Applied Surface Science, Vol.469, 804-810, 2019
A kinetic study of ZnO atomic layer deposition: Effects of surface hydroxyl concentration and steric hindrance
We present a kinetic model for the reaction of diethylzinc (DEZ) adsorption in atomic layer deposition (ALD) of ZnO from DEZ and water. The proposed model has been verified by comparing kinetic experimental data to the prediction of the model in the temperature range of 60-200 degrees C. In this model, DEZ molecules are molecularly adsorbed on the hydroxyl-terminated surface in the first elementary reaction. Then the molecularly-adsorbed DEZs either desorb from the surface, or undergo an irreversible ligand exchange reaction and form monoethylzinc (MEZ)-terminated surface by liberating ethane molecules. According to the integrated rate law of the model, as the exposure time of DEZ increases, the growth per cycle (gpc) of ALD, i.e., the thickness increment per cycle, rapidly increases and then saturates showing the self-limiting growth behavior. The required DEZ exposure time to reach the saturated gpc value is shortened when the chemical equilibrium between the molecular adsorption and desorption shifts toward the adsorption, as this leads to higher effective rate constant in the overall mechanism of the DEZ adsorption. Although the saturated gpc value is primarily governed by the hydroxyl concentration on the ZnO surface, it is also heavily influenced by the steric hindrance due to the bulkiness of the ethyl ligands. We have determined that the critical temperature at which the steric hindrance disappears is around 150-200 degrees C, by investigating the variation of the hydroxyl concentration with temperature. At temperatures lower than 150 degrees C, we have observed that the saturated gpc value is governed by the steric hindrance rather than the hydroxyl concentration. However, the saturated gpc value at 200 degrees C has been achieved purely by the hydroxyl concentration. In other words, all hydroxyl groups at 200 degrees C have been consumed and saturated by the MEZ molecules of which the steric hindrance-free concentration has been evaluated to be similar to 7.1/nm(2). In addition, the effective activation energy was estimated to be similar to 8.50 kJ/mol by using the effective rate constants of all temperatures investigated.