Since the 1960-s, chromia has been the subject of much experimentation as a functional material for semiconductor gas sensors. In general, the material has a p-type gas response, distinguished, e.g., by the decrease of conduction under the influence of reducing gases. Such behavior contrasts with the n-type response typical of metal oxides conventional in this field of application. The two cases differ in the surface chemistry. However, the details of the chemistry and the dependence of conduction of the chromia layers on the partial pressure of different gases are still poorly understood. It seems useful to tackle the problems by examining what happens in the compact thin films. Here we describe atomic layer deposition (ALD) of such films and give a brief survey of their properties.

The films were grown at 420 °C on ceramic-phase and (012) and (001) cut single-crystalline alumina substrates. Chromyl chloride and methanol served as precursors. 25–900 ALD cycles were applied. The growth per cycle was, on average, 0.08 nm. The samples were provided with Pt electrodes covering the films or being underneath the films. The current through the samples was measured at 450 °C in the flowing synthetic air and in the air with hydrogen, methane, or carbon monoxide admixtures.

According to RHEED data, the films grown on single-crystalline substrates were epitaxial. The films on ceramic substrates were polycrystalline without any preferred orientation. Micro-Raman analysis did not show the presence of phases other than chromia. All the films were well-conductive. In a thickness range of tens of nanometers, the thickness of the films had only a slight effect on their conduction. The films were responsive to 30 ppm of hydrogen and carbon monoxide, being unable to discriminate between them, and almost unresponsive to methane. Transition times approached seconds and minutes. The epitaxial films had better sensor properties than their polycrystalline counterparts.

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