Properties of transparent conducting oxides formed from CdO and ZnO alloyed with SnO2 and In2O3

In this article we examine the structural, electrical, and optical properties of several ternary alloy thin films. The alloys are zinc oxide and cadmium oxide, each of which was reacted with both indium oxide and tin oxide to form sputtering targets. The films were deposited by rf sputtering. X-ray diffraction spectroscopy showed that cadmium stannate, cadmium indate, and zinc stannate films were all polycrystalline spinel phase, but only when deposited at room temperature in pure oxygen and then annealed in argon/cadmium sulfide. The fourth alloy, zinc indate, exhibited a hexagonal phase when prepared under identical conditions. Cadmium stannate has one of the lowest resistivities of any transparent conducting oxide (TCO), has low absorbance in the visible spectrum, and is an excellent compromise between electrical and optical requirements. For this material, we show that a single phase is essential for the highest electrical conductivity and lowest optical absorbance. Zinc stannate has a resistivity more than one order of magnitude higher than cadmium stannate but, because of a larger band gap, has an even lower absorbance. We conclude that there is no ''best'' TCO, and the decision regarding which to use depends on the specific application and the weighting given to electrical versus optical properties. The zinc-bearing alloys are nontoxic, which may also be an attractive feature for many applications.