above a certain threshold voltage, a non-linear regime where the current density is proportional to the square of the applied voltage is observed. 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 μm are shown to be depleted of their oxygen vacancies for electric fields in the range 10 3–10 4 V cm −1. The electrical admittance presents a steep non-linear double-injection regime above a certain threshol voltage. In this bias range the current density varies like V n with n ≥ 7. The threshold voltage is sensitive to the ambient and decreases when the sensor is exposed to an atmosphere with low oxygen concentration. It is shown that the migration of ionized oxygen vacancies induced by the electric field actually controls the electronic transport through the metal-oxide-metal structure. These results provide new insights into the capabilities and limitations of Nb 2O 5 sensors.