Wake-up in a Hf0.5Zr0.5O2 film â a cycle-by-cycle emergence of the

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Wake-up in a Hf Zr O film – a cycle-by-cycle emergence of the remnant polarization via the domain depinning and the vanishing of the anomalous polarization switching Anastasia A Chouprik, Maxim Spiridonov, Sergei Zarubin, Roman Kirtaev, Vitalii Mikheev, Yury Lebedinskii, Sergey Zakharchenko, and Dmitrii Negrov ACS Appl. Electron. Mater., Just Accepted Manuscript • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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ACS Applied Electronic Materials

Wake-up in a Hf0.5Zr0.5O2 film – a cycle-by-cycle emergence of the remnant polarization via the domain depinning and the vanishing of the anomalous polarization switching

Anastasia Chouprik,* Maxim Spiridonov, Sergei Zarubin, Roman Kirtaev, Vitalii Mikheev, Yury Lebedinskii, Sergey Zakharchenko, and Dmitrii Negrov

Moscow Institute of Physics and Technology, 9 Institutskiy lane, Dolgoprudny, Moscow Region, 141700, Russia

KEYWORDS: ferroelectric hafnium oxide, wake-up process, anomalous polarization switching, oxygen vacancies, band excitation piezoresponse force microscopy, crystallographic texture, mechanical coupling in PFM

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Abstract The mechanism of the remnant polarization (Pr) growth during the first stage of ferroelectric HfO2based memory cell operation (the wake-up effect) is still unclear. In this work, we reveal the microscopic nature of the Pr growth in functional ferroelectric capacitors based on a polycrystalline 10-nm-thick (111) out-of-plane textured Hf0.5Zr0.5O2 film during electric cycling. We observe the cycle-by-cycle evolution of the domain structure with the piezoresponse force microscopy (PFM). During the early stage of the wake-up, three types of domains are found: (i) normal domains (polarization aligned along the applied electric field), (ii) non-switchable domains with upward and downward polarization, (iii) domains with anomalous polarization switching (polarization aligned against the applied electric field) that are commonly surrounded by non-switchable domains. Initially, non-switchable and “anomalous” domains are 200-300 nm in width, and they occupy ~70% of the capacitor area. During electric cycling, these domains reduce in area, which is accompanied by the Pr growth. We attribute the domain pinning and the anomalous polarization reversal to the internal bias field of the oxygen vacancies. The correlation of the PFM data with both the results of the structural analysis of fresh and cycled Hf0.5Zr0.5O2 film by transmission electron microscopy and the performance of the ferroelectric capacitor indicates that after the first cycle of the wake-up, the Pr growth is not associated with phase transformations, but only with the transformation of the domain structure. The obtained results elucidate the physical mechanism of the emergence of Pr during first switching cycles of ferroelectric HfO2-based memory cell.

Introduction The HfO2-based ferroelectric (FE) films have recently emerged as viable candidates for application in nonvolatile memory devices,1 because of their full compatibility with the modern Si-based technology. At standard conditions, the stable phase of HfO2 is non-polar monoclinic (m) P21/c phase. At high temperatures non-equilibrium non-polar tetragonal (t) P42/nmc and cubic Fm3m 2 ACS Paragon Plus Environment

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phases could be stabilized.2 In thin (3-30 nm) films doped with different materials (Al, Si, Y, Gd, La, Zr) the parent t phase can convert to a non-centrosymmetric orthorhombic (o) phase Pca21 during thermal processing. The capping by the top electrode plays an important role in ferroelectricity of HfO2.3,4 During the first thousands of cycles of operation of FE HfO2-based memory cells, the remnant polarization usually grows significantly (so-called “wake-up” effect5). The wake-up was observed both in hafnium oxide with different dopants (Si,6 Gd,7 Sr8) and for the Hf1-xZrxO2 system.9 Fresh HfO2-based capacitors usually demonstrate constricted hysteresis. Antiferroelectric (AFE)-like behavior (backswitching) observed at the first stage of the wake-up has been explained by the built-in fields due to a non-uniform distribution of charged defects and defect dipoles,10,11 but the genuine AFE contribution cannot be excluded due to the presence of the t-phase.12,13 The presence of the charged defects or the pinning sites has been considered at the interfaces in a ferroelectric doped-HfO2 thin film sandwiched between TiN and Pt electrodes.6 The charged defects are attributed to the oxygen vacancies/ions because HfO2-based devices are known to form vacancies due to redox reaction with electrode material.14-22 It was shown that during the alternating current (ac) electric field cycling a redistribution of the oxygen vacancies, which are initially accumulated at the interfaces, occurs.23,24 As a result, the built-in bias field decreases and become more uniform. The reduction of the defect concentration near the electrodes leads to the depinning of the domains, which did not contribute to remnant polarization initially. Further, Grimley et al.13 revealed the reduction of the tetragonal layer at the interfaces and explained this reduction by a decrease of the barrier for the transformation from a tetragonal phase to an orthorhombic phase due to the redistribution of oxygen vacancies within the ferroelectric layer. Lomenzo et al.25 and Kim et al.26 also suggested the t→o phase transition as the origin of the wake-up effect based on a concurrent increase of Pr and a decrease of the dielectric constant. In addition, with the transmission electron microscopy study Grimley et al.13, Pešić et al.24, and Martin et al.27 found the phase transition from the m- to the o-phase as a result of ac cycling. Moreover, by means of the P-V measurements and 3 ACS Paragon Plus Environment


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the piezoresponse force microscopy (PFM) study it was strongly suggested that most part of the phase transformation took place during the very first voltage pulses.13,28 Therefore, the reduction of the local charge density (or the redistribution of the interface-localized charges into the bulk) leads to: 1) a reduction of the pinning sites, 2) a phase transformation to an o-phase resulting in an increase of the volume fraction taking part in switching process, i.e. an increase of the measured remnant polarization, and 3) a reduction of the built-in fields resulting in the hysteresis opening. It should be noted that the constricted hysteresis is not the fingerprint of the wake-up in hafnium oxide. In particular, the initial accumulation of vacancies at the one interface leads to a unipolar built-in field and the alignment of as-grown domains in one direction.25 Fresh structures can demonstrate open hysteresis with strong imprint. Moreover, in this case, with insignificant content of a non-polar phase in the as-prepared film the remnant polarization does not increase during electric cycling. Therefore, the initial distribution of charged defects in the stack and the reduction of their local density during the wake-up affects the performance of the memory device. Understanding the role of the charged defects in the wake-up mechanism is crucial for the improvement of ferroelectric devices performance, and, in particular, for the increasing of their endurance. In addition, using PFM, it was found that the cycled structures with relatively low remnant polarization Pr = 17 C/cm2 demonstrated piezoelectric activity in every point of the studied area except the domain walls.28, 29 The grains span the whole film thickness13,28,30,31 and, considering high coercive field ~ 1 MV/cm, which is specific for FE HfO2, the domain structure is a singlelayer. Meanwhile, the maximum calculated remnant polarization of HZO is equal to 50 C/cm2.32 These data indicate that the value of the remnant polarization would strongly depend on the orientation of the o-grains of HZO film crystallized during thermal processing. Engineering of the orientation texture paves the opportunity to improve the reading charge of FE memory devices.33 4 ACS Paragon Plus Environment

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In this work, we revealed that the Hf0.5Zr0.5O2 (HZO)-based capacitor with (111) out-of-plane texture demonstrated remnant polarization Pr ~ 28 C/cm2 after the wake-up. As from the second cycle of the wake-up process, the remnant polarization growth is not associated with the phase transformation, but only with the transformation of the domain structure. The evolution of the domain structure of HZO during the wake-up process showed the presence of normal domains, static (non-switchable) domains and domains with anomalous polarization switching. As from the second operation cycle, the reduction of the fraction of “anomalous” and static domains and the increasing of the fraction of normal domains result in the growth of the remnant polarization. Experimental The 10-nm-thick HZO films with the 18-nm-thick bottom TiN electrode were grown with an atomic layer deposition (ALD) technique on the Si substrate (Supporting Information, Section S1). Top Pt electrodes 15 nm in thickness were grown with a pulsed laser deposition technique. Crystallization of the as-grown HZO films occurred during rapid thermal annealing in Ar at T = 500oC. To allow external electric biasing of the samples, the functional ferroelectric capacitor devices 120x120 μm2 were patterned on the Si chip and routed to the contact pads (details of the device fabrication are described in the Supporting Information, Section S1). The schematic view of the samples used for the microscopic study by means of the PFM technique are shown in previous work.28 To wake-up fresh HZO film, the ferroelectric capacitors were cycled 104 times by applying bipolar voltage double triangular pulses with ±3 V amplitude and 60 ms duration with the waveform generator/fast measurement unit (Supporting Information, section S3). Simultaneously, the I-V curves were measured. The P-V curves clear from dielectric response contribution were obtained as a difference between the integrated transient switching and non-switching current generated by



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the first and second unipolar pulses, respectively. Further, for simplicity the pair of unipolar pulses is called the pulse. The structural properties of fresh versus electrically cycled HZO films in the capacitors were investigated by transmission electron microscopy (TEM). For details, see Supporting Information, Section S2. A microscopic study of the ferroelectric properties of the HZO films was performed with an offfield resonance-enhanced band-excitation (BE) PFM technique,34,35 which is implemented using commercially available AFM Ntegra (NT-MDT) with specialized in-house built digital signal processor (Nanoscan Technologies). The electrical excitation of the ferroelectric layer was performed with the following waveform parameters: the central frequency near the contact resonance frequency ~670 kHz, the bandwidth 97.7 kHz with 1000 frequency bins, the peak-topeak value of exciting voltage was 0.6 V. To minimize the contribution of the parasitic electrostatic tip-surface interactions36,37 the PFM experiments were carried out at the patterned capacitors routed to the contact pads. Both the top electrode and probe were grounded, whereas switching and excitation voltage were applied to the bottom electrode. The experimental scheme and the details of the BE PFM implementation were described earlier.28 Results and discussion An averaged selected-area electron diffraction (SAED) patterns of a fresh and cycled TiN/HZO/Pt stack, which are the sum of SAED patterns acquired over the length of the stack in the TEM sample (approximately 5 µm for the fresh stack), are shown in Figure 1a and S2 and they confirm the polycrystalline nature of both fresh and cycled HZO films. The diffraction rings from polycrystalline TiN layers as well as point reflexes from the monocrystalline Si substrate were used for precise calibration. The presence of polycrystalline t- and/or o-phases is evident in both fresh and cycled structures. Considering the ratio of the o111/t101 and monoclinic m𝟏𝟏𝟏 and


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m111 grains, we conclude that the content of the m-phase was insignificant in the as-prepared capacitors and did not reduce after electric field cycling.

Figure 1. (a) An averaged SAED pattern of the fresh TiN/HZO/Pt stack. Dotted ovals highlight bright areas on the diffraction rings that demonstrate the film texture. (b) Heteroepitaxial Pt and HZO grains.

Bright areas in 111 rings parallel to the growth direction g and ones in 200 rings inclined by 54.7° to g highlighted with the dotted ovals demonstrated strong (111) out-of-plane texture of Pt and TiN layers in the fresh stack. The HZO film exhibited (111) and (200/002) out-of-plane texture. Also, the presence of heteroepitaxial Pt and HZO grains was detected in high-resolution TEM (HRTEM) images (Fig. 1b, S1b). It is well known that the size of grains of metal films depends on the growth temperature.38 During annealing, the coarsening of the grains occurs, because the grain boundaries migrate to reduce the total grain boundary energy.39 Usually, secondary grains have their atomic planes that provide the minimal surface energy oriented parallel to the substrate40-42 (e.g., for metals with a face-centered 7 ACS Paragon Plus Environment


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cubic lattice this is the (111) surface). The recrystallization and the grain growth in metal films are observed at temperatures that are considerably lower than those in the corresponding bulk materials.40 Therefore, observed texture of metal electrodes corresponds to the expected texture. Further, it was reported that a (111)-textured tetragonal ZrO2 film can be grown epitaxially on a (111)-textured Pt layer.43-45 Considering the structural similarities between ZrO2 and HfO2 as well as the bottom TiN and top Pt electrodes texture, a (111)-textured HZO film is expected to be crystallized during rapid thermal annealing. Being formed, the o-phase with (111) out-of-plane orientation would have the remnant polarization equal to 57.7% of the maximum remnant polarization along the c-axis of the o-phase.3 Considering the maximum remnant polarization of HZO 50 C/cm2,32 it is expected that the (111)-textured HZO film would have the remnant polarization ~ 30 C/cm2. The residuals of a non-polar phase and the o-phase grains with another orientation would contribute to the total value of the remnant polarization. To evaluate the remnant polarization Pr the cycled TiN/HZO/Pt capacitors were subjected to standard electrical characterization (Supporting Information, Section S3). “Relaxed” I-V curves measured during the first voltage pulse train were shifted with respect to “excited” I-V curves measured during further cycling irrespective of the polarity of the first pulse (Figure 2). By “excited” I-V curves, we mean those obtained at further electric field cycling with the second and subsequent voltage trains. We associate the shift of the I-V curve with the relaxation of traps that were driven out of equilibrium during electric field cycling (for some details, see the Supporting Information in our previous work28). However, we cannot exclude the influence of the charge diffusion or the depolarization field. In any case, the “relaxed” HZO-based structures are always under investigation both in TEM and in PFM. The characteristic time of the relaxation of the studied structures is ~1 s. The “relaxed” I-V curves and P-V hysteresis loop clear from dielectric response contribution are shown in Figure 2. Switchable polarization 2Pr ~ 56 C/cm2 was obtained for the “excited” structure, whereas the relaxed structure had lower remnant polarization 2Pr ~ 52 C/cm2 due to both AFE-like contribution and trapped charge. 8 ACS Paragon Plus Environment

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Figure 2. I−V curves and P−V hysteresis loop clear from dielectric response contribution. The mean coercive voltage for the “relaxed” cycled capacitors for upward and downward polarizations was found to be −1.75 V and 0.95 V, respectively. The observed imprint is directly explained by the contact potential difference (CPD) of electrodes, which is always the case for the asymmetric structure.3 In addition, the heteroepitaxy of Pt and HZO grains with significantly different lattice constant results in the mechanical strain gradient and, therefore, the local built-in field due to the flexoelectric effect.10,46 The residual asymmetry in the charged defects distribution across the ferroelectric layer41,47 and the non-filamentary redox process48 which is non-equivalent at the TiN and Pt interfaces, can also cause an imprint effect. The “relaxation” of coercive voltage strongly increased after thermal conditioning. The imprint test after baking the structure at 125 oC showed the appearance of the internal bias field of ~ 1.5 MV/cm (Figure S4). At the similar temperature conditions for TiN/HZO/TiN capacitors Fengler et al.49 reported the internal bias field ~0.2 MV/cm and explained this field by the charge diffusion process. In any case, this result indicates that the charge concentration for the studied capacitors was large compared with those of Fengler et al. A microscopic study of the ferroelectric properties of the cycled HZO films was performed with BE PFM. Large triangular and polygon grains are visible on the top electrode surface (Figure 3f).



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Such morphology is typical for the Pt film with (111) out-of-plane texture grown at elevated temperature33 or annealed,37 which corroborates with the TEM data.

Figure 3. PFM data for the cycled TiN/HZO/Pt structure: (a), (b) amplitude maps after application of the voltage pulse with -3 V and 3 V amplitude, respectively (the blue and orange eclipses represent the pieces of the uniform phase maps); (c) contact resonance frequency map; (d), (e) amplitude- and phase-frequency characteristics corresponding to points 1 and 2 in (a) and (b), respectively; (f) Pt surface morphology. In (a), (b) orange circles indicate the regions with permanently low piezoresponse, whereas orange arrows indicate the regions with permanently high piezoresponse. Ib. blue ellipses indicate the regions with a local built-in field. In (d), (e) the green and blue lines correspond to the experimental data, whereas the black and grey lines correspond to the fitting curves.

After application of the voltage pulse with -3 V and 3 V amplitude, the PFM phase for the cycled TiN/HZO/Pt structure was uniformly switched over the whole scan area (insets in Figure 3a, 3b), which corresponds to downward and upward polarization. The comparison of the amplitude maps reveals the regions with a low piezoresponse with the upward polarization (Figure 3b, marked by 10 ACS Paragon Plus Environment

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the blue ellipses) and the same regions with a higher piezoresponse at the downward polarization (Figure 3a, marked by the blue ellipses). These regions should be associated with a local built-in field resulted in the imprint at the P-V curves (Figure 2b). In addition, the regions with permanently low and high piezoresponse (marked by orange circles and arrows, respectively) can be observed. The regions with a permanently high piezoresponse in the given structure (for example, point 2 in Figure 3a, b, spectrum in Figure 3d, e) can be attributed to the (002) out-of-plane-oriented o-phase grains or (111) o-phase grains aligned closer to the vertical axis. Due to the mechanical coupling by means of the passive layer of the top electrode, the residual grains of the non-piezoelectric phase and the misaligned polar phase grains would looks as low piezoresponse regions. The PFM phase in these regions would be equal to the PFM phase of the surrounding domains (for example, point 1 in Figure 3a, b, piezoresponse spectra in Figure 3d, e). Indeed, by modelling of the mechanical coupling in the given ferroelectric capacitor (Supporting Information, Section S4) it was found that the non-piezoelectric grains up to ~40 nm in diameter and the domains with opposite polarization up to 20 nm in diameter would look like pits in the amplitude map, whereas the PFM phase would remain uniform. The measured value of the pit diameter in comparison with the actual diameter of the non-piezoelectric region would be higher by ~40 nm. Therefore, the pits in Figure 3a, 3b with the measured diameter of 50-70 nm corresponded to the passive region ~1030 nm in the actual diameter, e.g., individual grains of non-piezoelectric phase. The nonswitchable domains would contribute to the imprint effect and would manifest themselves as regions marked by blue ellipses in the amplitude map. Therefore, the individual m-phase grains, which were revealed with the TEM, were unobservable in the PFM phase maps and can only be suspected in the PFM amplitude maps due to their low concentration in the cycled HZO-based structure. The contact resonance frequency maps (Figure 3c) were fully associated with the morphology and were used here and further for compensation of the thermal drift that distorted the scans.



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The domain structure evolution during the wake-up of the cycled TiN/HZO/Pt ferroelectric capacitors was studied by local piezoresponse mapping after an alternative application of the pairs of the unipolar triangular voltage pulses with ±3 V amplitude and 60 ms duration (Figure 4). In the first instance, the piezoresponse was mapped for the fresh structure (Figure 4c). Due to a very weak piezoresponse, the vector averaging of the 400 response spectra was performed in every point of the scan. Averaged spectra in the point 1 is shown in Figure 4b. The signal-to-noise ratio in these amplitude-frequency curves was very low. The similar piezoresponse maps at the same excitation and acquisition parameters were obtained for amorphous HZO-based capacitor (Supporting information, Section S5). Therefore, we concluded that the obtained PFM maps for the fresh structure should be attributed mainly to the pseudo-piezoresponse due to the electrostatic interaction of the capacitor plates, because both dc field due to CPD at closed-circuit conditions and ac field due to excitation voltage were applied.50 Indeed, the pseudo-piezoresponse of the amorphous HZO decreased when the compensating dc voltage was additionally applied (Supporting information, Section S5). It should be noted that the bottom part of the amplitude map for the fresh structure (Figure 4a) and the whole amplitude map for the amorphous HZO (Figure S7a) were associated with the contact resonance frequency map (Figure 4a, S7c), e.g., with morphology. However, the top of the amplitude map in Figure 4a contained the mixture of the morphology and some other contribution, presumably, due to the developing domains with upward polarization. Considering extremely prolonged scanning of the fresh structure (20 hours) in the closed-circuit conditions, we associate the development of the response in the fresh structure with a slow drift of oxygen vacancies in the CPD field. It is well known that a layer of the t-phase exists at the electrode interface of fresh HfO2-based capacitor.11 We suppose that the initial pseudopiezoresponse was caused by the reduction of voltage across the ferroelectric layer due to the dividing of the applied ac excitation voltage by this thick dead (t) layer at the interface (Supporting Information, Section S6). Similar to the ac electric cycling with the switching voltage with the ±3


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V amplitude, the lower dc CPD field ~ 0.5 MV/cm resulted in the redistribution of oxygen vacancies51 and a local reduction of the barrier for the transformation from the t- to the o-phase.

Figure 4. The very first stage of the wake-up of the TiN/HZO/Pt capacitor: (a) PFM contact resonance frequency map corresponding to (c); (b) experimental (orange and blue) and fitted (black) amplitude- and phase-frequency characteristics corresponding to points 1 and 2 in map (c) and (d); PFM amplitude and phase maps for the fresh structure (c), after 0.5 cycle (d), after 1 cycle (e), after 1.5 cycles (f).

After the very first pulse -3 V the piezoelectric activity strongly increased in some regions (Figure 4d, dark regions on the amplitude hills is due to the overload in the acquisition system), and the second value of PFM phase appeared in these regions (Figure 4d). However, this value of the PFM phase is abnormal because it corresponds to the upward polarization at the cycled structure (inset in Figure 3b) that should be caused by positive switching voltage. It was recently suggested that the presence of non-ferroelectric regions in FE HfO2 can cause the anomalous PFM phase due to injected and trapped charges.29 However, a high piezoresponse in the regions of the anomalous PFM phase indicates the genuine ferroelectricity. Moreover, the injected charge in the capacitor based on 10-nm-thick HfO2 had to relax eventually. Meanwhile, the piezoresponse maps were stable for more than 2 days (except the minor evolution of the fresh structure described 13 ACS Paragon Plus Environment


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above). In addition, the time of the relaxation of P-V hysteresis loop was ~1 s (Figure 2, S4 and Supporting Information in28]). Therefore, we believe that the anomalous PFM phase was caused by the anomalous polarization switching. After the next pulses with 3 V amplitude (1 cycle) and -3 V amplitude (1.5 cycles), the whole area, except the domain walls, showed a high piezoresponse (Figure 4e, f) and the binary PFM phase. The I-V curves measured during switching of the capacitor before the PFM mapping are shown in Figure 5a, c. The significant increase of piezoresponse indicate the reduction of the dividing dead layer at the interfaces (Supporting information, Section S6). Therefore, we suppose that in our experiment the t→o phase transition occurred during the very first cycle of the wake-up process. Meanwhile, Lomenzo et al.25 and Kim et al.27 previously have found that the dielectric constant k decreased gradually with electric switching cycle and associated this phenomenon with gradual phase transition from the t- to o-phase. However, the absence of the saturation of C-V curves at high voltage may indicate the contribution of the domain walls to the measured capacitance (the C-V curves are presented in Supporting information, Section S7). This assumption confirmed by the evolution of the domain structure during electric cycling (Figure 5). However, we do not exclude another mutual contributions of t→o phase transition and domain transformation during the wake-up process with shorter pulses. To analyze the cycle-by-cycle evolution of the domain structure, we overlapped the PFM phase maps (Figure 5), preforming their alignment using the contact resonance frequency maps, so that topographical features coincided. The PFM phase map after the very first voltage pulse was not considered, because the part of scan corresponded to the pseudo-piezoresponse. In Figures 5, 7 for every pair of the overlapped PFM phase maps, the map obtained after +3 V was put under the maps obtained after -3 V of the same cycle (the legend of the used colors is presented in Figure 5b).


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At the first stage of the wake-up three types of the domains were observed: i) normal domains (colored by violet in Figure 5), in which the PFM phase was switching correctly, ii) static (nonswitchable) domains with downward and upward polarization (blue and orange, respectively), iii) “anomalous” domains (colored by sienna), in which the PFM phase was switching oppositely. The “anomalous” domains were surrounded by static domains (predominantly with upward polarization), but composite halos were also present. During electric field cycling, the content of the “anomalous” domains was gradually reducing with substituting by the static domains with upward polarization, and after 1000 cycles the “anomalous” domains disappeared. The I-V curves shown in Figure 5 correspond to the first (green) and second (blue) triangular voltage pulses. In addition to the I-V curves with two distinct peaks measured at the first voltage pulse, the second I-V curves also had the polarization switching peak caused by the AFE-like contribution, which were there due to the built-in fields and strong imprint. During electric field cycling the area of the static domains with upward polarization first increased due to substitution of the “anomalous” domains and then, after 10 cycles, gradually reduced (Figure 6c). This process accompanied by the reduction of the AFE-like contribution in I-V curves. At the later stage of the wake-up (after ~100 cycles) the reduction of static domains prevails. Two distinct current peaks at the first I-V curves moved toward each other during bipolar cycling, quickly merging. Since the P-V curve is the integral of the difference of transient currents at first and second pulses, this behavior represents a transition of the initially pinched hysteresis loop to an open hysteresis. The saturated polarization of the “relaxed” capacitor increased during electric field cycling (Figure 6a).



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Figure 5. The evolution of the domain structure of the TiN/HZO/Pt capacitor and I-V curves measured during the switching of the capacitor before PFM mapping: (a) the very first I-V curves as a legend for I-V curves in (c)-(q); (b) the legend of the colors of domains in c-q; (c-q) overlapped PFM phase maps obtained at the same cycle after indicated pulses with opposite polarity and the appropriate I-V curves. Green I-V curves correspond to the P and N pulses, whereas blue ones correspond to the U and D pulses. Numbers near I-V curves indicate the number of switching cycles. 16 ACS Paragon Plus Environment

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Figure 6. (a) Evolution of the ferroelectric remnant polarization during bipolar cycling (±3 V amplitude and 30-ms pulse slopes) measured at “relaxed” HZO-based ferroelectric capacitors; P, U, N, D mark the integrals of total transient switching current and of transient current during second pulse at positive and negative applied voltage, P-U, N-D mark the differences; in inset the dependence of the measured remnant polarization on the pulse voltage sweep is shown; (b) Evolution of the content of normal, “anomalous” and static domains with both polarization during cycling by the same pulses.

The dependence of the domain structure on the switching time was studied. Two cycles of voltage pulses were applied to the fresh capacitor. Then it was preset by 5 pairs of 60-ms pulses with amplitude of -3 V, and the piezoresponse was mapped. Further, it was switched by one pair of 60ms pulses with amplitude of 3 V (Figure 7a) and then pair-by-pair of the positive pulses was applied sequentially. After each pair of the positive pulses the piezoresponse was mapped and overlapped with the PFM phase map of the preset structure (Figure 7b-e).



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Figure 7. The evolution of the domain structure of the TiN/HZO/Pt capacitor during continuing voltage stress: the PFM phase maps obtained at (a) 1, (b) 2, (c) 3, (d) 4, (e) 7 pairs of voltage pulses of 3 V overlapped with PFM phase map obtained after voltage pulse of -3 V; the colors corresponds to the legend in Figure 5b; (f) Evolution of the content of normal, “anomalous” and static domains with both polarization during continuing voltage stress.

The domain structure stabilized suddenly after 4 pair of voltage pulses (Figure 7f). The content of the “anomalous” domains decreased from 44% to the 20%, mainly due to their transformation into static domains. The content of the normal domains increased from 11% to 18%. These results indicate that the remnant polarization would depend on switching voltage duration. Indeed, the increased values of the remnant polarization were obtained at longer voltage pulses (inset in Figure 6a). In addition, these results mean that the value of remnant polarization obtained by the P-V measurement (Figure 6a) cannot precisely match to the PFM data (Figure 6b). Indeed, the measured Pr value corresponds to the difference of the charges flowed at the consecutive voltage (P,U and N,D) pulses, whereas the PFM maps corresponds the domain structure after the full P,U or N,D pair of pulses. 18 ACS Paragon Plus Environment

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The off-field BE piezoresponse force spectroscopy (PFS, the voltage train is presented in Supporting Information, Section S9) in normal, static and “anomalous” domains was measured at three different capacitors, which were preset by 2 cycles, on the same chip. The rectangular single 10-ms voltage pulses were applied during measurements. Considering the evolution of the structure during the voltage application, the PFM spectroscopy data cannot match Figure 5 precisely due to different form and the duration of switching voltage pulses. However, the obtained piezoresponse spectroscopy curves are informative qualitatively. The piezoresponse was equal to -A cos where A is resonance PFM amplitude,  is non-resonance PFM phase at low frequency of the phase-frequency characteristics (left-hand-side phase values in Figure 3d,e, 4b). The raw A and  data are presented in Figure S12.

Figure 8. Local piezoresponse loops in (a) normally switching regions, (b) non-switching regions and (c) regions exhibiting anomalous switching. The local piezoresponse loops measured in the regions of the “anomalous” domains (Figure 8c) are out-of-phase by 180° relative to the loops measured in the normally switching area (Figure 8a). This means that after passing the voltage pulse with threshold amplitude the polarization of the grain under the AFM tip aligned in the direction opposite to the applied field and stayed this way after the field is turned off. We suppose that anomalous polarization switching occurred during falling slope of pulse at V → 0.



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In the non-switching regions the PFM phase did not switch. There are two possible reasons of this behavior, which cannot be resolved: 1) genuine non-switching of the studied grain, 2) switching during rising slope of switching pulse with further backswitching during falling slope. The PFM amplitude changed at coercive voltage due to the mechanical coupling with the neighbor switching regions. In the normal domains the piezoresponse decreased smoothly in the range of coercive voltage (Figure 8a) also indicating the mechanical coupling with the neighbor regions. In contrast, in the “anomalous” domains the PFM loop shown jumping behavior (Figure 8a, S12c). Both the counterclockwise rotation and the self-crossing of the PFM loop close to loop`s tips were specific for the “anomalous” domains. In general, the jumping behavior of the PFM amplitude in the range of coercive voltage may indicate the possible parasitic contribution during measurement, in particular, the parasitic phase shift. For example, any electrostatic interaction in the AFM experiment may lead to the phase deviation53 and apparent phase jumps due to wrap around. This effect is specific for the particular case of the cantilevers with small force constant. However, we exclude this parasitic contribution because measured phase took only two values which are exactly 180o apart both in the static PFM maps and on spectroscopy curves which not the case in the presence of electrostatic contribution. Moreover, we used the stiff cantilevers with force constant 3.5 N/m. Therefore, we believe that the measured phase corresponds directly to the direction of the local piezoresponse. Due to specific features of the HZO-based stack, some possible reasons for the “anomalous” PFM switching should be considered: i) the reduction of the charge density (both in the bulk of o-grains and at the grain boundaries, due to both the charge trapping-detrapping and the drift of the oxygen vacancies), ii) flexoelectric effect, iii) ferroelastoelectric switching. Regarding (i), the “anomalous” PFM switching and its evolution during electric cycling can be caused by the complex interplay of the following phenomena: the charge trapping-detrapping, the 20 ACS Paragon Plus Environment

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migration of the oxygen vacancies, and the reduction of mixed phases at the interfaces. Recently, Fengler et al.49,52 demonstrated that at room temperature only minor vacancy diffusion is possible and that higher temperatures are necessary to observe oxygen vacancy movement. Our imprint test (Figure 2 vs. S4) confirmed these conclusions. Therefore, we suppose that the charge trappingdetrapping play the dominant role in anomalous switching. The total electric field Etot in the HZO layer was composed by the applied external field Eext, the local imprint field Eimp, and the internal bias field of charged oxygen vacancies Ev: Etot = Eext + Eimp + Ev (Figure 9c). The imprint field was composed by constant and uniform field of CPD ECPD ~0.5 MV/cm: Eimp = ECPD.



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Figure 9. Sketch of possible explanation of the PFM wake-up results: (a) band diagrams at different parts of cycle; evolution of the domain structure and the electric field redistribution 22 ACS Paragon Plus Environment

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during the wake-up: (b) at the relaxed state at V = 0 after V < 0, (c) at V < 0, (d) at the excited state at V = 0 after V < 0, (e) at the excited state at V = 0 after V > 0, (f) at V > 0, (g) at the relaxed state at V = 0 after V > 0.

During the application of the pulse -3 V (Eext) the following processes were expected to occur (Figure 9c): 1) normal domains were switching to the downward polarization (colored by blue), 2) the field of the charged traps near TiN interface (associated with positively charged oxygen vacancies) compensated Eext and, probably, the charged traps slightly moved toward the TiN interface, 3) the charged traps near Pt interface were excited, i.e., they captured negative charge carriers and were neutralized (Figure 9a). At low field (in subcase 0 V, Figure 9d) in the area above the vacancies the field Ev was not compensated by the Eext + Eimp and caused the switching of the domains in the upward direction, i.e., aligned against previous external field. Indeed, at a given coercive field ~ 1 MV/cm, dielectric constant ~30 and a typical geometry of the o-grain in the studied HZO films (10 nm x 10 nm x 10 nm) ~10 elementary charges at the interface (corresponding to the local volume concentration of ~1019 cm-3, which is typical for HfO254) could reverse the polarization of the single o-grain. Indeed, it was previously suggested that the initial accumulation of the vacancies at the one interface leads to a unipolar built-in field and the alignment of the as-grown domains in the one direction.25 After some characteristic time (> 1 s for our capacitors) the traps at the Pt interface were relaxed (Figure 9b). The field of these charged traps was aligned along previous external field and did not cause additional switching. The domain structure was preserved. The similar process occurred at V > 0 (Figure 9f, e, b). The opposite (anomalous) switching manifests itself at the I-V (Figure 5) and P-V curves as an AFE-like contribution at low voltage. In the PFM phase maps (Figure 5), these regions looked like “anomalous” domains. During the wake-up, the area of “anomalous” domains and the AFE-like contribution reduced. 23 ACS Paragon Plus Environment


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The static domains localized in the regions between normal and “anomalous” domains and looked like non-switchable regions in the PFM phase maps. We assume that one part of these regions contribute to the I-V and P-V curves, whereas another part don’t contribute to these curves. Indeed, the central region above the oxygen vacancies did not switch (Figure 9c, d), whereas the right region undergone the backswitching and, therefore, contributed to the AFE-like of the I-V and P-V curves. During first stage of the wake-up, the static domains initially substituted the “anomalous” domains. At the later stage of the wake-up, the static domains transformed into normal domains. Therefore, initially the area of the static domains increased and then reduced. Time stability of the piezoresponse maps indicate that in the absence of the external field the energy of repulsive interaction of the charges was insufficient to overcome the migration energy barrier. Therefore, the charges did not spread along the ferroelectric layer by themselves. They have possibility to spread horizontally only due to drift from their locations caused by the applied external field. Electric field cycling serves to repetitively move the oxygen vacancies toward the interfaces and, therefore, gives repetitively them some freedom to spread. The dc field would result in single short-time motion followed by freezing the charges at the interface without possibility to move. As a result of cycling, the local density of the vacancies reduced during the wake-up, the internal bias field reduces and normal and static domains replace the static and “anomalous” domains correspondingly. The evolution of the domain structure of the TiN/HZO/Pt capacitor during continuing voltage stress can be also explained in the frame of suggested model (Supporting Information, Section S8). The continuing migration of the vacancies during the voltage application (Figure S10a vs. S10c) leads to consolidation of the charge closer to the interface and, simultaneously, to its scattering along the ferroelectric layer. Therefore, the vacancy field becomes weaker. Static domains substituted the “anomalous” domains, whereas normal domains substituted static domains (Figure S10b vs. S10d). This suggestion corroborates with accelerated wake-up at longer cycling pulses (inset in Figure 6a). 24 ACS Paragon Plus Environment

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The PFS data is also in agreement with the suggested model. Indeed, the off-field piezoresponse force spectroscopy data was obtained locally, i.e., each PFS curve corresponded to the polarization switching of the single grain under the AFM tip. The piezoresponse was excited and measured after passing the switching voltage pulse at zero dc voltage. We suppose that the normal polarization switching in the normal regions occurred during switching pulse or its rising slope, while the anomalous polarization switching in the “anomalous” regions occurred during the falling slope of the switching pulse or at zero bias (Figure S11). Considering the typical grain size ~10 nm and the mechanical coupling by the top electrode, the PFM would not allow revealing whether the charge was consolidated in the bulk of grains or at the grain boundaries. However, the typical lateral size of the anomalous switching regions ~100 nm indicates that the grain boundaries have no considerable contribution. We assume that the oxygen vacancies were formed during both the reduction reaction with TiN electrode14, 22 and the oxygen diffusion along Pt grain boundaries during thermal processing.16, 55 The observed nonuniformity can be caused by the non-uniform oxidation of the TiN interface due to different grain orientation as well as the localization and different transparency of Pt grain boundaries. We suppose that the mechanisms of the cycle-by-cycle local charge density reduction and, therefore, the origin of the wake-up is much more sophisticated than in the suggested model. We do not exclude the drift of the non-neutralized charges across FE layer at high applied voltage and the diffusion of the charges. Indeed, the dependence of the saturated polarization on the velocity of the voltage sweep during the wake-up (inset in Figure 6b) can indicate the presence of mobile defect states in transition metal oxides.23,54,56,57 Using the Mott-Gurney equation Starschich et al.23 estimated the velocity of the oxygen vacancies of 1.3 · 10-5 m/s and a migration distance of 6.5 nm during each cycle at room temperature. In our experiment, the significant frequency dependence of the saturated polarization was observed at lower frequencies (inset in Figure 6a), which can indicate the slower charge drift.



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Further, decrease of the vacancy concentration due to chemical reaction with electrodes during cycling can be also the reason of the charge density reduction. In addition, the interplay between the local coercive field of the grains and the instant local charge density should strongly influence on the domain structure and its evolution. The anomalous polarization reversal was previously observed for the FE single crystals58,59 and films.60-63 For the bare FE surface this behavior was explained by the charge injection at the AFMtip-enhanced field.58-61 For the FE capacitors it was also attributed to the charge injection62 and to the charge compensation effect at the boundaries of the oblique grains with specific structure.63 HZO-based capacitors shown some specific features of anomalous polarization switching: 1) the reduction of the fraction of the anomalous switching during electrical cycling, 2) significant frequency dependence of the remnant polarization, 3) minor charge diffusion at room temperature, 4) supposed local volume density of the oxygen vacancies of ~1019 cm-3, which is typical for HfO2.54 Considering these features, we believe that the oxygen vacancies and charge trapping play a dominant role for the anomalous polarization switching, whereas the local reduction of the vacancy density results in domain structure evolution. (ii) Another possible reason for the anomalous polarization switching is the flexoelectric effect.10 The oxygen vacancies induce local crystallite-volume expansion.64 The localization of the charged oxygen vacancies at the bottom and the top interfaces following negative and positive voltage pulses (in the same way as in Figure 9), respectively, leads to the upward and downward mechanical strain gradient and, therefore, to the upward and downward internal electric field. Thus, during the wake-up both at the P-V hysteresis and in the PFM maps, the flexoelectric effect would manifest itself in exactly the same way as direct switching of the domains by charges of the oxygen vacancies. In addition, in both heteroepitaxial Pt-HZO and TiN-HZO grains, a lattice mismatch of the FE and metal films could result in strain relaxation within the height of the HZO grain, inducing a large 26 ACS Paragon Plus Environment

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local upward and downward strain gradient, respectively, and complex behavior of switching during the wake-up. In particular, the mechanical strain gradient can facilitate or hamper the oxygen vacancy migration.65 The effect of the strain relaxation is constricted spatially by the single grain size and is localized in individual heteroepitaxial grains, i.e., not in groups of grains that constitute the domains in Figures 3-5. Therefore, it is expected that the effect of heteroepitaxy gets lost due to the mechanical coupling by the top electrode. (iii) Ferroelastoelectric switching66 due to the coupling between the applied electric field and the mechanical stress can also cause the domain inversion to the direction against the applied field. The antiparallel polarization alignment is favorable at the compressive strain in FE HZO, which is the case of heteroepitaxial Pt-HZO and TiN-HZO grains. This effect is also highly localized and would be non-detectable in the capacitor geometry of the studied sample. The reduction of the area of anomalous switching during the wake up could occur during the reduction of compressive stress in the grains, for example, due to the accumulation of the oxygen vacancies at the relaxed part of the grain. Such concurrence is unlikely. Meanwhile, the mechanical stress can lead to the self-crossing of the PFM loop close to the loop`s tips,66,67 which was observed in the regions of the anomalous switching (Figure 8c). However, the increased piezoresponse observed in the piezoelectric loops in the range of coercive voltages was previously reported to be associated with purely ferroelectric, 180° domain wall switching contributions.67,68 The last mechanism describes the effect of the non-linear behavior of susceptibility due to the transient domain wall motion during the polarization reversal on the macroscopic piezoelectric coefficient. It is also known that the electric field arising from incomplete compensation of the bound charge on the domain walls is able to enhance the dielectric and the piezoelectric response of the ferroelectrics with the phase transition of first order and a strong dielectric anisotropy.70 The enhancement of the piezoelectric response can manifests itself at the range of coercive fields.



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However, considering the fact that the enhanced piezoresponse was specific only for the “anomalous” domains and the microscopic nature of the PFM loops, we suppose that this effect was caused by a change of the total electric field due to the contribution of the oxygen vacancies field. Therefore, in addition to Eimp, which existed for normal domains too, the non-compensated field of space charge Ev was during the PFS measurement leading to involuntary local on-field measurements.36,37 Conclusions With the selected-area electron diffraction technique we revealed the (111) out-of-plane texture of an o/t phase at thin (10 nm) ferroelectric polycrystalline HZO layer, which corresponds to the same texture of TiN and Pt electrodes of a functional ferroelectric capacitor. The remnant polarization ~28 C/cm2 of the cycled capacitor corresponds to the value expected for (111) out-of-plane texture. The content of m-phase in the fresh HZO was small and did not reduce after the wake-up. Therefore, the m→o phase transition did not contribute to the remnant polarization growth during electric field cycling of the TiN/Hf0.5Zr0.5O2/Pt stack. The local piezoresponse of the HZO film in the fresh ferroelectric capacitor was very weak, which was attributed to the presence of the thick dead (tetragonal) layer dividing the ac excitation voltage. The piezoelectric activity of the HZO strongly increased after the first cycle of the wake-up. Therefore, we concluded that t→o phase transition occurred during the first switching cycle and remnant polarization growth during further electric cycling was attributed to the domain structure transformation. During the first stage of the wake-up process, three types of domains were observed: (i) normal domains (polarization aligned along the applied electric field), (ii) static (non-switchable) domains with upward and downward polarization, (iii) domains with anomalous polarization switching (polarization aligned against the applied electric field) that were commonly surrounded by the domains. Initially, non-switchable and “anomalous” domains are 200-300 nm in width, and they 28 ACS Paragon Plus Environment

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occupy ~70% of the capacitor area. During the electric field cycling the static and “anomalous” domains reduce their area. Specifically, at first cycles the static domains replaced the “anomalous” domains and at further cycles the normal domains replaced the static domains. This phenomenon was accompanied by the remnant polarization growth. We assume that the anomalous local polarization reversal was caused by the internal bias field of the charged oxygen vacancies at the both Pt and TiN interfaces. At the boundary of the anomalously switching area the charges had lower concentration, and their internal bias field results to the non-switching and back-switching of this region. The reduction of the density of the oxygen vacancies due to their redistribution along and/or across ferroelectric layer resulted to the reduction of the internal bias field, and, therefore to the reduction of the “anomalous” and static domains and the growth of the remnant polarization. Considering our previous wake-up data28 for fully ALD stack TiN/HZO/TiN with Pr ~ 17 C/cm2, we conclude that the role of the oxygen vacancies in the performance of the HfO2-based capacitors is huge and, therefore, the wake-up process is not the pure intrinsic effect of FE HfO2. The further study of the correlation between the performance of FE HfO2-based capacitors (Pr, texture) and the wake-up process (velocity, evolution of the domain structure, and Pr) is of great scientific interest and paves an opportunity to develop the memory devices with optimal functional characteristics.

ASSOCIATED CONTENT

Supporting Information. Additional information and figures related to the growth and electrophysical characterization of the ferroelectric TiN/Hf0.5Zr0.5O2/Pt devices, the technique of the collecting of the SAED patterns, the results of the modelling of



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mechanical coupling, the C-V data, the sketch of the electric field redistribution during continuing voltage stress and the details of PFS measurement are presented in the supporting information file (PDF). Corresponding Author *E-mail

address of the corresponding author: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS This work was performed using equipment of MIPT Shared Facilities Center with financial support from the Russian Foundation for Advanced Research Projects and the Ministry of Education and Science of the Russian Federation (Grant No. RFMEFI59417X0014). TEM study was supported by the Russian Science Foundation (Project No. 14-19-01645-P). Part of the work was carried out using equipment of VNIIOFI Shared Facilities Center for High-Precision Measuring in Photonics (ckp.vniiofi.ru). ABBREVIATIONS HZO, Hf0.5Zr0.5O2; ALD, atomic layer deposition; PR, piezoresponse; PFM, piezoresponse force microscopy; PFS, piezoresponse force spectroscopy; BE, band-excitation; FE, ferroelectric; TEM, transmission electron microscopy, SAED, selected-area electron diffraction; CPD, contact potential difference. 30 ACS Paragon Plus Environment

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