Paper TF2-ThM9
Using Crystallographic Space Group-Subgroup Relations to Analyze Phase Selection and Transition in HfO₂ and Hf-based Ternary Oxide Films

Thursday, November 3, 2011, 10:40 am, Room 110

HfO₂ and Hf-based ternary oxides are important candidates for ultrathin high permittivity dielectric applications. However, technological aspects of their use in actual devices far outstrips our knowledge of the fundamental science that governs phase selection and transition in these materials. The latter is important for predicting both initial device performance and long term stability. One big issue is that pure HfO₂ readily forms nanocrystallites in thin films. These crystallites exhibit finite size effects on two different length scales: (1) Two metastable phases initially form in crystallites < ~ 7nm in size [1] and transform to monoclinic (m) HfO₂, the standard state, as crystallites grow. (2) Upon transformation from the metastables, m-HfO₂ nanocrystallites whose size is <~ 11 nm exhibit a lattice expansion concurrent with surface dipole repulsion [2]. A second issue involves the stability of Hf-based ternaries that are either intentionally grown or inadvertently form as a result of cation mixing during thermal processing or heating upon device use. These questions are being addressed from an experimental viewpoint through controlled isochronal and isothermal annealing studies. In this paper, we use crystallographic space group-subgroup analysis to examine phase selection and transition in three sputter deposited nanolaminates, HfO₂-Al₂O₃, HfO₂-TiO₂, and HfO₂-ZrO₂. The goal is demonstrate how this tool connects phase transitions between seemingly unrelated structures by symmetry considerations. We show that several important transitions observed in these materials are 2^nd order and can be described by a simple relation between a parent group of higher symmetry and a daughter group of lower symmetry. Using the suite of programs in the Bilbao Crystallographic Server [3], first, conjugacy classes associated with the parent → daughter transition are identified, and then using the operations within each class, the general atom positions of the parent are decomposed into cosets of symmetry elements expressed the daughter's basis. Symmetry elements that are “lost” in the decomposition are used to identify a twin domain structure in the daughter resulting from the transition. Using these formalisms, we discuss metastable phase→m-HfO₂ transition in pure HfO₂, the robustness of an entropy-stabilized HfAl-oxide phase, and the initiation HfTiO₄demixing.

Support from UWM Foundation Catalyst Grant / Rockwell Automation Charitable Trust.

[1] E.E. Hoppe et al. APL 91, 203105 (2007); APL 92, 109903 (2008).

[2] M.C. Cisneros-Morales et al., APL 96, 191904 (2010).

[3] M.I. Aroyo et al., Z. Kristallogr. 221, 15 (2006); M. Nespolo, Acta Crystall. A64, 96 (2008).