Scientists achieve stable ferroelectricity of Y-doped hafnium oxide films

On June 27, 2022, Xiaoshan Xu and Alexei Gruverman from the University of Nebraska-Lincoln, in collaboration with the Evgeny Y. Tsymbal team, published a new study titled “Intrinsic ferroelectricity in Y-doped HfO2 thin films” at Nature Materials.

The research group prepared a high-quality hafnium oxide epitaxial film using a laser pulse deposition system, observed iron polarization close to the theoretical prediction, revealed the positive correlation between film crystallinity and grain size and iron polarization, characterized the orthogonal phase distortion and rhomboid phase distortion coexisting in the film, and explored the effect of rhodochrosonic phase distortion on ferroelectric properties.

The correspondence is Xu Xiaoshan, Alexei Gruverman, Evgeny Y. Tsymbal; The first authors are Yun Yu, Pratyush Buragohain, and Li Ming.

With the rapid development of science and technology and the continuous improvement of human needs, the miniaturization and high integration of devices is an inevitable trend in the development of electronic devices. As a kind of material with spontaneous polarization and can be reversed under the action of an applied electric field, ferroelectric materials have a wide range of application prospects in data storage, sensors, and drivers. However, the low compatibility of traditional perovskite ferroelectric materials with modern semiconductor processes and the high demand for deposition thickness have greatly limited the application and development of ferroelectric materials. In recent years, hafnium-based ferroelectric materials have attracted a lot of attention for their compatibility with modern semiconductor processes and stable ferroelectric properties at low thicknesses (less than 10 nanometers). However, the ferroelectric phase (orthogonal phase) of hafnium oxide is a metastable phase, which makes how to stabilize the ferroelectric phase of hafnium oxide a challenge. Among them, using the lower surface energy of the orthogonal phase to stabilize the ferroelectric phase by reducing the grain size is a common method. However, this method introduces more defects, which not only makes the iron polarization of hafnium oxide much lower than the theoretical value, but also casts a mysterious veil over the intrinsic properties of hafnium oxide.

Recently, the University of Nebraska Lincoln Xu Xiaoshan team, Alexei Gruverman team, Evgeny Y. Tsymbal team used laser pulse deposition method to synthesize YHO (5%-Y doped HfO2) epitaxial film, iron electrodeization value is the largest value reported, and is consistent with theoretical predictions. The study demonstrates the positive correlation between thin film crystallinity and ferroelectric properties, provides a method for measuring and analyzing orthogonal and diamond phases, and calculates the effect of diamond phase distortion on iron polarization using density functional theory (DFT).

Figure 1: Characterization of the structure and ferroelectric properties of the YHO film

Figure 2: Surface topography and PFM characterization of the film

Positive correlation between iron polarization and film crystallinity. YHO epitaxial thin films are grown on two substrates, STO (001) and STO (110), using laser pulse deposition method, and the bottom electrode is La0.7Sr0.3MnO3. Among them, the room temperature iron polarization (50 μC/cm2) measured by YHO film grown on STO (110) is the highest reported room temperature iron polarization value. Using the rocking curve of the film as a measure of film crystallinity, by changing the film growth temperature, the study showed that the crystallinity of the YHO film showed a positive correlation with iron polarization (also applicable at low temperatures), which is different from all previous reports. Unlike previous reports, the iron polarization of hafnium oxide did not decrease significantly with the decrease in temperature. At a temperature of 20 K, the iron polarization along the (111) orientation is still 37 μC/cm2, and the estimated value along the polarization axis (001) is 64 μC/cm2, which is consistent with theoretical predictions.

Figure 3: Ferroelectric properties versus temperature dependence

Figure 4: Structural characterization of YHO films, distinguishing between orthogonal phases and rhomboid phases

The orthogonal phase of hafnium oxide is distorted from the diamond phase. The {002}, {001}, and {110} surface spacing of the YHO film were measured by X-ray diffraction, and the lattice constant of hafnium oxide was obtained and the lattice distortion peculiar to the orthogonal phase was confirmed. By measuring the {111}/{11-1}/}/{-111}/}/{1-11} crystal plane spacing, the rhomboid lattice distortion in the YHO film was confirmed, and the angles of rhomboid distortion on STO (001) and STO(110) were calculated to be 0.41° and 0.25°, respectively. Using RHED (reflective high-energy electron diffraction) to monitor the surface morphology of the film in real time, the transition temperature of the YHO film from the quadral phase to the quadrature phase ferroelectric is obtained at about 450 °C. Density functional theory suggests that the impact on ferroelectric properties should be very limited in terms of the size of the observed diamond distortion. Therefore, the main source of iron polarization of YHO films is its orthogonal phase, and the rhodochrosin phase distortion that coexists with it is caused by the volume expansion of the film along the extra-surface (111) direction during the cooling process.

Figure 5: Density functional theory calculates the effect of diamond distortion on ferroelectric properties

(Source: Science Network)

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