Incipient Ferroelectricity in Al-Doped HfO2 Thin Films

Incipient ferroelectricity is known to occur in perovskites such as SrTiO3, KTaO3, and CaTiO3. For the first time it is shown that the intensively researched HfO2 thin films (16 nm) also possess ferroelectric properties when aluminium is incorporated into the host lattice. Polarization measurements on Al:HfO2 based metal–insulator–metal capacitors show an antiferroelectric‐to‐ferroelectric phase transition depending on annealing conditions and aluminium content. Structural investigation of the electrically characterized capacitors by grazing incidence X‐ray diffraction is presented in order to gain further insight on the potential origin of ferroelectricity. The non‐centrosymmetry of the elementary cell, which is essential for ferroelectricity, is assumed to originate from an orthorhombic phase of space group Pbc21 stabilized for low Al doping in HfO2. The ferroelectric properties of the modified HfO2 thin films yield high potential for various ferroelectric, piezoelectric, and pyroelectric applications. Furthermore, due to the extensive knowledge accumulated by various research groups regarding the HfO2 dielectric, an immediate relevance of ferroelectric hafnium oxide thin films is anticipated by the authors. Polarization measurements show antiferroelectric‐to‐ferroelectric transitions in 16 nm thin film capacitors depending on annealing conditions and the molar concentration of incorporated aluminium. The ferroelectricity is attributed to a centrosymmetric to non‐centrosymmetric phase transition with decreasing aluminium content in the HfO2 host lattice. The incipient ferroelectric properties of the investigated material system hold the potential for various ferroelectric, piezoelectric, and semiconductor applications.