Tailored TCOs

The metal-like electrical conductivity in combination with a high visual transmittance is the characteristic property that opens up a broad spectrum of applications to transparent conductive oxides (TCOs). To fulfill the manifold requirements in each individual case, especially the optical properties of TCOs have to be adapted. The transmittance in the near infrared spectral range can be tailored by a modification of the carrier concentration N and mobility µ. The theoretical description for this behavior is based on the well-known Drude theory. Highly conductive indium tin oxide films (ITO) have been prepared by pulsed DC magnetron sputtering. However, due to its excellent electrical properties, the plasma resonance of free carriers occurs near the visual spectral range which results in a very low transmittance in the NIR. In contrast, ITO films with a NIR transmittance of ca. 80% have been prepared by plasma ion assisted evaporation. The combination of high transmittance and low resistivity of p=7.4x10-6 omega was achieved by a decrease of the carrier concentration and a simultaneous enhancement of the electron mobility µ. Secondary, the transmittance of aluminum doped zinc oxide films (AZO) in the UV spectral range could be adapted by changing the doping concentration N. This is a direct consequence of the Burstein-Moss shift that leads to a band gap widening dependent on N. However, the comparison of the experimental data with theory has shown that the contrary effect of band gap narrowing is not negligible, too.