Role of interfacial aluminum silicate and silicon as barrier layers for atomic layer deposition of Al₂O₃ films on chemically cleaned InP(100) surfaces

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 surface silicon atoms through targeted attack of Si-O-Si backbonds on Si-treated surfaces, a major cause for interfacial carbon contamination. In contrast, the reactivity of water with SiCl₄- and Si(OCH₃)₄-modified surfaces and of silicon precursors with OH- and CH₃-terminated silicon is low. The formation of an ALD-grown interface barrier SiO_x buffer layer, using only one pulse of silicon precursors (such as SiCl₄, Si(OCH₃)₄, or Si₃H₈), is also investigated prior to Al₂O₃ deposition on InP(100). The adsorption of all three precursors occurs through the P-OH surface site to form Si-O-P linkages, albeit with varying efficiencies: Si(OCH₃)₄ displays the lowest reactivity, while Si₃H₈ displays the highest reactivity. The growth of the Al₂O₃ layer depends on the nature of the silicon interlayer: a film with more Al₂O₃ and SiO₂ is observed after the Si₃H₈ and SiCl₄ exposures, whereas a thin aluminum phosphate layer composed of P=O···Al and P-O-Al linkages is observed after the Si(OCH₃)₄ exposure. This latter behavior is also observed upon deposition of Al₂O₃ directly on epi-ready InP(100) surfaces (without buffer layer).