Understanding phase evolution of ferroelectric Hf0.5Zr0.5O2 thin films with Al2O3 and Y2O3 inserted layers

Abstract

This study investigates the insertion traits of the Al₂O₃ and Y₂O₃ insertion layers (ILs) and their effects on the phase evolution and electrical characteristics of polycrystalline Hf_0.5Zr_0.5O₂ (HZO) thin films grown by atomic layer deposition (ALD). The Al₂O₃ and Y₂O₃ ILs are located at the middle position along the HZO film. The thick Al₂O₃ IL, above 2–3 ALD cycles, forms a continuous layer, physically separating the upper and lower regions of the film. Conversely, the thin Al₂O₃ IL, below 2–3 ALD cycles, and all the Y₂O₃ IL diffuse into the nearby HZO layers, making a single Al- or Y-doped HZO layer. The most crucial finding is that the diffused trivalent Al and Y ions substitute the tetravalent Hf and Zr ions, creating oxygen vacancies for charge neutrality and changing the phase evolutions. The substituted Al and Y suppress the monoclinic phase and enhance the tetragonal phase. Ultimately, the study suggests a new perspective on doped HfO₂-based thin films, highlighting the crucial role of substitutional diffusion of dopants and charge neutrality in determining the formation of the tetragonal phase.