Interface passivation layers have recently been incorporated into InP/high-k dielectric stacks using different approaches, including atomic layer deposition (ALD) to grow thin aluminum silicate layers and deposition of a buffer silicon layer prior to aluminum oxide growth. While beneficial effects have been observed on the interface electrical properties, little is known about the chemical composition and formation mechanisms of these interfacial layers. In this work, in situ infrared measurements are complemented by first-principles calculations to investigate (i) the ALD growth of aluminum silicate layers, using two different silicon precursors [silicon tetrachloride (SiCl4) and tetramethyl orthosilicate (Si(OCH3)4)], and (ii) the dependence on the specific order of the precursor sequence involving trimethylaluminum (TMA), water, and the Si precursors. Aluminum is found to foster silicate deposition by enhancing the reaction of silicon precursors (reaction with Al–OH). TMA readily transfers CH3 to surfa...