It is commonly believed that the impact of the top electrodes on the ferroelectricity of hafnium-based thin films is due to strain engineering. However, several anomalies have occurred that put existing theories in doubt. This work carries out a detailed study of this issue using both theoretical and experimental approaches. The 10 nm Hf0.5Zr0.5O2 (HZO) films are prepared by atomic layer deposition, and three different top capping electrodes (W/MO/ITO) are deposited by physical vapor deposition. The electrical testing finds that the strain does not completely control the ferroelectricity of the devices. The results of further piezoelectric force microscopy characterization exclude the potential interference of the top capping electrodes and interface for electrical testing. In addition, through atomic force microscopy characterization and statistical analysis, a strong correlation between the grain size of the top electrode and the grain size of the HZO film has been found, suggesting that the grain size of the top electrode can induce the formation of the grain size in HZO thin films. Finally, the first-principles calculation is carried out to understand the impact of the strain and grain size on the ferroelectric properties of HZO films. The results show that the strain is the dominant factor for ferroelectricity when the grain size is large (>10 nm). However, when the grain size becomes thinner (<10 nm), the regulation effect of grain sizes increases significantly, which could bring a series of benefits for device scaling, such as device-to-device variations, film uniformity, and domain switch consistency. This work not only completes the understanding of ferroelectricity through top electrode modulation but also provides strong support for the precise regulation of ferroelectricity of nanoscale devices and ultrathin HZO ferroelectric films.
Keywords: Hf0.5Zr0.5O2 (HZO); capping electrode; ferroelectricity; grain size; strain engineering.