Enhanced Ferroelectricity in Hf‐Based Ferroelectric Device with ZrO2 Regulating Layer

HfAlO film‐based ferroelectric memory is a strong contender for the next‐generation nonvolatile memories. However, the remanent polarization intensity of HfAlO films is small compared to other Hf‐based ferroelectric films at low annealing temperatures. In order to further improve the remnant polarization of the device, the ferroelectric memory with metal‐ferroelectric‐metal structure using ZrO2 as the regulating layer (RL) is designed and fabricated. Experimental results show that the device with the ZrO2 regulating layer exhibits triple enhancement, which may be due to the fact that ZrO2 RL has an effect on the enhancement of the ferroelectric phase. In addition, the device with ZrO2 regulating layer exhibits a superior ON/OFF conductance ratio, endurance, and retention characteristics, demonstrating potential for application to memory. This work provides an effective way to improve the ferroelectricity in HfAlO films at low annealing temperatures.


Introduction
Due to the existence of "memory wall" problem, conventional electronics could not meet the storage and computing requirements of the growing artificial intelligence and big data. [1][2][3] As a kind of emerging nonvolatile devices, ferroelectric memory has received a lot of attention from researchers with advantages of fast operation speed and low power consumption. However, conventional ferroelectric materials, such as Pb[Zr x Ti 1−x ]O 3 (PZT), [4] polyvinylidene fluoride (PVDF), [5] etc., are not compatible with the complementary metal oxide semiconductor (CMOS) processes, limiting the further application of DOI: 10.1002/aelm.202300208 ferroelectric memory. [6][7][8] The discovery of the ferroelectricity of hafnium-based materials has paved the way for ferroelectric memories to step into the nanometer size. Recent studies have shown that HfAlO thin films have wide promise for thermal stability, [9] radiation resistance, [10] and neural computation. [11] However, its low remnant polarization intensity and high annealing temperature have restricted its further development. [12] In order to improve the remanent polarization intensity of Hf-based ferroelectric materials, researchers generally insert an interface layer between the electrode and the ferroelectric layer, such as TiO 2 , [13,14] Al 2 O 3 , [15,16,17] HfO 2 , [18] ZrO 2 , [19,20] [21] CeO x , [22] etc. Among them, there are reported works on the performance improvement of ZrO 2 layer in memory [23][24][25] and ZrO 2 has good lattice matching effect with HfO 2 , which was suitable as insert layer for improving performances of Hf-based devices.
In this work, we first proposed ZrO 2 as the regulating layer (RL) to solve the low remnant polarization intensity problem at low annealing temperatures in HfAlO film. The device exhibits superior ferroelectric properties and memory performance, including the remnant polarization 2Pr of up to ≈31.2 μC cm −2 and the ON/OFF conductance ratio of up to ≈21. Based on firstprinciples calculation, it was found that the ZrO 2 regulating layer could promote the formation of orthorhombic phase in HfAlO layer.

Results and Discussion
The energy dispersive spectroscopy (EDS) and cross-sectional transmission electron microscope (TEM) images of the H/Z/Hdevice are depicted as Figure 1b,c. The thicknesses of the HfAlO, ZrO 2 , and HfAlO films are about 5, 2, and 8 nm and the interfaces between HfAlO and ZrO 2 are sharp, indicating little interdiffusion at the interfaces. The vertical atomic arrangement pattern shows that the film has good crystallinity ( Figure 1c). As shown in Figure 1d, the binding energy located at 74.18 eV represents Al 2p. The spectra of the O 1s energy level peak is deconvoluted into two distinct fitted peaks. The binding energy of 530.18 and 531.88 eV is considered to be the bonding of O─Hf and the bonding of O─Al. [26,27] The above results indicate that HfAlO/ZrO 2 /HfAlO film has been successfully prepared.  curves for the device without ZrO 2 RL at different annealing temperature. At the annealing temperature of 400°C, HAO-device shows no significant ferroelectric properties even when an electric field of more than 4.2 MV cm −1 was applied to the device. As shown in Figure 2a, the device with ZrO 2 RL shows ferroelectric properties at 400°C annealing temperature, where 2Pr of H/Z/H-device is ≈3.9 μC cm −2 , indicating that the method of inserting layer plays a role in the back-end process. As the temperature increases to 450°C, the ferroelectric hysteresis curve of HAO-device appears with 2Pr of ≈10.7 μC cm −2 . Such enhancements can be attributed to that the higher annealing temperature facilitates the crystallization of the film and promotes the formation of ferroelectric phases. As shown in Figure 2c, the ferroelectricity of the H/Z/H-device with ZrO 2 RL improves significantly and 2Pr can reach to ≈31.2 μC cm −2 . The data from the current loop also prove that ZrO 2 RL contributes to the improvement of ferroelectricity. Such obvious enhancement of remanent polarization intensity demonstrates that the inserting method is useful to improve ferroelectricity of HfAlO-based memory. Under an electric field of about 4 MV cm −1 , the device with ZrO 2 RL has a nearly three times higher 2Pr than the device without ZrO 2 RL. The effect of inserting layer can be attributed to that ZrO 2 RL is partly consisted of an o-phase, which can facilitate the t-phase to o-phase transition and suppress the t-phase to m-phase transition in HfAlO films at annealing process, and thus contribute to better ferroelectricity. [20,28,29] Additionally, the device with a ZrO 2 RL exhibit better uniformity (Figures S1 and S2, Supporting Information). This can be attributed to the grown ZrO 2 film is rel-atively uniform, which exerts the same effect on the HfAlO film in different regions.
In order to further verify the contribution of ZrO 2 RL to the phase transition in HfAlO film, we performed first-principles calculation as theoretical support. The structures of HfAlO and HfAlO-ZrO 2 with a thickness ratio of 4:1 were constructed, where an Al concentration is set to ≈3% (Figure 3a,b). After geometric optimization with Materilas Studio software, the statistic for the proportion of different phases in different lattice structure is depicted as Figure 3c. More specifically, in the HfAlO structure with ZrO 2 layers, the proportion of o-phase (Pca2 1 ) and tphase increases by ≈20% and the proportion of m-phases decreases by ≈40%, respectively. In addition, the X-ray diffraction (XRD) patterns of H/Z/H-device and HAO-device also prove that the HfAlO film with ZrO 2 RL has more o-phase and less mphase, [30][31][32] showing a similar trend to the first-principles simulation (Figure 3d). Furthermore, factors that influence the ferroelectric performance of the device include defects and oxygen vacancies at the interface. [33,34] We have taken measures to avoid these issues during the device design stage. Such a significant increase in o-phase composition favors the promotion of ferroelectricity in the device. [35] The characteristics of endurance and retention are two essential functions for nonvolatile memory. We woke up the device prior to the performance test ( Figure S3, Supporting Information). The wake up operation can clearly change the remnant polarization intensity and is helpful in stabilizing the polarization intensity during endurance and retention. And the reason why the voltages of the two devices are different is to ensure that www.advancedsciencenews.com www.advelectronicmat.de   the electric field through the devices remains similar, at ≈2.7 MV cm −1 (Table S2, Supporting Information). In terms of retention characteristics, the H/Z/H-device shows a 21% decrease in remanent polarization intensity over 10 000 s. In contrast, the HAOdevice experiences a 35% decrease. Given that the H/Z/H-device has a higher remanent polarization intensity, it suggests that it has more room for degradation and can retain its polarization for a longer period. As shown in Figure 4b, the H/Z/H-device still maintains a 2Pr value of 11.5 μC cm −2 at 10 8 seconds, while the HAO-device no longer exhibits a polarization window at 10 7 s, possibly due to the stabilizing effect of the asymmetric HfAlO structure on polarization. [36] Regarding endurance characteristics, we extracted the endurance cycles for the H/Z/H-device and HAO-device at different initial polarization intensity ( Figure  S4 and Table S1, Supporting Information). It can be observed that the H/Z/H-device exhibits significantly higher endurance cycles at lower remanent polarization intensity compared to higher ones. When comparing with the HAO-device, two advantages can be identified: 1) The H/Z/H-device can endure up to 90 000 cycles without breakdown at higher remanent polarization intensity, thanks to the enhancement of polarization intensity provided by the ZrO 2 RL; 2) When endurance testing is performed at lower polarization intensity, the H/Z/H-device demonstrates a larger number of cycles compared to the HAO-device, capable of enduring more than 10 6 cycles. In summary, compared to the HAO-device, the H/Z/H device exhibits good endurance and retention characteristics.
The polarization state of the ferroelectric layer may affect the potential distribution between the top and bottom electrodes in ferroelectric device (FE device), which in turn affects the conductivity value. The specific research process of tunneling mechanisms is presented in Figures S5 and S6 (Supporting Information). Figure 4c,d shows the up state and down state of the H/Z/H-device, respectively. After applying a positive voltage to the device, the polarization direction within the ferroelectric layer is to the right, making the potential barrier between the electrodes lower and the electrons more likely to achieve tunneling during the reading process (up state). Similarly, negative voltage raises the potential barrier, leaving the device in the down state.
For FE device, ON/OFF conductance ratio is one of the key measures of device performance. Here, the ON/OFF conductance and conductance ratio versus different write amplitude and pulse width is demonstrated in Figure 5. Obviously, as the write amplitude and pulse width increase, the ON/OFF conductance ratio tends to increase at a reading voltage of −0.5 V. Importantly, due to the lower remanent polarization intensity, the voltage amplitude does not effectively regulate the changes in the barrier in HAO-device. Consequently, the HAO-device cannot achieve conductivity modulation by altering the voltage amplitude. When applying voltage of 4.5 V and a pulse width of 1000 ns, the device with ZrO 2 RL (21.21) has larger conductance ratio than the device without ZrO 2 RL (4.75), which matches the previous statement that ZrO 2 RL can increase the remanent polarization intensity of the device. Additionally, the more uniform H/Z/H-device also demonstrate uniform ON/OFF ratios. In all, the H/Z/H-device can respond to different stimuli accordingly and exists a certain recognition window, showing potential in FE device.

Conclusion
In conclusion, the ferroelectric memory with MFM structure using ZrO 2 as RL was designed and fabricated. Based on the enhancing effect of ferroelectric phase synthesis in HfAlO films by ZrO 2 films, the H/Z/H-device achieves the remnant polarization 2Pr of up to ≈31.2 μC cm −2 compared to HAO-device (≈10.7 μC cm −2 ) at 450°C annealing temperature. The experimental results show that the device with ZrO 2 RL also has good endurance characteristics, which can be repeatedly polarized more than 90 000 times without breakdown at a 2Pr value of ≈24 μC cm −2 ; and can be kept for 10 000 s at room temperature with small change in polarization intensity. In addition, the device with ZrO 2 RL can achieve an ON/OFF conductance ratio of up to ≈21 for different write amplitude and pulse width, showing potential as a future candidate for nonvolatile memory.

Experimental Section
The design process of the device can be found in the Supporting Information. And the structures of the two fabricated devices (H/Z/H-device and HAO-device) are shown in Figure 1a, which have the same thickness of the HfAlO film. The silicon substrate is p-type silicon with a resistivity of about 1-10 ohm cm. First, 10/70 nm Ti/Pt thin film as bottom electrode is deposited on the SiO 2 /p-Si substrate using physical vapor deposition (PVD). The ferroelectric layer consists of two HfAlO layers of different thicknesses and a ZrO 2 regulating layer. All the oxide films were deposited by atomic layer deposition at 250°C using tetrakis-(ethylmethylamino)hafnium (TEMAH) as Hf precursor, trimethylaluminum (TMA) as Al precursor, tetrakis-(ethylmethylamino)-Zirconium (TEMAZ) as Zr precursor and H 2 O as O source. The doping concentration is controlled by the cycle ratio. The ratio of Hf to Al is 34:1 in HfAlO film. Then, TiN was deposited by sputtering as the capping layer. The ferroelectric phase crystallization was achieved by 450°C for 30 s in N 2 , which is important to form the orthorhombic phase. After that, the TiN layer is etched by wet etching in a NH 4 OH: H 2 O 2 : H 2 O solution (1:1:5). Finally, top electrodes of 10 nm Ti/70 nm Au with an area of 80 × 80 μm 2 are fabricated after lithography. H/Z/H-device and HAO-device require "wake up" operation prior to endurance, retention, and conductance testing. For the H/Z/H-device, "wake up" operation is 300 consecutive scans of the device with a voltage of ±5.5 V. And the HAO-device uses the voltage of ±5 V to achieve the "wake up" operation in the similar electric field and scan cycles.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.