# Macro and nanoscale properties of (001)-oriented Bi0.5Na0.5TiO3 lead-free piezoelectric thin films grown by sputtering on LaNiO3/Si substrates

Saj Mohan Mohandas Moolayil, Arthur Hamieh, Antonio da Costa, Anthony Ferri, Rachel Desfeux, Denis Rémiens

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Saj Mohan Mohandas Moolayil, Arthur Hamieh, Antonio da Costa, Anthony Ferri, Rachel Desfeux, et al.. Macro and nanoscale properties of (001)-oriented Bi0.5Na0.5TiO3 lead-free piezoelectric thin films grown by sputtering on LaNiO3/Si substrates. Journal of Alloys and Compounds, 2024, 976, pp.172909. 10.1016/j.jallcom.2023.172909. hal-04343163

HAL Id: hal-04343163

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Submitted on 13 May 2025

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Macro and Nanoscale Properties of (001)-Oriented
Bi<sub>0.5</sub>Na<sub>0.5</sub>TiO<sub>3</sub> Lead-Free Piezoelectric Thin Films
Grown by Sputtering on LaNiO<sub>3</sub>/Si Substrates

Saj Mohan Mohandas Moolayil <sup>a,1,*</sup>, Arthur Hamieh <sup>b</sup>, Antonio Da Costa <sup>a</sup>, Anthony Ferri <sup>a</sup>,
Rachel Desfeux <sup>a</sup>, Denis Rémiens <sup>b</sup>

<sup>a</sup>Univ. Artois, CNRS, Centrale Lille, Univ. Lille, UMR 8181 – UCCS – Unité de Catalyse et Chimie du Solide, F-62300 Lens, France

<sup>b</sup>Univ. Polytechnique Hauts-de-France (UPHF), CNRS, UMR 8520, Institut d'Electronique, de Microélectronique et de Nanotechnologie (IEMN), Site de Valenciennes, F-59300, Valenciennes, France

Corresponding Author:

*Saj Mohan Mohandas Moolayil; Email: s.mohandasmoolayil@utwente.nl

Present Address:

<sup>1</sup>Industrial Focus Group XUV Optics, MESA+ Institute for Nanotechnology, Universiteit Twente, P.O Box 217, Enschede, Overijssel 7500 AE, The Netherlands
In this paper, the effect of the post-annealing temperature on the structural and electrical properties at macro and nano scale of Sodium Bismuth Titanate - Bi<sub>0.5</sub>Na<sub>0.5</sub>TiO<sub>3</sub> (BNT) films is reported. Stoichiometric BNT thin films were deposited by ex-situ radio frequency (rf) sputtering at substrate temperature of 200°C. The as-deposited films were amorphous and post-annealing treatment was necessary to crystallize the film in the perovskite phase. 400-nm-thick stoichiometric BNT films were annealed at various temperatures from 400°C up to 700°C. The BNT film annealed at 400°C remains amorphous. Film starts to crystallize at 450°C and then the crystallization increases with the post-annealing temperature to reach an optimal at 650°C without any secondary phase. Dielectric, ferroelectric and piezoelectric properties were also improved with the increasing of the post-annealing temperature. The piezoelectric coefficient d<sub>33eff</sub>, measured at macroscale, reaches a maximum value of 57 pm/V for 400 nm-thick film post-annealed at 650°C. On nanoscale, measurements performed by piezoresponse force microscopy are in perfect concordance with the performances obtained at macroscale. The efficient polarization reversal for domains and their retention when locally manipulated are observed, combined to a strong piezo-activity.

KEYWORDS: Lead-free, BNT, thin films, sputtering, annealing, electric properties at
macro/nanoscale, piezoresponse force microscopy.

**1. Introduction**

Lead based materials such as lead zirconate titanate Pb(Zr<sub>x</sub>Ti<sub>1-x</sub>)O<sub>3</sub> (PZT) and lead magnesium niobate-lead titanate PMN-PT in thin film form, have been widely investigated and used in
Micro Electro Mechanical Systems (MEMS) because of their excellent piezoelectric properties as can be ferrites, another class of oxide ceramic samples, for their remarkable dielectric properties [1-4]. However, due to the growing concerns about environmental issues caused by the toxicity of lead contained in the traditional piezoelectric materials, the development of lead-free piezoelectric materials have attracted increasing attention since 2000s [5,6]. In this way, many Bi-based oxides crystallizing in the perovskite structure have been investigated including multiferroic $BiFeO_3$-based or lead-free $(1-x)BiFeO_3-xBaTiO_3$ ceramics [7-12]. A strong development was also observed on potassium-based compounds like $(K,Na)NbO_3$ [13,14]. Among these lead-free piezoelectric materials, $(Bi_{0.5}Na_{0.5})TiO_3$ (BNT) is one of the promising candidate to replace the PZT in various applications like sensors, actuators and energy harvesters [15,16]. BNT has been widely studied in ceramics form and its properties are relatively known [17]. However, the synthesis of BNT in thin film form is still under development and has even attracted increasing interest over the past 20 years.

Indeed, BNT, unlike its homologue PZT, is not a solid solution of two compounds and can only exist in a single composition with Na/Ti and Bi/Ti ratios that are equal to 0.5 and any deviation from this composition results in degradation of the electrical properties [18,19]. Pure BNT and BNT-based thin film have been prepared by various chemical and physical methods including sol-gel, Pulsed Laser Deposition (PLD), and ex-situ/in-situ sputtering [20-22] while many others techniques exist such as electrochemical deposition, spray technique, atomic layer deposition, etc. [23-29], the main objective being to obtain a material at low temperature that exhibits excellent properties as well in terms of structure, microstructure, and electrical/magnetic properties. Although different types of processes have been used by many researchers, most of BNT films exhibit poor electrical performances associated with high conductivity [30]. These
results are mainly due to a non-stoichiometric composition generally caused by the volatility of Bi and Na during the growth processing or the annealing treatment at high temperature [31,32]. Thus, the synthesis and the control of the composition of BNT (exactly 50:50 for the Bi : Na ratio) has become a challenge to obtain BNT films with good electrical properties and in particular piezoelectric behavior. The same low temperature constraints exist for other Bi-based compounds like (Na₂O-Bi₂O₃-MoO₃), as reported by S-Z. Hao *et al*. in order to obtain optimal dielectric properties for highly integrated devices [33]. Typically, the deviation of the concentration of the original cations from a given value or oxygen stoichiometry can lead to a change in the charge state of the transition metal cations, which in turn will greatly change the magnetic and electrical parameters [34-39].

In our previous work, we have established a process to control the composition of BNT films using ex-situ rf magnetron sputtering [40]. With this procedure, called ex-situ process, the film is amorphous after deposition. We observed that, using a stoichiometric target, a Bi-excess is obtained in BNT film when it is deposited at 140°C (heated naturally by the plasma). Such an excess of Bi in the film was already showed regarding the growth of Bi₂212 high-Tc superconducting thin films from a Bi₂Sr₂CaCu₂O₈ stoichiometric target by PLD [41]. To reduce the Bi-excess present in the film, we took advantage of the high sensitivity of the sticking coefficient of bismuth with the temperature and we showed that a slight heating of the substrate during the deposition could accurately control the Bi-content in the deposited film. Therefore, when the substrate is heated at 200°C a stoichiometric BNT film is obtained. At this low growth temperature, the BNT films are amorphous and a post-annealing treatment is necessary to crystallize the BNT in the pure perovskite phase. In this context, in order to finely determine the
suitable temperature for optimized structural and electrical properties, a systematic study of the effects of the annealing temperature was carried out in the present work.

## 2. Experimental method

In this work, LaNiO₃ (LNO) bottom electrodes and BNT films were deposited by rf magnetron sputtering (Plassys MP600 system). (001)-oriented LNO bottom electrodes (200-nm-thick) were deposited on a silicon substrate (Si n-type silicon with (001) orientation) heated at 450°C and then annealed in air at 700°C to improve its conductivity. Details on the manufacturing process and properties of LNO are given elsewhere [42]. For BNT deposition, 3-inch stoichiometric BNT target was fabricated by cold pressure of mixed oxide powders (Bi₂O₃, Na₂Ti₃O₇ and TiO₂) with high purity (> 99.99% - NEYCO supplier). BNT thin films were sputtered on LNO/Si substrates in a pure Ar gas, at a pressure of 1.3 Pa, with a rf power of 100W. The substrate was fixed at a distance of 8 cm from the target and heated at 200°C. The choice of these parameters is discussed in a previous work [40]. After the deposition, the films are annealed in air with a ramp of 3°C/min from room temperature to the annealing temperature at which it was maintained for 1h.

The evolution of the crystalline structure of BNT film during the annealing was followed by in-situ high temperature X-ray diffraction (in-situ HT-XRD) analyses performed with a Rigaku Smartlab diffractometer using a heating plate (Anton Paar DHS 1100). The in-situ HT-XRD was carried out in an air atmosphere with a 5°C/min heating ramp from room temperature up to 750°C. The sample was analyzed every 50°C after a stabilization of 1 min. On the other hand, to determine the crystalline structure of annealed films, conventional X-Ray Diffraction (XRD) using Cu Kα radiation (θ - 2θ configuration) has been realized. The microstructure was performed
by scanning electron microscopy (SEM) with JEOL JSM-IT100 used at low voltage (5 kV). The surface morphology was investigated by atomic force microscopy (AFM) using a MFP-3D microscope (Asylum Research/Oxford Instruments) operating at room temperature and under ambient conditions. Energy Dispersive Spectroscopy (EDS) from Oxford Instrument was used to determine the chemical composition of BNT films.

For the electrical properties of BNT films at the macroscopic scale, circular Au top electrodes with a diameter of 150 µm were evaporated through a photoresist mask with a lift-off process. Dielectric constant and loss tangent were measured at 10 kHz using a HP4192A impedance analyzer. Ferroelectric properties and leakage current were analyzed using a TF analyzer 200 from aixACCT Systems. Piezoelectric responses were investigated using a homemade system based on a laser Doppler vibrometer (Polytec OFV-5000) dedicated to the measurement of the $d_{33\text{eff}}$ piezoelectric coefficient by a single beam method. A detailed description of this method is reported by Herdier *et al.* [43]. All electrical measurements were achieved at room temperature.

Local electrical properties were studied by using the piezoresponse force microscopy (PFM) technique available on the MFP-3D microscope. Pt/Ir-coated conductive tips (nanosensors PPP-EFM probes with a nominal ~ 2.8 N.m⁻¹ spring constant) were used. The ferroelectric domains pattern of as-annealed BNT film was revealed by PFM imaging. Switching properties of domains were also evidenced by PFM imaging after poling experiments performed over some square micron large areas at the free surface of the film by applying DC voltages of ± 20 V to the probe tip. Both local switching properties and local electromechanical activities were measured by recording in-phase and in-amplitude piezoelectric hysteresis loops using the spectroscopic tool of PFM. For such measurements, a driving voltage of 1 V was applied to the local probe while the PFM loops were recorded in the remnant mode to promote electromechanical activity against
electrostatic interaction [44,45]. To enhance the detected electromechanical signal, PFM
analyses were carried out at the contact resonance frequency of the cantilever by using the Dual
AC Resonance Tracking (DART) method [46]. The nanoscale electric performances of the
sample were given by both measuring the coercive voltage V<sub>c</sub> [V<sub>c</sub> = (V<sub>c</sub><sup>+</sup>+|V<sub>c</sub><sup>-</sup>|)/2] on the PFM
loops and by estimating the effective piezoelectric coefficient d<sub>33eff</sub> deduced from the
displacement measured at saturation on the in-amplitude PFM butterfly-shape loops. To obtain
such a d<sub>33eff</sub> quantitative value, the PFM probe was preliminary calibrated by using the GetReal
function in the Asylum Research software in order to get spring constant of the cantilever and
inverse optical lever sensitivity (InvOLS) at the same time. This is performed by scanning the
noise through the air to get the thermal resonance frequency of the cantilever when the tip is not
in contact. Then, our reference sample [i.e. Pb(Mg<sub>1/3</sub>Nb<sub>2/3</sub>)O<sub>3</sub>-PbTiO<sub>3</sub> piezoelectric thin film]
with known d<sub>33</sub> piezoelectric constant is probed to quantify the performance of the PFM setup.
The detected amplitude signal A is amplified while operating at the contact resonance through
the DART-PFM method; so, this needs to take into consideration the quality factor Q in the
relation A = d<sub>33eff</sub> x V<sub>ac</sub> x Q to access to the d<sub>33eff</sub> value. The Q value, given during experiment,
depends on the mechanical properties of the probed sample. On the other hand, to locally probe
electron-transport properties (i.e. conductivity, conductivity variations, leakage currents over the
surface of BNT film), the conductive AFM (c-AFM) technique was used. Current mappings
were recorded by applying a -5 V DC bias voltage between the grounded tip and the LNO
conductive bottom electrode. For these experiments, Ti/Ir coated-silicon tips with stiffness for
cantilever of ~ 2.7 N.m<sup>-1</sup> (Asylum Research ASYLEC-01 probes) were used.

**3. Results and discussions**
As already described in the introduction, the BNT films used for this work were produced using a particular ex-situ sputtering process [40]. Consequently, before annealing, these BNT films are stoichiometric (Bi/Ti = Na/Ti = 0.5) but amorphous. So, a post-annealing treatment was necessary to crystallize the BNT films in the desired perovskite structure with no secondary phases. In order to determine the optimal annealing temperature, the evolution of structural properties of BNT films during the heating was studied by an in-situ HT-XRD. Figure 1 presents the spectra obtained on BNT film during the heating at different temperatures. Until 400°C no change was observed and the film remains amorphous (spectra recorded for temperatures lower than 350° are not shown for clarity) while the LNO layer is perfectly well crystallized with a preferential orientation (001).

Above 400°C, the film starts to crystallize and a pure perovskite phase of BNT is formed. The crystallization of the film increases with rising temperature to reach a maximum at 650°C as shown in Figure 1. Above 650°C, the crystallization of the film decreases with increasing temperature.
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Evolution of structural properties of a BNT film as a function of the annealed temperature obtained by in-situ HT-XRD.</figcaption></figure>

The previous results show that BNT starts to crystallize at low temperature (~ 450°C) and that only the pure perovskite phase was formed over the entire studied temperature range. To confirm the influence of the annealing temperature on the structural and electrical properties and to determine the optimal annealing temperature, 400-nm-thick BNT films were annealed in air for 1 hour at temperatures between 400°C and 700°C. Figure 2 shows an example of the XRD patterns of the films. At 400°C, only LNO peaks are present which means that the film is still amorphous even for an annealing for 1 h at this temperature. The films annealed at a temperature of 450°C and above were crystallized in pure perovskite structure, i.e. without the appearance of secondary phases. Crystallized BNT films show a (001)-preferential orientation, the same orientation as the LNO bottom electrode. Similar to the HT-XRD results, the intensity of (001) diffraction peaks
increases as the annealing temperature is raised and the maximum of crystallization was obtained for an annealing at 650°C. As the film is amorphous at the beginning, the evolution of BNT peaks intensities reflects the gradual transformation of BNT from amorphous to crystallized phase with increasing annealing temperature. It is important to note that no secondary phases appeared whatever the annealing temperature. Therefore, to conclude these analyses it can be highlighted that when the film is stoichiometric before annealing, no element in the range of temperatures studied evaporates and only the desired phase is present.

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** XRD pattern of annealed BNT film at different annealing temperature.</figcaption></figure>

Figure 3-a shows the SEM image of the cross section and the surface of BNT films annealed at 650°C. The film exhibits a dense structure with a granular microstructure resulting from the good crystallization of the film at this annealing temperature. The grain size is in the order of 100 nm and we don't observe any variation between 650°C and 750°C. It is well known that the
effect of the average crystallite size and crystallite size distribution have a strong effect on the
dielectric properties of the obtained samples [47-52]. The size effect is evident for ceramic
material (bulk form) but in thin film form the effect is less pronounced. For example, in our case
the grain size is constant (100 nm) across the entire film surface (typically 10 cm x 10 cm) and
so the effect is completely neglected. The surface morphology imaged by AFM on the BNT film
annealed at 650°C confirms the SEM results (see Figure 3-b): a dense and uniform granular-like
surface constituted of well-connected rounded grains with an average diameter of 80 nm is
showed while a root-mean-square roughness R<sub>rms</sub> of about 5.8 nm is calculated over 10µm x
10µm scan area. This R<sub>rms</sub> value for BNT can be compared to the R<sub>rms</sub> value of 3.3 nm measured
over a similar 10µm x 10µm scan area for the LNO bottom electrode with small faceted grains.
The EDS analyzes realized on different films deposited at 200°C were carried out before and
after annealing at 650°C and no change on the films composition has been observed. Even if no
information could be precisely given about the exact oxygen content in our films, the electrical
properties obtained, in particular the loss factor and the leakage current, closely associated with
oxygen vacancies, prove that the oxygen stoichiometry is almost reached.
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** a) SEM and b) AFM surface morphology images of a BNT film annealed at 650°C (the scan size of the AFM image is 10 µm x 10 µm while the surface morphology of the LNO bottom electrode before BNT deposition is showed in the top right corner of Figure b).</figcaption></figure>

Figure 4-a and b present the relative dielectric constant and the loss tangent of different films annealed at various temperatures between 450 and 700°C as a function of the applied electrical field. Note that the amorphous as-deposited film and the film annealed at 400°C (not represented in this figure) exhibit a paraelectric behavior with a low relative dielectric constant of 60. For a post-annealing treatment at temperatures between 450°C and 700°C, all the resulting cycles exhibit a butterfly shape, which is characteristic of ferroelectric behavior. The permittivity
measured at zero electric field grows with the increasing of the annealing temperature from 225
at 450°C to reach a maximum of 545 at 650°C and a small decrease is observed until 700°C.
Concerning the dielectric losses, the general trend is an increase with temperature with a
significant jump from 3% to 4.5% when annealing temperature increased from 650°C to 700°C
respectively. The change in the permittivity as a function of the applied electric field also called
tunability grows with the increasing of annealing temperature. For example, for a film annealed
at 450 ° C, the permittivity goes from 225 to 180 when the electric field increases respectively
from 0 to 550 kV/cm, which represents a tunability of 15%. In case of the film annealed at
650°C, the permittivity measured under the same conditions varies from 545 to 245, i.e. a
tunability of 55%. This property makes BNT a serious competitor of Barium Strontium Titanate
(BST), which is a reference for its excellent tunability (61% between 0 kV/cm and 570 kV/cm
biasing field for a 350 nm thick film) and in particular at high frequency [53, 54]. It is necessary
in the next step to test BNT films with coplanar electrodes for example to measure the tunability
at microwave frequency.
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Evolution of the dielectric properties: a) relative dielectric constant and b) loss tangent as a function of the applied electric field for different films annealed at various annealing temperature.</figcaption></figure>

The ferroelectric properties of these films were also analyzed. Figure 5 shows the P(E) loops measured on the different films. As specified before, the amorphous film exhibits a paraelectric behavior. The films annealed at 450°C show ferroelectric properties with a difficulty to be poled despite the onset of the crystallization of BNT. With increasing annealing temperature, an
improvement of ferroelectric properties can be seen. The maximum polarization was increased from 12 to 27 µC/cm² between 450°C and 650°C respectively under an applied field of 540 kV/cm. Above 650°C, the maximum polarization dropped to 25 µC/cm². Note that the obtained P(E) loops are not fully saturated at maximum applied electrical field which can be explained by the conductivity of pure BNT [55,56]. Thus, the best ferroelectric properties were obtained with the optimal crystallization achieved with an annealing temperature of 650°C. The film annealed at 650°C presents a coercive field of 65 kV/cm and a low remnant polarization of 4 µC/cm². The low remnant polarization value is often reported in pure BNT thin films [40,57].

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Evolution of ferroelectric properties as a function of the applied electric field for different films annealed at various annealing temperatures.</figcaption></figure>

The same investigation was carried out on the piezoelectric properties. Figure 6 presents the evolution of the piezoelectric responses of BNT films annealed at various temperatures as a function of the applied electric field. Similar to the ferroelectric properties evolution, no piezoelectric activity was observed on the amorphous film annealed at 400°C. However, at low
annealing temperature of 450°C, a non-negligible piezoelectric response is obtained: $d_{33}$ reaches
a value of 27 pm/V. By increasing the annealing temperature, the piezoelectric response was
improved and a maximum $d_{33}$ of 57 pm/V was obtained for an annealing temperature of 650°C.
At annealing temperature of 700°C, a small decrease of $d_{33}$ was observed where a maximum $d_{33}$
of 52 pm/V is obtained.

BNT does not exhibit magnetic properties and therefore it cannot be directly used for advanced
magnetic/electric field driven microwave device applications such as BaFeO<sub>19</sub>, SrFeO<sub>19</sub> and M-
type substituted hexaferrites which reveal optimal intrinsic magneto-electric properties [3, 58-
62]. However, due to its piezoelectric properties, BNT can be coupled to a magnetic material to
obtain artificial magnetoelectric heterostructure, *i.e.* piezoelectric film/magnetic film. By the
action of an electric field it is possible to modify the dielectric properties of the magnetic film
like its permeability. Such structures have already been produced with
ferroelectric/antiferroelectric and magnetic films [63-67].
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Evolution of $d_{33}$ piezoelectric coefficient as a function of the applied electric field for different films annealed at various annealing temperatures.</figcaption></figure>

At the local scale, PFM measurements including imaging and hysteresis loops were performed to obtain further information about domains pattern, switching properties and electromechanical activity. The BNT film annealed at 650°C was preferentially investigated since previously identified as displaying the better piezo-/ferroelectric properties at the macroscale. Ferroelectric domain pattern was observed by out-of-plane PFM imaging. In-phase PFM image was simultaneously recorded with the corresponding AFM image revealing the surface morphology of the area probed (Figure 7). As shown on the Figure 7-b, the in-phase PFM image evidences the existence of bright and dark contrasts traducing the presence of polarized nano-regions with “upward” (away from the substrate) and “downward” (toward the substrate) component for
polarization, respectively [68]. If we take into account that BNT in thin film cristallizes in the
rhombohedral structure with the *R*3c space group as reported for single-crystal [69,70] and that,
for simplicity, BNT can be described using a pseudo-cubic cell with *a*<sub>p</sub> = 3.89 Å taking into
consideration the very small distortion of the rhombohedral cell (α<sub>p</sub> = 89.8°) [32,71], we can
conclude that the polarization in the BNT pseudo-cubic cell is along the [111]<sub>pc</sub> direction due to
the fact that the Bi<sup>3+</sup> and Na<sup>+</sup> ions as well as the Ti<sup>4+</sup> ions are not well-centered in the
cubooctahedral and octahedral cavities, respectively. Then, the bright contrasts observed on the
in-phase PFM image would correspond to the “upward” normal component of the ~35°-
disoriented polarization vector within the pseudo-cubic BNT cell with respect to the plane of the
substrate while the dark contrasts would correspond to the “downward” normal component of the
~35°-disoriented polarization vector with respect to the plane of the substrate. Such an angle of
~35° can be calculated from the relative [111] direction in the pseudo-cubic BNT cell with
respect to the (100) plane [72]. However, the bright and dark contrasts don’t correspond to a
well-defined bimodal distribution; indeed, slightly different contrats for both “bright” and “dark”
can be observed. In fact, the interpretation of the ferroelectric domain pattern observed in such a
BNT film is much more complex than the one to consider “stricto-sensus” a bimodal distribution
of the polarization vector in the film. In a way, we consider that compressive strains are induced
by the bottom electrode in the film (a<sub>p</sub> LNO = 3.84 Å and a<sub>pc</sub> BNT = 3.89 Å), leading to a change
of symmetry for BNT, and then to affected contrasts. It is well known that many groups led
works to define precisely the structural properties of BNT and that some findings were made in
the last years: we can cite for example the works of Gorfman *et al*. who state that BNT would
cristallize in a monoclinic structure with the space group *Cc* [73,74].
Besides, by comparing the out-of-plane PFM in-phase and AFM images, we can conclude that most of the grains are monodomain with uniform polarization. In some cases, domains also extend to the adjacent grains [75].

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** a) AFM surface morphology and b) corresponding out-of-plane PFM phase image recorded at the surface of BNT thin film annealed at 650°C. The scan size of the images are 1µm x 1µm.</figcaption></figure>

PFM imaging was also used to study the switching properties of BNT films at larger scale after polarization and reverse polarization experiments. On a 6 µm x 6 µm square area, a DC voltage of + 20 V was applied to the tip while, inside this one, a DC voltage of - 20 V was applied to the tip on a 3 µm x 3 µm square area. Such values of ± 20 V were intentionally chosen to overcome the polarizing voltage coercivities deduced from the local hysteresis piezoelectric loops (see after) allowing for efficient switching of the domains. Out-of-plane PFM images were then recorded in-phase and in-amplitude on a 10 µm x 10 µm square area including the two previous manipulated square areas. As shown on the in-phase PFM image of Figure 8-b, we evidence that the polarization switching of the domains is successfully achieved, considering the well-defined dark and bright squares areas obtained after polarization and reverse polarization experiments.
These results clearly point out the poled domains with “downward” and “upward” polarizations and that the switching process is completely reversible. On the other hand, no modification of the surface is observed on the corresponding AFM image excluding surface reactions during the polarizing process. The image of the amplitude signal is depicted on the Figure 8-c. The bright contrasts traduce the electromechanical vibration of the manipulated domains and, as expected, appear similar for the two opposite polarized areas.

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** a) AFM surface morphology, b) out-of-plane PFM phase and c) out-of-plane PFM amplitude images simultaneously recorded after polarization and reverse polarization experiments using DC voltages of ± 20V. The scan size of the images are 10 µm x 10 µm.</figcaption></figure>

In order to get a better insight on the local switching behavior of the domains and to measure electric performances of these films, local hysteresis piezoelectric loops were recorded in phase and in amplitude. The figure 9-a displays typical piezoloops recorded over the center of grains observed at the free surface on the film. As a first result, the 180° phase difference measured on the in-phase PFM loop confirms the well-defined ferroelectric hysteresis of the BNT film with its two opposite states. On the other hand, the mean coercive voltage $V_c$ determined from 50 loops is about 9.4 V (value ranges from 6.3 to 12.6 V). Taking into account that $A = d_{33\text{eff}} \times V_{ac} \times Q$, a
mean value for d33eff of 38 pm/V (value ranges from 28 to 46 pm/V) was calculated considering a
driving voltage Vac of 1 V and a quality factor Q in the range 40-80 depending on the grain
probed. The variation in these parameters at 50 different locations of the sample is shown in the
corresponding histograms (see Figure 9-b and c). We can remark that this mean value is slightly
lower than the one measured at the macroscale level (38 pm/V vs 57 pm/V). However, such a
slightly lower value for nanoscale measurement can be explained by the electric field beneath the
AFM probing tip which is inhomogeneous in such PFM experiments, leading to particularly
difficult quantitative measurements of signal [76] although a careful calibration has been
performed. Finally, a slight asymmetry in the tip vibration-amplitude along the verical axis is
observed. This may be explained by the difference between the work functions of the probing tip
and the conductive substrate, resulting in a internal bias field as commonly observed for classical
and innovative ferroelectric materials [77-79].
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 9.** a) Typical remnant in-phase and in-amplitude PFM hysteresis loops recorded at the center of grains of BNT film grown on LNO/Si substrate and annealed at 650°C, b) histogram showing the variation in coercive voltage values for the BNT sample for 50 different measurements and c) histogram displaying the variation in $d_{33\text{eff}}$ values for the BNT film for 50 different measurements.</figcaption></figure>

Results described above have shown a correlation between structural and electrical properties. The structural analysis demonstrates that BNT remains amorphous up to 400°C and starts to crystallize at annealing temperatures about 450°C. It can be considered that at this temperature the film starts to crystallize at the film-substrate interface because the LNO can be considered as a seed layer (nucleation sites) thanks to the lattice parameters matching between LNO and BNT
[80-82]. The analysis reveals also an improvement in crystallization with increasing annealing temperature to reach a maximum of crystallization at 650°C. The optimal crystallization obtained at 650°C can be explained by the crystallization of the entire film in the perovskite structure and the disappearance of the amorphous phase. Above 650°C, a weak degradation of the crystallization is observed which could be probably due to the evaporation of volatile elements of Na or Bi but we have not measured.

The electrical properties as a function of annealing temperature seems to have the same evolution as the structural properties. Figure 10 compares the evolution of the intensity of (00l) peaks of perovskite phase, the permittivity at zero electrical field and the $d_{33max}$ coefficient (as determined at the macroscale) as a function of annealing temperature. For an annealing temperature of 400°C and below, the film remains amorphous and shows a low permittivity of 60 with no piezoelectric response. For an annealing temperature of 450°C, the permittivity increases from 60 (permittivity of the amorphous film and the 400°C annealed film) to 220. A significant electro-mechanical response with a $d_{33max}$ of 27 pm/V appears in the film annealed at 450°C, higher than that measured on other lead-free piezoelectric materials such as zinc oxide (ZnO), Aluminum Nitride (AlN), Lithium Niobate (LiNbO$_3$),.... The advantage here is a low crystallization temperature compatible with silicon integration. This improvement of the electrical properties between 400°C and 450°C is due to the beginning of the crystallization of the piezoelectric perovskite phase of BNT. Above 450°C, the electrical properties gradually increase with the improvement in the crystallization of the films as the annealing temperature increases and reach its maximum at 650°C with a permittivity of 545 and a $d_{33max}$ of 57 pm/V. This improvement in the electrical properties can be explained by the phase change of the film from amorphous to perovskite phase as a function of the annealing temperature. The film goes
gradually from amorphous state with lower permittivity (~ 60 as shown before) even with an
annealing at 400°C to totally crystallized in perovskite structure with a high permittivity of 545
for an annealing temperature of 650°C. Between these temperatures, a mixture of amorphous and
perovskite phases occurs in the film. Whatever the annealing temperature, a pure perovskite
phase was obtained with no secondary phases.

<figure><img src="image_14.png" /><figcaption>**Fig. 10.** Comparison of the evolution of structural and electrical properties as a function of the annealing temperatures.</figcaption></figure>

The mixture of amorphous/perovskite can explain the evolution of ferroelectric properties presented in Figure 5. As the film is formed from amorphous and perovskite phases, a major part of the applied electrical field goes into the amorphous part of the film that presents a low permittivity (60 for amorphous phase vs 545 for perovskite phase). This effect can explain the
low polarization and the paraelectric behavior obtained on films with a small amount of material crystallized in the perovskite phase and the improvement of the polarization with the disappearance of amorphous phase from the film with increasing annealing temperature.

With our deposition and annealing conditions, the best annealing temperature is at 650°C. Figure 11-a, relating to the evolution of the current density as a function of the electric field applied to a film annealed at 650°C, testifies to this property. The leakage current density of BNT thin film is found to be about 2 x 10⁻⁷ at 0 kV/cm and 2 x 10⁻⁵ A/cm² at 450 kV/cm.

<figure><img src="image_15.png" /><figcaption>**Fig. 11.** a) Leakage current density as a function of applied electric field of BNT thin film annealed at 650°C. b) Typical AFM surface morphology and c) corresponding c-AFM current mapping recorded on a 5µm x 5 µm square area.</figcaption></figure>
The obtained leakage current density is comparable to the best one reported in the literature for pure BNT films (1 x 10⁻⁶ A/cm² at 200 kV/cm) [80] or measured by T. Takenaka et al. (6.10⁻⁵ A/cm² measured at an applied electric field of 100 kV/cm) [17]. It is better than that measured on BNT-BT [27,83], in the order of 5.2 x 10⁻⁴ A/cm² at 100 kV/cm. Different conduction mechanisms, such as the space-charge-limited current (SCLC), the Schottky emission or the Frenk-Poole emission, may be employed to explain the leakage current in the Au/BNT/LNO capacitor [80]. Besides, at the local scale, current mapping was recorded by c-AFM when applying a bias voltage as high as 5 V through the conductive AFM tip on a 400-nm-thick BNT film annealed at 650°C (such a 5 V voltage would correspond to an electric field of 125 kV/cm by assuming a homogeneous electric field) and presented on Figure 11-c. As a result, the film is showed to be insulating, taking into account the observation of non-significative current as displayed on the scale bare of the c-AFM image of Figure 11-c along with the absence of leakage current channels and any nanoscale conductivity variations.

Finally, the structural and electrical results reveal that 650°C is the optimal temperature for BNT annealing. BNT films annealed at this temperature are well crystallized in a pure perovskite phase with (00l) preferential orientation. It exhibits also good electrical properties with a permittivity of 545 and dielectric loss of 3%, a coercive field of 65 kV/cm with low remnant polarization of 4 µC/cm² and a d₃₃ of 57 pm/V. At the sub-micron level, well-defined ferroelectric domains are imaged. The dark and bright domains observed are in good agreement with the different polar directions existing inside the (00l)-oriented pseudo-cubic BNT while a coercive voltage about 9.4 V and an estimated mean d₃₃ piezoelectric coefficient of 38 pm/V from piezoloops are in good concordance with performances measured on macroscale.
To our knowledge, these properties are among the best one reported in the literature on pure BNT thin film [22,55,56,84]. Compared to other lead-free materials such as Potassium Sodium Niobate (KNN) which is the most studied material in thin film form, our 400-nm-thick film presents promising piezoelectric properties when the best values obtained on pure KNN are between 40 and 80 pm/V for much thicker films (1-3 µm) [84]. On the other hand, films annealed at lower temperature also exhibit acceptable electrical properties. In particular, the BNT film annealed at 500°C shows a permittivity of 335 and a $d_{33}$ piezoelectric coefficient of 37 pm/V. These later values, although they are not the optimal ones in the case of our study, are superior to some results reported in the literature and obtained at higher temperature [84,85].

To note that the piezoelectric and ferroelectric properties of BNT films can be further improved, as reported in the literature by considering various approaches. For example, BNT can be doped with Mn to reduce the conductivity by compensating the oxygen vacancies formed due to the volatility of Bi and Na [31,86] or by forming BNT-based solid solution with a Morphotropic Phase Boundary (MPB) as BNT-BT [87, 88] and BNT-BKT [89]. These different ways will be the topic of our future works.

## 4. Conclusion

In summary, we have demonstrated the effect of the annealing temperature on structural and electrical properties of BNT thin films deposited by ex-situ sputtering at substrate temperature of 200°C. By varying the annealing temperature between 400°C and 700°C, we observed a strong correlation between the structural, electrical and piezoelectric properties. BNT starts to crystallize at an annealing temperature of 450°C and by increasing the annealing temperature, we observed an improvement of the structural and electrical properties and the optimal properties
were obtained with an annealing at 650°C. Indeed, the effects observed are closely related to the
amount of crystallized material in the film. The film goes from a completely amorphous form
with an annealing at 400°C to a completely crystallized film in the pure perovskite structure at
650°C with no change in the film composition. BNT films annealed at temperatures lower than
650°C also present good electrical properties. The 400-nm-thick BNT films annealed at 500°C
and 650°C present a $d_{33max}$ piezoelectric coefficient of 37 pm/V and 57 pm/V respectively at 550
kV/cm. At the sub-micron level, the ferroelectric domain pattern is imaged, highlighting the
existence of well-defined dark and bright contrasts, which traduce the direction of ferroelectric
polarization in the pseudo-cubic BNT cell. From PFM spectroscopy, a coercive voltage about 9.4
V is measured while a strong piezo-activity is detected for 400-nm-thick BNT film annealed at
650°C, the local piezoelectric hysteresis loops confirming the electrical performances of such
insulating samples. These results show the potential of BNT as an alternative to lead-based
piezoelectric materials as well as it can be synthetized at low processing temperature, which is
compatible with Si-CMOS technologies.

Acknowledgments

This work was supported through grant ANR-16-CE08-0025, ANR NanoPiC from French
National Research Agency. Chevreul Institute (FR 2638), Ministère de l'Enseignement
Supérieur, de la Recherche et de l'Innovation, Hauts-de-France Region, Fonds Européen de
Développement Régional (FEDER) and Major Domain of Interest (DIM) "Eco-Energy
Efficiency" of Artois University are acknowledged for supporting and funding partially this
work. Région Hauts-de-France and FEDER are gratefully acknowledged for funding the MFP-
3D microscope under Program "Chemistry and Materials for a Sustainable Growth".
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