We report a large ferroelectric polarization of Al:HfO2 films in metal-ferroelectric-semiconductor structures with a top-electrode free annealing configuration. Annealing an uncapped film at the Al concentration of 7.7 mol. % shows a large remnant polarization up to 50.5 μC/cm2. The film has a unique microscopically laminar distribution of dopant atoms. We find that the formation of the paraelectric monoclinic phase is suppressed in films with laminar distribution. The uniaxial confinement due to the microscopic stress introduced by the Al-rich strip structures is suggested. It is regarded as a possible explanation for enhancing the remnant polarization of the ferroelectric Al:HfO2 film by increasing the atomic layer deposition cycles for dopant layers. The results elucidate a growth procedure to produce high performance ferroelectric Al:HfO2 nanofilms without the postcapping process.

The Al:HfO2 ferroelectric nanofilms with different total thicknesses and distributions of Al-rich strips are prepared using atomic layer deposition (ALD) in an uncapped configuration. The synergistic interplay between the number of Al-rich layers and the thickness of total film offers the additional flexibility to boost the ferroelectricity of the resulting Al:HfO2 nanofilms. By carefully optimizing both the ALD cycles for dopant layer and the total film thickness in the preparation, the HfO2 nanofilms in post-deposition annealing can exhibit excellent ferroelectricity. The highest remanent polarization (2Pr) of 51.8 μC cm−2 is obtained in a 19.4 nm thick Al:HfO2 nanofilm at the dopant concentration of 11.1 mol% with a three ALD cycles for Al-rich strips. Remarkable remanent polarization value observed in the uncapped electrode clamping film paves a new way to explore the origin of ferroelectricity in hafnium oxide nanofilms. The observed ferroelectricity of the nanofilm is affected neither by the presence of an interface between the upper electrode and the film nor the choices of the materials of upper electrode in the measurement, ensuring a high flexibility in the designing and fabrication of the relevant devices in the future.

Cross-sectional TEM images of (a) the as-deposited Al:HfO 2 film before annealing and (b) the Al:HfO 2 film after annealing and (c) GIXRD spectra of the Al:HfO 2 films with a cycle ratio of 12:1 annealed at 750 °C to 1000 °C.

Combining the experimental characterization with the large-scale density functional theory calculations based on finite-element discretization (DFT-FE), we address the stabilization of polar orthorhombic phases (o-HfO2) in Al:HfO2 nanofilms by means of the atomic registry distortions and lattice deformation caused by Al substitutional defects (AlHf) and Schottky defects (2AlHf+VO) in tetragonal phases (t-HfO2) or monoclinic phases (m-HfO2). The phase transformation directly from the t-HfO2 into polar o-HfO2 are also elucidated within a heterogeneous distribution of Al dopants in both t-HfO2 bulk crystal structure and Al:HfO2 nanofilm. It is revealed using large-scale DFT calculations that the Al substitutional defects (AlHf) or the Schottky defect (2AlHf+VO) could induce the highly extended atomic registry distortions or lattice deformation in the t- and m-HfO2 phases, but such effects are greatly diminished in ferroelectric orthorhombic phase. By purposely engineering the multiple AlHf defects to form dopant-rich layers in paraelectric t-HfO2 nanofilm or bulk crystal, the induced extended lattice distortions surrounding the defect sites exhibit the shearing-like atomic displacement vector field. The large-scale DFT calculations further predicted that the shearing-like microscopic lattice distortions could directly induce the phase transformation from the t-HfO2 into polar orthorhombic phase in both Al:HfO2 bulk crystal and nanofilms, leading to the large remanent polarization observed in Al:HfO2 nanofilms with the presence of Al-rich layers. The current study demonstrates that the ferroelectricity of HfO2 bulk crystal or thin film can be optimized and tuned by delicately engineering both the distribution and concentration of Al dopants in ALD without applying the top capping electrode, providing the extra flexibility for designing the HfO2 based electronic devices in the future.

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