On the growth and texturing of ultra-thin zinc oxide films in spin coating

The growth and texturing of zinc oxide thin films in sol-gel spin-coating method are re-visited for different rotation speeds of the coater, molarity of the solution and annealing temperature of the films. We report the formation of highly textured ultra-thin films oriented along a-axis on glass substrates and the change in the texture coefficient as a function of the molarity of the sol and the annealing temperature. All the deposited films were found to have a thickness between 15 nm to 70 nm with an optical band-gap of similar to 3.33 eV. Highly textured, a-axis oriented films were formed at 400 degrees C annealing temperature at an angular speed of 4000 rotations per minute by using a 0.4 molar solution and the orientation preference changes to c-axis above 450 degrees C for a similar thickness of the coating. It appears that the growth of the planes is determined by the annealing temperature and the texture coefficient is determined by the speed of the coater. A phenomenological model is proposed to explain the effect of different parameters on the structure formation and texturing by connecting the properties of the spinning liquid to the ultimate film structure by taking into account the difference in the distribution of nucleation sites at different rotation speeds, the subsequent lattice stress and the formation energy of different planes in the framework of competitive capture of diffusing particles by the growth/nucleation centres.