Crystallization behaviors of ultrathin Al-doped HfO₂ amorphous films grown by atomic layer deposition

In this work, ultrathin pure HfO₂ and Al-doped HfO₂ films (about 4-nm thick) are prepared by atomic layer deposition and the crystallinities of these films before and after annealing at temperatures ranging from 550℃ to 750℃ are analyzed by grazing incidence x-ray diffraction. The as-deposited pure HfO₂ and Al-doped HfO₂ films are both amorphous. After 550-℃ annealing, a multiphase consisting of a few orthorhombic, monoclinic and tetragonal phases can be observed in the pure HfO₂ film while the Al-doped HfO₂ film remains amorphous. After annealing at 650℃ and above, a great number of HfO₂ tetragonal phases, a high-temperature phase with higher dielectric constant, can be stabilized in the Al-doped HfO₂ film. As a result, the dielectric constant is enhanced up to about 35. The physical mechanism of the phase transition behavior is discussed from the viewpoint of thermodynamics and kinetics.