Atomic layer deposition of lithium metaphosphate from H₃PO₄ and P₄O₁₀ facilitated via direct liquid injection: Experiment and theory

We report the use of $H3PO4$ as a reactant in atomic layer deposition (ALD) of lithium metaphosphate. The ALD growth cycle starts by injection of the lithium tetramethyl heptadionate (⁠$LiTMHD$⁠) precursor followed by injection of the $H3PO4$ reactant. Both the reactant and the precursor are injected into the ALD chamber via direct liquid injection, which allows us to achieve ALD without plasma or high temperatures. The liquid $H3PO4$ solution, injected 10 s after the precursor, evaporates and decomposes into the gaseous mixture of $H3PO4$⁠, $P4O10$⁠, and $H2O$⁠. The $H3PO4$ and $P4O10$ molecules finally react with the $LiTMHD$ molecules adsorbed at the sample substrate, which results in the film growth. The obtained films are amorphous, and the x-ray photoelectron spectroscopy measurements reveal the $LiPO3$ composition. The growth process exhibits the features of the ALD, namely, the saturation of the growth rate with cycle duration and the maximum growth rate when the number of the injected precursor/reactant molecules reaches a critical value. We show theoretically that the growth rate is governed by the gas-phase equilibrium between $H3PO4$ and $P4O10$⁠, both of which are reactive but to different degrees. Depending on the temperature and other conditions, we obtain a reactive gas adjustable at will between pure $H3PO4$ and pure $P4O10$⁠. Our theory explains essential features of the observed ALD growth. It determines which of the two reactants (⁠$H3PO4$ or $P4O10$⁠) causes the growth and which of them provides a faster growth.