Heterogeneous Local-Epitaxial Growth Behavior of Ultrathin HfO ₂ /Al-Doped TiO ₂ Bilayer Dielectrics for Dynamic Random-Access Memory Capacitor Applications

This study systematically examines the growth behavior and interfacial properties of a HfO₂/Al-doped TiO₂ (ATO) bilayer deposited on a Ru thin-film bottom electrode. Local-epitaxial growth of ATO into the rutile structure proceeds in two distinct, thickness-dependent stages that ultimately influence the crystallinity and interfacial integrity of the subsequently grown HfO₂ capping layer. At low ATO thicknesses (<4-4.5 nm), a coherent interface with strained rutile ATO is stabilized by the dominant influence of the underlying in situ-formed RuO₂. Conversely, at higher ATO thicknesses (>4-4.5 nm), an incoherent interface with relaxed rutile ATO becomes energetically preferred due to the increased volumetric energy contribution of the ATO. On coherently grown rutile ATO, the HfO₂ capping layer stabilizes in the higher-k tetragonal phase by efficient local lattice matching between aligned tetragonal HfO₂ and rutile ATO planes. In contrast, incoherent rutile ATO promotes disorder/intermixing, and monoclinic HfO₂ can emerge as the HfO₂ thickness increases. An appropriate r-ATO thickness region preserves insulating integrity and minimizes interfacial low-k layer effects. A Pt/HfO₂/ATO/Ru capacitor achieves a minimum equivalent oxide thickness of 0.62 nm at a minimum physical thickness of 5.9 nm, while maintaining sufficiently low leakage current level (<10^-7 A/cm² at 0.8 V).