Sol-gel synthesis of highly transparent and conducting Cadmium Oxide

Among metal oxides CdO has a very high electron mobility of >100 cm²/Vs with high electron concentration and is therefore a candidate as transparent conductors for applications in a wide spectral range of 400-2000 nm. In this work, In doped CdO (CdO:In) thin films were synthesized by low-cost sol-gel spin coating method. By adjusting the precursor solutions molarity, spin coating parameters, as well as sintering and annealing conditions, smooth, uniform films with high conductivity and transmittance were obtained. With In doping <10%, CdO:In films are conducting even without sintering, having electron concentration 10¹⁹-10²⁰ cm^-3 and mobility 0.1-1 cm²/Vs. The roughness for optimized films of thickness ∼100nm is ∼2nm. The transmittance also improvs from 75% to 85% after In doping for CdO over the spectral range of 400 to 1600 nm. Varying the annealing temperature in the range of 200-600°C results in a significant increase in grain size and a drastic improvement in the resistivity to low 10^-4Ωcm. These electrical and optical properties are comparable to those of the transparent conducting oxides grown by physical vapor deposition at elevated temperature.