Gas sensing capability of spray deposited Al-doped ZnO thin films

Aluminium-doped zinc oxide thin films were deposited from zinc acetylacetonate and aluminium acetylacetonate at various aluminium concentrations and deposition temperatures by ultrasonic spray pyrolysis. The structural and morphological properties and elemental composition of these films were studied by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy, respectively. The gas sensing properties were studied by a two-point probe in O-2/99% N-2 and H-2/97% Ar at operating temperatures of 60, 80, and 100 degrees C. The films were composed of hexagonal wurtzite-type zinc oxide. The surface morphology of the films depends on the amount of aluminium doping. The hydrogen sensing capability of aluminium-doped zinc oxide thin films deposited from 5 at% Al/Zn solution decreased from 11% to 5.5% as deposition temperature was increased from 280 to 400 degrees C, but the stability improved by an order of magnitude. The highest sensor response (8%) to 3 vol% H-2 in Ar was observed at an operating temperature of 100 degrees C in the film deposited at a substrate temperature of 400 degrees C from the solution containing 2 at% Al/Zn. Aluminium doping above 2 at% Al/Zn in the solution reduced film resistance by up to two orders of magnitude and sensor response decreased to 3% at 100 degrees C. Response times as low as 3 s were observed when detecting hydrogen at operating temperatures 60-100 degrees C.