In situ study of phase transition in HZO ferroelectric thin films via TEM electron beam irradiation

HZO ferroelectric capacitors and cross-sectional TEM sample preparation

Structure characterization and ferroelectric performance testing of the fabricated HZO ferroelectric film

The phase composition of the W/HZO/W ferroelectric capacitor was characterized using a grazing incidence X-ray diffractometer (Smartlab 9 kW, Rigaku^TM). The grazing incidence angle was 0.5°, the scan step was 0.002°, and the scan range was 2θ from 25° to 55°.

The HZO ferroelectric thin film was tested for dynamic hysteresis [polarization-voltage (P-V)] using a ferroelectric analyzer (axiACTT TF analyzer 3000, Germany). P-V curve test conditions: 1 kHz triangular wave, 4 V pulse voltage. Wake-up conditions: 100 Hz square wave, 3.5 V pulse voltage, 1,000 cycles.

HZO ferroelectric film TEM microstructure characterization and in situ electron beam irradiation

A TEM (FEI Talos F200X, Thermo Fisher Scientific^TM) was used for TEM characterization of the cross-sectional samples of the HZO ferroelectric thin film and electron beam irradiation with an accelerating voltage of 200 keV. The electron beam spot diameter was scaled down to about 20 nm for continuous irradiation of the HZO thin film. At this time, the electron beam current density was approximately 40,000 e/Ų s, and HRTEM images were obtained every 3 min during irradiation. Elemental dispersive X-ray spectroscopy (EDX) elemental distribution characterization was performed on the same area of the HZO thin film before and after electron beam irradiation. The relative ratio (atomic ratio) of Hf atoms to O atoms along the growth direction of the HZO thin film was chosen as the evaluation standard for semi-quantitatively analyzing the V_O concentration in the thin film.

Characterization of ALD fabricated HZO thin films

The HZO ferroelectric films with Metal-Ferroelectric-Metal (MFM) structures were fabricated through an ALD method as shown in Figure 1A. The specific fabrication process is described in the Section “MATERIALS AND METHODS”. Figure 1B displays the cell structure models of the centrosymmetric t-phase and the o-phase of the HZO thin film, showing that the two structures are very similar. In this figure, the solid orange-yellow spheres represent hafnium (Hf) or zirconium (Zr) ions. In the t-phase, all oxygen ions are located at the center of the nearest Hf/Zr ions, represented by gray solid spheres, as four-coordinated oxygen ions. However, in the o-phase, in addition to the tetra-coordinated oxygen ions located at the center position of the nearest Hf/Zr ions, there are non-centrosymmetric triple-coordinated oxygen ions offset along the c-axis direction, represented by red solid spheres. This is widely believed to be the origin of the ferroelectricity in HfO₂-based thin films^[30]. To determine the composition of the fabricated HZO ferroelectric thin film, Figure 1C presents the results of the grazing incidence X-ray diffraction (GIXRD) tests conducted at a grazing incidence angle of 0.5°. The main phase of the thin film is the o-phase/t-phase, with a prominent peak at approximately 30.5°, corresponding to O(111)/T(101), and a weaker peak near 28.3°, corresponding to M($$\overline{1}$$11). This reveals that the thin film primarily consists of the o-phase and the t-phase. The structures of the o- and the t-phase are similar, resulting in their diffraction peaks at 30.5° being very close, making it hard to distinguish them through GIXRD^[31]. Meanwhile, the P-V hysteresis loop and the current (I)-V curve were shown in Figure 1D with a 1 kHz and 4 V pulse voltage. The wake-up was tested with 100 Hz, 3.5 V pulse voltage at 1,000 cycles. The prepared HZO ferroelectric thin film showed good ferroelectric performance. After being awakened, the residual polarization value P_r was 22.9 µC/cm² under the test voltage of 4 V. At the same time, its polarization switching current peak was visible, and there was no leakage current under this voltage.

Figure 1. (A) Fabrication process of HZO ferroelectric capacitor, (B) Structural models of a non-polar t-phase with space group P4₂/nmc and a polar o-phase with space group Pca2₁. (C) GI-XRD pattern of W/HZO/W ferroelectric thin film, (D) P-V and I-V curves of HZO ferroelectric thin film.

Electron beam-induced V_O in HZO films inside TEM

The cross-sectional view of the fabricated HZO film is shown in Figure 2A. The polycrystal HZO layer with a thickness of 14 nm was sandwiched between two W layers (top layer around 15 nm), as shown in the high-resolution TEM (HRTEM) image in Figure 2A. The interfaces between the HZO thin film and the top and bottom electrodes were smooth. In situ electron beam irradiation was carried out inside TEM on the fabricated ferroelectric film as illustrated in Figure 2B, and the V_O concentration was analyzed during electron beam irradiation through EDX. By adjusting the diameter of the electron beam, the dosage of the electrons bombarded on the sample area was tuned, and HRTEM images during the irradiation process were captured every minute.

Figure 2. (A) HRTEM image of the cross-sectional morphology of the HZO ferroelectric capacitor, (B) Schematic diagram of electron beam irradiation, scale bar 5 nm, (C) Semi-quantitative analysis of V_O concentration before and after electron beam irradiation of the TiN/HZO/TiN ferroelectric thin film, (D) EDX element mapping of the TiN/HZO/TiN ferroelectric thin film before electron beam irradiation, scale bar 10 nm, (E) EDX element mapping of the TiN//HZO/TiN ferroelectric thin film after electron beam irradiation, scale bar 10 nm.

A semi-quantitative analysis of the V_O concentration changes in the HZO ferroelectric thin film before and after electron beam irradiation was performed using EDX analysis, with the results presented in Figure 2C. The relative ratio (relative fraction) of Hf to O atoms was selected as the function for the vertical axis coordinates. A higher relative fraction indicates a higher concentration of V_O. The intensity ratio of Hf and O in the red line after irradiation was increased than before in the black line within the HZO layer denoted with green color; thus, the V_O in the HZO thin film was increased after an hour of electron beam irradiation, especially in the region near the top electrode layer. This may be attributed to the asymmetry fabrication process of the ferroelectric capacitor. The Knotek-Feibelman mechanism for radiolysis is suggested as the mechanism of the introduction of V_O. First, the electron beam creates an inner-shell vacancy on the metal site. Next, an electron from a nearby oxygen atom has an interatomic Auger decay to the metal inner shell hole, and further Auger electrons are ejected from the oxygen atom. This results in a neutral or positive oxygen atom that is repelled by the surrounding metal ions and ejected into the vacuum^[32,33]. The V_O concentration induced by electron beam irradiation depends on irradiation parameters such as electron beam energy, dose rate, exposure time, etc. Under controlled conditions, the V_O concentration remains relatively stable during experiments. A moderate concentration of vacancies lowers the switching barrier and enhances polarization, whereas excessive vacancy formation can lead to defect clustering, increased leakage, and domain wall pinning^[2,19,22]. To exclude the influence of the material of the electrode layer, the W/HZO/W ferroelectric capacitor was also fabricated and irradiated; the EDX results were shown in Supplementary Figure 1, which demonstrated a similar increasing trend of V_O. These results confirmed that electron beam irradiation can lead to the increase of the V_O in the sample.

The element distribution along the growth direction of the HZO thin film was characterized with line-profile EDX analysis^[31]. Before irradiation, as shown in Figure 2D, the high-angle annular dark field (HAADF) image demonstrated the sample area before exposure, with Hf, Zr, and O elements distributed evenly in the middle layer. Titanium (Ti) and nitrogen (N) were distributed in the top and bottom electrode layers. The diffusion of elements across different layers was not obvious. In Figure 2E, the elements showed diffusion across different layers which may be attributed to the joule heating phenomenon of the electron beam irradiation^[34]. The joule heating phenomenon caused the local temperature rise in the irradiated area, thus leading to the diffusion of the elements.

Ferroelectric phase transition in HZO thin films by electron beam irradiation

Electron beam irradiation-induced phase transition from m-phase to o-phase was observed inside TEM. The focused-ion-beam (FIB) thinned cross-sectional HZO ferroelectric film was irradiated with Talos F200X TEM, under 200 kV accelerating voltage. The irradiated area of the HZO thin film was confined to a circular region with a diameter of about 20 nm, and the electron beam current density was about 410⁴ e/(Ų s). The HRTEM image was captured every 3 min, as shown in Figure 3. The pristine HZO ferroelectric film was a m-phase in the red rectangle area of Figure 3A. The area was the o-phase after being irradiated for 30 min under the electron beam denoted with a green rectangle in Figure 3D. The irradiated area underwent phase transition from m-phase with zone axis [1$$\overline{1}$$0] to po phase with zone axis [010], which was identified in the fast Fourier transformation (FFT) image. The increased V_O concentration during the irradiation process contributes to the phase transition; this has been confirmed in previous research such as O vacancy engineering, helium (He) ion irradiation-induced phase transformation, and ferroelectric transition under biasing voltage, all of which proved the V_O are beneficial to the phase transition^[2,35,36]. From the above EDX results in Figure 2, the V_O concentration increased after the electron beam irradiation for 60 min. The high energy electrons collided with O atoms and left with V_O in the original location. The charged V_O in the pristine m-phase increases, thereby lowering the phase transition potential barrier from the m-phase to the o-phase, which was proved from DFT in previous research^[37]. As the electron beam irradiation continues, the m-phase absorbs the energy of the energetic electrons and converts it into its internal energy. Thus, at some critical point, the phase transition potential barrier from the m-phase to the o-phase is crossed and the phase transition is finally completed.

Figure 3. (A) HRTEM image of the HZO ferroelectric thin film before electron beam irradiation, scale bar 5 nm. (B) Magnified image of the m-phase in the red box area, scale bar 2 nm. (C) FFT image of the red box area, (D) HRTEM image of the HZO ferroelectric thin film after electron beam irradiation, scale bar 5 nm. (E) Magnified image of the o-phase in the green box area, scale bar 2 nm. (F) FFT image of the green box area.

Similarly, the phase transition from the t-phase to the o-phase was also observed through in situ electron beam irradiation experiments on HZO thin films, as shown in Supplementary Figure 2. The ferroelectric phase transition occurred just 90 s after irradiation, indicating that compared to the phase transition from the m-phase to the o-phase, the phase transition from the t-phase to the o-phase requires a smaller transition barrier, which is consistent with previous research because increase in the concentration of the V_O lowers its phase transition barrier^[37,38]. In addition, this type of phase transition has also been widely observed in related studies of thermal fields^[39], wake-up^[40], and strain engineering^[41]. The wake-up effect is characterized by an increase in remnant polarization (Pr) after a number of electric field cycles, which may be attributed to the redistribution of V_O and field-driven phase transformation from t-phase to o-phase during electric field cycling^[42]. In addition, some reports also pointed out that the wake-up effect is related to the suppression of disorder^[11]. Herein, we are focused on investigating the V_O concentration and phase transition of HZO thin films during in situ TEM electron beam irradiation. In this process, it is difficult to observe the suppression of disorder. When irradiated for 690 s, a crystal zone axis flip occurred, from [1$$\overline{2}~\overline{1}$$]o to [001]o, at which point the irradiated area presented an in-plane polarization state, as shown in Supplementary Figure 2C. Continuing irradiation, a crystal zone axis flip occurred, and the in-plane domain changed to an out-of-plane domain, i.e., [100]o and [010]o, as shown in Supplementary Figure 2D and E. This behavior is one of the mechanisms of the wake-up effect of HfO₂-based ferroelectric thin films^[43]; hence, this result provides new experimental evidence and control methods for the explanation of the wake-up effect mechanism of HfO₂-based ferroelectric thin films.

Strain analysis of ferroelectric phase transition processes

During the phase transition of the HfO₂-based thin films, the strain distribution in the film was analyzed by the GPA method. Figure 4 shows the strain distribution during the phase transition of m-phase to o-phase. The obtained ɛ_xx and ɛ_yy images correspond to the stress distribution along the x-direction (i.e., the horizontal axis) of the analyzed region, and the strain in the y-direction, respectively (as shown in the FFT images of Figure 3C). It can be seen that there is almost no strain along the x-direction in the red box area, while there is a tensile strain on the y-direction, which can be seen in the yellow box area. The stress state of the m-phase within the red box area in Figure 4A at 0 min of electron beam irradiation is used as a reference. Upon the start of electron beam irradiation, the strain state change in the ɛ_yy image is substantial. After irradiation for 31 min [Figure 4E], a phase transition occurs; the crystal plane changes from m(11$$\overline{2}$$) to o(002), causing the yellow box area to change from tensile strain to compressive strain. As the irradiation time continues to increase, the compressive strain state in the red box area becomes more pronounced as shown in Figure 4F and G.

Figure 4. (A-G) HRTEM images of the HZO ferroelectric thin film during electron beam irradiation and the GPA analysis results of the corresponding red box area, scale bar 5 nm. (H) Schematic diagram of electron beam irradiation-induced ferroelectric phase transition in HZO thin film.

Moreover, the area of the unit cell projection plane corresponding to the [1$$\overline{1}$$0]m and [010]o crystal orientations was calculated before and after the phase transition. The cell parameters were obtained from the optimization calculation results of the DFT^[44]. The cell parameters of the m-phase area = 5.33 Å, b = 5.20 Å, c = 5.15 Å, α = 90°, β = 90°, γ = 99.7°, and the cell parameters of the o-phase area = 5.27 Å, b = 5.05 Å, c = 5.08 Å, α = 90°, β = 90°, γ = 90°. The area of the unit cell projection plane corresponding to the [1$$\overline{1}$$0]m crystal orientation is Sm = 37.80 Ų, and the area of the unit cell projection plane corresponding to the [010]o crystal orientation is So = 26.77 Ų (as illustrated in Supplementary Figures 3 and 4). The calculation results show that the area of the unit cell projection plane corresponding to the [010]o crystal orientation after the phase transition is smaller than that corresponding to the [1$$\overline{1}$$0]m crystal orientation before the phase transition. This also confirms that there exists compressive strain during the phase transition process from the parent phase [1$$\overline{1}$$0]m to the new phase [010]o, consistent with the GPA results.

Ferroelectric crystallization induced by electron beam irradiation of amorphous HZO thin films

During the semi-quantitative analysis of V_O concentration of HZO ferroelectric films before and after e-beam irradiation in Figure 2, it was found that electron beam irradiation was able to induce crystallization of amorphous HZO films, which is the electron beam heating effect, producing an effect similar to rapid thermal annealing (RTA)^[45]. For the HZO thin film without RTA, its crystallinity was analyzed using GIXRD testing, with specific results shown in Supplementary Figure 5. Compared with the HZO ferroelectric thin film after RTA, it was determined that the HZO film without RTA exhibits strong amorphous characteristics. Therefore, the crystallization process of the HZO film under electron beam irradiation through in situ HRTEM is shown in Figure 5. First, after 10 s of high-energy electron beam irradiation, two nuclei begin to appear in the smooth and uniform amorphous HZO film in Figure 5A, being the ferroelectric o-phase and disordered polycrystalline, respectively. The nuclei continued to grow with prolonged irradiation accompanied by the formation and fusion of new nuclei in Figure 5B and C. Multiple o-phases and m-phases were observed in the HZO sample area. As the irradiation time increased to about 100 s in Figure 5D, the amorphous HZO area completely crystallized, and the phase transition to the o-phase and the m-phase occurred, which is similar to the phenomenon in RTA^[46]. When the irradiation time was further extended to 110 s (see Figure 5E), the two o-phase grains experienced mutual squeezing. This interaction led to the generation of dislocations at the grain boundaries, as indicated by the purple box in Figure 5E. These dislocations may cause V_O to accumulate at these locations, which can subsequently lead to breakdown fatigue in the HZO ferroelectric film^[47]. Until 200 s of irradiation, a part of the ferroelectric o-phase grains near the dislocations underwent a crystal zone axis flip. The interplanar spacing decreased from 3.033 to 1.515 Å, corresponding to the crystal plane changing from o(111) to o(131), driven by the thermal effect of electron beam irradiation.

Figure 5. Electron beam irradiation-induced crystallization of amorphous HZO thin film: (A) nucleation begins in the o-phase, (B) growth of the o-phase nucleus, (C) growth of the nucleus in both the o-phase and m-phase, (D) complete crystallization of the amorphous HZO thin film, (E) generation of lattice dislocations at the interface, (F) rotation of the o-phase crystal axis. The scale bars are all 10 nm.