Thermally Stable and Radiation Hard Ferroelectric Hf0.5Zr0.5O2 Thin

Article Cite This: ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

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Thermally Stable and Radiation Hard Ferroelectric Hf0.5Zr0.5O2 Thin Films on Muscovite Mica for Flexible Nonvolatile Memory Applications Wenwu Xiao,† Chen Liu,† Yue Peng,‡ Shuaizhi Zheng,*,† Qian Feng,‡ Chunfu Zhang,*,‡ Jincheng Zhang,‡ Yue Hao,‡ Min Liao,*,† and Yichun Zhou† †

Downloaded by UNIV AUTONOMA DE COAHUILA at 00:47:16:578 on May 27, 2019 from https://pubs.acs.org/doi/10.1021/acsaelm.9b00107.

Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, School of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, People’s Republic of China ‡ State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi’an 710071, People’s Republic of China S Supporting Information *

ABSTRACT: Metal−ferroelectric−metal (MFM) capacitors on flexible substrates are promising for flexible nonvolatile memory applications, while the insufficient scalability of perovskite-based ferroelectric thin films and the difficulty of direct integration of high performance ferroelectric thin films on current conventional flexible substrates are the most serious obstacles to their practical applications. Meanwhile, performance under harsh conditions (such as high temperature and high total ionized dose (TID) radiation) is highly demanded due to the growing applications for nonvolatile memory. Here, we integrate highly scalable ferroelectric Hf0.5Zr0.5O2 (HZO) thin films on potential flexible mica substrates using atomic layer deposition (ALD) to form flexible MFM capacitors and investigate the ferroelectric properties, the retention behaviors, and endurance characteristics of the TaN/HZO/TaN/mica flexible MFM capacitors under various tensile and compressive bending radii. In addition, these characteristics of the HZO-based MFM flexible capacitors are explored in a wide temperature range from 25 up to 125 °C. The ferroelectricity of the film can be retained under a bending radius down to 7.5 mm after 1000 bending cycles, and the retention properties can be reserved under a bake time of 104 s at 125 °C, with an extrapolated retention time-to-failure longer than 10 years. Furthermore, the flexible devices display robust ferroelectric performance against radiation of 60Co γ-rays with a total dose of 1 Mrad (Si). Our work represents a critical step in HfO2-based ferroelectric memory implemented on mica toward flexible nonvolatile memory operated under harsh conditions. KEYWORDS: ferroelectric, Hf0.5Zr0.5O2 (HZO), flexible, mica, thermally stable, radiation hard

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dose (TID) radiation7) desired for applications is gaining interest. Furthermore, conferring flexibility to the electronics could dramatically enhance those applications for the emerging Internet of Things (IoT) fields.8,9 To realize this aim, one crucial aspect is to fabricate the device on a flexible substrate. In the development of flexible devices, PZT ferroelectric capacitors were demonstrated on platinum (Pt) foil by using the transfer technique. However, Pt is not suitable for monolithic integration, and the achieved devices were not examined under harsh conditions.10 Besides, PZT ferroelectric memories were fabricated on ultrathin flexible silicon (Si) substrates using soft-backside etching, focusing on harsh environment applications, yet the bending radius was 1.25 cm with the experimented devices, since an anomaly in

INTRODUCTION Ferroelectric random access memories (FeRAMs) are nonvolatile ferroelectric memories that use ferroelectric films to store information. FeRAMs have attracted considerable attention because of their nonvolatility, low power consumption, high operation speed, and almost unlimited read/ write endurance.1−3 Although mass production of FeRAMs that use lead zirconium titanate (PZT) integrated on rigid bulk silicon substrate commenced in 1992,4 they are restricted to a niche market mainly including industrial applications, factory automation equipment, measuring equipment, power meters, bank terminals, and so on.5 To take full advantage of FeRAMs, commercial vendors are putting great effort on developing new markets, where FeRAMs have found much broader applications in automotive systems, robotic systems, medical devices, contactless smart cards, and food industry. As FeRAMs are becoming more widely used, the demand for them is expected to increase; especially their performance under harsh conditions (such as high temperature6 and high total ionized © XXXX American Chemical Society

Received: February 21, 2019 Accepted: May 10, 2019

A

DOI: 10.1021/acsaelm.9b00107 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Electronic Materials

Figure 1. (a) GIXRD patterns of the as-fabricated and the annealed TaN/HZO/TaN/mica structures. (b) TEM image and (c) HRTEM image of the annealed structures; the inset of (c) shows the enlarged HRTEM image of the selected area marked by a red frame. (d) P−V hysteresis loops between 1 and 105 bipolar switching cycles. (e) Retention and (f) endurance of the annealed TaN/HZO/TaN/mica flexible capacitor.

in HfO2 films brings great opportunities to resolve the bottlenecks.5,24−28 HfO2-based thin films are the most widely used gate dielectrics for sub-45 nm (CMOS) technologies, and they can maintain obvious ferroelectricity even with the thickness scaled down to 3 nm.29 Recently, the ferroelectric HfO2-based thin film was grown on plastic polyimide substrate and manifested for the first time its application in flexible nonvolatile memory devices.30 However, the thermal degradation of the polyimide substrate resulted in additional effort in fabrication and inhibited further improvement in ferroelectricity of the ferroelectric films. In this work, we report a fabrication of ferroelectric Hf0.5Zr0.5O2 (HZO) thin films on mica substrates by atomic layer deposition (ALD) for flexible nonvolatile memory applications. The effects of bending on the performance such as polarization−voltage (P−V) properties, retention behaviors, and endurance characteristics of the TaN/HZO/TaN/mica flexible MFM capacitor were investigated. In addition, the thermal stability of the TaN/HZO/TaN/mica flexible MFM capacitor was also examined in a wide temperature range from 25 up to 125 °C. Furthermore, the flexible capacitor was demonstrated to be stable under radiation with a dose of 1 Mrad (Si). Our work represents an important step in HfO2based ferroelectric memories implemented on mica toward flexible electronic devices operated under harsh conditions.

retention behaviors occurred at a smaller radius at room temperature.11,12 Alternatively, because of the relatively small thickness of ferroelectric thin films, if deposited on flexible substrates, these devices become naturally flexible. Therefore, naturally flexible plastic substrates have been adopted. However, PZT’s crystallization temperature (>600 °C) is much higher than the melting temperature of most polymeric organic substrates; thus, the PZT film was built on silicon and then transferred to polymeric flexible substrates. Despite that stable operation under an 8 mm bending radius could be achieved, operation under harsh conditions was not explored.13 On the other hand, ferroelectric polymers14−16 or organic crystals17−19 on the plastic substrates have been demonstrated, while the thermal and operation stability limit their application.20 In fact, the thermal budget of the plastic substrate itself may become a serious obstacle in the applications under high temperature. Interestingly, PZTbased ferroelectric memories were built up on flexible muscovite mica, exhibiting robust operation against mechanical stress, temperature evaluation, and radiation exposure.21,22 Therefore, mica with the key advantages of atomically smooth surface, high thermal stability (e.g., the melting temperature over 1000 °C), chemical inertness, and mechanical flexibility has provided a new platform for flexible inorganic electronics.21−23 Although the PZT-based flexible memory presents a valuable concept, however, they usually contain heavy metals and are not environmental friendly.24 Moreover, they suffer from limited complementary metal−oxide−semiconductor (CMOS) compatibility and insufficient scalability, which result in the bottlenecks on memory density and cost for FeRAM development.5,25 Significantly, the discovery of ferroelectricity

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RESULTS AND DISCUSSION For HfO2-based thin films, the emerging ferroelectric performance is attributed to a noncentrosymmetric phase. Therewith, a structural characterization for the films is conducted with grazing incidence X-ray diffraction (GIXRD), and the results are shown in Figure 1a. Apparently, annealing at 500 °C B


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Figure 2. (a) P−V hysteresis loops. (b) Retention and (c) endurance of the annealed TaN/HZO/TaN/mica flexible capacitor at various temperatures.

switching cycles, we started using automatic pulse program to flip the ferroelectric domain under a pulse amplitude of 3.5 V and a frequency of 1 kHz and added the P−V test after completing the specified number of bipolar switching cycles. To clearly describe the test process, the test sequences are listed in Figure S1. As shown in Figure 1d, the well-saturated ferroelectric hysteresis loops could evidently advocate the structural analyses. In addition, as the bipolar switching cycles increased to 105, an expanding of the polarization and coercive field could be observed, which is termed the “wake-up” effect and is common in HfO2-based ferroelectric thin films.25 Furthermore, a remnant polarization (Pr), 2Pr ∼ 21 μC/cm2, and a coercive electric field (Ec), 2Ec ∼ 2.1 MV/cm, can be extracted from the saturated P−V hysteresis loop after 105 bipolar switching cycles. The 2Pr value is comparable to the general results of HZO films on rigid substrates30,32,33 and ferroelectric polymers on flexible substrates.20,30,41,42 It should be noted that the 2Pr value is related to many factors, such as film thickness, capping electrode, and annealing condition; thereby, further optimization would provide opportunities for improved ferroelectricity of the films. For practical applications in the field of FeRAM, HfO2-based ferroelectric thin films should achieve superior performance on the data retention and endurance. Figure 1e displays retention properties of the capacitors that are programmed/erased at 3.5 V. These nonvolatile memory (NVM) cells exhibit excellent retention after 105 s, with an extrapolated retention time to failure longer than 10 years. Figure 1f illustrates the summarized results of the pulse amplitude dependence of the endurance cycling. It should be noted that the frequency (100 kHz) used here is different from that used in Figure 1d. The endurance test is performed by adding a continuous bidirectional flipping square pulse to the sample and inserting a pulse polarization test into it. Once the polarization value is measured, it will automatically enter the next stage of endurance until all the preset pulse cycles have been completed. The endurance test sequence is shown in Figure S2. Obviously, the endurance cycles decrease when the pulse voltage increases from 2.5 to 4 V. In the endurance test, the HZO ferroelectric thin films usually broke down, especially at higher voltages. The pulse amplitude dependence of the endurance process is also observed in TiN/HZO/TiN ferroelectric capacitors on silicon substrates.43 For a voltage of 3.5 V, the 2Pr slightly decreases from 32.4 to 28 μC/cm2 after 105 bipolar switching cycles, while only 59% (19 μC/cm2) is observed after 106 bipolar switching cycles. This endurance

induces an obvious peak at 30.5°, which is absent in the asfabricated HZO film, suggesting that the film is crystallized after the annealing process. In general, this peak could be ascribed to the mixed phases of cubic, tetragonal, and orthorhombic in HfO2 thin films. However, because of their similar lattice parameters, it is difficult to unambiguously and quantitatively identify these phases.31−40 Among these phases, the orthorhombic phase is considered to be the cause of ferroelectricity.31−40 Particularly, during thermal annealing process, the ferroelectric orthorhombic phase could be stabilized or promoted in HfO2-based films, and this process normally requires high crystallization temperature. For HZO films, annealing at ∼500 °C is required to achieve feasible ferroelectricity.24 As mentioned before, polyimide was utilized as the substrate for flexible HfO2-based films. However, even after optimization of the annealing process, temperatures higher than 430 °C could not be applied, and prolonged annealing at 380 °C for 150 s resulted in a decrease in the ferroelectricity of remnant polarization.30 Besides, HfO2 films doped with elements other than Zr demand an even higher temperature to attain ferroelectricity.24 Rationally, insufficient annealing to form the ferroelectric phase would weaken the polarization.30 Consequently, mica with high thermal stability proves to be a promising substrate for high temperature induced orthorhombic phase formation in HfO2-based thin films. Furthermore, given the limit in GIXRD to distinguish the above-mentioned coexistent phases, transmission electron microscopy (TEM) is performed. Figure 1b shows the crosssectional TEM image. The MFM capacitor structure exhibits clear interfaces, and the HZO film thickness is ∼18 nm. Figure 1c illustrates the cross-sectional high resolution TEM (HRTEM) images, and the inset figure shows the enlarged HRTEM image of the selected area marked by a red frame. The lattice fringes of the selected HZO areas reveal the orthorhombic phase (111) plane with an interplanar distance of 2.94 Å, demonstrating the formation of the ferroelectric phase of the HZO film in the annealed TaN/HZO/TaN/mica structure. Combining these structural analyses, ferroelectricity could be predicted for the capacitor with the annealed HZO film. The polarization−voltage (P−V) hysteresis loops between 1 and 105 bipolar switching cycles of the annealed TaN/HZO/TaN/mica flexible capacitor under an applied voltage of 3.5 V are evaluated and displayed in Figure 1d. Because of the pulse frequency limitation to 1 kHz of the test equipment, we tested the P−V loops 10 times manually to obtain the data of 10 bipolar switching cycles. After 103 bipolar C


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Figure 3. (a) Flex-in/compressive strain mode and (b) flex-out/tensile strain mode of the thin film. (c) P−V hysteresis loops under various compressive bending radii. (d) P−V hysteresis loops under various tensile bending radii. (e) Retention and (f) endurance for the TaN/HZO/ TaN/mica capacitors in unbent and compressive and tensile bending conditions after 1000 bending cycles. (g) GIXRD patterns of the TaN/HZO/ TaN/mica structure before and after bending.

at a high temperature of 125 °C. This performance can benefit from HZO film quality and high-temperature resistance of the underlying mica.22,23 Figure 2c shows the endurance at different temperatures from 25 to 125 °C, and the test process is the same as that in Figure 1f. The degradation rate accelerates with the increasing cycling temperature, which is attributed to the enhanced oxygen vacancy defect generation and diffusion with elevated temperature as compared with that of the device cycled at a lower temperature.49 Figures 3a and 3b demonstrate the flex-in/compressive strain and flex-out/tensile strain modes, while Figures 3c and 3d illustrate the P−V characteristics of the HZO ferroelectric thin film under compressive stress and tensile stress, respectively. The 2Pr and 2Ec values show no significant change after 1000 bending cycles, and the 2Pr values are all higher than those of the HfO2-based ferroelectric films on polyimide.30 Furthermore, a slight increase in 2Pr is observed until the bending radius is further reduced from 12.5 to 7.5 mm. In consistence, Park et al. reported that large in-plane tensile stress can promote the formation of the orthorhombic phase,50 which may account for the observed increase in 2Pr

characteristics is comparable to that of the reported ferroelectric HfO2-based FeRAM.44 In fact, it is reported that with very high density of bits an endurance of 104 cycles could be sufficient for many applications.24 Moreover, by optimization on operation conditions, film thickness, and so on, it is possible to achieve an improved endurance.24,45 Figure 2a shows the P−V curves of the annealed flexible MFM capacitor at different temperatures from 25 to 125 °C. The ferroelectric capacitor demonstrates a 2Pr ∼ 18 μC/cm2 and a 2Ec ∼ 2.1 MV/cm at 25 °C. With elevated temperatures, a slight positive shift in the coercive field for P−V loop appears. Probably, higher temperature is more helpful to stimulate electron−hole pairs, oxygen vacancies, and subsequent charge trapping, which lead to a built-in field. Consequently, the builtin field affects the voltage imposed on ferroelectric domains and thus induces the voltage shift.46−48 Therefore, the slight shift may suggest a low built-in field due to the large bandgap and stability of the HZO film in the capacitor. The temperature dependence of the retention is also examined in this work. These flexible MFM capacitors sustain excellent retention, as shown in Figure 2b, even after 104 s at 25 °C and D


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Figure 4. (a) P−V hysteresis loops. (b) Retention and (c) endurance of the annealed TaN/HZO/TaN/mica flexible MFM capacitors under tensile and compressive bending of 7.5 mm for 1000 bending cycles after radiation. (d) GIXRD patterns of the TaN/HZO/TaN/mica structure before and after radiation.

whereas bending beyond 2000 cycles caused severe and even complete loss of ferroelectricity.30 Moreover, metallographic micrographs of the flexible capacitor after different bending cycles with a bending radius of 7.5 mm are shown in Figure S5. There is no obvious cracking or peeling off after 700 bending cycles, while after 900 bending cycles, some cracking and peeling off start to appear. Importantly, according to Hussain et al., the mechanical stress of nominal strain is given by εnominal = t/2R, (t is substrate thickness and R is the bending radius), which indicates an inverse proportionality between the substrate’s thickness and its flexibility.12 Thus, reducing the substrate thickness may help to further improve the bending performances of the film.12,21 Figure 3f displays stable endurance of the capacitors irrespective of physical bending states and radii up to 105 bipolar switching cycles, and the test process is the same as that in Figure 1f. For instance, with a bending radius of 7.5 mm, the 2Pr value decreases slightly from 35 to 32 μC/cm2 after 105 bipolar swithcing cycles. After 106 bipolar switching cycles, the polarization starts to decrease dramatically. Noteworthy, the flexible capacitors show similar fatigue behavior when without mechanical stress and under various temperatures; i.e., in all conditions 2Pr values can be kept without obvious decrease after 105 bipolar switching cycles, and dramatic degradation appears after 106 bipolar switching cycles. After bending, the structure is further

when the bend radius is reduced to 7.5 mm. In addition, for the capacitor under bending state with 7.5 mm radius after 1000 bending cycles, the P−V hysteresis loops at an applied voltage of 3.5 V between 1 and 105 bipolar switching cycles were measured and are shown in Figure S3. The test process is the same as that in Figure 1d. Similarly, an expanding of polarization and coercive field with the increasing bipolar switching cycles could be observed. Figure 3e shows the retention of the annealed TaN/HZO/TaN/mica flexible capacitors in unbent, flex-in/compressive, and flex-out/tensile strain modes. Here, the applied voltage is fixed at 3.5 V, and the bending radius is changed from 10 to 7.5 mm. Noteworthy, the flexible capacitors exhibit excellent retention in both compressive and tensile bending conditions even after 1000 bending cycles. Furthermore, under bending conditions, the mica-based flexible capacitor could still reserve excellent retention properties after 104 s, with an extrapolated retention time to failure longer than 10 years. Additionally, for the capacitor under bending state with 7.5 mm radius after 1000 bending cycles, a test with an extended retention time of 105 s was carried out, and it still shows no polarization degradation, as shown in Figure S4. However, retention is not available with the flexible HZO film on polyimide after bending, and it was reported that with a bending radius of 8 mm the ferroelectricity endures repetitive bending up to 1000 cycles, E


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ACS Applied Electronic Materials Table 1. Properties of Several Flexible Ferroelectric Memory Devicesa ferroelectric material

HZO

HZO

PZT20

PLZT

BLT

BFOMnTi

PZT48

P(VDF-TrFE)

substrate deposition technique thickness of ferroelectric material (nm) Pr (μC/cm2) Ec (kV/cm)

mica ALD 18

PI ALD 30

mica PLD N/A

mica PLD 100

mica PLD 100−300

mica CSD 300

silicon sol−gel 280

PDMS spin-coating 200 340

∼11 1150

10 N/A

60 100

∼41 ∼290

∼10 ∼80

18 60

∼5.4 ∼1200

∼9 ∼415

retention (years) fatigue (bipolar switching cycles) minimum bending radius (mm) bending cycles

>10 108

N/A N/A

>10 >1010

>105 s 1010

>10 109

66 Ec+ ∼ 844, Ec− ∼ 276 105 s 109

N/A N/A

7.5

5

2.5

5

1.4

2

>10 109 (unbent), >1010 (bent) 5

N/A

N/A 1000 at 5 mm 2000 ns 175 N/A 22

1000 at 5 mm N/A N/A 5 23

10000 at 1.4 mm N/A 200 N/A 21

1000 at 4 mm

1000 at 5 mm

N/A

N/A

N/A N/A N/A 54

500 ns N/A N/A 11, 12

N/A N/A N/A 55

N/A N/A N/A 56

switching time working temp (°C) radiation dose (Mrad (Si)) reference

Si

a

PZT20: PbZr0.2Ti0.8O3; PLZT: Pb0.9La0.1(Zr0.7Ti0.3)O3; BLT: Bi3.25La0.75Ti3O12; BFOMnTi: Bi(Fe0.93Mn0.05Ti0.02)O3; PZT48: Pb1.1Zr0.48Ti0.52O3; P(VDF-TrFE): poly(vinylidene−trifluoroethylene); PI: polyimide; PDMS: polydimethylsiloxane; ALD: atomic layer deposition; PLD: pulsed laser deposition; CSD: chemical solution deposition.

properties of the HZO thin film on flexible mica seem to play a decisive role in the ferroelectric performance of the capacitor. The structural analysis with GIXRD after irradiation and mechanical stress is shown in Figure 4d. Clearly, the peak at 30.5° associated with orthorhombic phase is reserved, indicating that the irradiation does not induce significant phase transition, which keeps pace with the previous GIXRD obtained after bending. To show the progress in flexible ferroelectric memories, comparisons of some basic parameters are made between this work and several reported ones,11,12,21−23,30,54−56 as shown in Table 1. It should be noted that our work represents a critical step in HfO2-based ferroelectric memory implemented on mica toward flexible nonvolatile memory operated under harsh conditions. The thickness of HfO2-based ferroelectric films is beneficial when compared to those traditional inorganic and organic ferroelectrics, with which the polarization of ferroelectric films tends to degrade as the thickness is scaled down. Moreover, HfO2-based ferroelectric films could be grown by the highly matured atomic layer deposition (ALD) technique, which enables the fabrication of devices with 3D structures or on curved surfaces.31,57

characterized by GIXRD, and the results are demonstrated in Figure 3g. Similarly, the peak at 30.5° stays nearly unchanged before and after bending. In combination with the P−V characteristics, the ferroelectric orthorhombic phase could be verified. The stable retention and endurance performance against mechanical stress may profit from the ultrathin thickness of the ferroelectric HZO film, which is also suggested in the flexible HZO-based capacitor with polyimide substrate.30 Therefore, the fabricated HZO NVM capacitors are demonstrated to be suitable for highly desirable flexible device applications. Figure 4a shows the P−V hysteresis loops of the TaN/ HZO/TaN/mica capacitor after exposing to 1 Mrad (Si) radiation. In addition, the ferroelectricity under compressive and tensile strain modes after radiation is explored. The values of the 2Pr and 2Ec are slightly larger than those of the pristine state, which is consistent with previous results in the bending test and may be attributed to the promotion from the large inplane tensile stress to form the orthorhombic phase.50 After 1 Mrad (Si) radiation, the ferroelectric properties do not show noticeable change regardless of the mechanical constraints. This stable performance is substantially better than traditional ferroelectric materials such as PZT and SBT reported so far51,52 and is consistent with the reported results of Y-HfO2based ferroelectric memory.48,53 Figure 4b displays the constant retention behaviors of the TaN/HZO/TaN/mica capacitors in unbent and bent states after 1 Mrad (Si) radiation. Deserving of emphasis, Figure 4c again exhibits the similar endurance characteristics to the above-mentioned results, with which serious degradation shows up after 106 bipolar switching cycles. Taking the endurance of the capacitor with a bending radius of 7.5 mm for an example, after radiation and then 1000 bending cycles, the 2Pr decreases from 32.4 to 30 μC/cm2 after 105 bipolar switching cycles, and it drops to 26 μC/cm2 (80%) after 106 bipolar switching cycles. On the basis of these observations, it is suggested that external factors as temperature, mechanical stress, and γ-ray radiation in this work have a negligible influence on the performance of the flexible HZO-based capacitor. Significantly, the intrinsic

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CONCLUSIONS In conclusion, the HfO2-based ferroelectric capacitor is demonstrated on a mica substrate toward the realization of the flexible nonvolatile memory. Promisingly, the flexible capacitor retains its stable electric performance against temperature elevation, bending stress, and radiation exposure in this work. Therefore, the intrinsic properties of the HZO thin film on flexible mica seem to play a decisive role in the ferroelectric performance of the capacitor. This work sheds light on the utilization of ferroelectric HfO2-based films for flexible nonvolatile memories applied under harsh conditions.

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EXPERIMENTAL SECTION

Mica substrates were sequentially cleaned with acetone, isopropanol, and ultrapure water in an ultrasonic bath for 10 min. Next, the 80 nm thick TaN was sequentially deposited on the substrate via reactive magnetron sputtering. Immediately after sputtering, the sample was F



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moved to an atomic layer deposition (ALD) chamber for growth of ferroelectric HZO films. [(CH3)2N]4Hf (TDMAHf) was used for Hf precursor, and [(CH3)2N]4Zr (TDMAZr) was used for Zr precursor. H2O was used for oxygen source in the ALD chamber. An alternative cycle formulation of HfO2 and ZrO2 is used to achieve a doping ratio of Hf:Zr = 1:1. After deposition of the HZO film, 100 nm thick TaN electrodes were deposited by reactive magnetron sputtering and patterned by photolithography and reactive ion etching techniques, and the top electrode area for the TaN/HZO/TaN/mica structures was 4 × 10−4 cm2. Finally, the whole device was annealed by using a rapid thermal annealing at 500 °C for 30 s in ambient N2. The ferroelectric properties of the TaN/HZO/TaN/mica were measured by an RT66A measurement system. A heating stage was used to change the measurement temperature to estimate the ferroelectric performance along with the temperature ranging from 25 to 125 °C. The structural properties of the HZO thin films were investigated by grazing incidence X-ray diffraction (GIXRD), and transmission electron microscopy (TEM). For the bending test, predesigned Teflon molds with different fixed bending radii were used to implement a flex-in mode (thin film under compression) and flexout mode (thin film under tension). It is noted that all bending results of the flexible MFM capacitor were measured under the bending state after 1000 bending cycles. The radiation source of this work is 60Co γrays, and irradiations were performed at dose levels: 1 Mrad (Si) with a radiation rate of 127 rad/s. During the radiation, TaN/HZO/TaN/ mica MFM flexible capacitors were in open circuit state, and γ-rays are from a uniform radiation field. It is noted that all irradiated results of the flexible MFM capacitor were measured after the absorbed radiation.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 51702273), the “Huxiang Young Talents Plan” Support Project of Hunan Province (Grant No. 2018RS3087), and the Science and Technology Innovation Project of Hunan Province (Grant No. 2017XK2048).

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaelm.9b00107.

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Article

Test sequences used for measuring the polarization− voltage (P−V) hysteresis loops between 1 and 105 bipolar switching cycles of the annealed TaN/HZO/ TaN/mica flexible capacitor (Figure S1); test sequences for endurance measurement of the annealed TaN/ HZO/TaN/mica flexible capacitor (Figure S2); P−V hysteresis loops of the TaN/HZO/TaN/mica flexible MFM capacitor under bending state with 7.5 mm radius after 1000 bending cycles (between 1 and 105 bipolar switching cycles) (Figure S3); retention of the TaN/ HZO/TaN/mica flexible MFM capacitor under bending state with 7.5 mm radius after 1000 bending cycles (Figure S4); metallographic micrographs of the flexible MFM capacitor after different bending cycles with a bending radius of 7.5 mm (Figure S5) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (S.Z.). *E-mail [email protected] (M.L.). *E-mail [email protected] (C.Z.). ORCID

Shuaizhi Zheng: 0000-0003-0209-7769 Chunfu Zhang: 0000-0001-9555-3377 Jincheng Zhang: 0000-0001-7332-6704 Author Contributions

W.X. and C.L. contributed equally to this work. Notes

The authors declare no competing financial interest. G


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Thermally Stable and Radiation Hard Ferroelectric Hf0.5Zr0.5O2 Thin

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