The discovery of ferroelectricity in HfO2
Résumé
HfO2 is known to exist in three different crystal phases at normal pressure: monoclinic phase at 300K, a tetragonal phase above 2050K, and finally a cubic phase above 2803K. The stable region of the tetragonal phase extends to lower temperatures in nanoscale crystallites due to the surface energy effect. As a consequence, the crystallization in thin films tends to proceed by nucleation in a tetragonal phase and a martensitic transformation to the monoclinic phase during crystal growth. This phase transformation involves volume expansion and shearing of the unit cell. The admixture of sufficient SiO2 (between 5 and 10 mol.%) has been found to stabilize the tetragonal phase in HfO2 but in 2011, it was reported that also the presence of for the formation of ferroelectric and antiferroelectric crystalline phases in SiO2 doped HfO2 thin films. Based on X-ray diffraction measurements, it was argued that the ferroelectric phase is orthorhombic with a Pbc21 space group. The phase is formed due to inhibition of the tetragonal->monoclinic transformation by mechanical confinement. The occurrence of ferroelectricity in Si:HfO2 is remarkable as it represents one of very few metal oxides which are thermodynamically stable on silicon, leading to enable a number of device concepts relying on silicon/ferroelectric heterostructures. The different ways to stabilize the ferroelectricity in HfO2, the main properties of the material and the remaining issues will be presented.