Microwave-Assisted Hydrothermal Synthesis of BiFexCr1âxO3

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Microwave-assisted hydrothermal synthesis of BiFeCr O ferroelectric thin films Gitanjali Kolhatkar, Fabian Ambriz Vargas, Reji Thomas, and Andreas Ruediger Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00603 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017

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Crystal Growth & Design

Microwave-assisted hydrothermal synthesis of BiFexCr1-xO3 ferroelectric thin films Gitanjali Kolhatkar,1 Fabian Ambriz-Vargas,1 Reji Thomas,1 Andreas Ruediger1* 1

Institut Nationale de la recherche scientifique, centre Énergie, Matériaux, Télécommunications, 1650 Boulevard Lionel-Boulet, Varennes, Québec, J3X 1S2, Canada.

ABSTRACT We report the deposition of ferroelectric BiFexCr1-xO3 thin films by microwave-assisted hydrothermal synthesis. BiFeO3, a multiferroic material, has been receiving a lot of interest due to its polar and magnetic ordering at room temperature, which makes it suitable for non-volatile semiconductor memories. The addition of chromium enhances the materials magnetic properties through ferromagnetic ordering with iron. The synthesis technique used here is extremely cost efficient as compared to the more commonly used physical and chemical vapor deposition tools. By studying the effect of deposition time on the topography and the ferroelectric properties of the film, the deposition parameters are optimized and a ferroelectric thin film is obtained after 10 min at 120 W of nominal microwave output power (Figure A (a)). As the deposition time increased beyond completion of the reaction, the surface of the film became rougher and its ferroelectric properties weakened rapidly. Further analysis reveals that the incorporation of Cr deteriorates the ferroelectric retention, as shown in Figure A. In an attempt to improve the retention in BiFexCr1-xO3 thin films, the effect of annealing under various conditions on the topography and the ferroelectric properties of the films is investigated in depth, revealing that annealing improves significantly the topography.

Figure A. Piezoelectric force microscopy phase images of BiFexCr1-xO3 (a) 0 h, (b) 1 h30 min and (c) 3 h after poling.

*

Corresponding author: Andreas Ruediger, Institut Nationale de la recherche scientifique, centre Énergie, Matériaux, Télécommunications, 1650 Boulevard Lionel-Boulet, Varennes, Québec, J3X 1S2, Canada. Email : [email protected]

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Microwave-assisted hydrothermal synthesis of BiFexCr1-xO3 ferroelectric thin films Gitanjali Kolhatkar,1 Fabian Ambriz-Vargas,1 Reji Thomas, 1 Andreas Ruediger1* 1

Institut Nationale de la recherche scientifique, centre Énergie, Matériaux, Télécommunications, 1650

Boulevard Lionel-Boulet, Varennes, Québec, J3X 1S2, Canada. *Corresponding author e-mail: [email protected]

ABSTRACT We synthesize ferroelectric BiFexCr1-xO3 thin films by microwave-assisted hydrothermal synthesis on (111) oriented Nb doped SrTiO3 substrates. Bi(NO3)3, Fe(NO3)3, Cr(NO3)3 and (KOH) are used in the precursor solution, along with deionized water. Combination of Raman and X-ray photoelectron spectroscopies confirmed the incorporation of chromium into the rhombohedral structure of BiFeO3. The effect of deposition time on the surface morphology and the ferroelectric properties of the films are analyzed. This reveals that if the deposition goes on beyond completion of the reaction (10 min at 120 W), the ferroelectric properties of the film deteriorate due to stoichiometry changes. Through piezoresponse force microscopy, we show that Cr significantly deteriorates the film retention, which cannot be recovered by thermal annealing.


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1. INTRODUCTION Multiferroic materials are a class of materials that exhibit both ferroelectric and ferromagnetic polarizations.1,2 Those properties make them highly suitable for non-volatile semiconductor memories.3 One of the most interesting multiferroic materials is bismuth ferrite (BiFeO3), due to its polar and magnetic ordering at room temperature. BiFeO3 belongs to the R3c symmetry group. It has a rhombohedrally distorted perovskite structure with lattice parameters a=b=c= 5.63 Å and α=β=γ=59.4° that displays a spontaneous polarization along one of the [111] pseudo-cubic axes.4 Its Curie and Néel temperatures are high, with values of ~1100 K and ~640 K, respectively.5 In addition to being multiferroic, it is also magnetoelectric, meaning that is shows magnetization under the action of an electric field and vice versa.6 Its ferroelectricity and magnetism have different origins. The former arises from the dangling bonds of the Bi3+ stereo-chemically active lone pairs,7 while the latter comes from G-type ordering, where the Fe3+ are surrounded by six nearest neighbors of opposite spin.8–10 While BiFeO3 is a mineral that occurs naturally, it can also be synthesized. To do so, various methods can be employed, such as molecular beam epitaxy (MBE),11 radio frequency (RF)-magnetron sputtering,12 pulsed laser deposition (PLD),13 metalorganic chemical vapor deposition (MOCVD),14 chemical solution deposition,15 conventional hydrothermal synthesis,16 and microwave-assisted hydrothermal synthesis17. Conventional hydrothermal synthesis presents many advantages compared to other deposition techniques as it uses a low deposition temperature, it is inexpensive, does not require any sophisticated equipment, and provides a good stoichiometry control.18 However, regarding the conventional hydrothermal synthesis of BiFeO3, days are needed for a reaction to be complete without any secondary phase. To avoid these long 3 ACS Paragon Plus Environment


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reaction times and thus reduce the energy consumption, microwave-assisted hydrothermal synthesis can be employed to accelerate the crystallization kinetics.19 Furthermore, other atoms, such as lanthanum (La) or manganese (Mn) can be introduced as dopant to improve the material’s ferroelectric properties through a reduction of the oxygen vacancies, and therefore, the leakage current.20–22 Chromium (Cr3+) can be incorporated to enhance the magnetic properties through ferromagnetic ordering with iron.23 Double-perovskite Bi2FeCrO6 can be obtained through the addition of Cr. This material has attracted a lot of attention as its double-perovskite nature enhances the ferroelectric and ferromagnetic properties of the material. Furthermore, Bi2FeCrO6 displays interesting photovoltaic properties.24–26 So far, most research previously reported on BiFexCr1-xO3 used pulsed laser deposition27 or chemical solution deposition15. In this work we report on the synthesis of BiFexCr1-xO3 thin films using a microwaveassisted hydrothermal process. Starting from a 1 millimolar solution of the precursors, we optimize the deposition time through topography and piezoresponse force microscopy (PFM) measurements to obtain a ferroelectric thin film. This is followed by a comparative retention study between BiFexCr1-xO3 and BiFeO3, and the effect of annealing on the material’s surface topography and ferroelectricity. This study demonstrates the potential of the microwave-assisted hydrothermal method for the synthesis of BiFexCr1-xO3 ferroelectric thin films.

2. EXPERIMENTAL SECTION


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Substrate preparation. The thin films were deposited on one side polished (111) oriented Nb (0.5 wt. %) doped SrTiO3 single crystal substrates from Crystec GmbH using a microwaveassisted hydrothermal technique. A detailed description of the experimental setup can be found in literature 28. Prior to deposition, the (111) Nb-doped SrTiO3 substrates were etched using the microwave assisted thermal process described elsewhere:17 the substrates were placed inside a polymer autoclave, which was inserted into a hydrothermal reactor filled with deionized water before being placed in the microwave oven. The microwave-assisted hydrothermal treatment performed on those substrates consisted in 4 min at 360 W, after which they were left in the microwave oven for 2 h to cool down. The substrates where then annealed at 1000°C for 10 min under an oxygen flow of 80 sccm in a programmable MTI Corporation tube furnace. The (111) orientation of the substrates was chosen as it enhances the ferroelectric properties of the BiFeO3 29

. The atomic force microscopy (AFM) images of surface morphology of the substrate after the

microwave-assisted hydrothermal treatment revealed a flat surface with an rms roughness of ~0.15 nm and terraces steps having an average height of 0.22 nm, as presented in a previous paper.17 Microwave-assisted hydrothermal synthesis. For the thin film synthesis, Sigma-Aldrich 98% bismuth nitrate pentahydrate (Bi(NO3)3)·5H2O, 98% iron nitrate nonahydrate (Fe(NO3)3)·9H2O and 99% chromium nitrate nonahydrate (Cr(NO3)3)·9H2O were used as precursors. A low concentration of potassium hydroxide (KOH) was introduced in the solution to act as a mineralizer ((KOH/Bi) ratio of 2.5), similarly to Komarneni et. al.19 The deposition was performed for a given number of cycles, where 1 cycle consisted in 10 min of microwave irradiation at 120 W of nominal microwave output power. In these conditions, the deposition temperature was estimated to ~280°C, according to the vapor pressure diagram of water and



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knowing the hydrothermal reactor has a pressure limit of 83 bar. To avoid overpressure which would activate the safety valve and terminate the experiment, we left a delay of 2 h between each consecutive cycle. This 2 h delay was introduced by keeping the hydrothermal reactor inside the microwave oven. Nanoscale characterization. Both topography and ferroelectric switching were characterized with an AIST-NT SmartSPM-1000 system. The topography was imaged using AFM measurements, during which the system was ran in tapping mode using Nanosensor AFM probes with a silicon (Si) tip, at a scan rate of 0.5 Hz. The Si tip had a radius of ~10 nm, a resonance frequency in the 200-400 kHz range and a spring constant in the 25-95 N/m range. Ferroelectric switching was analyzed with PFM measurements, by a conductive ~30 nm radius Pt-Ir coated Si tip (resonance frequency in the 43-81 kHz range and a spring constant in the 1-5 N/m range) acting as a mobile top electrode, in contact the film’s surface, while the conductive substrate acted as a bottom electrode. An AC voltage was applied through the tip. Local hysteresis loops off-resonance were collected as a function of DC switching bias at a given position on the sample, while for the PFM images, ferroelectric square shaped domains were written by an electrical bias potential. Chemical characterization. The chemical composition of the films was characterized by Raman spectroscopy. The confocal Raman spectra were measured using a Horiba system with a Synapse Back-Illuminated DeepDepletion 1024 × 256 CCD from Horiba Scientific and a 473.2 nm solid state blue Cobolt 04-01 laser. The Fe and Cr atomic concentrations were determined by X-ray photoelectron spectroscopy (XPS) using a VG Escalab 220i XL system operated with a 1486.6 eV Al Kα source at 15 kV and 20 mA in normal emission geometry with a pass energy of 100 eV for the general survey, and 20 eV for the high resolution scans. The pressure in the 6 ACS Paragon Plus Environment

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analytical chamber was kept at 3 × 10-9 Torr during the measurements and the reported binding energies were all referenced to the 285.0 eV adventitious hydrocarbon C1s peak. To calibrate the spectrometer energy scale, the Au 4f5/2 (87.0 eV) and 4f7/2 (84.0 eV) lines were used, and an uncertainty of 0.05 eV was determined. 3. RESULTS AND DISCUSSION The BiFexCr1-xO3 films (equimolar, 0.001 M) were synthesized for different deposition times. Typical topography images are illustrated in Figure 1(a). After 1 cycle, a polycrystalline surface morphology is observed, indicating that the film growth process started with the nucleation of small clusters on the substrates that grew into 3D features, with an rms of ~3 nm. The size of those grains increases with the synthesis time, resulting in a higher roughness after 2 cycles (rms of ~8 nm) and 3 cycles (rms of ~10 nm), as the particles have more time to coalesce. For all three samples, no atomic step-terraces were observed due to a complete coverage of the substrate surface with polycrystalline layers. The ferroelectric behavior of the films was verified by PFM. Typical PFM phase maps of the films and local hysteresis curves are shown in Figure 1(b) and (c), respectively. To obtain the PFM image patterns, the films were poled, first with a negative voltage (green squares), and then with a positive voltage (blue squares). To ensure reproducibility, the measurements were repeated at different positions on the films. Figure 1(b), shows that after 10 min of synthesis, the regions poled with a negative voltage (black regions) can be clearly distinguished from the regions poled with a positive voltage (brown-yellow regions). The antiparallel polarization is further confirmed by the 180° phase contrast between the two regions. The remaining area corresponds to the virgin state which depicts a preferential downward polarization. However, the polarization switching diminishes as the number of cycle increases. After 2 cycles, a very weak 7 ACS Paragon Plus Environment


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phase-contrast is observed, while after 3 cycles, no phase-contrast can be seen in the PFM image. This suggests that the precursor’s reaction is complete after 10 minutes at 120 W. Beyond that time, the loss in ferroelectricity could be due to stoichiometry changes. This hypothesis is further confirmed by the increase in the grain size observed by AFM. This higher deposition time could also result in etching occurring after the reaction is complete or in an additional incorporation of hydrogen.


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Figure 1. Typical (a) 1 µm × 1 µm AFM image (b) PFM phase-contrast image and (c) Local phase-hysteresis loops of BiFexCr1-xO3 samples synthesized for 1, 2 and 3 cycles, respectively. The local hysteresis curves obtained for the three different samples (Figure 1(c)) reveal changes in the ferroelectric properties. A typical hysteretic curve is observed for the film deposited for 1 cycle, with coercive voltages of +6 V (polarization downward) and – 8 V (polarization upward) as estimated from the amplitude loop. This indicates a larger coercive field than the one of BiFeO3 synthesized with the microwave-assisted hydrothermal method.17 Even 9 ACS Paragon Plus Environment


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though those measurements do not allow us to accurately measure the coercive field,17 they give us qualitative information concerning its behavior variation between different samples. As the number of cycles increases to 2, the coercive field increases, with coercive voltages of +7 V and – 9 V, indicating an increase of energy necessary to complete the 180° ferroelectric switching. In the case of the sample produced with 3 cycles of microwave irradiation, a decrease in both coercive field and amplitude, indicates a degradation of the ferroelectric properties.30 Furthermore, after 3 cycles of deposition, the local hysteresis curve is not centered at zero, but shows pronounced imprint due to the presence of local charges on the surface. The atomic concentration of Fe and Cr in the BiFexCr1-xO3 film deposited for 1 cycle of microwave irradiation was determined by XPS. This film was selected for XPS measurements as it showed the strongest ferroelectricity. A survey scan taken in the 1200-0 eV range is presented in Figure 2(a). It reveals the presence of Bi, Fe, Cr, O and C at the film surface. No potassiumrelated peaks are observed, indicating that the ferroelectricity shown in Figure 1(b) does not come from a potassium-related secondary product, such as potassium nitride (KNO3), but from the BiFexCr1-xO3 film itself. Additional high resolution scans were acquired in the selected binding energy ranges of Bi4f, Fe2p, and Cr2p, as shown in Figure 2(b)-(d). Bi4f (Figure 2(b)) consists in two broad Bi4f5/2 (165.0±0.05 eV) and Bi4f7/2 (160.0±0.05 eV) peaks, and two low intensity satellites peaks of Bi4f5/2 at 155.5±0.05 eV and 153.5±0.05 eV, all four of which can be attributed to the Bi-O bonds. The two peaks observed at 725.5±0.05 eV and 711.7±0.05 eV in Figure 2(c) are due to Fe2p1/2 and Fe2p3/2, and can be linked to the Fe-O bonds. Finally, Cr2p consists in two peaks seen at 587.2±0.05 eV and 577.6±0.05 eV (Figure 2(d)), attributed to Cr2p1/2 and Cr2p3/2, respectively, which originate from Cr-O bonds. These results indicate a


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Fe:Cr ratio of ~2:1. Using transmission electron microscopy (TEM), the film thickness was determined to be ~5 nm (see Supporting Information, section 1).

Figure 2. XPS spectra of the BiFexCr1-xO3 thin film deposited for 1 cycle of microwave irradiation showing (a) the general survey, (b) the Bi4f, (c) Fe2p, and (d) Cr2p signals. The background curves (dotted lines) based on the Shirley method are shown for each element.

The structural properties were studied by X-ray diffraction (XRD), and the presence of the BiFexCr1-xO3 (222) peak was confirmed,31 attesting to the crystallinity of our films (see



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Supporting Information, section 2). The chemical composition of the films was analyzed with Raman spectroscopy. Only the Raman signal of the SrTiO3 substrate could be seen and no information regarding the composition of the film was obtained. However, during the microwave-assisted hydrothermal synthesis, homogenous nucleation in the liquid medium results in the formation of BiFexCr1-xO3 particles and their sedimentation at the bottom of the reactor. Therefore, to avoid the strong contribution of the substrate, we performed Raman spectroscopy measurements on the powders formed in the hydrothermal reactor simultaneously with each thin films, illustrated in Figure 3. Even though an indirect method, this still gives information regarding the chemical composition of the thin films. According to literature, the BiFexC1-xO3 phase with rhombohedral R3c structure has 18 optical modes (4A1 + 5A2 + 9E). As the A2 modes are Raman-inactive, the BiFexCr1-xO3 phase can be represented by 13 Raman modes (4 + 9), but only a fraction of them is usually observed at room temperature. The Raman spectra of the BiFexCr1-xO3 powders show peaks at 84 cm-1, 139 cm-1, 210 cm-1, 259 cm-1 and 350 cm-1 which are characteristics of the BiFexCr1-xO3 phase.32–34 The two peaks at 139 cm-1 and 210 cm-1 correspond to the A1(LO) modes of the Bi-O bond, the peaks at 259 cm-1 and 350 cm-1 are related to the different ( ) modes of the Fe-O bonds and the peak that appears at ~825 cm-1 correspond to the Cr-O bond.34,35 Apart from the BiFexCr1-xO3, no other phases such Bi2O3, Fe2O3 or any other Bi compound were observed. As the number of cycle increases, we can see that the peaks become wider and less defined, indicating lower crystallinity. After 1 cycle, the peak at 350 cm-1, attributed to the E-5(TO) mode of the Fe-O bond is more intense than after 3 cycles. Regarding the Cr-O peak at 825 cm-1, its intensity decreases as the number of cycle increases, while its width increases. This reinforces the hypothesis of a deterioration in the


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crystallinity with the deposition time, which was already suggested by the AFM and PFM measurements.

Figure 3. Typical Raman spectra of BiFexCr1-xO3 powders after (a) 1 cycle (black line), (b) 2 cycles (red line) and 3 cycles (green line) of microwave irradiation; (d) presents a typical Raman spectrum of BiFexCr1-xO3 powders after 1 cycle in the low wavenumbers.

The ability of maintaining a poled polarization state with time, referred to as ferroelectric retention), has long been one of the most important reliability issues hindering the commercialization of semiconductor memories.36 Therefore, we studied the retention in our BiFexCr1-xO3 film deposited for 1 cycle of microwave irradiation. To do so, PFM images, illustrated in Figure 4, were taken at the same position on the sample but at different times: right after poling (Figure 4(a)), 1 h 30 min after poling (Figure 4(b), and after 3 h after poling (Figure 4(c)). We can see that after 1 h 30 min, some ferroelectric domains are already switching back to their initial state, as indicated by the nucleation of small bright spots in the negatively biased 13 ACS Paragon Plus Environment


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regions. After 3 h, the poling has almost completely disappeared, attesting to the poor retention of the film. The retention mechanisms are associated to an imperfect screening at the electrodefilm interface caused by the presence of charges on the surface.36 This excess in charges can partially be attributed to the presence of Cr in the film. Indeed, the substitution of Fe3+ ions by Cr4+ ions results in the formation of free electrons (e’) that will affect the ferroelectric retention according to + → + + (1 ) + 3 + .

(1)

Figure 4. PFM phase images of BiFexCr1-xO3 (a) 0 h, (b) 1 h30 min and (c) 3 h after poling. To verify this hypothesis, the same study was performed on a BiFeO3 sample synthesized with the microwave-assisted hydrothermal process, as shown in Figure 5. We can see that after 3 h, the polarization is preserved, attesting to the better retention of the BiFeO3 thin film. This confirms that the poor retention in BiFexCr1-xO3 is indeed due to the incorporation of Cr into the rhombohedral unit cell of BiFeO3.


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Figure 5. PFM phase images of BiFeO3 (a) 0 h, (b) 1 h 30 min and (c) 3 h after poling.

To improve the retention in BiFexCr1-xO3, we performed a series of annealing on the same sample to re-arrange the surface structure and improve the film crystallinity. Indeed, annealing increases the surface atoms’ mobility and causes them to reorganize into a lowest energy state through diffusion. The topography obtained after each annealing is illustrated in Figure 6. After annealing at 500°C under O2 flow (Figure 6(a)), the surface roughness increases compared to the as deposited surface, and an rms of 3.8 nm is measured. The grain density observed on the surface decreases, but the grains are larger as the reorganization of the atoms led to the coalescence of small grains. After annealing at 600°C under O2 flow (Figure 6(b)), the grains appear to have coalesced to form a smooth film. On top of this film, grains are still present, but their density is significantly lower. A triangular shaped structure can be seen in the bottom left corner, due to the (111) orientation of the substrate. The rms of the film between the grains is 2 nm, lower than the value of the as deposited surface. After annealing at 650°C under O2 flow (Figure 6(c)), the surface remains smooth, with an rms of 1.6 nm, or 0.4 nm between the grains, and the grains are much fewer and smaller than after annealing at 600°C. More triangular



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structures appear on the surface as their density increases while their size decreases at higher annealing temperature. After annealing at 600°C under vacuum (Figure 6(d)), the surface is flat, with an rms value of 0.4 nm, and almost no grains are observed.

Figure 6. 1 µm × 1 µm topography images of the BiFexCr1-xO3 sample synthesized for 1 cycle of microwave irradiation after annealing for 30 min (a) under O2 at 500°C, with an rms of 3.8 nm, (b) under O2 at 600°C, with an rms of 2 nm, (c) under O2 at 650°C, with an rms of 1.6 nm and (d) under vacuum at 600°C, with an rms of 0.4 nm. The effect of annealing on the ferroelectric retention of the films is presented in Figure 7. After annealing at 500°C under O2 (Figure 7(a)), ferroelectric retention is very poor, and greatly deteriorated compared to the as synthesized surface. This could be due to the increase in the grain size and therefore a higher surface roughness. As the retention is already very poor at 0 h,


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the measurement was not repeated after 2 h. After annealing at 600°C under O2 (Figure 7(b)), the PFM image was taken in a flat region. The PFM map thereby obtained is similar to the one acquired on the as-deposited surface, due to the grain density reduction. However, the retention remains low, as shown by the PFM image taken after 2 h. Indeed, even after such a short time period, the poling has almost disappeared. After annealing at 650°C under O2 (Figure 7 (c)), and at 600°C under vacuum (Figure 7 (d)), the retention deteriorates even more. Furthermore, after those last two annealing steps, the PFM maps acquired after 0 h reveal a weakening of the material’s ferroelectric properties, potentially because of the creation of oxygen vacancies (

!

)

and free charges ( ) during the annealing, following:

"##"$#% → + ! → + & & ('()) +

!

+ 2 .

(2)

This change in the oxidation state is enforced by XPS measurements performed on the annealed sample, which showed a shift from the Fe3+ state on the as deposited sample, to the Fe2+ state, a possible charge compensation for Cr4+, after annealing (see Supporting Information, section 3). While it is known that annealing under vacuum increases the oxygen vacancies and the leakage current,37 thereby deteriorating the ferroelectricity of the material, annealing under O2 at high temperatures seems to not sufficiently counteract this effect. This study reveals that even though annealing significantly decreases the surface roughness and leads to the disappearance of grains measured on the initial as-deposited surface, it does not improve the retention. To improve the retention in BiFexCr1-xO3 thin films, alternative approaches should be explored, such as using different precursors and scavengers for typical aliovalent contaminations. While such a poor retention is not yet sufficient for memory devices, the material system qualifies for printable electronics.38



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Figure 7. PFM phase images of the BiFexCr1-xO3 sample synthesized for 1 cycle of microwave irradiation after annealing for 30 min (a) under O2 at 500°C (b) under O2 at 600°C, 0 h and 2 h 00 after poling, (c) under O2 at 650°C, 0 h and 2 h 00 after poling, (d) under vacuum at 600°C, 0 h and 2 h 00 after poling.

4. CONCLUSION In summary, this study demonstrates the potential of microwave-assisted hydrothermal synthesis to produce ferroelectric BiFexCr1-xO3 thin films. The films were synthesized on (111) oriented Nb-doped SrTiO3 using a low concentration of KOH as mineralizer. The effect of the deposition time on the morphology and the ferroelectric switching was analyzed. After 1 cycle of


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microwave irradiation, a relatively smooth BiFexCr1-xO3 ferroelectric thin film was obtained. Yet, as the deposition time increased beyond completion of the reaction, the surface of the film became rougher and its ferroelectric properties declined rapidly. The presence of both Fe and Cr even though in different valence states in the layers was confirmed by Raman spectroscopy and XPS. Moreover, we show that the presence of Cr4+ deteriorates the ferroelectric retention in the film, which could not be recovered through annealing. Nevertheless, this study shows that microwave-assisted hydrothermal synthesis is a very promising route for the cost effective development BiFexCr1-xO3 thin films. Such a material has many applications for non-volatile random access memories, energy harvesting devices, as well as printable electronics.

ACKNOWLEDGEMENTS The authors would like to thank the Nanoelectronics-Nanophotonics team at INRS-EMT for fruitful discussions. G. K. is grateful for an FRQNT Postdoctoral scholarship and F. A. V. is thankful for individual FRQNT MELS PBEEE 1M scholarship and for the financial support of CONACyT (National Council of Science and Technology-Mexico). A. R. gratefully acknowledges generous support through an NSERC discovery grant (RGPIN-2014-05024).

ASSOCIATED CONTENT Supporting Information



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Thickness measurements performed by TEM are presented in section 1. XRD measurements are depicted in section 2. An XPS study of the effect of annealing on the chemical composition is provided in section 3. REFERENCES (1)

Safi, R.; Shokrollahi, H. Prog. Solid State Chem. 2012, 40 (1–2), 6.

(2)

Schmid, H. Ferroelectrics 1994, 162 (1), 317.

(3)

Ramesh, R.; Spaldin, N. A. Nat. Mater. 2007, 6 (1), 21.

(4)

Wang, J.; Neaton, J. B.; Zheng, H.; Nagarajan, V.; Ogale, S. B.; Liu, B.; Viehland, D.; Vaithyanathan, V.; Schlom, D. G.; Waghmare, U. V; Spaldin, N. A.; Rabe, K. M.; Wuttig, M.; Ramesh, R. Science 2003, 299 (5613), 1719.

(5)

Kubel, F.; Schmid, H. Acta Cryst. 1990, B46 (6), 698.

(6)

Velev, J. P.; Jaswal, S. S.; Tsymbal, E. Y. Philos. Trans. A. Math. Phys. Eng. Sci. 2011, 369 (1948), 3069.

(7)

Thomas, R.; Scott, J. F.; Bose, D. N.; Katiyar, R. S. J. Phys. Condens. Matter 2010, 22 (42), 423201.

(8)

Smolenski, G. A.; Chupis, I. E. Sov. Phys. Uspekhi 1982, 25, 475.

(9)

Ederer, C.; Spaldin, N. A. Phys. Rev. B 2005, 71 (6), 060401–1.

(10)

Khomskii, D. Physics (College. Park. Md). 2009, 2, 20.

(11)

Laughlin, R. P.; Currie, D. A.; Contreras-Guererro, R.; Dedigama, A.; Priyantha, W.; Droopad, R.; Theodoropoulou, N.; Gao, P.; Pan, X. J. Appl. Phys. 2013, 113 (17), 20 ACS Paragon Plus Environment

Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17D919-1. (12)

Li, Y.; Sritharan, T.; Zhang, S.; He, X.; Liu, Y.; Chen, T. Appl. Phys. Lett. 2008, 92 (13), 132908.

(13)

Palai, R.; Katiyar, R. S.; Schmid, H.; Tissot, P.; Clark, S. J.; Robertson, J.; Redfern, S. A. T.; Scott, J. F. Phys. Rev. B 2008, 77, 14110.

(14)

Yang, S. Y.; Zavaliche, F.; Mohaddes-Ardabili, L.; Vaithyanathan, V.; Schlom, D. G.; Lee, Y. J.; Chu, Y. H.; Cruz, M. P.; Zhan, Q.; Zhao, T.; Ramesh, R. Appl. Phys. Lett. 2005, 87 (10), 102903.

(15)

Murari, N. M.; Thomas, R.; Melgarejo, R. E.; Pavunny, S. P.; Katiyar, R. S. J. Appl. Phys. 2009, 106 (1), 14103.

(16)

Velasco-Davalos, I. A.; Moretti, M.; Nicklaus, M.; Nauenheim, C.; Li, S.; Nechache, R.; Gomez-Yanez, C.; Ruediger, A. Appl. Phys. A Mater. Sci. Process. 2014, 115 (3), 1081.

(17)

Velasco-Davalos, I.; Ambriz-Vargas, F.; Kolhatkar, G.; Thomas, R.; Ruediger, A. AIP Adv. 2016, 6 (6), 65117.

(18)

Byrappa, K.; Yoshimura, M. Handbook of Hydrotermal Technology. A Technology for Crystal Growth and Materials Processing; Byrappa, M. Yoshimura-Noyes Publications: Park Ridge, NJ, 2001.

(19)

Kormarneni, S.; Menon, V. C.; Li, Q. H.; Roy, R.; Ainger, F. J. Am. Ceram. Soc. 1996, 79, 1409.

(20)

Das, S. R.; Bhattacharya, P.; Choudhary, R. N. P.; Katiyar, R. S. J. Appl. Phys. 2006, 99 (6), 97. 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(21)

Singh, S. K.; Ishiwara, H.; Maruyama, K. Appl. Phys. Lett. 2006, 88 (26), 262908–1.

(22)

Singh, S. K.; Ishiwara, H.; Sato, K.; Maruyama, K. J. Appl. Phys. 2007, 102 (9), 94109.

(23)

Murari, N. M.; Thomas, R.; Winterman, A.; Melgarejo, R. E.; Pavunny, S. P.; Katiyar, R. S. J. Appl. Phys. 2009, 105 (2009), 84110.

(24)

Nechache, R.; Harnagea, C.; Licoccia, S.; Traversa, E.; Ruediger, A.; Pignolet, A.; Rosei, F. Appl. Phys. Lett. 2011, 98 (20), 202902.

(25)

Nechache, R.; Harnagea, C.; Carignan, L. P.; Gautreau, O.; Pintilie, L.; Singh, M. P.; Menard, D.; Fournier, P.; Alexe, M.; Pignolet, A. J. Appl. Phys. 2009, 105 (6), 061621–1.

(26)

Baettig, P.; Ederer, C.; Spaldin, N. A. Phys. Rev. B 2005, 72, 214105.

(27)

Nechache, R.; Nauenheim, C.; Lanke, U.; Pignolet, A.; Rosei, F.; Ruediger, A. J. Phys. Condens. Matter 2012, 24 (14), 142202.

(28)

Velasco-Davalos, I.; Ambriz-Vargas, F.; Gómez-Yáñez, C.; Thomas, R.; Ruediger, A. J. Alloys Compd. 2016, 667, 268.

(29)

Li, J.; Wang, J.; Wuttig, M.; Ramesh, R.; Wang, N.; Ruette, B.; Pyatakov, A. P.; Zvezdin, A. K.; Viehland, D. Appl. Phys. Lett. 2004, 84 (25), 5261.

(30)

Nanoelectronics and information technology, 3rd ed.; Waser, R., Ed.; Wiley-VCH: Weinheim, 2012.

(31)

Singh, M. K.; Katiyar, R. S. J. Appl. Phys. 2011, 109 (7), 109–112.

(32)

Yang, Y.; Sun, J. Y.; Zhu, K.; Liu, Y. L.; Wan, L.; Yang, Y.; Sun, J. Y.; Zhu, K.; Liu, Y. L.; Wan, L. J. Appl. Phys. 2008, 103, 93532.


Page 22 of 31

Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(33)

Chen, Z.; Jin, W.; Lu, Z. J. Mater. Sci Mater Electron. 2015, 26, 1077.

(34)

Deng, H.; Deng, H.; Yang, P. J. Mater. Sci Mater Electron. 2012, 23, 1215.

(35)

Weckhuysen, B. M.; Wachs, I. E. J. Chem. Soc. Faraday Trans. 1996, 92, 1969.

(36)

Lou, X. J. J. Appl. Phys. 2009, 105, 94107.

(37)

Yang, H.; Wang, Y. Q.; Wang, H.; Jia, Q. X.; Yang, H.; Wang, Y. Q.; Wang, H.; Jia, Q. X. Appl. Phys. Lett. 2010, 96, 12909.

(38)

Leenen, M. A. M.; Arning, V.; Thiem, H.; Steiger, J.; Anselmann, R. Phys. Status Solidi 2009, 206 (4), 588.



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For Table of Contents Use Only Microwave-assisted hydrothermal synthesis of BiFexCr1-xO3 ferroelectric thin films Gitanjali Kolhatkar,1 Fabian Ambriz-Vargas,1 Reji Thomas,1 Andreas Ruediger1† 1

Institut Nationale de la recherche scientifique, centre Énergie, Matériaux, Télécommunications, 1650 Boulevard Lionel-Boulet, Varennes, Québec, J3X 1S2, Canada.

BiFeCrO3 films were synthetized using the micro-wave assisted hydrothermal method. The ferroelectric BiFeCrO3 phase is obtained after 10 min of hydrothermal deposition. Longer synthesis time deteriorate the film ferroelectricity due to crystallinity changes. The presence Cr into the BiFeO3 crystal structure affects its ferroelectric retention. Annealing improves the surface morphology but not the ferroelectric retention.

†

Corresponding author: Andreas Ruediger, Institut Nationale de la recherche scientifique, centre Énergie, Matériaux, Télécommunications, 1650 Boulevard Lionel-Boulet, Varennes, Québec, J3X 1S2, Canada. Email : [email protected]


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Figure 1. (a) 1 µm × 1 µm AFM image (b) PFM phase-contrast image and (c) Local phase-hysteresis loops of BiFexCr1-xO3 samples synthesized for 1, 2 and 3 cycles, respectively. 149x136mm (300 x 300 DPI)

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Figure 2. XPS spectra of the BiFexCr1-xO3 thin film deposited for 1 cycle of microwave irradiation showing (a) the general survey, (b) the Bi4f, (c) Fe2p, and (d) Cr2p signals. The background curves (dotted lines) based on the Shirley method are shown for each element. 139x118mm (300 x 300 DPI)


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Figure 3. Typical Raman spectra of BiFexCr1-xO3 powders after (a) 1 cycle (black line), (b) 2 cycles (red line) and 3 cycles (green line) of microwave irradiation; (d) presents a typical Raman spectrum of BiFexCr1xO3 powders after 1 cycle in the low wavenumbers. 72x31mm (300 x 300 DPI)



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Figure 4. PFM phase images of BiFexCr1-xO3 (a) 0 h, (b) 1 h30 min and (c) 3 h after poling. 39x18mm (300 x 300 DPI)


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Figure 5. PFM phase images of BiFeO3 (a) 0 h, (b) 1 h 30 min and (c) 3 h after poling. 36x16mm (300 x 300 DPI)



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Figure 6. 1 µm × 1 µm topography images of the BiFexCr1-xO3 sample synthesized for 1 cycle of microwave irradiation after annealing for 30 min (a) under O2 at 500°C, with an rms of 3.8 nm, (b) under O2 at 600°C, with an rms of 2 nm, (c) under O2 at 650°C, with an rms of 1.6 nm and (d) under vacuum at 600°C, with an rms of 0.4 nm. 129x102mm (300 x 300 DPI)


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Figure 7. PFM phase images of the BiFexCr1-xO3 sample synthesized for 1 cycle of microwave irradiation after annealing for 30 min (a) under O2 at 500°C (b) under O2 at 600°C, 0 h and 2 h 00 after poling, (c) under O2 at 650°C, 0 h and 2 h 00 after poling, (d) under vacuum at 600°C, 0 h and 2 h 00 after poling. 109x142mm (300 x 300 DPI)


Microwave-Assisted Hydrothermal Synthesis of BiFexCr1âxO3

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