Info

the film. At the same time, higher calcination temperature results in the grain growth, Figure 9, which may decrease the gas sensitivity. Hence, due to the optimum balance between the amount of crystallization and the degree of grain growth, the maximum sensitivity value is observed at the intermediate calcination temperature of 450 °C. Decrease in the film thickness with increasing calcination temperature would also tend to enhance the gas sensitivity.

The dependence of the gas sensitivity on the operating temperature, within the range of 100-500 °C, is shown in Figure 16 [5, 8, 9, 12-14, 17, 21, 22, 25]. The gas sensitivity increases with increasing operating temperature, reaches the maximum value at 320-350 °C, and then decreases with further rise in the operating temperature. The electrical resistance of the sensor is also known to follow similar behavior as a function of operating temperature [82]. Within the lower operating temperature range of 100-320 °C, the adsorption of the oxygen ions dominates, which decreases the conduction electron density in the film, which favors the high gas sensitivity. On the other hand, in the higher temperature range, 350-500 °C, desorption of the oxygen ions dominates, thus reducing the conduction electron density, which reduces the gas sensitivity. The low activation energy for the reaction between the reducing gases and the adsorbed oxygen ions at higher operating temperatures is also a major factor for increased gas sensitivity at higher operating temperatures. On the other hand, instability in the microstructure at high operating temperatures can lead to reduced gas sensitivity at higher operating temperatures.

The variation in the gas sensitivity as a function of the film thickness is shown in Figure 17 [5, 9, 13, 17, 21]. The graph shows that the maximum gas sensitivity appears at the film thickness of ~110 nm, while above and below this thickness, the gas sensitivity decreases. To explain the thickness dependence, a single-crystal thin-film model has been suggested [87], which excludes the effects of microstructure.

Pore sno2

200 300 400

Operating Temperature (0C)

Figure 16. Variation in the gas sensitivity as a function of operating temperature [5, 8, 9, 12-14, 17, 21, 22, 25].

Figure 18. Model of SnO2 thin film having straight channel structure [55].

0 0

Post a comment