Introduction
Ferroelectric materials have a non-centrosymmetric crystal structure with spontaneous polarization that can be flipped by external electric fields, and have promising applications in computer memory, energy storage capacitors, and sensors. Chalcogenide oxide (perovskite) is the most common ferroelectric material, however, it is difficult to achieve a high degree of integration with complementary metal oxide semiconductor (CMOS) circuits, which is one of the industrial bottlenecks of conventional ferroelectric materials for memory devices. Compared with chalcogenide ferroelectrics, the new fluorite (fluorite) structure of hafnium oxide (HfO2) has high CMOS compatibility and can achieve ferroelectricity in films less than 10 nm thick, bringing light to the industrial application of next-generation high-density, non-volatile ferroelectric memories. the thermodynamically stable phases of HfO2 crystals are all centrally symmetric crystal structures, and the current experimentally The experimentally determined ferroelectric phases are all sub-stable phases, including non-centrosymmetric orthogonal phasesPca21and the rhombic phase. These two sub-stable phases are mostly present in nanometer-thick doped-state films and are prone to change to paraelectric or antiferroelectric phases with the change of growth parameters or external excitation, see Schematic 1, which seriously affects the ferroelectric properties of the devices. In recent years, a large amount of literature has reported the modulation of HfO2 ferroelectricity by growth parameters/external excitation and the thermal/kinetic laws for stabilizing the ferroelectric phase. However, most of the existing reviews are on HfO2 ferroelectric properties and device physics, and almost no reviews focus on the relationship between HfO2 structure evolution and property modulation. In view of this, this paper summarizes the correspondence between different microstructural features and ferroelectricity in HfO2 thin films from the perspective of structure-property integration, as well as the role of growth parameters/external excitation on the internal microstructure evolution of HfO2 thin films. Finally, some prospects are given for the scientific problems that still exist in the structure-property correspondence of HfO2 so far.
Schematic diagram 1. Phase structure evolution and ferroelectric properties of HfO2 films under different growth parameters or external excitation
Results
Recently, Dou Zhao (first author), a PhD student in the group of Professor Xiaozhou Liao (co-corresponding author) at the University of Sydney, Australia, and Assistant Professor Zibin Chen (co-corresponding author) at the Hong Kong Polytechnic University, published in the international journalMicrostructuresA review titled Microstructural evolution and ferroelectricity in HfO2 films was published in the journal. The main points of the review are summarized as follows.
Point I HfO2 thin film ferroelectric phase identification Since the thermodynamically stable phases of HfO2 are all crystalline structures with centrosymmetry, the observation of ferroelectricity in HfO2 thin films initially seems to be an incredible thing. The ferroelectricity of HfO2 polycrystalline films was attributed to a non-centrosymmetric orthorhombic phase by X-ray diffraction (XRD) characterization and density flooding theory (DFT) calculationsPca21, which is a thermodynamically sub-stable phase that can exist by modulating the conditions of grain size, doping, and thermal stress. Since the HfO2Pca21phase is very close to the structure of the other orthogonal phases, see Table 1 and Fig. 1(a-c), and only the O-atom occupancy differs between the different phases, which makesPca21The experimental confirmation of the phase requires direct observation of the O-atom location, which poses great difficulties for the experiment. This problem was finally broken through by researchers at the Institute of Microelectronics, CAS, who used spherical aberration-corrected scanning transmission electron microscopy (STEM) to realize Hf/Zr and O-atom arrays imaging of zirconium (Zr)-doped HfO2 (Hf0.5Zr0.5O2, HZO) thin film ferroelectric phases by high-angle annular dark field (HAADF) and annular bright field (ABF) imaging techniques, respectively. , see Fig. 1(d-e), was determinedPca21phase is one of the ferroelectric phases. The rhombic phase is another ferroelectric phase, which is currently observed only in HZO epitaxially in thin films on lanthanum strontium manganese oxide (LSMO)/strontium titanate (STO) substrates, and its structural features are determined by transmission electron microscopy combined with diffraction. Table 1. Various crystal structures and cell parameters of HfO2 predicted by experimental measurements or calculations
Figure 1. ferroelectric phasePca21Structural characterization and identification (a-c) Projections of different orthorhombic phases along the [010] crystal band axis, green atoms are Hf atoms and red atoms are O atoms, (d-e) STEM-HAADF and STEM-ABF images of HZO films under the [010] crystal band axis, scale bar represents 1 nm.
Point II HfO2 polycrystalline film structure evolution-ferroelectric properties characteristics of HfO2 ferroelectric polycrystalline films can be prepared by a variety of methods, including ALD, and the process parameters are mature, so many related studies have been conducted in the early stage. The results show that the HfO2 ferroelectric polycrystalline films inPca21Ferroelectric phase often with paraelectric phaseP21/cand the antiferroelectric phaseP42/nmcCoexist and can transform each other under the action of external factors. Among them, theP21/cphase is a room temperature stable phase.P42/nmcis a high temperature stable phase.Pca21is the in-between sub-stable phase, generally consisting ofP42/nmcThe phase is formed by annealing. The dopant, film thickness, and thermal stress all have an effect on thePca21The stability of the phase is affected. In general, as the dopant (e.g., Zr) concentration increases in a certain range, the HfO2 films show a paraelectric-ferroelectric-antiferroelectric property transition, see Fig. 2(a). Corresponding to the change in ferroelectricity, the film internalP21/cThe proportion of the phase (cis-electric phase) decreases gradually, thePca21phase (ferroelectric phase) gradually increases; when a certain concentration is exceeded, theP42/nmcphase (antiferroelectric phase) dominates, see Figure 2(b). The dopant can reduce thePca21phase of the bulk Gibbs free energy, different dopants (atomic radius, concentration) will play different roles in the regulation of HfO2 ferroelectric properties, see Fig. 2(c), but generally cannot independently stabilizePca21phase, a synergy of interface energy is also required. In HfO2 polycrystalline films, the interfacial energy depends on the grain size and is controlled by the film thickness. For polycrystalline films prepared by the ALD method, the residual polarization intensity (PrIn addition to the growth parameters, external excitations such as temperature and electric field will affect the phase structure and ferroelectricity of HfO2 polycrystalline films. As the temperature increases, thePca21The phase gradually evolves into a high temperature stable phaseP42/nmc, corresponding to this, the material shows a transition from ferroelectricity to antiferroelectricity, see Figure 3. The electric field plays a more important role than the temperature. Ferroelectric memory devices need to be stable under cyclic electric fields during their use. In the case of HfO2 ferroelectric films, the cyclic electric field causes firstPrenhancement, called the "wake-up effect", which in turn causesPrThe HfO2 ferroelectric film has a significant wake-up effect, which is related to the phase transformation, defect redistribution, and polarization charge injection inside the polycrystalline film under the action of electric field; and the electric fatigue phenomenon may be related to the accumulation of oxygen vacancies inside the film along the grain boundaries to form The electrical fatigue phenomenon may be related to the accumulation of oxygen vacancies along the grain boundaries to form conductive channels, see Figure 5.
Fig. 2. Paraelectric-ferroelectric-antiferroelectric evolution with increasing Zr content (a) and (b) the correspondingPrandP21/cphase ratio decreases, (c) the effect of dopant atomic radius size and concentration onPrThe impact of the
Figure 3. Evolution of phase structure and ferroelectricity of HfO2 films due to temperature change
Figure 4. Arousal effect and electrical fatigue due to electric field cycling
Figure 5. Phase structure evolution, defects and migration and accumulation of polarized charges in HZO thin films due to electric field cycling
Point III HfO2 single-phase epitaxial film structure evolution-ferroelectric properties characteristics in HfO2 polycrystalline films, multi-phase coexistence, grain boundaries and other issues have a great impact on the ferroelectric properties, giving rise to the study of epitaxial HfO2 ferroelectric films. In epitaxial films, the ferroelectric phase can be stabilized by epitaxial lattice mismatch stress between the film and the substrate. Yttrium (Y)-doped HfO2 films with different orientations can be epitaxially applied to ITO/YSZ substrates, and HZO films can be epitaxially applied to LSMO/STO substrates. By modulating the epitaxial stress and interfacial chemistry between the HfO2 film and the substrate, the internal phase structure/orientation of the film is changed, which in turn affects the ferroelectric properties. In general, epitaxial films are generally single phase with uniform orientation and low number of internal grain boundaries, see Figure 6, which helps to reduce the wake effect due to phase transition and fatigue effect due to oxygen vacancy enrichment at grain boundaries in polycrystalline films, see Table 2.
Figure 6. HZO epitaxial film structure and epitaxial orientation relationship between the film and the substrate LSMO/STO Table 2. Comparison of ferroelectric properties of polycrystalline and epitaxial HfO2 films
Outlook
Finally, the authors look ahead to the scientific issues that remain in the study of HfO2 ferroelectric thin films. For example, doping is a simple and effective way to modulate the ferroelectricity of HfO2 thin films, and studies suggest that dopants may modulate the evolution of the ferroelectric phase through complex interactions with point defects inside the films, but the exact mechanism of action is not yet clear, which poses certain difficulties in material design. This problem needs to be solved with the help of nanoscale structural characterization tools, therefore, the authors believe that in situ/ex situ electron microscopy will play a great role in the field of HfO2 ferroelectric thin film research.
