Morphology of sol-gel porous In-Ga-Zn-O thin films as a function of annealing temperatures

The physics of solution-processed thin films is studied with the example of In-Ga-Zn-O. After annealing at 450°C, the films become fully inorganic and the pore distribution is modeled by a hexagonal close-packed based structure. The surface porosity is approximately 0.23 and the volume porosity deduced from small-angle X-ray scattering is approximately 0.26. The corresponding specific surface area is in the range of 65m2g−1. An instability model allows to successfully describe the film morphology. The solution diffusion coefficient, estimated from the rate of thinning as a function of temperature, follows an Arrhenius behavior with an activation energy of approximately 9.1×103Jmol−1 and a pre-exponential coefficient D0 of approximately 1.8×10−8m2s−1. Moreover, the surface tension-to-viscosity ratio of the solution is determined from the surface morphology. In addition, the observed phase separation between ZnO and Ga2O3 may come from the solubility difference of these oxides in the solvent. This separation has a major consequence on the electronic material properties. Finally, during cooling, the large tensile stress occurring between the film and the substrate is relaxed by the pores which adopt an oblate spheroid shape. The surface energy of In-Ga-Zn-O is then estimated to 1.5Jm−2 from the pressure on the spheroidal pore.