Highly transparent and conducting In doped CdO synthesized by sol-gel solution processing

Abstract

Cadmium oxide (CdO) is a much-studied wide gap semiconductor with an inherent high mobility of > 100 cm2/Vs, high electron concentration of > 1021 cm−3 and a wide optical transparency window of > 1800 nm. These unique properties make CdO a potential transparent conductor for full spectrum photovoltaics. However, in order to achieve optimum material properties for optoelectronic applications, CdO was grown by vacuum-based physical or chemical vapor deposition methods. In this work, we explored the application of a low-cost sol-gel spin coating method to achieve highly conducting and transparent CdO thin films doped with 0–10% In (CdO:In). We find that while as-grown CdO:In films are nanocrystalline/amorphous with a high resistivity of ~ 1 Ω-cm, polycrystalline and highly conducting films can be obtained after optimized annealing at ≥ 400 °C. However, the electron concentration n saturates at ~ 5 × 1020 cm−3 for In concentration > 5% (or NIn ~ 1.9 × 1021 cm−3). This low activation of In may be attributed to the high density of native defects and/or impurities incorporated in the sol-gel process. With 5% In doping, we obtained a low resistivity of ρ ~ 2.5 × 10–4 Ω-cm and a high mobility μ ~ 50 cm2/Vs. These values of σ and µ are better than those reported for other TCOs synthesized by solution processes and comparable to conventional commercial TCOs grown by physical vapor deposition methods. Benefiting from their high mobility, these sol-gel CdO:In films are optically transparent over a wide spectral range up to λ > 1800 nm, making them promising as transparent conductors for optoelectronic devices utilizing the infrared photons.