Development of CeO2-HfO2 Mixed Oxide Thin Films for High Performance Oxygen Sensors

In this work, the authors report the fabrication and characterization of CeO₂ -HfO₂ mixed oxide thin film based oxygen gas sensors. The atomic concentrations of the individual elements Ce and Hf in the mixed oxide (CeO₂ -HfO₂) thin films were controlled and tuned using a novel method in RF sputtering to achieve better oxygen sensing characteristics. The characteristics of the sensing film were evaluated using various characterization techniques such as TEM-EDS, FESEM-EDS, XPS and XRD. The XPS and EDS data revealed that the Hf concentration increases with an increase in size as well as number of the HfO₂ pellets that are placed on a 3-inch CeO₂ target during sputtering. From the XRD and XPS analysis, it was found that the mixed oxide film with 10-11% Hf atomic concentration has the best sensing characteristics. The superior sensing characteristics of the CeO₂ -HfO₂ film can be attributed to the existence of a highly reactive plane (200) with the highest surface energy and a strongly reduced surface with oxygen vacancy formation due to the presence of Ce³⁺ ions and HfO_x, x <; 2 on the surface of the mixed oxide film. The sensor film detected the presence of oxygen gas even at low temperatures (<; 400°C); however, the response time and recovery time were slightly higher. The sensor film of thickness 220 nm with Hf concentration between 10-11% showed excellent sensitivity (~15), fast response and recovery times of 8 s and 10 s respectively at an operating temperature of 400°C, which are the best values reported till date for CeO₂ based oxygen sensors.