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Heterogeneous Local-Epitaxial Growth Behavior of Ultrathin HfO2/Al-Doped TiO2 Bilayer Dielectrics for Dynamic
Dae Seon Kwon1,2, Kun Hee Ye1,3, In Soo Lee1
1Department of Materials Science and Engineering and Inter-University Semiconductor Research Center, Seoul National University, Seoul 08826, Republic of Korea.
ACS Applied Materials & Interfaces
|June 6, 2026
Summary
This study reveals how Al-doped TiO2 (ATO) thickness impacts HfO2 crystallinity and interfacial properties on a Ru electrode. Optimizing ATO thickness is key for high-k dielectric performance in capacitors.
Area of Science:
- Materials Science
- Thin Film Technology
- Dielectric Materials
Background:
- Understanding the growth and interfacial properties of complex oxide heterostructures is crucial for advanced electronic devices.
- Hafnium oxide (HfO2) and aluminum-doped titanium dioxide (ATO) are key materials for high-k dielectrics.
- Ruthenium (Ru) is a common bottom electrode material in thin-film devices.
Purpose of the Study:
- To systematically investigate the growth behavior and interfacial characteristics of HfO2/ATO bilayers on a Ru thin-film electrode.
- To determine the influence of Al-doped TiO2 thickness on the crystallinity and integrity of the HfO2 capping layer.
- To correlate interfacial properties with the dielectric performance of Pt/HfO2/ATO/Ru capacitors.
Main Methods:
- Deposition of HfO2/Al-doped TiO2 bilayers on Ru thin-film electrodes using sputtering techniques.
- Analysis of local-epitaxial growth stages of ATO based on thickness.
- Characterization of interfacial structures and crystallinity of HfO2 and ATO layers using advanced microscopy and diffraction techniques.
- Electrical characterization of Pt/HfO2/ATO/Ru capacitors to assess dielectric performance.
Main Results:
- Al-doped TiO2 exhibits thickness-dependent growth stages (local-epitaxial) on Ru, influencing subsequent HfO2 growth.
- A critical ATO thickness (~4-4.5 nm) separates coherent (strained rutile ATO) and incoherent (relaxed rutile ATO) interfaces.
- HfO2 stabilizes in the tetragonal phase on coherent ATO due to lattice matching, while incoherent ATO promotes monoclinic HfO2 and intermixing.
- Optimized ATO thickness preserves insulating integrity, minimizing low-k interfacial effects.
- A Pt/HfO2/ATO/Ru capacitor achieved an equivalent oxide thickness of 0.62 nm with low leakage current (<10^-7 A/cm^2 at 0.8 V).
Conclusions:
- The thickness of the Al-doped TiO2 layer critically controls the interfacial structure and phase of the HfO2 capping layer.
- Achieving a coherent interface between HfO2 and ATO is essential for stabilizing the high-k tetragonal phase of HfO2.
- Optimized HfO2/ATO/Ru structures offer promising potential for next-generation high-performance dielectric applications.
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