Transparent Conducting Oxides Based on the Spinel Structure

The structural characteristics of spinel (from both theoretical and experimental viewpoints) that are advantageous for producing transparent conducting oxide (TCO) materials are considered. A working hypothesis is presented for finding new TCO materials with spinel structure. Results of tight-binding energy-band calculations reveal the formation of extended conduction bands in selected TCOs. This is a necessary criterion for n-type TCOs containing cations with d10s0 electronic configuration. Discovery of some transparent conducting spinels, namely MgIn2O4, CdGa2O4, and ZnGa2O4, provides experimental confirmation for the formation of extended conduction bands. Li+-, He+-, and H+-ion implantations into MgIn2O4 films have been performed to explore a new technique to generate electron carriers. Carrier generation in implanted MgIn2O4 films has been verified from electrical conductivity and optical absorption measurements. Implanted Li+ ions have been observed to occupy vacant tetrahedral cation interstices in the spinel lattice. Each Li+ ion releases an electron to the conduction band upon occupying this tetrahedral vacancy. Observations suggest that the ion implantation technique is effective and feasible for carrier generation in TCO materials.

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