Stress Engineering in the Optimization of Next-Generation Hafnium-Based Ferroelectric Memory

Abstract

Hafnium oxide thin films have been extensively investigated for high-speed and low-power memory applications. Herein, we investigated the influence of oxygen vacancies and external stress on the ferroelectric characteristics of Al-doped HfO₂ (HfAlO). Compared with HfAlO with 14% oxygen vacancies, films with 21% oxygen vacancies could lower the polarization switching barrier and increase the fraction of the ferroelectric phase. Furthermore, significant external stress promotes ferroelectric phase formation, thereby enhancing ferroelectric characteristics. The remanent polarization achieved with W electrodes (2Pr = 38 µC/cm²) is about 18 times that of Au electrodes, owing to the lower thermal expansion coefficient of W electrodes. Density functional theory calculations and finite element analysis provide theoretical insights corroborating the experimental results, helping to pave the way for developing hafnium-based materials for next-generation in-memory computing applications.

1. Introduction

With the rapid advancement of artificial intelligence, there is an unprecedented demand for highly efficient information processing in the field of integrated circuits [1,2,3]. Consequently, the development of high-performance memory devices has emerged as a foundational innovation frontier for next-generation information technologies [4,5,6]. Among the candidates for next-generation memory, ferroelectric memory has gained significant attention due to its high-speed operation and low power consumption [7,8,9]. Conventional ferroelectric materials exhibit severe limitations, including heavy metal toxicity, incompatibility with standard CMOS processes, and complex integration schemes [10,11,12]. The discovery of ferroelectricity in hafnium-based materials in 2011 fundamentally addressed these issues [13]. However, HfO₂-based ferroelectric materials still face challenges, such as the elusive modulation mechanisms of their ferroelectric properties.

Recent studies have shown that oxygen defects have a significant impact on device performance [14,15]. For example, Islamov et al. observed that a wake-up phase is required to increase the remanent polarization at low oxygen vacancy densities, while excessive oxygen vacancies will suppress the ferroelectricity in hafnium-based films [16]. Huang et al. demonstrated that interfacial oxygen injection could reduce the coercive field of hafnium-based ferroelectric thin films by decreasing the oxygen vacancy concentration [17]. Additionally, out-of-plane stress also affects the ferroelectric characteristics of hafnium-based thin films [18,19]. Liu et al. reported that the hysteresis loop contracts with increasing compressive stress [20]. Song et al. suggested that stress plays a crucial role in optimizing the ferroelectric response [21]. Although the above studies have well illustrated empirical strategies for modulating hafnium-based ferroelectric thin films, the underlying modulation mechanisms remain elusive.

In this work, we systematically investigate the effects of oxygen content and mechanical stress on the ferroelectricity of Al-doped HfO₂ (HfAlO) and its underlying physical mechanisms. The oxygen content of HfAlO is modulated by alternating the oxygen source during deposition. The oxygen vacancies play a crucial role in determining the polarization behavior and stability of HfO₂. The stress induced in the films is tuned by employing different top electrodes. Stress can significantly alter the microstructure arrangement of HfO₂-based thin films. Theoretical analyses are conducted to elucidate the mechanisms governing how oxygen content and stress enhance the ferroelectric characteristics.

2. Experimental

Initially, SiO₂/Si substrates (with a 300 nm-thick SiO₂ layer) were sequentially cleaned using acetone, absolute ethanol, and deionized water [22]. Subsequently, W bottom electrodes were deposited via physical vapor deposition (PVD). Then, 10 nm-thick aluminum-doped HfO₂ (HfAlO) thin films were deposited on the W/SiO₂/Si substrates using atomic layer deposition (ALD). Tetrakis(ethylmethylamino) hafnium (TEMAH) and trimethylaluminum (TMA) were employed as precursors, which were alternately pulsed into the ALD chamber at 250 °C using an N₂ carrier gas (with a TEMAH:TMA cycle ratio of 34:1). H₂O or O₂ plasma was used as the oxidant. For the Al₂O₃, the ALD temperature was 280 °C. The ALD pulse and purge time were 2 and 5 s, respectively. The growth rate of the Al₂O₃ was 1.05 Å for each cycle. For the HfO₂, the ALD temperature was 280 °C. The ALD pulse and purge time were 5 and 5 s, respectively. The growth rate of the HfO₂ was 1 Å for each cycle. Finally, W top electrodes (80 × 80 µm²) were patterned via a standard lift-off process. A rapid thermal annealing (RTA) treatment was then performed in a nitrogen atmosphere at 500 °C for 30 s.

Based on density functional theory (DFT) calculations using the Materials Studio software 2019, we evaluate the effect of oxygen vacancy on the ferroelectricity of hafnium-based film. The Perdew–Burke–Ernzerhof (PBE) functional of generalized gradient approximation (GGA) is used to optimize the geometric stricture until the residual force on each atom is smaller than 0.02 eV/Å. The crystal structure is a 2 × 2 × 2 supercell with 96 atoms.

3. Results

Figure 1a schematically illustrates the structure of the HfAlO ferroelectric devices. Figure 1b shows that the ALD cycle of the HfAlO thin films consists of 34 cycles of HfO₂ followed by one cycle of Al₂O₃ at a chamber temperature of 250 °C. Cross-sectional scanning electron microscopy (SEM) images reveal distinct interfaces between the device layers, as demonstrated in Figure 1c,d. The thickness of the as-grown HfAlO thin film is approximately 10 nm. Through high-resolution transmission electron microscopy (HRTEM), the HfAlO thin films demonstrate high crystallinity after the RTP process. The measured d-spacing value is approximately 2.6 Å, which is consistent with the X-ray diffraction (XRD) results (Figure 1e). The XRD patterns of the HfAlO film display diffraction peaks at 30.5° and 36°, corresponding to the HfO₂ orthorhombic (O) (111) and (200) phases, respectively [23,24]. Furthermore, Figure 1f shows that the Piezoelectric Force Microscopy (PFM) response is symmetrical for the +P and –P poled regions with an approximate 27° phase change, indicating robust ferroelectric switching [25,26]. As shown in Figure 1g, the X-ray photoelectron spectroscopy (XPS) survey spectra of the HfAlO films confirm the presence of Hf, Al, and O. Figure 1h shows that the Hf 4f core-level spectrum displays two peaks at 16.9 eV and 18.6 eV, corresponding to Hf 4f_7/2 and Hf 4f_5/2, respectively [17,27,28]. Figure 1i shows a characteristic peak located at approximately 74.1 eV, corresponding to the Al 2p core level, which indicates the Al³⁺ chemical state [29]. The O 1s spectrum was deconvoluted into three sub-peaks (Figure 1j). The main peak at approximately 530 eV is attributed to Hf–O bonding, and the 531 eV peak corresponds to Al–O bonding [30,31]. The remaining peak at a higher binding energy is ascribed to the presence of oxygen vacancies, as shown in Figure 1j. Furthermore, the oxygen content of the HfAlO thin films deposited using an O₂ plasma is higher than that of the films deposited using an H₂O oxidant, as shown in Figure S1.

Figure 2a illustrates the atomistic model, where red and blue spheres represent oxygen and hafnium atoms, respectively. As depicted, oxygen atom displacement drives macroscopic polarization switching at room temperature. Figure 2b shows the P-E hysteresis curves of the HfAlO ferroelectric devices under different voltage sweep ranges. As the voltage sweep increases, a larger hysteresis window is obtained. Conversely, Figure 2c demonstrates the degradation of the P-E loops with increasing sweep frequency. As shown in Figure 2d, the current–voltage (I–V) curves exhibit a typical increase in leakage current with applied voltage. Figure S2 displays the capacitance–voltage (C–V) curves measured with bias voltages ranging from −4 V to +4 V at 1 KHz. These collective electrical characterizations indicate robust ferroelectric characteristics and high film quality. Furthermore, we systematically compared the ferroelectric characteristics of devices with different top electrodes and oxygen sources. As shown in Figure 2e, the devices with W top electrodes show well-saturated P-E hysteresis curves, yielding a remanent polarization of approximately 10 µC/cm². In contrast, devices with Au top electrodes display poor P-E hysteresis curves. For the devices fabricated using an H₂O oxygen source, the P-E hysteresis curves are shown in Figure S3. Figure 2f corresponds to the O₂ plasma, and shows that both the remanent polarization and coercive field increase as the thermal expansion coefficient (CTE) of the top electrode decreases. For the HfAlO films with an oxygen vacancy concentration of 21.5% corresponding to water source, Figure S4 shows that the remanent polarization similarly increases with decreasing CTE. However, the coercive field exhibits an opposite trend, decreasing with reduced CTE. Figure 2g and Figure S5 show distinct switching current peaks for the devices with W top electrodes. For samples deposited with a H₂O source, the devices show highly robust P-E curves, as shown in Figure 2h. The devices with HfO₂ show a high remanent polarization of over 20 µC/cm². From the results, it can be concluded that devices with W top electrodes display superior remanent polarization owing to mechanical confinement provided by their lower thermal expansion coefficient. Additionally, the remanent polarization is further enhanced for devices deposited with an H₂O source, as summarized in Figure 2i.

To systematically investigate the effects of top electrodes and oxygen sources on device characteristics, endurance and retention properties were evaluated as two essential reliability indicators for the HfAlO ferroelectric devices. The applied pulse waveforms for the endurance and retention tests are illustrated in Figure 3a. The endurance characteristics of the devices were tested with a 1000 ns-wide write pulse of 3.5 V and a 1000 ms-wide read pulse of 3.5 V. For the W electrode devices fabricated with an H₂O oxygen source, Figure 3b shows that the devices exhibit an endurance of over 10⁴ cycles, without significant fatigue degradation. Meanwhile, the retention time is maintained for over 10⁴ s for the ferroelectric devices with W top electrodes, as shown in Figure 3c.

For W electrode devices with an O₂ plasma source, Figure 3d shows that the devices demonstrate a substantially enhanced endurance of over 10⁶ cycles. Their corresponding retention times are shown in Figure S6a. Notably, compared with the samples derived from H₂O, the endurance characteristics are improved by two orders of magnitude. Other devices show distinct fatigue behaviors prior to failure. As shown in Figure 3e, the devices with TiN top electrodes achieve an endurance of over 10⁸ cycles, with retention times shown in Figure S6b. The device’s operational lifespan is clearly divided into three distinct stages: wake-up, stabilization, and fatigue. Similarly, the three-stage degradation behavior is also observed for Pt top electrode devices, which yield an endurance of over 10⁷ cycles, as shown in Figure 3f. The underlying physical mechanisms by which the top electrode and oxygen sources modulate the ferroelectric properties will be further analyzed below.

To elucidate the top electrode effects on the ferroelectric characteristics of the devices, finite element analysis (FEA) was performed using ABAQUS. A two-dimensional model was constructed based on the stacked film structure. Figure 4a shows that the W top electrode can induce large in-plane stress within the HfAlO ferroelectric films owing to the low thermal expansion coefficient (CTE) of metal W. However, other top electrodes predominantly induce out-of-plane stress in the HfO₂ thin films. As shown in Figure 4b, the TiN electrode applies out-of-plane stress to the ferroelectric thin films, and the stress distribution exhibits three distinct spatial zones: a region of high stress, a transfer region, and a bulk stress zone. Because of the small CTE mismatch between the top electrodes (W as well as TiN) and HfO₂, no large local deformation occurs during the thermal annealing process, thereby preventing severe stress relaxation. However, Pt and Au electrodes undergo significant deformation during the annealing process due to their high thermal expansion, resulting in pronounced stress dispersion. Figure 4c shows that film deformation may lead to the disappearance of transfer regions. With further increases in the thermal expansion coefficient, Figure 4d shows that the region of high stress disappears, and stress dissipation regions appear. This effectively reduces the stress exerted on the ferroelectric HfAlO films, ultimately leading to the degradation of their ferroelectric characteristics. Figure 4e illustrates that W top electrodes apply in-plane stress to the HfO₂ thin films, whereas other electrodes exert out-of-plane stress on the functional thin films. Among them, the TiN top electrodes apply the maximum out-of-plane stress to the ferroelectric thin films. A similar stress distribution can also be observed along the vertical depth profile of the films (Figure 4f). Collectively, these simulations elucidate the influence of the electrode thermal expansion coefficient on the ferroelectric properties of the devices.

To better explore the effect of oxygen vacancies on the ferroelectric characteristics of the HfAlO devices, density functional theory (DFT) calculations were performed to determine the polarization switching barrier and the spontaneous polarization of the O-phase in HfO₂. The DMol3 code within the Materials Studio suite was utilized to elucidate the oxygen vacancy−induced ferroelectric characteristics of HfAlO thin films. A 2 × 2 × 2 supercell with 96 atoms was built for Brillouin zone integration. To obtain an equilibrium structure, geometry optimization was conducted using the Local Density Approximation (LDA) and Projector Augmented Wave (PAW) methods. The structures were optimized until the residual force on each atom was smaller than 0.004 Ha/Å.

Figure 5a−c depict the minimum energy paths for switching between the two ferroelectric states, illustrating that the energy barrier is greatly reduced with increasing oxygen vacancy concentration. The switching barrier decreases by 0.5 eV relative to the pristine structure at an oxygen vacancy concentration of 7.6 × 10²¹ cm⁻³ (Figure 5b). At a higher oxygen vacancy concentration of 1.52 × 10²² cm⁻³, Figure 5c shows that the switching barrier is further reduced to less than 2.1 eV. The above analysis can explain the macroscopic coercive field reduction owing to the increase in oxygen vacancies.

Meanwhile, the electron density distributions and local geometric effects were investigated. Figure 5d shows slices of the charge density isosurfaces in the (100) plane for the pristine lattice. With oxygen vacancy increase, the electrode density decreases around the hafnium atoms, which may make it easier for oxygen atoms to move, i.e., the reduction in the coercive field (see Figure 5e). As the oxygen vacancies increase, the electron density decreases around the hafnium atoms, which facilitates oxygen atom displacement, i.e., the reduction in the coercive field (Figure 5e). With further increases in oxygen vacancies, the charge density continues to decrease around the hafnium atoms, as shown in Figure 5f, monotonically decreasing the coercive field. However, the HfO₂ polarization also decreases when the oxygen vacancy concentration reaches 1.52 × 10²² cm⁻³ (Figure 5g). Conversely, for HfO₂ cells with an oxygen vacancy concentration of 7.6 × 10²¹ cm⁻³, the remanent polarization reaches a maximum of 34.8 µC/cm².

Figure 5h illustrates the pristine HfO₂ cells. With an increase in polarization switching cycles, the O-phase structure may be damaged, leading to M-phase formation and the generation of massive oxygen vacancies. When these oxygen vacancies accumulate in the films, the devices lose efficacy (Figure 5i). For HfO₂ thin films with excessive oxygen vacancies, the probability of conductive filament formation is higher, resulting in poor fatigue characteristics. Thus, precisely modulating a suitable content of oxygen vacancies is beneficial for optimizing the ferroelectric characteristics of HfO₂ thin films.

4. Conclusions

In this study, we systematically investigated the influence of oxygen content and mechanical stress on the ferroelectricity of HfAlO thin films. The P-E curves demonstrate that an appropriate oxygen vacancy concentration can enhance the polarization properties and decrease the coercive field of HfAlO thin films. Specifically, the HfAlO films with a 21% oxygen vacancy concentration display a remanent polarization of approximately 20 µC/cm². First-principle calculations reveal that an optimal level of oxygen vacancies can lower the polarization switching barrier of the HfO₂ ferroelectric films and promote the formation of the ferroelectric phase. However, excessive oxygen vacancies degrade the endurance of the thin films (<10⁵ cycles) due to the accelerated generation of defects during cycling. Furthermore, significant external stress can enhance macroscopic film polarization owing to the stabilization of the ferroelectric phase. Our work provides a solid experimental foundation and profound theoretical insights for the optimization of HfAlO ferroelectric thin films, paving the way for their application in next-generation non-volatile memory devices.