Mechanistic Study of Lithium Aluminum Oxide Atomic Layer Deposition

There is significant interest in developing lithium conductive thin films that have potential applications as lithium-permeable electrode barrier coatings and as solid electrolytes in thin film batteries. In this work, we demonstrate the atomic layer deposition (ALD) of lithium aluminum oxide (LiAlOx) thin films and provide a thorough characterization of the growth mechanism. LiAlOx thin films were deposited by combining the ALD processes for Al2O3 (trimethylaluminum and water) and LiOH (lithium tert-butoxide and water). The composition of the films was controlled by adjusting the percentage of LiOH cycles. Both the pure LiOH process and the combined LiAlOx process were characterized by a combination of quartz crystal microbalance, Fourier transform infrared spectroscopy, and film deposition studies. These studies revealed a complex growth mechanism that is strongly affected by the hygroscopic and reactive LiOH component. Stable ALD with a constant growth rate as a function of ALD cycles was only achieved at <= 50% LiOH cycles. Within this stable regime, a maximum Li cation percentage of 55% and a growth rate of 1.5 angstrom/cycle were observed. LiAlOx films with >50% LiOH cycles exhibited greater Li cation percentages and stable growth only for the initial 20-30 cycles. This narrow window of stable LiAlOx ALD may restrict the deployment of this process in battery applications.