Three-zone model for Ti, Al co-doped ZnO films deposited by magnetron sputtering

Al-doped ZnO is well known as an alternative Transparent Conductive Oxide (TCO) to Indium Tin Oxide (ITO). Recently, co-doped ZnO with Ti and Al (TAZO) have been explored to improve the conductivity while maintaining moisture durability and opto-electronic properties. In this context, the structural, morphological, and optical properties of TAZO films deposited by reactive magnetron are investigated by experiments and Density Functional Theory (DFT) simulations. The results show that TiO₂ units substitute Zn atoms in the ZnO wurtzite structure and, whatever the concentration of Ti, the formation of Al₂O₃ is taking place. In addition, a three-regime model as a function of Ti content is proposed to explain the evolution of the properties. First, for Ti content 〈 2.2 at%, TiO₂ is incorporated by substitution of Zn by TiO₂ units in the ZnO network leading to a more oxidized Zn state and a hybridization between Zn_4s and Ti_3d orbitals. This opens the bandgap and increases the compressive stress of TAZO films. Between 2.2 at% and 7.0 at% of Ti content, cauliflower shape grains appear due to the compressive stress induced by small clusters of TiO₂. For Ti content 〉 7.0 at%, amorphous TAZO films are observed and are composed of TiO₂, Al₂O₃ and ZnO.