Effect of monovalent dopant ionic radius and concentration on the growth orientation and optical properties of the sol-gel-derived ZnO thin films

Pure and doped ZnO thin films with different monovalent elements (Li+, Na+, Ag+ and Cs+) are deposited onto glass substrates using a sol-gel spin coating method. The physical properties of the films are investigated for various ionic radius and concentrations of the dopant. X-ray diffraction results confirm clearly that the synthesized films exhibited hexagonal wurtzite structure without any secondary phases. The Scherer formula reveals that the average size of the crystallites is ranged between 35 and 55 nm. The main finding of this work is to show that the control of the crystalline growth orientation is possible basically by varying the monovalent dopant ionic radius and concentration. This control is therefore considered as a key factor when seeking to promote the ZnO-based transducers. In fact, the ZnO thin films doped by Cs+ (1.67 angstrom) are preferentially oriented along the c-axis; meanwhile, when decreasing the dopant ionic radius form 1.26 angstrom (Ag+) to 0.97 angstrom (Na+), other preferential orientations appear especially (100). However, in the case of Li (0.68 angstrom), no change in the preferential orientation has been observed until the concentration reaches 10%. Noticing that, the lattice parameters are strongly affected when the ionic radius and/or the dopant concentration change. The SEM micrographs of all the samples exhibit homogeneous and dense grains distribution with a quite smooth surface. UV visible transmittance spectra indicate a high transparency in the visible region with a slight dependence on the dopant radius. The evaluated optical bandgap varies from 3.29 to 3.2 eV.