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

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 Hf0.5Zr0.5O2 (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.