Simulation of the grain size effect in gas-sensitive SnO2 thin films using the oxygen vacancy gradient distribution model

The model of gradient-distributed oxygen vacancies is utilized in simulating the grain size effects of gas-sensitive SnO2 thin films. The distribution profile of oxygen vacancies has a grain size effect and the profile gradient correlates positively with the radius of the grains. The simulation results show that the grain size is a fundamental factor dominating the gas-sensing properties of thin films. The potential barrier height and resistivity have significant grain size effects when m is between 0.1 and 0.5 nm-1. The size effects on sensor response to stimulant gases can be enhanced by increasing the value of m or the absolute value of a. Two expressions are used to simulate the grain size effect of the sensor response. The expressions act similarly when a < 0.2. The simplified response provides a neat function to quantitatively explain the sensor performance on gases with low partial pressure. Although the accurate response is complicated, it is applicable to the entire concentration range. A small power-law exponent n is calculated from the accurate response expression when a high gas concentration is employed, illustrating a “saturation effect” of the response.