Tunable gas sensing properties of p- and n-doped ZnO thin films

This work focuses on the gas sensing properties of p- and n-type ZnO films obtained by reactive sputtering. Variations of the deposition conditions (sputtering gas and substrate temperature) enabled tailoring the carrier type (electrons or holes) and concentration through tuning the Zn/O ratio to produce oxygen-deficient n-type or oxygen-rich p-type films, respectively. Thus, deposition in oxygen-lean atmospheres (inert gas) or at elevated temperatures (200 °C) lead to n-type ZnO films whereas deposition in ambient temperature and oxygen-rich atmospheres resulted in p-type films. The carrier type and concentration were determined by Hall Effect measurements.The gas sensing properties of the films were investigated by measuring the changes of their DC electrical resistance upon exposure to different concentrations of O2 in an inert gas (N2 or argon) or H2 in inert gas or synthetic air (79% N2, 21% O2). The sensor response to O2 or H2 was found to be profoundly dependent on the carrier type and concentration providing means to tailor the sensitivity to these gases.Finally, the effects of short- and long-term exposures to different gas environments on the sensors performance were studied. It was furthermore found that exposure of p-type ZnO films to relatively high H2 concentrations (>1000 ppm) inverts the majority carriers from holes to electrons in a reversible way (i.e. the sensor regains its p-type nature when the H2 environment is removed). The origin of the doping effect, the gas sensing mechanisms and the surface and bulk processes underlying the variations of the carrier type and concentration and affecting the sensing properties are highlighted.