Large remnant polarization and great reliability characteristics in W/HZO/W ferroelectric capacitors

In this work, the effect of rapid thermal annealing (RTA) temperature on the ferroelectric polarization in zirconium-doped hafnium oxide (HZO) was studied. To maximize remnant polarization (2P_r), in-plane tensile stress was induced by tungsten electrodes under optimal RTA temperatures. We observed an increase in 2P_r with RTA temperature, likely due to an increased proportion of the polar ferroelectric phase in HZO. The HZO capacitors annealed at 400°C did not exhibit any ferroelectric behavior, whereas the HZO capacitors annealed at 800°C became highly leaky and shorted for voltages above 1 V. On the other hand, annealing at 700 °C produced HZO capacitors with a record-high 2P_r of ∼ 64 μC cm⁻² at a relatively high frequency of 111 kHz. These ferroelectric capacitors have also demonstrated impressive endurance and retention characteristics, which will greatly benefit neuromorphic computing applications.

Introduction

Ever since the first report of ferroelectricity in hafnium oxide (HfO₂) in 2011 (Böscke et al., 2011), considerable efforts have been made to elucidate the origins of ferroelectricity and enhance its performance for emerging applications. The excellent compatibility of HfO₂ with the complementary metal-oxide-semiconductor (CMOS) process and its suitability as a popular high-k gate dielectric has rapidly enabled applications such as ferroelectric random-access memories (FeRAMs) (Francois et al., 2019; Okuno et al., 2020) and ferroelectric field-effect transistors (FeFETs) (George et al., 2016; Dünkel et al., 2017; Dutta et al., 2020). Specifically, FeFET structure-based devices have found their niche in applications such as neuromorphic computing, where they form the basic unit of neural network circuits that can implement brain-inspired algorithms to solve various problems (Jerry et al., 2017; Mulaosmanovic et al., 2017; Mulaosmanovic et al., 2018; Ni et al., 2018; Dutta et al., 2019). They have been reported to mimic synaptic behavior and key neuronal functions. In particular, HfO₂-based FeFETs with analog threshold voltage (Mulaosmanovic et al., 2017) (V_t) and channel conductance tuning (Jerry et al., 2017; Ni et al., 2018) have been employed in synaptic applications as well as leaky integrate-and-fire (LIF) neuron models (Mulaosmanovic et al., 2018; Dutta et al., 2019). These neuromorphic applications have been realized through the partial polarization domain switching in ferroelectric HfO₂ thin films (Jerry et al., 2017; Ni et al., 2018). HfO₂-based ferroelectrics have been applied in ferroelectric tunnel junctions (FTJs) (Chen et al., 2018; Kobayashi et al., 2018; Cheema et al., 2020; Goh et al., 2020; Ma et al., 2020), negative capacitance field-effect transistors (NCFETs) (Hoffmann et al., 2016; Si et al., 2018; Kim et al., 2019), sensors (Smith et al., 2017; Jachalke et al., 2018; Mart et al., 2018), and energy storage (Ali et al., 2017; Kühnel et al., 2019) and conversion (Hoffmann et al., 2015). A high remnant polarization (2P_r) of the ferroelectric thin films that can sustain numerous switching cycles without degradation generally benefits memory applications such as FeRAMs (Kondo et al., 2007; Park et al., 2018b). However, a high remnant polarization is detrimental for FeFET and FTJ applications due to the highly concentrated electric field in the interfacial layers resulting from the high depolarization field in the ferroelectric layer. Nevertheless, large 2P_r not only reflects high asymmetric ionic displacement from the center of mass and higher nonvolatile charge density in the capacitor, but also indicates microstructural uniformity in the HfO₂-based ferroelectrics.

Doping HfO₂ with elements such as silicon (Böscke et al., 2011; Mart et al., 2018; Kühnel et al., 2019), aluminum (Mueller et al., 2012b; Müller et al., 2013; Zhou et al., 2020), gadolinium (Mueller et al., 2012a; Sang et al., 2015), zirconium (Zr) (Karbasian et al., 2017; Smith et al., 2017; Chen et al., 2018; Francois et al., 2019; Kim et al., 2019; Zacharaki et al., 2019; Goh et al., 2020; Okuno et al., 2020; Kashir et al., 2021), lanthanum (La) (Chernikova et al., 2018; Schroeder et al., 2018; Kozodaev et al., 2019; Song et al., 2020), etc., has been proven to improve its ferroelectric polarization by stabilizing the non-centrosymmetric orthorhombic phase (Pca2₁). Among others, zirconium has been shown to be a very effective dopant at $\sim 50$ cat% concentration because it reduces the crystallization temperature and improves the ferroelectric properties of HfO₂. Additionally, HfO₂ − ZrO₂ solid solution has a wider Zr concentration range for favoring the ferroelectric phase compared to other dopants. This system is commonly referred to as hafnium zirconium oxide (HZO). Electrode materials sandwiching the ferroelectric film has also been found to play a role in stabilizing the orthorhombic phase (o-phase) (Karbasian et al., 2017; Goh et al., 2020). Although titanium nitride (TiN) is the most widely used electrode, it was recently reported that materials like tungsten (W) (Goh et al., 2020) with a relatively low thermal expansion coefficient (α_W = 4.5 × 10^–6 °C⁻¹, α_TiN = 9.4 × 10^–6 °C⁻¹) improved remnant polarization by suppressing the formation of the monoclinic phase (m-phase).

In this work, the influence of varying rapid thermal annealing (RTA) temperatures after top electrode (TE) deposition on the 2P_r of W/HZO/W ferroelectric capacitors was investigated. To stabilize large ferroelectricity, an appreciable tensile stress was induced during RTA by sandwiching HZO with W electrodes. A record-high 2P_r was achieved with the capacitors operated at a particularly high frequency of 111 kHz. These HZO capacitors endured 10⁸ positive-up-negative-down (PUND) cycles while exhibiting a high 2P_r. Finally, a promising polarization retention was observed at various temperatures (25, 85, and 125°C) with very little polarization degradation over the monitored duration of 10³ min.

Materials and methods

To fabricate the capacitors, a ∼10 nm HZO film was deposited on the bottom electrode (BE) via the atomic layer deposition (ALD) process at 200°C, with tetrakis (dimethylamino)hafnium (TDMA–Hf) and tetrakis (dimethylamino)zirconium (TDMA–Zr) as the Hf and Zr precursors, respectively, and water (H₂O) as the oxidant. To maintain the Hf:Zr ratio as close to 1 : 1 as possible, each cycle consisted of one sub-cycle of TDMA–Hf and H₂O, and another sub-cycle of TDMA–Zr and H₂O.

For the BE, a blanket thin film of 70-nm thick W was sputter-coated on a silicon wafer with a 300 nm SiO₂ layer grown thermally. Another 45-nm thick W layer was sputter-deposited as the TE and a 50-nm thick platinum (Pt) was used to cap-off the capacitors. A device with a size of 30 μm × 30 µm was defined by TE pads patterned via photolithography and liftoff processes. The capacitors were then subjected to RTA for 1 min with five different annealing temperatures (400, 500, 600, 700, and 800°C) under the nitrogen atmosphere. All electrical characterization tests were performed using Keysight’s B1530A waveform generator/fast measurement unit module at room temperature (25°C).

Results and discussion

The ferroelectric hysteresis curves of the capacitors were obtained using a PUND voltage scheme with triangular pulses at 100 kHz frequency and a voltage amplitude of 4 V. The HZO capacitors were first woken up by using one million trapezoidal PUND pulse cycles with an amplitude of 4 V at a frequency of 111 kHz before obtaining the polarization data in Figure 1A. The capacitors annealed at 500 and 600°C exhibited a 2P_r of about 42 μC cm⁻² and 57 μC cm⁻², respectively. In particular, the capacitors annealed at 700°C resulted in a 2P_r of ∼ 64 μC cm⁻², which is among the largest reported values and the highest ever achieved at such a high frequency of 100 kHz. Figure 1B shows the bar plot of how 2P_r of the capacitors increased with an increase in RTA temperature. This could be attributed to an enhanced formation of polar o-phase and a greater ferroelectric polarization at higher RTA temperatures (Fig. Supplementary Figure S1) (Kashir et al., 2021). In addition, previous studies have demonstrated that the m-phase, which is responsible for attenuating ferroelectricity, can almost entirely be suppressed at high annealing temperatures (Pal et al., 2017; Park et al., 2018a; Kashir et al., 2021). These phenomena might be the reasons why the capacitors annealed at 700°C have shown such a large 2P_r. However, when annealed at 800°C, the capacitors became highly leaky and shorted when a voltage greater than 1 V was applied. A possible explanation for this could be the excessive amount of oxygen vacancies generated in the HZO through oxygen scavenging by the W electrodes during annealing (Lomenzo et al., 2016; Pešić et al., 2016; Kashir et al., 2021). On the other hand, HZO capacitors annealed at 400°C did not display any ferroelectric behavior, even after annealing at the same temperature twice (Supplementary Figure S2). A low deposition temperature of 200°C coupled with weak oxidant like water is known to facilitate increased carbon impurity in HZO thin films, which may have resulted in the late appearance of ferroelectricity (from 500°C onwards) (Materano et al., 2020; Hsain et al., 2022).

FIGURE 1

FIGURE 1. (A) Polarization-voltage plots of HZO capacitors annealed at various temperatures obtained using triangular PUND voltage scheme at 100 kHz. (B) Variation of 2P_r with respect to RTA temperature. The error bars represent standard deviation.

Ferroelectric hysteresis measurement using a dynamic hysteresis mode (DHM) can lead to an overestimation of the polarization charge. That is, the bipolar triangle pulses applied with a delay in between two excitations in DHM do not account for the non-ferroelectric contributions accumulated along with the ferroelectric component. The PUND technique, however, was designed to cancel out such non-ferroelectric contributions (Scott et al., 1988). Although this method can almost truly extract the ferroelectric contribution, it still fails to eliminate leakage current in the low-frequency regime (Fina et al., 2011). Therefore, the higher the frequency of the pulse scheme used, the closer the results are to the true ferroelectric polarization values. A clear trend of the remnant polarization is presented in Figure 2A, wherein the 2P_r value decreased as the frequency of the PUND scheme increased for all the measured HZO capacitors. For example, the average 2P_r of capacitors annealed at 700°C is 87.6 ± 5.1 μC cm⁻² at 1 kHz. This value was reduced to 71.3 ± 0.6 μC cm⁻² at 10 kHz and further decreased to 63.0 ± 0.1 μC cm⁻² at 100 kHz. Evidently, the values at lower frequencies revealed a larger spread, indicating the presence of leakage contributions. In Figure 2B, the same observation is shown in terms of polarization-voltage hysteresis curves. As the frequency decreased, the loop opened, and the curves became rounded around ±4 V.

FIGURE 2

FIGURE 2. (A) Dependence of 2P_r on the frequency of PUND scheme. (B) Polarization-voltage plots obtained using different frequencies of PUND scheme in HZO capacitors annealed at 600°C.

To characterize the endurance of all three HZO capacitors with meaningful 2P_r, a PUND voltage scheme with trapezoidal pulses of 1.5 µs width, rise and fall times of 1.5 µs each (i.e., frequency of 111 kHz), and a voltage of ±4 V was employed for both cycling and measuring 2Pr as shown in the inset of Figure 3. The 2P_r value was measured five times for every decade of endurance cycles, as plotted in Figure 3. A jump in 2P_r was observed from the pristine cycle (data points plotted at 10^–1 of the endurance cycles axis) to the first cycle in all three cases. The jumps were ∼ 8 μC cm⁻², ∼ 7 μC cm⁻², and ∼ 11 μC cm⁻² for capacitors annealed at 500, 600, and 700°C, respectively. After this initial jump, the 2P_r value started to increase steadily. This observation is due to the wake-up effect generally seen in HZO-based ferroelectrics (Mehmood et al., 2020; Jiang et al., 2021) and even in traditional ferroelectrics like lead zirconate titanate (Carl and Hardtl, 1977). In the case of the capacitors annealed at 500°C, 2P_r increased to ∼ 43 μC cm⁻² (95.5%) after just 100 cycles. Similarly, after 100 cycles, 2P_r of capacitors annealed at 600 and 700°C increased to ∼ 58 μC cm⁻² (96.0%) and ∼ 62 μC cm⁻² (96.5%), respectively. These results reveal commendable wake-up behavior in all three HZO capacitors. We believe that these findings could be attributed to the large electric field (4 MVcm⁻¹) used during the endurance tests, which is known to accelerate the number of cycles required for wake-up (Zhou et al., 2013). Remarkably, the capacitors annealed at 700°C maintained a large 2P_r of ∼ 64 μC cm⁻² without detectable fatigue or breakdown, even after 10⁸ cycles under such relatively large cycling voltage. The capacitors annealed at 600°C also exhibited a comparable endurance performance with ∼ 60 μC cm⁻² remnant polarization. However, the capacitors annealed at 500°C showed an onset of fatigue after ∼6.3 × 10⁷ cycles. The use of high frequency cycling could have resulted in improved endurance behavior while exhibiting such high 2P_r (Liao et al., 2021). These results are encouraging, as previous studies reported either a high 2P_r value with a low endurance (Schroeder et al., 2018; Zacharaki et al., 2019; Kashir et al., 2021) or a high endurance with a low 2P_r value (Chernikova et al., 2018; Kozodaev et al., 2019).

FIGURE 3

FIGURE 3. Endurance tests of the different HZO capacitors performed using trapezoidal PUND pulses with an amplitude 4 V and a frequency of 111 kHz frequency (i.e., t_r = t_f = 1.5 µs, t_h = 1.5 µs).

To further probe the endurance of fabricated HZO capacitors, voltage pulses with an amplitude of ±1.5 V were applied at a frequency of 1.67 MHz for 10¹⁰ cycles (Supplementary Figure S3). Under these test parameters, all three capacitors manifested a lower 2P_r value that started declining at around 10⁷ and 10⁸ cycles. Lower 2P_r values were obtained because the applied voltage amplitude was not sufficient to achieve a complete polarization switching of all the available domains. This phenomenon is referred to as unsaturated polarization switching or subcycling behavior (Schenk et al., 2014; Li et al., 2020). The decline in the 2P_r can also be attributed to the subcycling effect where the domains segregate into groups that require different voltages for switching. Subcycling is more pronounced at reduced cycling voltages and higher frequencies (Li et al., 2020). On the other hand, the application of larger voltages at such high frequencies resulted in the premature breakdown of the capacitors. Nonetheless, all capacitors were still switchable after 10¹⁰ cycles, even under a fast-testing mode.

To further characterize the reliability of the HZO capacitors, retention tests were conducted at different temperatures (25, 85, and 125 °C) with baking times of 1, 10, 100, and 1,000 min. The capacitors were cooled down to room temperature (25°C) for read and write operations. Two pulse waveforms (PUND and NDPU) with a voltage amplitude of ±4 V at 111 kHz were applied to measure the retention of the same state (SS+ and SS-) and opposite state (OS+ and OS-) polarization. Five million PUND cycles, also with ±4 V at 111 kHz, were used at room temperature (25°C) to fully wake-up the capacitors before starting the retention tests. Figure 4 shows the results of the retention tests on six capacitors (2 capacitors each for the three RTA temperatures) where the polarization monitored was normalized. One capacitor was subjected to PUND pulse waveform to monitor SS+ and OS- polarization whereas the other was subjected to NDPU pulse waveform to monitor SS- and OS+ polarization. Details of the retention measurement procedure can be found in Supplementary Figure S4. In most cases, no discernible retention loss was observed, which is most likely due to the polarization states being close to full saturation (Mueller et al., 2012c). However, a small retention loss was detected in the capacitors annealed at 500°C, which may be attributed to a larger depolarization field in the HZO thin film. This field results from a greater proportion of the non-polar tetragonal phase formed at such a lower annealing temperature (Mehmood et al., 2019). Although the HZO capacitors suffered from imprint (Supplementary Figure S5), the findings from the retention tests hold great potential for neuromorphic computing applications.

FIGURE 4

FIGURE 4. Retention tests of the different HZO capacitors performed at 25, 85, and 125°C.

To compare the performance of fabricated HZO capacitors with previous literature featuring large 2P_r values in doped HfO₂ capacitors after RTA, a radar chart showing the impact of various experimental conditions was generated, as shown in Figure 5. Generally, the values of each parameter increase from the center towards the perimeter of the chart. However, the electric field (MV cm⁻¹) and RTA temperature (°C) axes were inverted so that the desirable values are located towards the edge, just like other performance parameters. In other words, a low operating voltage and thermal budget are desired. In their recently published work, Kashir et al. (2021) reported a similar 2P_r of ∼ 64 μC cm⁻² while annealing the HZO capacitors at 700°C for 5 s. Employing a 3 V - 1 kHz triangular pulse PUND technique for the endurance tests, they were only able to achieve a maximum of 10⁴ cycles before encountering breakdown. To improve the leakage behavior of their HZO capacitor, they inserted a Pt layer between the W and HZO interface, which increased the potential barrier and resulted in a lower current density. Zacharaki et al. (2019) utilized an epitaxial HZO thin film grown on germanium to achieve a 2P_r of 61.2 μC cm⁻². Despite a promising achievement, they were only able to cycle their epitaxial HZO capacitors for 10³ cycles, after which the leakage current increased significantly and resulted in a device breakdown. Their best endurance was reported to be over 10⁵ cycles with a reduced 2P_r at ∼ 40 μC cm⁻² under an electric field of 2.3 MV cm⁻¹ Schroeder et al. (2018) reported decent endurance results at 10⁹ cycles in lanthanum-doped hafnia (HLO) with a La concentration of over 10 cat% but with a trade-off in 2P_r. After performing RTA at 800°C, their HLO capacitors with a maximum 2P_r of 55.4 μC cm⁻² were able to achieve 2 × 10⁵ endurance cycles at 4 V - 100 kHz. Remarkably, the fabricated HZO capacitors in this work, specifically those which were annealed at 700°C, have either performed similarly or outperformed the previous works with better endurance while preserving a large 2P_r for the endurance measurements performed at 111 kHz frequency.

FIGURE 5

FIGURE 5. Performance comparison of this study with other works (Schroeder et al., 2018; Zacharaki et al., 2019; Kashir et al., 2021) with high 2P_r.

Conclusion

In summary, our fabricated HZO-based ferroelectric capacitors demonstrated a 2P_r value as high as 64 μC cm⁻², an endurance of 100 million cycles, and promising retention performance. We discovered that the 2P_r value increased with the RTA temperature up to 700°C. We also emphasized that the frequency of the applied PUND voltage scheme in measuring the polarization is crucial to avoid overestimated results. Our HZO ferroelectric films with a large remnant polarization and great reliability characteristics can potentially be used to engineer both synaptic and neuronal devices for neuromorphic computing applications.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions

JY conceived the concept. JY, YC, and SA designed the experiments. SA fabricated the devices and performed electrical and X-ray characterizations. JP carried out RTA. YZ wrote a custom code for endurance and retention measurements. TM, RM, DG, JL, QW, MB, SG, RK, YC, QX, and JY helped with experiments and data analysis. SA, JP, JY, YC, and RK prepared, reviewed and edited the manuscript. All the authors participated in discussing the results and commented on the manuscript at every stage.

Funding

This work was partially supported by the Air Force Office of Scientific Research (AFOSR) through the Multidisciplinary University Research Initiative (MURI) program under contract no. FA9550-19-1-0213 and the United States Air Force Research Laboratory (AFRL) under grant no. FA8750-18-2-0122 and FA8650-21-C-5405. This work was also partially supported by the National Science Foundation under contract no. 2023752. The opinions, findings, and conclusion, or recommendation expressed in this material are those of the authors and do not necessarily reflect the views of AFRL.

Acknowledgments

RK appreciates the faculty start-up fund from UMass Amherst.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2022.969188/full#supplementary-material

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