Paper EM-TuA1
Understanding Metastable Structures in Sputter Deposited Hafnia-Alumina, Hafnia-Zirconia, and Hafnia-Titania Nanolaminates

Tuesday, October 21, 2008, 1:40 pm, Room 210

The formation of metastable nanocrystalline phases during reactive sputter deposition of HfO₂-Al₂O₃, HfO₂-ZrO₂, and HfO₂-TiO₂ nanolaminates on unheated substrates is discussed. In addition to being technologically useful, these nanolaminates are archetypical because their bulk pseudobinary phase diagrams predict three different modes of interfacial mixing: complete immiscibility (HfO₂-Al₂O₃), complete miscibility (HfO₂-ZrO₂) and limited miscibility without a common end-member lattice (HfO₂-TiO₂). Of these individual constituents, all but Al₂O₃ (which has structural complexity) form both intralayer and interfacial nanocrystalline phases. However, these are often not bulk equilibrium phases and are termed here metastable. This paper addresses two questions: (1) By what mechanisms do these metastable phases arise? (2) How thermally stable are they at temperatures that a device may see during routine processing? We discuss metastable phases resulting from finite crystal size effects (tetragonal and orthorhombic HfO₂ in HfO₂-Al₂O₃) and heteroepitaxy (tetragonal Hf_1-xZr_xO₂ in HfO₂-ZrO₂). We discuss the formation of a complex interface in HfO₂-TiO₂ nanolaminates that includes monoclinic Hf_1-xTi_xO₂, a metastable phase which here results from a second order phase transition of orthorhombic HfTiO₂ (a stable interfacial phase) to accommodate the larger Hf atom at aHfO₂-on-TiO₂ interface. In all cases, these metastable structures represent self-assembly into the lowest possible energy structures in the absence of long-range diffusion.