Resistive oxygen gas sensors stand out among various types of sensors due to their simplicity, low cost, portability, measurement circuit simplification, and low power consumption. Rare earth materials such as pure and doped ceria are potential candidates for resistive oxygen gas sensors due to their unique ability to lose or gain oxygen in response to ambient oxygen concentration. We have recently observed that 6 atom % Sm doping is optimum for obtaining better conductivity using samaria doped ceria (SDC) films. In order to develop oxygen gas sensors based on SDC thin films, it is important to understand the influence of film thickness on the electrical properties.

 

In order to achieve this goal, we have grown SDC thin films on sapphire, Al2O3 (0001) substrates by using oxygen plasma-assisted molecular beam epitaxy. The 6 atom% Sm doped SDC films with thickness ranging from 50 – 300nm have been grown and their conductivity was studied. The resistance of these films, obtained by two probe measurement capability under various oxygen pressures (1mTorr-100Torr) and temperatures (473K to 973K) at a fixed voltage of 2V, will be discussed. The differences in the electrical properties, thereby the oxygen ion conductivity, will be explained based on the chemical and structural characteristics of various films. Structural and chemical characteristics of the as grown films were analyzed by various in situ and ex situ, surface and bulk sensitive techniques.

 

In this study we observed that, overall conductivity increases with the film thickness at each temperature and oxygen pressure value. We also observed saturation in the conductivity at film thicknesses above 200nm. With the increasing interest for miniaturized oxygen sensors for high temperature uses, the nano-scale thin film doped ceria sensors may have a significant role to play in various future functional applications. In that sense it is vital to undertake fundamentals studies such as this.