Atomic Layer Deposition (ALD) in a supercyclic fashion is a common approach to synthesize tailor-made ternary materials, to prepare nanolaminates and to distribute a doping species into a host material. Hence, the initial growth of the individual materials onto each other is of particular interest. Due to nucleation inhibition or enhancement, the initial GPC for ALD processes of a given material system onto a specific substrate may differ from its (steady-state) literature value, since the GPC is mostly noted as an average value of at least a few hundred cycles. However, the knowledge of the growth behavior/GPC of the individual materials within the first few cycles is of particular importance for a precise dosage/doping and tailor-made synthesis.
Herein, precise quartz crystal microbalance (QCM) studies were used to investigate the initial growth of titania (TiO2 by utilizing titanium(IV) isopropoxide and water as ALD precursors) onto alumina (Al2O3 deposited by ALD of trimethylaluminum and water) (1) and vice versa the initial growth of Al2O3 onto TiO2 (2). In case (1), an enhanced initial GPC of the TiO2 deposition was observed that was close to the equilibrium value of Al2O3 deposition. In the 2nd case, the initial GPC of Al2O3 was found to be close to the equilibrium value of TiO2 deposition. Actually, the growth process itself can be simply but generally modeled by a superposition of parallel growth onto itself and onto the foreign species. Furthermore, we attribute our observations to an ALD process intrinsic inhibition of sticky isopropyl-groups during the TiO2-growth.
We will show that care has to be taken by deposition of nanolaminates or compound materials since a simple combination of two (or more) well-established processes might end up with unintended results regarding stoichiometry, properties, and film thickness. However, knowledge of a process intrinsic deviation of the nucleation leads to the development of model systems—as demonstrated above—which enable the manufacturer for exact calculations of pulse ratio to get the desired doping level and compound stoichiometry.