The ferroelectric properties and crystal structure of doped HfO₂ thin films were investigated for different thicknesses, electrode materials, and annealing conditions. Metal-ferroelectric-metal capacitors containing Gd:HfO₂ showed no reduction of the polarization within the studied thickness range, in contrast to hafnia films with other dopants. A qualitative model describing the influence of basic process parameters on the crystal structure of HfO₂ was proposed. The influence of different structural parameters on the field cycling behavior was examined. This revealed the wake-up effect in doped HfO₂ to be dominated by interface induced effects, rather than a field induced phase transition. TaN electrodes were shown to considerably enhance the stabilization of the ferroelectric phase in HfO₂ compared to TiN electrodes, yielding a P_r of up to 35 μC/cm². This effect was attributed to the interface oxidation of the electrodes during annealing, resulting in a different density of oxygen vacancies in the Gd:HfO₂ films. Ab initio simulations confirmed the influence of oxygen vacancies on the phase stability of ferroelectric HfO₂.

Size Affects Shape: Porous molecular framework materials can adopt a different phase when guest molecules absorb and uniformly distort the framework. Usually the framework returns to its original shape when the guests desorb. Sakata et al. (p. 193 ) noted that because surface stress drives this process, it might be avoided in smaller crystals. Indeed, a flexible porous coordination polymer, [Cu ₂ (dicarboxylate) ₂ (amine)] _n , could retain the structure induced by guest molecules such as methanol if crystallites were made sufficiently small (submicrometer scale) and did so to a greater degree as the crystallite dimensions decreased.