Enhanced Piezoresponse and Dielectric Properties for Ba_1-XSr_XTiO₃ Composition Ultrathin Films by the High-Throughput Method

Abstract

The stacked single-unit cell Ba_1-xSr_xTiO₃ (BSTO) thin film designed by the high-throughput method is fabricated by layer-by-layer deposition by laser molecular beam epitaxy, and its ferroelectric and dielectric characteristics as a function of Sr concentration are comprehensively investigated. The permittivity of BSTO exhibits a monotonous increase by Sr with a plateau in the region of 14% < Sr < 85%. Meanwhile, at the low Sr doping regime, the piezoelectric response has been discovered, and the maximum piezoresponse and d₃₃ can reach approximately 139.05 pm and 88 pm/V once an appropriate Ba/Sr ratio is formed, exhibiting a coexistence of a dielectric property and giant piezoresponse. This effective piezoelectric constant d₃₃ value is significantly larger than the conventional chemical doping scenarios, suggesting that the intra-plane interaction is crucial for designing future promising dielectric and ferroelectric thin films via high-throughput technologies.

1. Introduction

In recent years, Ba_1-xSr_xTiO₃ (BSTO) films have been employed for the development of various multifunctional devices, such as microwave filters, piezoelectric sensors, access memory, and integrated circuits and microelectromechanical systems (MEMS) [1,2,3,4]. Not surprisingly, all these developments are dependent on a good fundamental understanding of the BSTO; as is known, the tunable competition of local lattice symmetry via the crystal field is extremely influenced by the Sr doping in BaTiO₃ (BTO) [5,6,7]. Conventionally, this investigation interest can be approached on BSTO by chemical ion doping via the solid reaction method, which can offer an accurate doping level [8]. Furthermore, the BSTO thin films can be obtained by several modern fabrication technologies, such as rf-sputtering, atom layer deposition, and pulsed laser deposition, etc. [9,10]. Abe et al. have grown different Ba/Sr ratio films using double-target rf-sputtering [11]. Costa et al. have prepared Ba_1-xSr_xTiO₃ films by the conventional solid reaction method [12]. Le et al. have grown Ba_1-xSr_xTiO₃ films using the atomic layer deposition technique (ALD) [13]. However, the conventional methods only focus on fabricating BSTO thin films with a single Ba/Sr ratio [14,15]. Moreover, lots of computational studies make use of a limited number of experimental points for comparison [16,17]. Local variations such as microstructural changes and stress variations cannot be easily observed, which may miss the “sweet spot” of the response surface [18]. Therefore, the multilayer ferroelectric thin films with composition gradients have attracted reasonable attention [19,20]. Compared with the traditional thin films, a high-throughput solution for a complex oxide can make the component and structure of the composition gradient thin films continuously modulable in the lateral direction, and the collection of multi-dimensional data drives new material to be discovered, then efficiently optimizes newly identified materials through combinatorial processing [21,22]. Therefore, it is necessary to further study their microstructure and properties to open up a new field in the application of ferroelectric thin films. The LMBE (laser molecular beam epitaxy) technique has been widely applied for the formation of high-quality composition-spread thin films, which is reliable and can predict film thickness and components, and it possesses the advantages of maintaining control of stoichiometry, crystal structure, and crystal orientation [23]. Not only the substitution effect, but also the high-throughput method seems to be a promising way to achieve a fundamental understanding of inter- or intra-interaction in the complex functional oxide. Therefore, we have designed an ultrathin composition BSTO film (<10 nm) in this way, which is different from the previous reports about the thick composite films [24]. Although the ultrathin ferroelectric films may not present interesting domain structures, their polarization properties would play an important role in modern electronics, such as the tunneling interlayer in the ferroelectric tunnel junctions, a typical information storage device. Meanwhile, we believe that the fundamental understanding of the ferroelectric thin films is driven to the nanometer scale; for instance, the critical thickness of the ferroelectric/piezoelectric thin films, etc. Intriguing phenomena in ultrathin films may offer new opportunities for the design of microdevices.

In this study, we have successfully fabricated continuous composition-spread ultrathin films and studied how the substitution affected the piezoelectric characteristics and dielectric properties of the BSTO thin film. The BSTO permittivity and loss increase and reach the maximum at the Sr-rich side, with a plateau in the region of 14% < Sr < 85%. Tracking of the changes in piezoelectric characteristics as functions of substitution concentration by the piezoresponse force microscope (PFM) can be revealed. The composition BSTO ultrathin film enables us to achieve the ferroelectricity at room temperature. The piezoelectric response and effective piezoelectric constant d₃₃ can reach the maximum of 139.05 pm and ~88 pm/V respectively, at the appropriate Ba/Sr ratio region. Our investigation provides correlations between components, structures, and properties, and can be used for future material design.

2. Experimental Details

BSTO composition thin films were epitaxially grown on (001) single-crystalline Nb:SrTiO₃ (Nb-STO, Nb: 0.7 wt.%) substrates by LMBE using a KrF excimer laser (Coherent COM PexPro 201). Prior to the deposition, the Nb-STO substrates were etched by NH₄F-buffered HF solution and then annealed at 960 °C for 60 min in flowing O₂ to form a TiO₂ single-terminated step-terrace surface (Supplementary Figure S1). The BSTO films were deposited at 2 Hz repetition, keeping the substrate temperature at 750 °C and the O₂ pressure at 0.1 Torr. Before depositing composition thin film, BaTiO₃ (BTO) and SrTiO₃ (STO) fully covered thin films were deposited on respective single-crystal substrates. The thickness of each layer is controlled by the number of laser pulses at a known growth rate d/p, where d and p are the thickness and number of the laser pulses, respectively. The thickness was mainly calibrated by XRR (X-ray Reflectometry, Smartlab 3 kw, Rigaku, Tokyo, Japan), and thus BTO and STO growth rates were confirmed at about 27 and 29 pulses per unit. During depositing composition thin film, BTO and STO targets switch and the mask motion was programmed to complete a cycle of STO thickness gradient from zero to one unit cell, and BTO from one unit cell to zero, in which Sr contains a range from 0 to 1, and the overall film thickness is about 96 Å. We characterized the local structure of the BSTO film (80 nm) with X-ray diffraction (XRD, Smartlab 3 kw, Rigaku, Tokyo, Japan), initially to confirm that the gradient composition was fabricated successfully. The Sr content variation was characterized by Scanning Electron Microscopy coupled energy dispersive X-ray spectroscopy (SEM–EDX, Regulus 8100, Hitachi, Tokyo, Japan). The piezoelectric and ferroelectric characteristics of the BSTO composition thin film were examined by PFM (Asylum Research MFP-3D, Asylum Research, Santa Barbara, CA, USA). The dielectric characteristics were measured using an Agilent 4294A meter (Agilent Technologies Inc., Santa Clara, CA, USA).

3. Results and Discussion

We have successfully grown continuous composition-spread thin film by LMBE with a moving shutter, and the schematic diagram is shown in Figure 1a–c. The composition thin film was divided into 7 regions, as shown in Figure 1d. According to the EDX results (Figure 1e), we calculated the Sr/(Ba + Sr) atom ratio, which indicates that the composition gradient varies almost linearly from 0 to 1, along the x-direction. From this measurement, we roughly estimated the Sr percentage of each region (RGN). Furthermore, the XRD patterns, as shown in the Supplementary Materials, evolve as a function of the composition of the BSTO. With the increase of Sr concentration, the peaks of BSTO approach that of Nb–STO. The surface morphology of the thin film was examined with atomic force microscopy (AFM). Sr-substituting thin films exhibited atomically flat surfaces. The scan for three parts of the entire film was shown in Figure 2, for the Ba–rich (RGN1), Ba/Sr equivalent (RGN4), and the Sr–rich sides (RGN7). The root-mean-square roughness determined at a scan size of 25 µm² was approximately 239 pm for the three regions of the BSTO composition thin film.

The domain structure and ferroelectric and piezoelectric characteristics of the BSTO/Nb–STO composition thin films were examined by PFM, as shown in Figure 3. We revealed that the first four regions of BSTO composition thin film were in the ferroelectric phase at room temperature. This fact was confirmed by writing an artificial ferroelectric domain pattern, as presented in Figure 3a–d. These regions of the thin film had an external voltage of ±8 and 10 V applied to polarize in two adjacent areas. Clear contrast of the PFM amplitude and phase images with a downward spontaneous polarization (pointing from BSTO to Nb:STO) can be observed. The contrast becomes affected when x is large, and we found that the latter three regions (RGN5, RGN6, and RGN7) of the BSTO composition thin film have weak contrast (not shown), indicating the absence of ferroelectricity. Both the out-of-plane amplitude image and the local hysteresis loops confirmed the piezoelectric and ferroelectric response of four regions (RGN1–RGN4) of the film. Domains observed in the four regions of the composition thin film were similar to those already reported for BSTO films. For example, the (Ba, Sr)TiO₃ (Sr content about 30%) film was in the ferroelectric phase at room temperature [25]. The Ba_0.8Sr_0.2TiO₃ film was measured ferroelectricity by its hysteresis loops, domain writing, and reading experiments [26]. A paraelectric phase appeared in the Ba_1-xSr_xTiO₃ thin films only at x > 0.6, as reported in [27]. The substitution-induced evolution of the structure–functional property correlation strongly depends on the ionic size of the element [22,28]. It is noted that the ferroelectric polarization of the BSTO composition thin film is related to the Ti⁴⁺ displacement in the TiO₆ octahedra. The volume of the TiO₆ octahedra decreases with an increase of Sr²⁺ content, and Ti⁴⁺ has less space to move off-center. This is because the lattice constants decrease with Sr concentration due to the fact that the ionic radius of Sr²⁺ (≈1.16 Å) is smaller than that of Ba²⁺ (≈1.36 Å) [29], which is consistent with experimental measurements as well as the theoretical studies [30,31]. Thus, the ferroelectricity of the BSTO composition thin film is expected to decrease with an increase of Sr content, and with increased Sr, the clamping imposed by the substrate may also affect the ferroelectricity [32].

RGN5, RGN6, and RGN7 of the sample exhibited low piezoelectric activity. The PFM switching spectroscopy revealed that the local hysteresis loops were obtained for the former four regions (RGN1–4) of compositions, as shown in Figure 4a, which implies that the latter regions of the sample (RGN5–7) are not ferroelectric at ambient temperature. Measurements were performed on various areas, and the most representative phase hysteresis loops of the particular composition are shown in Figure 4b. The relationship between Sr concentration and coercive voltage of the BSTO composition thin film is shown in Figure 4c. The coercive voltage of various regions of the composition thin film was extracted from the amplitude signal. The results show that the fourth region of the BSTO composition thin film has the largest average coercive voltage (8.72 V), revealing its relative difficulty of electrical domain switching. These results show that the higher the Sr concentration is, the more difficult polarization switching is. Due to the imprint voltage, the symmetric piezoelectric butterfly curves were shifted to be asymmetric. This could be caused by asymmetric electrostatic boundary conditions resulting from the different work functions between the two electrodes or by the defect dipole accumulation at the BSTO/Nb:STO interface due to interfacial diffusion [33].

Furthermore, we analyzed the local piezoresponse and effective d₃₃ extracted from these local loops. Local piezoresponse and effective d₃₃ are defined and calculated by two formulas with different variables [34]:

P = A × cos (θ)

(1)

Ed₃₃ = A/V_ac

(2)

where P is the piezoresponse, A is the amplitude value, θ is the phase value, Ed₃₃ is the effective d₃₃, and V_ac is the applied ac voltage. The effective d₃₃ enhanced a lot in RGN3, as shown in Figure 4d. The effective piezoelectric constant d₃₃ is high at an appropriate Ba/Sr ratio. The maximum values of the local piezoelectric response and effective d₃₃ can reach 139.05 pm and 88 pm/V, respectively. This is possibly due to the coexistence of different phase structures, which is similar to what has been reported in [35,36]. In our case, Sr content led to the distortions of TiO₆ octahedral caused by the instability of Ti⁴⁺, and the distortions of TiO₆ octahedral led to the lower-symmetry structure, which is possibly due to the coexistence of tetragonal-like and orthorhombic-like phase structures, such as at the morphotropic phase boundary-enhanced piezoelectric response. Although BSTO composition thin films have been intensively investigated in terms of ferroelectric and dielectric properties, their piezoelectric characteristics were only theoretically studied [37].

We have found interesting features in the dielectric behavior of composition BSTO ultrathin films. Figure 5a,b show the frequency dependence of the dielectric constant and dielectric loss of the BSTO composition thin film as a function of Sr contents. We deposited arrays of Au as electrodes (diameter = 200 μm) along with Sr contents’ increasing direction through a shadow mask. The schematic of capacitance measurement is shown in Figure 5d. In addition, the relative permittivity, ε_r, of the BSTO composition thin film was calculated from the mean capacitance, C, according to the following equation:

ε_r = Cd/ε₀S

(3)

where S represents the area of capacitors, ε₀ is the dielectric permittivity of the vacuum, and d is the thickness of the films. All the regions exhibited a relatively high dielectric constant at low frequency, which slowly fell and attained a level of stability. The maximum value of the dielectric constant was ~44 in RGN7 and the minimum was in RGN1, about ~5. The increasing trend is shown in Figure 5c in ε_r values of the composition thin film at 100 kHz. The result clearly shows that by increasing the Sr²⁺ content, a significant improvement in dielectric constant is possible, and the dielectric constant appeared to have reached a plateau from RGN2 to RGN6. This is possibly due to the coexistence of different phase structures due to the distortions of TiO₆ octahedral with increasing Sr content, and the low symmetry may enhance the dielectric constant values [38,39]. Therefore, systematic studies on tracking changes of the structure are required. On the other hand, the electrode properties, such as poor contact interface state and interfacial charge, can influence the films’ properties [40]. As others have reported, the dependence of the permittivity as a function of the Sr content increased first and reached the maximum around an Sr content of 0.5 [24]. However, in our scenario, the trend was presented differently, which may be caused by the large proportion of the interfacial effect due to the ultrathin thickness of our sample (9.6 nm). We hope to clarify the mechanism of the interface in our further research. Remarkably, the loss tangent shown in Figure 5b is very small for all different Sr content positions, remaining at 0.01–0.05 in the high-frequency range. The dielectric properties of BTO films can be improved by adding Sr²⁺ into the films. The controlled dielectric response of ultrathin films by substituting the A-site makes it possible to use in the microelectronics industry. The impact of Sr-substituting on the properties of BSTO is a crucial determining factor, and the film size effect and defect chemistry or constraints such as electrode interactions can also be performed on properties in a complex manner [41,42]. However, it is yet not clear what the mechanism of substituted ultrathin perovskites’ electronic properties is, due to the limited thickness, which still needs further research.

4. Conclusions

In this study, we showed that lateral compositionally graded BSTO ultrathin film was successfully grown by LMBE with the mask technique, and the piezoelectric characteristics and dielectric properties were investigated. Although each composition along the gradient exists only in a narrow part of the sample, scanning probe microscopy techniques make it possible to separately probe the properties at each composition. The local hysteresis loops revealed that the ferroelectric characteristics at room temperature to some extent also existed in composition films with higher Sr content. The piezoelectric response was obtained for the composition thin film with low Sr content. At the low Sr doping regime, the maximum piezoresponse and the effective d₃₃ can reach approximately 139.05 pm and 88 pm/V once an appropriate Ba/Sr ratio is formed. Lower piezoelectric activity with an increase in x indicates that substitution influences the domain structure and polarization switching. The local permittivity measurements revealed a monotonous increase of Sr with a plateau in the middle region, showing the dependence on Sr concentration of the BSTO composition ultrathin film. These data are based on correlations between components and properties in order to open up a new field of ferroelectric thin films.