Low Temperature Fabrication of BiFeO3 Films on Aluminum Foils under an N2-Rich Atmosphere

1. Introduction

BiFeO₃ (BFO) has been extensively investigated as a promising multiferroic material in recent years [1,2,3,4,5,6,7,8]. Studies have reported on enhancing the ferroelectric properties of BFO films, such as introducing buffer layers, doping the isovalent or aliovalent ions, and domain engineering [9,10,11,12,13,14,15,16,17]. However, obtaining P-E hysteresis loops with high rectangularity in polycrystalline BFO films using chemical solution deposition methods remains a challenge, especially under low annealing temperature (< 500 °C). Most efforts have focused on improving the quality of BFO films [9,10,11,12,13,14,15,16,17,18,19], however, few studies have focused on the bottom electrode as well as the interface between the bottom electrode and film, though these factors remain equally important. In general, obtaining desired contact at the interface between the traditional substrate (such as Si) and film may be hardly achieved due to slight thermal deformation during low temperature (<500 °C) annealing treatment. Recently, Kingon et al. successfully obtained high quality Pb(Zr_0.52Ti_0.48)O₃ films directly on base metal copper foils, providing a new strategy for choosing base metal foils as the bottom electrodes [20]. As a common base metal, aluminum (Al) foil may serve as an alternative electrode for BFO ferroelectric films, as Al₂O₃ can readily form a very dense, stable, and extremely thin (~5 nm) layer. This may effectively reduce the leakage current and lower the risk of breakdown. Furthermore, the thermal expansion coefficient of Al (23.8 ×10⁻⁶ /°C) [21] is much higher than that of Si (3.6 × 10⁻⁶/°C) [22], thus the tight contact interface between the BFO film and Al substrate can be expected even at a low annealing temperature. In this work, aluminum foils were adopted as substrates for preparation of BFO films using the metal organic decomposition (MOD) method. High temperatures often cause serious problems, including interdiffusion, charged defects, phase decomposition, and valence fluctuations, and these issues can damage a film’s electrical properties and performance stability [23]. To enhance the performance of BFO films and provide complementary metal oxide semiconductor (CMOS)-compatibility, a low processing temperature below 500 °C was required. The annealing atmospheres on properties of BFO films on Al substrates were discussed, it was found that an N₂-rich atmosphere can facilitate the crystallization of BFO films and thus lower the annealing temperature. Therefore, low temperature preparation for BFO films were attempted. By adopting an N₂-rich atmosphere and the aluminum foil substrates, decent ferroelectric properties (P_r~70 μC/cm²@ 1118.1 kV/cm) were obtained at a very low annealing temperature of 365 ±5 °C.

2. Materials and Methods

BFO films (~800 nm) were fabricated on mirror aluminum foils (Surface roughness~0.02±0.005 μm, thickness~0.3 mm. Size~10mm×10mm) using the MOD process. The precursor solution was prepared by dissolving bismuth nitrate and iron nitrate in acetic acid and ethylene glycol according to the stoichiometric ratio. 5 mol% excess bismuth was used to compensate the volatilization of Bi₂O₃. The ratio of acetic acid and ethylene glycol was 3:1, and the solution concentration was 0.1mol/L. The films were deposited onto the Al foils by spin coating and then annealed layer by layer for 10 min at 365~440±5°C in N₂ -rich (BFO_N) or O₂ -rich (BFO_O) atmospheres (During the annealing process, N₂ or O₂ is introduced at a rate of 1.5sccm.) Each layer of film was deposited onto the substrate by spin coating at 4000 rpm for 30 s. Au top electrodes were deposited using a sputtering system through a shadow mask with a diameter of 0.2 mm on the films. Crystallographic characteristics of BFO films were analyzed by using standard X-ray diffraction (XRD) 2θ-scans in a Dmax-2500PC diffractometer (Rigaku, Japan) equipped with a Ni-filtered Cu-Kα radiation source (λ = 1.54184 Å). Surface morphologies were characterized using a SU-70 thermal field emission scanning electron microscope (SEM) (Hitachi, Japan). The chemical bonding states of the constituent elements were analyzed using a Thermo Scientific™ K-Alpha™ (hν = 1486.6 eV) X-ray Photoelectron Spectrometer (Thermo Fisher Scientific, USA). The energy resolution and the spatial resolution of the spectrometer were 0.5 eV and 50 μm, respectively. A Precision Premium II ferroelectric tester (maximum voltage of 99.9 V, Radiant Technology, USA) was adopted to measure the ferroelectric properties and leakage currents. The soaking time was 500ms during leakage test under the unswitched linear mode.

3. Results and Discussion

3.1. Performance of BiFeO₃ under Different Atmospheres Annealed at 440 ± 5 °C

3.1.1. Microstructure Analysis

The polycrystalline perovskite structures with a bulk-like rhombohedral phase were obtained in both the BFO_N and BFO_O films at 440 ± 5°C [23], as shown in Figure 1. No secondary phase was observed. This demonstrated that the Al foil was a suitable substrate material for the preparation of BFO films. The global average grain size calculated via Scherrer formula for BFO_N and BFO_O films were shown in table1. The grain size of BFO_N film was larger than that of BFO_O film. Furthermore, the local average grain sizes via SEM analysis (Figure 2, from a statistical analysis of 100 grains via the Nano Measurer software) have also been shown in Table 1. The similar results can be indicated that the reduced atmosphere could facilitate grain growth. The difference between the global average grain size and the local one may be related to the grain size nonuniformity.

Figure 2 shows the surface morphologies of the BFO films. A compact surface and larger grains were observed in the BFO_N films compared to the BFO_O films at 440 ± 5°C, possibly due to the higher content of oxygen vacancies under a reducing atmosphere than that of O₂ rich atmosphere. Reports have demonstrated that oxygen vacancies can facilitate the diffusion of ions during the annealing process [24]. As shown in Figure 2(b), the BFO_N film was mainly composed of large block-like grains, which could be ascribed to the presence of the grain merger phenomenon during the grain growth process. Notably, the observed white fine grains in surface of the BFO_N film may be Bi₂O₃ grains, because the relatively high content of oxygen vacancies in the BFO_N film possibly accelerated the diffusion of Bi₂O₃. As a result, significantly more Bi₂O₃ potentially migrated from the interior to the surface. From Figure 2 (a), the granular surface morphology with uniform small grains were observed in BFO_O film. Obvious voids were located at the grain boundaries, deteriorating the densification of the BFO_O film and its electric properties.

The X-ray photoelectron spectroscopy (XPS) spectrum of O1s core levels of BFO_O and BFO_N thin films are shown in Figure 3. A strong peak at ∼529.5 eV was observed for BFO_O and BFO_N thin films, which corresponds to oxygen in the perovskite lattice (O_L). The peak at ∼531.2 eV was the surface-absorbed oxygen (O_A) [23]. To verify this, the surface of BFO_O and BFO_N films were etched with a thickness of ~ 3nm. No peaks can be obtained at ∼531.2 eV in the interior BFO_O and BFO_N thin films. Furhemore, no obvious oxygen vacancies peak was observed in BFO films, which maybe related to the low amount of oxygen vacancies in BFO films annealing in a rapid thermal annealing furnace in air.

3.1.2. Room Temperature Electrical Properties

Figure 4 shows the electrical properties of BFO_O and BFO_N films at 440 ± 5°C. Figure 4(a) and (b) presents the typical P-E loops of the BiFeO₃ films. When the applied electric field was sufficiently large, well rectangular-shaped, saturated P-E hysteresis loops were observed in the BFO_N and BFO_O film. A large remanent polarization (P_r~78.1 μC/cm²) and high rectangularity (~91.3%) of the hysteresis loop was observed in the BFO_N film at the applied electric field of 1165.2 kV/cm. A high remanent polarization (P_r~79.6 μC/cm²) and decent rectangularity (~86.8%) of the hysteresis loop was observed in the BFO_O film at the applied electric field of 915.4 kV/cm. Compared to the BFO_N film, the P-E loops of the BFO_O film displayed a somewhat roundish shape, indicating contributions from the leakage current induced by the voids and more grain boundaries [25]. The normalized pulsed polarization（ΔP=P* (switched polarization)-Pˆ (nonswitched polarization）with retention time and switching cycles were shown in Figure 4 (c) and (d). Both the BFO_N and BFO_O films exhibited good charge-retaining ability for up to 1.0 × 10³ s. The improved charge-retaining ability (~1% loss) in the BFO_N film compared to that (~4% loss) for the BFO_O film was attributed to the reduced domain backswitching induced by the grain boundary defects and voids in the former than the latter [25]. As shown in Figure 4(d), good fatigue resistance up to 1.0 × 10⁹ switching cycles was observed in both the BFO_N and BFO_O films, which could be attributed to the improved interface by using Al substrates as well as the low oxygen vacancies as discussed in Figure 3.

The leakage performances for BFO_O and BFO_N films are shown in Figure 5, the soaking time was 500 ms during leakage test under the unswitched linear mode [26]. From Figure 5 (a) and (b), we can see that the leakage current density (J) of BFO_N film was slightly higher than 1 time that of BFO_O film but less than 2 times, which can be ascribed to the slightly lower oxygen vacancy concentration in BFO_O film as discussed in Figure 3. To clarify the dominant leakage mechanism involved in BFO films, the linear fitting of leakage behaviors are shown in Figure 5(c)-(f) based on leakage mechanisms formulas [27]. From Figure 5(c) and (e), the Fowler-Nordheim tunneling (FN tunneling) [ln (J/E²)∝1/E] mechanism induced by the interface electrons activated into the conduction band by tunneling involved in BFO_N film under the positive electric field (<251.0 kV/cm) [28,29]. The FN tunneling mechanism observed at low electric field maybe related to the good crystallization of BFO_N film. The Ohmic conduction mechanism (log J∝log E) can be observed at high electric field (>251.0 kV/cm), which should be related to the free oxygen vacancies in the film under the positive electric field. With increasing the electric field, the electrons transitioning from the interface to the body may form defect complexes with oxygen vacancies. The formation of defect complexes suppressed the electron transition from the interface to the conduction band and changing the leakage mechanism [30]. During the negative electric field, FN tunneling mechanism (<326.3 kV/cm) and space-charge-limited conduction (SCLC) (log J∝2log E) were involved in the leakage behavior of BFO_N film. From Figure 5(d) and (f), the SCLC mechanism (< 100.4 kV/cm) and the FN tunneling mechanism (>100.4 kV/cm) were predominant under positive electric field. The Ohmic conduction mechanism (<150.6 kV/cm) and FN tunneling mechanism (>150.6 kV/cm) can be observed under the negative electric field.

According to the above discussion, it can be concluded that by adopting an N₂-rich atmosphere, improved crystalline quality could be achieved in the BFO films at 440 ± 5°C, resulting in better ferroelectric properties. Furthermore, a lower annealing temperature was expected for the BiFeO₃ films with Al substrates and an N₂-rich atmosphere. Therefore, an attempt was made to fabricate the BFO films below the annealing temperature of 400 °C. Finally, BFO films with decent electric properties were achieved a low temperature of 365 ±5°C (BFO_N365) on Al substrates under an N₂-rich atmosphere. The detailed properties of the BFO_N365 film are shown in Figs. 6 and 7.

3.2. Performance of BiFeO₃ Film under an N₂-Rich Atmospheres Annealed at 365 ± 5 °C

3.2.1. Microstructure Analysis

The XRD 2θ-scan pattern and surface SEM image of the BFO_N365 film are shown in Figure 6. A bulk-like random polycrystalline structure with obvious (100), (110)/(104) peaks was observed, indicating that the BFO films were crystallized even at a low annealing temperature of 365 ±5°C. A dense morphology with fine nanograins were clearly revealed, indicating that the film has crystallized at this low annealing temperature.

3.2.2. Room Temperature Electrical Properties

Typical P-E loops were observed in Figure 7(a) for the BFO_N365 film, and a sizable remnant polarization (P_r~70 μC/cm²) and decent rectangularity (~75.0%) of the hysteresis loop at the applied electric field of 1118.1 kV/cm were obtained. These observations indicated that Al foil served as a suitable substrate for fabricating ferroelectric BiFeO₃ films under low annealing temperatures. As shown in Figure 7(b), the leakage current density of BFO_N365 was lower than ~1×10⁻⁴ A/cm² under the electric field of 353.5 kV/cm. Figure 6(c) and (d) illustrates that good fatigue resistance with a low loss (~5.8%) up to 1.0 × 10⁹ switching cycles for positive polarization was observed, and the loss for negative polarization under the same testing conditions was ~27%. Retention testing was carried out for BFO_N365 films, a degradation of ~29% for the negative polarization was observed after 1.0 × 10³ s, which is much higher than that for the positive polarization (~14%). The improved fatigue and retention performance at the positive side could be ascribed to the reduced interfacial defects (e.g. vacancies, grain boundaries or amorphous regions) at the bottom electrode under a long exposure to a processing temperature. The achieved ferroelectric characteristics of the BFO film directly deposited on the substrate without a buffer layer under the temperature of 400 °C have not yet been reported. This was attributed to the N₂-rich atmosphere, the Al foil substrate, as well as the homogeneous precursor solution.

4. Conclusions

In summary, the BFO films were successfully fabricated at low annealing temperature (< 450 °C). A common base metal, aluminum foil was adopted as the bottom electrode and substrate for BFO ferroelectric film. Compared to an oxygen-rich atmosphere, a nitrogen-rich atmosphere was more conducive to optimizing the performance of BFO films. Along with excellent retention and fatigue properties, P-E loops with a large P_r value (~78.1 μC/cm²) and high rectangularity (~91.3%) at the applied electric field of 1165.2 kV/cm were obtained in the BFO films deposited under an N₂-rich atmosphere at 440 ± 5°C. By using an N₂-rich atmosphere as well as the Al substrates, BFO films with decent electric properties were achieved a very low temperature of 365 ±5°C. This offers a new strategy for lowering the annealing temperature of BFO films.