Interfacial area between hetero-epitaxial γ-Al₂O₃ and silicon

The interfacial region between hetero-epitaxial γ-Al₂O₃ and Si (111) substrates is studied in detail. The purpose is to address many open questions regarding the growth of γ-Al₂O₃ grown on Si, such as the atomic stacking orders, strain relaxation modes, and observed thin-film qualities. The cross-sectional atomic stacking order is directly evidenced to be a cubic spinel structure, with a possible in-plane stacking order proposed. A 1.5 nm defect-rich transition layer is found at the interface, in which the lattice structure transitions from Si to γ-Al₂O₃ arrangement. The thin-film quality, in terms of crystallinity and low film roughness, is observed to improve with increasing thickness up to ≈8 nm. For thicknesses above 8 nm, grain boundaries are observed along with the appearance of pinholes, due to the large lattice constant and thermal expansion coefficient differences between γ-Al₂O₃ and Si. Polycrystalline islands form in these pinholes and gradually replace the initial layer-by-layer growth of monocrystalline γ-Al₂O₃, leading to a mainly polycrystalline material at large thicknesses. The insights gained on the hetero-epitaxy of γ-Al₂O₃ on Si will be useful for future work looking to exploit this hetero-epitaxial materials system.