Conductance Model for Single-Crystalline/Compact Metal Oxide Gas Sensing Layers in the Non-Degenerate Limit: Example of Epitaxial SnO$_2$(101)

Semiconducting metal oxide (SMOX)-based gas sensors are indispensable for safety and health applications, e.g. explosive, toxic gas alarms, controls for intake into car cabins and monitor for industrial processes. In the past, the sensor community has been studying polycrystalline materials as sensors where the porous and random microstructure of the SMOX does not allow a separation of the phenomena involved in the sensing process. This lead to conduction models that can model and predict the behavior of the overall response, but they were not capable of giving fundamental information regarding the basic mechanisms taking place. The study of epitaxial layers is the definite prove to clarify the different aspects and contributions of the sensing mechanisms that are not possible to do by studying a polycrystalline sample. A detailed analytical model for n and p-type single-crystalline/compact metal oxide gas sensors was developed that directly relates the conductance of the sample with changes in the surface electrostatic potential. Combined DC resistance and work function measurements were used in a compact SnO2 (101) layer in operando conditions that allowed us to check the validity of our model in the region where Boltzmann approximation holds to determine surface and bulk properties of the material.