Effect of aluminum dopant concentration on the growth orientation control of sol-gel-derived AZO thin films

In the present study, undoped and aluminum-doped ZnO films have been synthesized by sol-gel process with different Al concentrations (1, 5 and 10%). The effect of Al concentrations on the growth orientation, morphological and optical properties of ZnO thin films has been investigated in detail. The X-ray diffraction reveals that all the obtained films exhibit a pure hexagonal wurtzite structure without any Al phases. Meanwhile, it seems that the crystal quality and preferential growth orientation are deeply influenced by changing the Al concentration. In fact, the preferential growth orientation changes from (002) to (100), when increasing Al content from 1 to 10%. This orientation (100) makes as well the acoustic transverse mode to be excited. These results find a direct application in the improvement of the acoustic transducers technology. Likewise, doping with Al also affects equally the lattice parameters and the mean crystallite size. A smooth surface with uniform spherically shaped nano-metric grains are observed through scanning electron microscopy. However, optical characterizations show that increasing the Al content results in a decline of the optical transmittance, attributed to the preferential growth orientation change, which strongly affects the optical gap value.