Impact of Al and In co-doping on transmittance, bandgap energy, and electrical characteristics of ZnO films for TCO applications

The modification of morphology, electrical parameters, transmittance, and optical bandgap energy in ZnO films doped with donor impurities: Al and In atoms, has been investigated. The In content in the films was kept constant equal to 2.0 at.%, but the Al contents varied from 1.0 to 3.5 at.%. The films were then annealed for oxidation and crystallization in a flow of nitrogen at 400 degrees C for 4 h. A non-monotonic variation of all parameters has been detected, such as chemical compositions, surface morphology, transmittance, and energy bandgap with varying Al contents in the ZnO:Al:In nanocrystal (NC) films. The transmittance, measured in the visible and near-ultraviolet spectral ranges, was estimated to be almost 80-90%. The elastic stresses in the films, related to the doping process, were reduced by using elements with smaller and bigger ionic radii than those of Zn ions. The latter allows moving the beginning of the generation of self-compensating defects in the films toward higher donor contents. Finally, the electrical resistivity in the ZnO:In:Al films was achieved as 2.2 x 10-4 Omega cm. Non-monotonic variation of film characteristics is discussed.