Enhancement of the hydrogen gas sensitivity by large distribution of c-axis preferred orientation in highly Ga-doped ZnO polycrystalline thin films

The hydrogen gas sensing properties of highly Ga-doped ZnO (GZO) polycrystalline thin films deposited by radio-frequency magnetron sputtering have been studied. The relationship between the microstructural properties of preferred c-axis oriented thin films and the hydrogen gas sensing properties is described. The crystallite size and the preferred orientation distribution were characterized by X-ray diffraction. The crystallite size increased and the preferred orientation distribution decreased with increasing film thickness. In order to control the crystallite size and the c-axis orientation separately, a highly oriented ZnO template layer with different thickness was employed for deposition of 30-nm-thick GZO films. The c-axis orientation of these films were nearly comparable each other, while the crystallite size increased significantly with increasing thickness of the ZnO templates. The hydrogen gas sensitivity at an operating temperature of 330 °C increased slightly with decreasing crystallite size, while the sensitivity was dramatically enhanced by increasing the preferred orientation distribution. It is therefore proposed that the c-axis orientation plays an important role in determining the sensitivity of the hydrogen gas sensor.