Sublayer thickness dependence of nanolaminated HfO₂–Al₂O₃ films for ferroelectric phase stabilization

In this study, to understand the effect of sublayer thickness of doped HfO2 films with limited dopant solubility on ferroelectric phase stabilization, nanolaminated HfO2–Al2O3 films with various sublayer thicknesses were prepared through atomic layer deposition (ALD), and the phase evolution behavior of these films with increasing post-metallization annealing (PMA) temperature was investigated. A narrow optimal range of the HfO2 sublayer thickness was required to achieve facile crystallization into a tetragonal phase, followed by orthorhombic phase transformation through sufficient Al diffusion. Because the Al2O3 sublayer cannot be completely dissolved, it should be as thin as possible so that it can easily agglomerate to provide an effective connection between the HfO2 sublayers during the PMA process. When stabilizing the ferroelectric phase of HfO2 films by mixing with dopants with limited solubility, the thicknesses of the HfO2 and Al2O3 sublayers in the nanolaminated form were revealed to be more critical than the nominal doping concentration inferred from their thickness ratios (ALD cycle ratios).