Transmittance, structure and resistivity of ZnO films doped with Ga and In elements for TCO applications

The transmittance, structure and electrical characteristics of ZnO crystalline films obtained by spray pyrolysis and doped with Ga and In elements have been studied. ZnO films were prepared with the permanent In content of 2.0 at% and various Ga contents of 0.5–3.0 at%. The thermal treatment at 400 °C was applied for the crystallization and oxidation of the films. The XRD patterns of studied films demonstrate the crystalline structure of wurtzite and high transmittance of 85–90%. The non-monotonic variation of the surface morphology, the grain sizes, and the residual stresses versus the variation of the Ga content has been revealed. Using for doping the atoms with larger (In) and smaller (Ga) ionic radii compared to Zn ions allows obtaining the ZnO lattice parameters in doped films much closer to the bulk stochiometric ZnO parameters and reduced residual stresses. The latter allows increasing the donor concentrations and reaching the smallest electrical resistivity of 6.0 × 10⁻⁴ Ω cm in ZnO:In:Ga films. The results presented have shown that doping with two elements (Ga and In) allows to significantly improve the transmission in the visible spectral range and the electrical conductivity of ZnO films, which are important for the application of ZnO films as transparent conductive oxides in optoelectronic devices. Furthermore, the reduced stress and minimized distortion of the ZnO crystal lattice with high donor doping have been obtained.