Synthesis and characterization of ZnO and Au/ZnO thin films for ethanol gas sensing application

Undoped zinc oxide (ZnO) and Au/ZnO thin films have been deposited on glass substrates using spray pyrolysis and sputtering methods. The impact of substrate temperature and Au thickness on the structural, electrical, optical, and ethanol sensing properties of the deposited films has been investigated. X-ray diffraction (XRD) patterns reveal a polycrystalline structure across all samples. Notably, for pure ZnO deposited at 500 °C, the (002) orientation exhibits a substantial increase in intensity compared to samples deposited at other temperatures and the layer reaches its superior crystalline state. Substrate temperature demonstrates a significant effect on the electrical resistance of ZnO. Increasing the substrate temperature from 350 to 550 °C results in a notable decrease in sheet resistance, ranging from 34.4 [MΩ/cm²] to 54.2 [KΩ/cm²]. The ZnO film deposited at a substrate temperature of 500 °C displays the lowest sheet resistance. Sputter-depositing a 10 Å of gold (Au) on top of the ZnO layer results in a simultaneous decrease in sheet resistance from 54.2 [KΩ/cm²] to 7.02 [KΩ/cm²], while it doesn’t have considerable effect on the transparency of layer. The effect of substrate temperature on ethanol sensing characteristics of ZnO has been investigated. According to the results, at the temperature of 350 °C, ZnO layer has a superior response compared to the other samples. As the substrate temperature increases, the response of the layers decreases. The film deposited at 500 °C showing predominant orientation (002), demonstrates a weak ethanol sensing feature compared to other layers. Ethanol sensing characteristics of the Au/ZnO double layer were also examined, encompassing parameters such as dynamic response, sensitivity, response/recovery times, and operational temperature. Sputtering of Au on the top of ZnO exhibits a significant improvement in gas sensitivity. Further investigation indicates that increasing the Au thickness to 40 Å contributes to sensitivity increase. However, beyond this thickness, a diminishing trend in sensitivity has been observed. According to our search, there is no report for synthesis and characterization of Au/ZnO double layer by employing two methods of spray pyrolysis and sputtering. The layers exhibit superior homogeneity and crystallization compared to other reports. Also the layers exhibit excellent gas sensing properties toward ethanol gas. The response and recovery time have low values. The gas sensitivity of Au/ZnO thin film has high value of 5 toward 200 ppm ethanol gas. According to the selectivity graph by sputtering Au on top of ZnO layer the gas selectivity toward ethanol gas enhances. The layers show good stability.