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Highly conductive films of Al-doped ZnO have been prepared by rf magnetron sputtering of a ZnO target with Al2O3 dopant of 1–2 wt% in content added. Films with resistivity as low as 2×10-4 Ωcm and transmittance above 80% at the wavelength between 400 and 800 nm can be produced on low temperature substrate with a relatively high deposition rate. It is shown that a stable resistivity for use in various ambients at high temperature can be attained for the films. The characteristic features of Al-doped ZnO films are their high carrier concentration and low mobility in comparison with non-doped ZnO films.

The detailed study of electrical properties in group III impurity doped ZnO thin films prepared by rf magnetron sputtering is described. The resistivity is lowered by doping of B, Al, Ga and In into ZnO films. The characteristic features of ZnO films doped with group III elements except for B are their high carrier concentration and low mobility. Variation of the mobility with the impurity content is roughly governed by the ionized impurity scattering. It is shown that the doped ZnO films exhibit the resistivity dependence on film thickness below 300 nm.

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A sensor with a high sensitivity and an excellent selectivity for ammonia gas was prepared by using sputtered ZnO thin films. The sensor exhibited an increase of resistance for exposure to ammonia gas whereas it exhibited a decrease of resistance for exposure to many other gases such as inflammable and organic gases. The resistance change and the selectivity of the sensor were enhanced by doping group III metal impurities such as Al, In, and Ga. The lower limit of the detection for ammonia gas was about 1 ppm at a working temperature of 350 °C.