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Rapid epitaxial growth of magnetoelectric thick BiFeO3 films by hybrid liquid-phase epitaxy
ARTICLE IN PRESS
Journal of Crystal Growth 293 (2006) 128–135 www.elsevier.com/locate/jcrysgro
Rapid epitaxial growth of magnetoelectric thick BiFeO3 films by hybrid liquid-phase epitaxy Meicheng Lia,b,, Ahmed Kursumovica, Xiaoding Qia, Judith L. MacManus-Driscolla a
Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK b Department of Materials Physics and Chemistry, Harbin Institute of Technology, Harbin 150001, China Received 11 February 2006; received in revised form 30 March 2006; accepted 7 April 2006 Communicated by M. Kawasaki Available online 27 June 2006
Abstract Epitaxial magnetoelectric BiFeO3 thin films have been prepared on SrRuO3-buffered (0 0 1) SrTiO3 substrates using a novel hybrid liquid-phase epitaxy (HLPE) approach, which is a solid–liquid growth process with delivery from a vapour source by pulsed laser deposition (PLD). The HLPE process enables rapid ‘‘liquid assisted’’ growth of epitaxial films of many functional materials without some of the disadvantages of classical liquid-phase epitaxy (LPE) and of PLD. High growth rates up to 2 nm/s were demonstrated in dense films of 350 nm thickness. The method has great promise for fabrication of device materials where thick, dense epitaxial films are required. r 2006 Elsevier B.V. All rights reserved. PACS: 81.15.Aa; 81.40.Rs; 81.70.Ex Keywords: A1. Hybrid liquid-phase epitaxy; A2. Thin films; B1. BiFeO3; B2. Magnetoelectric materials
1. Introduction Coupling different combinations of electric, magnetic, and structural order parameters results in so-called multiferroics possessing ferroelectricity, ferromagnetism, or ferroelasticity [1]. For simultaneous ferroelectricity and ferromagnetism (magnetoelectrics) there are potential applications in information storage, spintronics, and in magnetic or electric field sensors [2]. The perovskite BiFeO3 (BFO) is antiferromagnetic below the Neel temperature of 647 K and ferroelectric with a high Curie temperature of 1043 K [3,4]. It exhibits weak magnetism at room temperature (RT) due to the residual moment from a canted spin structure [5]. Bulk BFO has a smaller spontaneous polarization value than the expected value for a ferroelectric with such a high Tc. Recently Corresponding author. Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK. Tel.: +44 1223 767140; fax: +44 1223 334567. E-mail address: [email protected] (M. Li).
0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.04.108
strong magnetoelectric behaviour was reported in thin films with ferromagnetism (strong RT saturation magnetization of 1 mB/Fe) and ferroelectricity (polarization saturation polarization of 90 mC/cm2) [2]. Following this report, the study of BFO thin films has attracted much recent attention. In fact, not only is BFO potentially useful as a magnetoelectric but also as a piezoelectric to replace Pbcontaining Pb(ZrxTi1x)O3 (PZT). Most BFO thin films reported so far have been grown by pulsed laser deposition (PLD) [6–9] although metalorganic chemical vapour deposition (MOCVD) [10] and liquidphase epitaxy (LPE) [11,12] have also been used successfully. The effects of deposition conditions and epitaxial constraints on crystallization and properties of BFO thin films have been extensively studied [6–8]. In addition, the influence of different ionic substitutions on the structure and magnetoelectric properties of films has been investigated [11,13]. Electric leakage is a remaining issue to be solved before BFO is practically implemented in any devices at RT. Another issue for potential piezoelectric applications is the requirement to grow thick (41 mm) films.
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It is likely that nonstoichiometry and second-phase formation are the factors which cause leakage in BFO. It has been suggested that oxygen nonstoichiometry leads to valence fluctuations of Fe ions in BFO, resulting in high conductivity [14]. However, our recent research work using coulometric titration has showed that BFO is only very slightly oxygen nonstoichiometric [15] and so it is likely that charge carries are induced by cation vacancies. The thickness dependence of the polarization, piezoelectric coefficient, and dielectric constant of the epitaxial BFO films has been studied by Wang et al. [2]. The piezoelectric constant d33 shows a dramatic increase with thickness and values of 85 pm/V have been achieved in 400 nm-thick films, which is comparable to around 25–500 pm/V in PZT. Thus, thicker BFO films should have even larger piezoelectric coefficients, making them useful for applications in micro-electro-mechanical system (MEMS) and actuators. A fast, reliable growth method with the ability to maintain film stoichiometry and a dense microstructure is required for this purpose. It has been proposed that higher leakage in BFO can be greatly suppressed by the very high heating rate, short sintering period, and liquid-phase sintering [16]. Here, we consider a novel approach for growth of BFO films which combines a liquid growth process with vapour growth. The technique is called hybrid liquid-phase epitaxy (HLPE) [17], and the method involves growth under a thin liquid layer. BFO is an ideal system to grow by HLPE because a low melting point eutectic liquid is formed at around 785 1C [18]. Using HLPE, we demonstrate growth of highly crystalline, oriented epitaxial BFO films on SrTiO3 (STO). In all kinds of methods used for growth of epitaxial films, the primary factor determining the nucleation frequency and growth mode is the thermodynamic driving force, or relative supersaturation s. A key characteristic of HLPE is the presence, during film growth, of a thin layer of liquid flux which is fed with appropriate chemical components to maintain the constant supersaturation necessary for steady-state growth. The self-consistent adjustment of the steady-state supersaturation to match growth rate and feed rate makes the description of the mechanisms that control the process easy to quantify. This is not the case for classical LPE or the various physical vapour deposition (PVD) methods. Classical LPE is regarded as a low supersaturation, near-equilibrium growth process. Whereas, PVD is commonly treated as a high supersaturation growth process [19–21]. So, for deposition from a thin flux layer, diffusion through the liquid is no longer the limiting constraint as it is for LPE and so a larger range of s can be explored and maintained by varying the deposition conditions. In this work, the liquid flux was fed from the vapour using high rate PLD under a variety of oxygen pressures. During HLPE growth, a thin liquid layer can be stabilized below the bulk melting temperature by a free energy contribution coming from the interface [22,23]. However, the growth was only explored at temperatures or
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at just above the bulk equilibrium temperature for eutectic melting of the flux. Feeding from a vapour source is incidental to film growth through the liquid layer, and hence the term vapour–liquid–solid (VLS) growth is not broad enough to cover the general case for HLPE. Liquid–solid (LS) growth is, in fact, the correct terminology. In HLPE of BFO, growth occurs by BFO diffusive transport through the thin liquid Bi2O3–Fe2O3 surface flux layer to the BFO seed film. The growth is controlled by adjusting the rate of feeding of the liquid layer with Bi2O3rich material at a constant temperature; in this case s, and the corresponding undercooling adjust self-consistently to match the growth rate to the feeding rate. Since the liquid flux layer can be as thin as 100 nm, the flux cannot dissolve a large amount of the substrate or seed layer, and the transit time for diffusion of BFO through the layer is of the order of 104 s, defining a time constant for equilibration of s. Thus HLPE embodies all the advantages of LPE without the disadvantages of difficult nucleation and a large volume of aggressive liquid flux. The thermodynamic driving force Dm/kBT controls both the nucleation and the growth rates, where Dm is the change in chemical potential and kB is the Boltzmann constant. Dm/kBT is defined as [24,25] Cd Dm=kB T ¼ ln , Ce where Ce and Cd are the equilibrium concentration and actual solute concentration in the flux at the growth temperature, respectively. When growth occurs under a flux liquid layer with the background solute concentration Cd, a solute concentration gradient forms across some thickness d due to finite diffusion constant D. In the case pffiffiffiffiffi ffi of stationary solution, d increases with time t as Dt. For the process with flux steering, this d can be made rather small (tens of micrometers) and kept constant, resulting in so-called diffusion boundary layer [26]. For the LPE process, supposing an existence of a finite surface coefficient the growth rate through a liquid layer of thickness d can be expressed as [26,27] F¼
D 1 ðC d C e Þ , d 1þR C xt
where the dimensionless parameter R reflects the relative importance of diffusion coefficient and kinetic coefficient as the rate-limiting process [27]. However, in HLPE process the solution is still, and the flux layer is thin (50 nm), henceafter the initial transient d becomes equal to the flux thickness. In this case, solute supersaturation depends on the feeding rate and on the deposition temperature. The effects of composition, temperature and oxygen partial pressure on solid–liquid equilibria are required to assess the optimum flux composition and growth conditions for the films: for growth of films in multilayered structures usually a low growth temperature is desirable to minimize the diffusion of impurities.
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Two of the phase boundary lines determine the theoretical limits of LPE growth: the BFO stability and the flux solidification lines. Both of these boundaries are sensitive to the flux composition (primarily the Bi: Fe molar ratio) and the oxygen partial pressure p(O2). The stability of BFO in the flux is also related to the peritectic temperature and oxygen partial pressure p(O2) [15]. 2. Experimental procedure The targets for PLD growth of BFO by HLPE were made in-house using high-purity commercial Bi2O3 and Fe2O3 powders. Flux targets were prepared with a Bi:Fe molar ratio of 4:1. Standard PLD was carried out using a KrF excimer laser (Lambda Physik Compex 205) with radiation at l ¼ 248 nm, with energy density of 12 J cm2, and with repetition rates from 5 to 50 Hz. The BFO films were grown on a 50 nm underlayer of SrRuO3 (SRO) on 5 10 mm2 (0 0 1) STO substrates. The SRO films were grown at 650 1C and under 8 Pa oxygen pressure using a stoichiometric target. The substrates were maintained at temperatures from 700 to 805 1C during deposition using a heater of a tubular form, ensuring a uniform substrate temperature by radiation rather than by thermal contact. An epitaxial BFO seed film of 20 nm thickness was deposited onto the SRO-buffered substrate. A flux layer of 100 nm thickness was then deposited onto the SRO at just above its melting point (by 15 1C). This was immediately followed by deposition of BFO. The BFO feeds the flux layer, and the Bi and Fe chemical constituents diffuse to the surface of the BFO seed layer to nucleate BFO. As the BFO grows, the phase layer is continuously pushed away from the substrate so that it always remains on the surface. The process is illustrated in Fig. 1. After deposition, films were cooled to 450 1C, and annealed in situ for 1 h in 20 kPa oxygen. To explore the optimized process of HLPE for BiFeO3 growth, experiments were carried out over a range of temperatures between 700 and 805 1C at a range of oxygen pressures from 0.8 to 16 Pa O2.
We note that the Bi2O3–Fe2O3 flux layer was not deposited at a temperature below its melting point and then heated to the melting point or above followed by feeding of the flux with BFO. The reason of this is that the BFO concentration in the flux would build gradually up from zero, and then nucleation and initial growth would take place at low s, which would mean slower nucleation and growth, allowing time for the flux to attack the seed layer substrate. Field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectrometry (EDS) were used for surface morphology and composition measurements of the deposited films. The crystal phases present and the inplane and out-of-plane crystal orientation were determined by X-ray diffraction (XRD) and high-resolution XRD (HRXRD). The magnetic properties of the films were studied using a Princeton Measurements Corporation (Princeition, NJ Model) vibrating sample magnetometer (VSM). Ferroelectric measurement were carried out on some of the films using a commercial ferroelectric tester (Radiant Technologies, Inc.). 3. Results and discussions Firstly, a flux film grown on a BFO-seeded SRObuffered STO substrate was deposited and measured. A surface SEM image of the sample quenched from 795 1C is shown in the inset of Fig. 2. Solidified flux regions are apparent on the surface of the BFO film. EDS analysis shows that the Bi:Fe ratio in this region is approximately 4:1, which is equal to that of the flux target. The Bi:Fe ratio in-between the flux regions was approximately 1:1, as expected, since this is the composition of the seed layer. The phases present in this flux sample were analysed by XRD (Fig. 2). (0 0 l) diffraction peaks from BFO, SRO, and STO were observed. The peaks of the 30–50 nm thick SRO were too weak to be resolved from the tails of the intense STO peaks. Meanwhile, additional secondary-phase peaks, e.g., (2 0 1), (0 0 2), (2 2 0) b-Bi2O3 (tetragonal P4¯ 21c(1 1 4), a ¼ 0.7741(3) nm, c ¼ 0.5634(2) nm) BFO Vapour
Flux layer remains on surface during growth BFO nucleates on BFO seed layer
~ 100nm ~ 350nm ~ 20nm ~ 50nm
BFO seed layer Conductive SRO layer
STO Substrate
Fig. 1. The schematic diagram of HLPE growth process.
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Flux layer with Seed layer on SRO/STO
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SrTiO3 (002) Bi2O3 (002)
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Fe2O3 (214)
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Bi2O3 (201)
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Bi2O3 (220)/ Fe2O3 (104)
BiFeO3(00/)
SrTiO3 (001)
Intensity (log/a.u.)
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Fig. 2. XRD patterns of flux layer grown on SrRuO3/SrTiO3; inset is SEM image of the sample quenched from 795 1C.
and (1 0 4), (2 1 4) a-Fe2O3 (rhombohedral , R3¯ c, a ¼ 0.5112 nm, c ¼ 1.382 nm) peaks were observed. These Fe2O3 phases arise from the surface flux layer. At first set of BFO films were grown to study the influence of deposition temperatures on the quality of film epitaxy. Fig. 3(a) shows the XRD patterns of three BFO samples deposited on SRO/STO for 400 s at 785, 795, and 805 1C, respectively at a pO2 of 8 Pa and 50 Hz repetition rate. The composition and structure of the films were found to be quite sensitive to the deposition temperature. For the sample deposited at 785 1C (the peritectic temperature of flux) diffraction peaks from (0 0 1), (0 0 2) BFO and (0 0 1), (0 0 2), (0 0 3) STO were observed with a weak peak attributable to (2 1 1) Bi2O3 and two very weak peaks that may be indexed as (0 1 2) and (0 4) a-Fe2O3. For the sample deposited at 795 1C (15 1C higher than the flux eutectic temperature), (0 0 1), (0 0 2), and (0 0 3) BFO diffraction peaks were found, while the diffraction peaks from aFe2O3 were much weaker, and the Bi2O3 peaks stronger. Upon increasing the deposition temperature further to 805 1C, the (0 1 2) and (0 2 4) a-Fe2O3 diffraction peaks became stronger again, while the peaks from BFO were weaker and the peak of (0 0 3) BFO peak was barely above the background level, as for the 785 1C sample. In summary, the optimum films were grown at 795 1C at just above the flux melting temperature. Another set of samples was grown to study the influence of oxygen on the film quality. The XRD patterns of BFO thin films grown at 50 Hz repetition rate on SRO-buffered STO at 795 1C (optimized growth conditions from above) under various oxygen deposition pressures are shown in Fig. 3(b). As the deposition pressure decreased from 16 to 8 Pa, the (1 0 l) peaks of the BFO films were enhanced, while the peaks of Bi2O3 decreased in intensity. We note
that while the deposition conditions for the samples of Fig. 3(a) and Fig. 3(b) (795 1C) were nominally identical, in fact small differences in the actual deposition conditions led to small differences in the amount of residual Bi2O3 from the solidified flux. When the deposition pressure decreased to 0.8 Pa, the diffraction peaks from Bi2O3 disappeared. However, two peaks identified as (0 1 2) and (0 2 4) a-Fe2O3 were then found. These analyses indicate that at a fixed deposition temperature the crystallization of BFO thin films is affected considerably by the oxygen partial pressure. No Bi2Fe4O9, which is commonly found in PLD films of BFO [28,29] was observed. Fig. 4 shows XRD patterns (linear scale) for the films grown on SRO-buffered (0 0 1) STO substrates by HLPE under optimized processing conditions of 795 1C, 8 Pa pO2, and laser repetition rate of 50 Hz. The BFO shows a high degree of epitaxial growth with stronger (0 0 l) diffraction peaks from BFO and STO. The left inset curve in Fig. 4 shows the rocking curve of (0 0 1) BFO, with full-width at half-maximum (FWHM) of 0.371. Reciprocal space mappings using HRXRD revealed that the BFO film by HLPE has a rhombohedral structure, the lattice parameters are: a ¼ 3.9638, a ¼ 89.52. Hence, the film by HLPE was oriented along the c-axis of the unit cell, and the degree of BFO epitaxy was found to be comparable to classical LPE. An XRD pole figure of the 1 2 0 reflections from a film grown on a (0 0 1) STO substrate at 795 1C, 8 Pa pO2, with 350 nm thickness is shown in Fig. 5. There are two sets of peaks, one from (1 2 0) BFO, and the other from (1¯ 2 0) BFO. Each set includes four peaks at tilt angles c of 26.221 and 63.781, respectively, with the four peaks being 901 apart from each other. The HLPE film is highly biaxially textured and has a four-fold symmetry, indicating either cubic or pseudo-cubic symmetry.
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Cu/Kβ
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Bi2O3(203)
Cu/Kβ
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Bi2O3(002) Bi2O3(211)
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(a)
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Fe2O3(024)
Cu/Kβ Cu/Kβ
Bi2O3(211) Bi2O3(211)
Fe2O3(012)
Bi2O3(111)
Fe2O3(012)
Intensity (log/a.u.)
Fe2O3(012)
BiFeO3(001) and (002) SrTiO3(00/)
20
805°C
Fe2O3(024)
132
50 2θ (degree)
795°C pO2=8Pa
795°C pO2=8Pa
60
70
Fig. 3. XRD pattern of BiFeO3 thin film grown by HLPE at different deposition temperatures (a) and at different oxygen pressures (b).
The degree of in-plane orientation of the films was also assessed by examining XRD f-scans (not shown). The peaks for the (1 2 0) reflection of the film occurs at the same azimuthal y angle (to within 0.71) as that for the (1 0 0) STO reflection, indicating a ‘‘cube-on-cube’’ epitaxial growth on STO. The FWHM of the peaks is around 21, indicating good epitaxy. To demonstrate the essential features of the HLPE process, Fig. 4 (right top insert) shows a cross-sectional SEM view of a 350 nm-thick BFO film grown on seeded STO, under a layer of flux 100 nm thickness. The film was grown at 795 1C, pO2 ¼ 8 Pa, for 300 s (growth rate 1.2 nm/ s). The image clearly shows that growth occurred under the flux layer. Generally, the film growth rate increases with supersaturation. The growth rate (up to 2 nm/s) is enhanced compared to standard PLD because the sticking coefficient increases when deposition is onto a liquid
surface. This is to be expected since a liquid surface can be considered to be ideally rough [30]. In addition, the mobility of the constituent species in BFO is more rapid in the dense liquid than in the vapour phase. A wide range of deposition rates and corresponding values of s have been investigated for values of substrate temperature ranging from 805 1C down to 700 1C. The conditions have resulted in growth rates of 0.5–2 nm s1. Hence, it is clear that the increase in s leads to a much increased growth rate and better epitaxy. To confirm the multiferroic properties of the BFO thin films, the RT magnetization as a function of magnetic field and ferroelectric polarization as a function of electric field were measured. Fig. 6 shows two magnetic hysteresis loops measured on two 350 nm-thick BFO films deposited under 0.8 and 26 Pa oxygen pressures. The loops were measured at 293 K with
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Intensity (arbitrary units)
Intensity (a.u.)
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M. Li et al. / Journal of Crystal Growth 293 (2006) 128–135
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2x105
FWHM=0.37
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100nm Flux layer BFO 350nm
5
SRO 11.0 11.5 Ω (degree)
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40
STO
12.0
100nm
(003) BFO (003) STO
10.5
(002) BFO
0 10.0
(001) STO
(001) BFO
1x10
50 2θ (degree)
60
70
Fig. 4. XRD pattern of BiFeO3 thin film grown under optimal conditions, left insert shows the rocking curve of the film and right top inset shows the cross-sectional FE SEM image.
Fig. 5. XRD pole figure scan of the (1 2 0) reflection of the HLPE BFO film.
Fig. 6. Out-of-plane magnetization–field (M–H) curves of the 350 nmthick BFO films deposited at oxygen pressures of 0.8 Pa (red curve) and 26 Pa (blue curve).
the field applied perpendicular to the substrate plane. The films show a saturated, weak ferromagnetic response at RT. The saturation magnetization increased from 5.1 emu/ cm3 for the film deposited at 26 Pa to 26.5 emu/cm3 for the film deposited at 0.8 Pa. For the sample deposited at 0.8 Pa, there was some tetragonal, nonmagnetic b-Fe2O3 impurity present as shown by XRD. However, we cannot completely exclude the existence of some additional g-Fe2O3 in the residual flux layer surface. g-Fe2O3 is ferrimagnet with a saturation magnetization, Ms, of 450 emu/cm3 [31]. For the sample deposited at 26 Pa, the Ms was 5.1 emu/cm3, which is calculated to be 0.03 mB/Fe. In this sample there is a small amount of Bi2O3 in the residual flux, but no Fe2O3 as
detected by XRD. Therefore, the magnetic contribution is presumed to come from the BFO. The weak net moment of 5.1 emu/cm3 is comparable to the value of bulk BFO, and is consistent with the findings of Wang et al. [2] BFO has a G-type [32] or even more complicated [33,34] antiferromagnetic structure with weak ferromagnetic ordering due to spin canting in the unit cell [35]. Magnetic interactions between Fe2+ and Fe3+ could also produce weak ferromagnetism in BFO [36]. It might be expected that Fe2+ ions would be present in the films because they were grown in the presence of a melt at a higher temperature (and hence a more reducing environment) than by standard PLD growth. Indeed, considerable
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Fig. 7. Ferroelectric polarization–electric (P–E) hysteresis loops of a typical HLPE BFO film grown on (0 0 1) SRO/STO, measured at 1.25 kHz at room temperature.
amounts of Fe2+ have been found in other ferrite single crystals grown from high-temperature melts [37]. However, since the Ms value are similar to samples grown by standard methods, we can conclude that either any possible additional Fe2+ in the HLPE film does not yield enhanced ferromagnetism or there is simply no additional Fe2+ in the HLPE films. The conductive SRO layer was used as the bottom electrode and gold electrodes were evaporated onto the top of the film. RT polarization–electric field (P–E) hysteresis loops of a sample grown at 795 1C, p(O2) ¼ 8 Pa by HLPE is shown in Fig. 7, measured at frequencies of 1.25 kHz on 250 mm diameter capacitor structures. The range of applied fields was 70 to 270 MV/m, which indicates that high voltages are required to obtain saturated P–E curves. Under the 270 MV/m applied field, the spontaneous polarization (Ps), remanent polarization (Pr), and coercive field (Ec) are 42.2, 0.9 mC/cm2, and 90 MV/m, respectively. The remanent polarization of 2Pr ¼ 1.8 mC/cm2 is similar to that (3.5–6.1 mC/cm2) of single-crystal BFO [38], but it is much smaller than the highest reported values in some thin films [2,7]. As we have shown, the sample grown under the optimal (in terms of epitaxy and phase purity) HLPE conditions has an unconsumed surface flux layer with the presence of Bi2O3 in this layer. With the improvement of the process to consume the flux layer or to remove it completely, it is anticipated that the saturation polarization values will increase significantly. There is also no inherent limitation to the growth of much thicker (3 mm) films by HLPE and
the thickness limitation is worth exploring further once the ferroelectric properties are optimized. 4. Conclusions Epitaxial BiFeO3 (BFO) thin films have been grown on single-crystal substrates of SrTiO3 (STO) by a novel hybrid liquid-phase epitaxy (HLPE) method. During film growth, a thin liquid flux layer was fed with appropriate components to maintain a constant supersaturation necessary for the solid–liquid growth. The HLPE films grown on (0 0 1) STO were single phase, highly textured and highly oriented along the c-axis of the pseudo-cubic cell with an in-plane texture of 21 and an out-of-plane texture of 0.371. The growth rate of HLPE films is enhanced considerably compared to standard PLD film (i.e. up 2 nm/s compared to 0.1 nm/s) because the sticking coefficient increases when deposition is onto a liquid surface, and because there is a high supersaturation which is maintained during growth. HLPE provides a potential method for depositing thick (41 mm) BFO films for practical applications in MEMS and actuators. Further work is now required to reduce or eliminate the residual surface flux layer, which likely interferes with the polarization measurements. Acknowledgements The support for Dr. Meicheng Li by a UK Royal Society China Fellowship is gratefully acknowledged.
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Journal of Crystal Growth 293 (2006) 128–135 www.elsevier.com/locate/jcrysgro
Rapid epitaxial growth of magnetoelectric thick BiFeO3 films by hybrid liquid-phase epitaxy Meicheng Lia,b,, Ahmed Kursumovica, Xiaoding Qia, Judith L. MacManus-Driscolla a
Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK b Department of Materials Physics and Chemistry, Harbin Institute of Technology, Harbin 150001, China Received 11 February 2006; received in revised form 30 March 2006; accepted 7 April 2006 Communicated by M. Kawasaki Available online 27 June 2006
Abstract Epitaxial magnetoelectric BiFeO3 thin films have been prepared on SrRuO3-buffered (0 0 1) SrTiO3 substrates using a novel hybrid liquid-phase epitaxy (HLPE) approach, which is a solid–liquid growth process with delivery from a vapour source by pulsed laser deposition (PLD). The HLPE process enables rapid ‘‘liquid assisted’’ growth of epitaxial films of many functional materials without some of the disadvantages of classical liquid-phase epitaxy (LPE) and of PLD. High growth rates up to 2 nm/s were demonstrated in dense films of 350 nm thickness. The method has great promise for fabrication of device materials where thick, dense epitaxial films are required. r 2006 Elsevier B.V. All rights reserved. PACS: 81.15.Aa; 81.40.Rs; 81.70.Ex Keywords: A1. Hybrid liquid-phase epitaxy; A2. Thin films; B1. BiFeO3; B2. Magnetoelectric materials
1. Introduction Coupling different combinations of electric, magnetic, and structural order parameters results in so-called multiferroics possessing ferroelectricity, ferromagnetism, or ferroelasticity [1]. For simultaneous ferroelectricity and ferromagnetism (magnetoelectrics) there are potential applications in information storage, spintronics, and in magnetic or electric field sensors [2]. The perovskite BiFeO3 (BFO) is antiferromagnetic below the Neel temperature of 647 K and ferroelectric with a high Curie temperature of 1043 K [3,4]. It exhibits weak magnetism at room temperature (RT) due to the residual moment from a canted spin structure [5]. Bulk BFO has a smaller spontaneous polarization value than the expected value for a ferroelectric with such a high Tc. Recently Corresponding author. Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK. Tel.: +44 1223 767140; fax: +44 1223 334567. E-mail address: [email protected] (M. Li).
0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.04.108
strong magnetoelectric behaviour was reported in thin films with ferromagnetism (strong RT saturation magnetization of 1 mB/Fe) and ferroelectricity (polarization saturation polarization of 90 mC/cm2) [2]. Following this report, the study of BFO thin films has attracted much recent attention. In fact, not only is BFO potentially useful as a magnetoelectric but also as a piezoelectric to replace Pbcontaining Pb(ZrxTi1x)O3 (PZT). Most BFO thin films reported so far have been grown by pulsed laser deposition (PLD) [6–9] although metalorganic chemical vapour deposition (MOCVD) [10] and liquidphase epitaxy (LPE) [11,12] have also been used successfully. The effects of deposition conditions and epitaxial constraints on crystallization and properties of BFO thin films have been extensively studied [6–8]. In addition, the influence of different ionic substitutions on the structure and magnetoelectric properties of films has been investigated [11,13]. Electric leakage is a remaining issue to be solved before BFO is practically implemented in any devices at RT. Another issue for potential piezoelectric applications is the requirement to grow thick (41 mm) films.
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It is likely that nonstoichiometry and second-phase formation are the factors which cause leakage in BFO. It has been suggested that oxygen nonstoichiometry leads to valence fluctuations of Fe ions in BFO, resulting in high conductivity [14]. However, our recent research work using coulometric titration has showed that BFO is only very slightly oxygen nonstoichiometric [15] and so it is likely that charge carries are induced by cation vacancies. The thickness dependence of the polarization, piezoelectric coefficient, and dielectric constant of the epitaxial BFO films has been studied by Wang et al. [2]. The piezoelectric constant d33 shows a dramatic increase with thickness and values of 85 pm/V have been achieved in 400 nm-thick films, which is comparable to around 25–500 pm/V in PZT. Thus, thicker BFO films should have even larger piezoelectric coefficients, making them useful for applications in micro-electro-mechanical system (MEMS) and actuators. A fast, reliable growth method with the ability to maintain film stoichiometry and a dense microstructure is required for this purpose. It has been proposed that higher leakage in BFO can be greatly suppressed by the very high heating rate, short sintering period, and liquid-phase sintering [16]. Here, we consider a novel approach for growth of BFO films which combines a liquid growth process with vapour growth. The technique is called hybrid liquid-phase epitaxy (HLPE) [17], and the method involves growth under a thin liquid layer. BFO is an ideal system to grow by HLPE because a low melting point eutectic liquid is formed at around 785 1C [18]. Using HLPE, we demonstrate growth of highly crystalline, oriented epitaxial BFO films on SrTiO3 (STO). In all kinds of methods used for growth of epitaxial films, the primary factor determining the nucleation frequency and growth mode is the thermodynamic driving force, or relative supersaturation s. A key characteristic of HLPE is the presence, during film growth, of a thin layer of liquid flux which is fed with appropriate chemical components to maintain the constant supersaturation necessary for steady-state growth. The self-consistent adjustment of the steady-state supersaturation to match growth rate and feed rate makes the description of the mechanisms that control the process easy to quantify. This is not the case for classical LPE or the various physical vapour deposition (PVD) methods. Classical LPE is regarded as a low supersaturation, near-equilibrium growth process. Whereas, PVD is commonly treated as a high supersaturation growth process [19–21]. So, for deposition from a thin flux layer, diffusion through the liquid is no longer the limiting constraint as it is for LPE and so a larger range of s can be explored and maintained by varying the deposition conditions. In this work, the liquid flux was fed from the vapour using high rate PLD under a variety of oxygen pressures. During HLPE growth, a thin liquid layer can be stabilized below the bulk melting temperature by a free energy contribution coming from the interface [22,23]. However, the growth was only explored at temperatures or
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at just above the bulk equilibrium temperature for eutectic melting of the flux. Feeding from a vapour source is incidental to film growth through the liquid layer, and hence the term vapour–liquid–solid (VLS) growth is not broad enough to cover the general case for HLPE. Liquid–solid (LS) growth is, in fact, the correct terminology. In HLPE of BFO, growth occurs by BFO diffusive transport through the thin liquid Bi2O3–Fe2O3 surface flux layer to the BFO seed film. The growth is controlled by adjusting the rate of feeding of the liquid layer with Bi2O3rich material at a constant temperature; in this case s, and the corresponding undercooling adjust self-consistently to match the growth rate to the feeding rate. Since the liquid flux layer can be as thin as 100 nm, the flux cannot dissolve a large amount of the substrate or seed layer, and the transit time for diffusion of BFO through the layer is of the order of 104 s, defining a time constant for equilibration of s. Thus HLPE embodies all the advantages of LPE without the disadvantages of difficult nucleation and a large volume of aggressive liquid flux. The thermodynamic driving force Dm/kBT controls both the nucleation and the growth rates, where Dm is the change in chemical potential and kB is the Boltzmann constant. Dm/kBT is defined as [24,25] Cd Dm=kB T ¼ ln , Ce where Ce and Cd are the equilibrium concentration and actual solute concentration in the flux at the growth temperature, respectively. When growth occurs under a flux liquid layer with the background solute concentration Cd, a solute concentration gradient forms across some thickness d due to finite diffusion constant D. In the case pffiffiffiffiffi ffi of stationary solution, d increases with time t as Dt. For the process with flux steering, this d can be made rather small (tens of micrometers) and kept constant, resulting in so-called diffusion boundary layer [26]. For the LPE process, supposing an existence of a finite surface coefficient the growth rate through a liquid layer of thickness d can be expressed as [26,27] F¼
D 1 ðC d C e Þ , d 1þR C xt
where the dimensionless parameter R reflects the relative importance of diffusion coefficient and kinetic coefficient as the rate-limiting process [27]. However, in HLPE process the solution is still, and the flux layer is thin (50 nm), henceafter the initial transient d becomes equal to the flux thickness. In this case, solute supersaturation depends on the feeding rate and on the deposition temperature. The effects of composition, temperature and oxygen partial pressure on solid–liquid equilibria are required to assess the optimum flux composition and growth conditions for the films: for growth of films in multilayered structures usually a low growth temperature is desirable to minimize the diffusion of impurities.
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Two of the phase boundary lines determine the theoretical limits of LPE growth: the BFO stability and the flux solidification lines. Both of these boundaries are sensitive to the flux composition (primarily the Bi: Fe molar ratio) and the oxygen partial pressure p(O2). The stability of BFO in the flux is also related to the peritectic temperature and oxygen partial pressure p(O2) [15]. 2. Experimental procedure The targets for PLD growth of BFO by HLPE were made in-house using high-purity commercial Bi2O3 and Fe2O3 powders. Flux targets were prepared with a Bi:Fe molar ratio of 4:1. Standard PLD was carried out using a KrF excimer laser (Lambda Physik Compex 205) with radiation at l ¼ 248 nm, with energy density of 12 J cm2, and with repetition rates from 5 to 50 Hz. The BFO films were grown on a 50 nm underlayer of SrRuO3 (SRO) on 5 10 mm2 (0 0 1) STO substrates. The SRO films were grown at 650 1C and under 8 Pa oxygen pressure using a stoichiometric target. The substrates were maintained at temperatures from 700 to 805 1C during deposition using a heater of a tubular form, ensuring a uniform substrate temperature by radiation rather than by thermal contact. An epitaxial BFO seed film of 20 nm thickness was deposited onto the SRO-buffered substrate. A flux layer of 100 nm thickness was then deposited onto the SRO at just above its melting point (by 15 1C). This was immediately followed by deposition of BFO. The BFO feeds the flux layer, and the Bi and Fe chemical constituents diffuse to the surface of the BFO seed layer to nucleate BFO. As the BFO grows, the phase layer is continuously pushed away from the substrate so that it always remains on the surface. The process is illustrated in Fig. 1. After deposition, films were cooled to 450 1C, and annealed in situ for 1 h in 20 kPa oxygen. To explore the optimized process of HLPE for BiFeO3 growth, experiments were carried out over a range of temperatures between 700 and 805 1C at a range of oxygen pressures from 0.8 to 16 Pa O2.
We note that the Bi2O3–Fe2O3 flux layer was not deposited at a temperature below its melting point and then heated to the melting point or above followed by feeding of the flux with BFO. The reason of this is that the BFO concentration in the flux would build gradually up from zero, and then nucleation and initial growth would take place at low s, which would mean slower nucleation and growth, allowing time for the flux to attack the seed layer substrate. Field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectrometry (EDS) were used for surface morphology and composition measurements of the deposited films. The crystal phases present and the inplane and out-of-plane crystal orientation were determined by X-ray diffraction (XRD) and high-resolution XRD (HRXRD). The magnetic properties of the films were studied using a Princeton Measurements Corporation (Princeition, NJ Model) vibrating sample magnetometer (VSM). Ferroelectric measurement were carried out on some of the films using a commercial ferroelectric tester (Radiant Technologies, Inc.). 3. Results and discussions Firstly, a flux film grown on a BFO-seeded SRObuffered STO substrate was deposited and measured. A surface SEM image of the sample quenched from 795 1C is shown in the inset of Fig. 2. Solidified flux regions are apparent on the surface of the BFO film. EDS analysis shows that the Bi:Fe ratio in this region is approximately 4:1, which is equal to that of the flux target. The Bi:Fe ratio in-between the flux regions was approximately 1:1, as expected, since this is the composition of the seed layer. The phases present in this flux sample were analysed by XRD (Fig. 2). (0 0 l) diffraction peaks from BFO, SRO, and STO were observed. The peaks of the 30–50 nm thick SRO were too weak to be resolved from the tails of the intense STO peaks. Meanwhile, additional secondary-phase peaks, e.g., (2 0 1), (0 0 2), (2 2 0) b-Bi2O3 (tetragonal P4¯ 21c(1 1 4), a ¼ 0.7741(3) nm, c ¼ 0.5634(2) nm) BFO Vapour
Flux layer remains on surface during growth BFO nucleates on BFO seed layer
~ 100nm ~ 350nm ~ 20nm ~ 50nm
BFO seed layer Conductive SRO layer
STO Substrate
Fig. 1. The schematic diagram of HLPE growth process.
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Fig. 2. XRD patterns of flux layer grown on SrRuO3/SrTiO3; inset is SEM image of the sample quenched from 795 1C.
and (1 0 4), (2 1 4) a-Fe2O3 (rhombohedral , R3¯ c, a ¼ 0.5112 nm, c ¼ 1.382 nm) peaks were observed. These Fe2O3 phases arise from the surface flux layer. At first set of BFO films were grown to study the influence of deposition temperatures on the quality of film epitaxy. Fig. 3(a) shows the XRD patterns of three BFO samples deposited on SRO/STO for 400 s at 785, 795, and 805 1C, respectively at a pO2 of 8 Pa and 50 Hz repetition rate. The composition and structure of the films were found to be quite sensitive to the deposition temperature. For the sample deposited at 785 1C (the peritectic temperature of flux) diffraction peaks from (0 0 1), (0 0 2) BFO and (0 0 1), (0 0 2), (0 0 3) STO were observed with a weak peak attributable to (2 1 1) Bi2O3 and two very weak peaks that may be indexed as (0 1 2) and (0 4) a-Fe2O3. For the sample deposited at 795 1C (15 1C higher than the flux eutectic temperature), (0 0 1), (0 0 2), and (0 0 3) BFO diffraction peaks were found, while the diffraction peaks from aFe2O3 were much weaker, and the Bi2O3 peaks stronger. Upon increasing the deposition temperature further to 805 1C, the (0 1 2) and (0 2 4) a-Fe2O3 diffraction peaks became stronger again, while the peaks from BFO were weaker and the peak of (0 0 3) BFO peak was barely above the background level, as for the 785 1C sample. In summary, the optimum films were grown at 795 1C at just above the flux melting temperature. Another set of samples was grown to study the influence of oxygen on the film quality. The XRD patterns of BFO thin films grown at 50 Hz repetition rate on SRO-buffered STO at 795 1C (optimized growth conditions from above) under various oxygen deposition pressures are shown in Fig. 3(b). As the deposition pressure decreased from 16 to 8 Pa, the (1 0 l) peaks of the BFO films were enhanced, while the peaks of Bi2O3 decreased in intensity. We note
that while the deposition conditions for the samples of Fig. 3(a) and Fig. 3(b) (795 1C) were nominally identical, in fact small differences in the actual deposition conditions led to small differences in the amount of residual Bi2O3 from the solidified flux. When the deposition pressure decreased to 0.8 Pa, the diffraction peaks from Bi2O3 disappeared. However, two peaks identified as (0 1 2) and (0 2 4) a-Fe2O3 were then found. These analyses indicate that at a fixed deposition temperature the crystallization of BFO thin films is affected considerably by the oxygen partial pressure. No Bi2Fe4O9, which is commonly found in PLD films of BFO [28,29] was observed. Fig. 4 shows XRD patterns (linear scale) for the films grown on SRO-buffered (0 0 1) STO substrates by HLPE under optimized processing conditions of 795 1C, 8 Pa pO2, and laser repetition rate of 50 Hz. The BFO shows a high degree of epitaxial growth with stronger (0 0 l) diffraction peaks from BFO and STO. The left inset curve in Fig. 4 shows the rocking curve of (0 0 1) BFO, with full-width at half-maximum (FWHM) of 0.371. Reciprocal space mappings using HRXRD revealed that the BFO film by HLPE has a rhombohedral structure, the lattice parameters are: a ¼ 3.9638, a ¼ 89.52. Hence, the film by HLPE was oriented along the c-axis of the unit cell, and the degree of BFO epitaxy was found to be comparable to classical LPE. An XRD pole figure of the 1 2 0 reflections from a film grown on a (0 0 1) STO substrate at 795 1C, 8 Pa pO2, with 350 nm thickness is shown in Fig. 5. There are two sets of peaks, one from (1 2 0) BFO, and the other from (1¯ 2 0) BFO. Each set includes four peaks at tilt angles c of 26.221 and 63.781, respectively, with the four peaks being 901 apart from each other. The HLPE film is highly biaxially textured and has a four-fold symmetry, indicating either cubic or pseudo-cubic symmetry.
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Cu/Kβ
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20
805°C
Fe2O3(024)
132
50 2θ (degree)
795°C pO2=8Pa
795°C pO2=8Pa
60
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Fig. 3. XRD pattern of BiFeO3 thin film grown by HLPE at different deposition temperatures (a) and at different oxygen pressures (b).
The degree of in-plane orientation of the films was also assessed by examining XRD f-scans (not shown). The peaks for the (1 2 0) reflection of the film occurs at the same azimuthal y angle (to within 0.71) as that for the (1 0 0) STO reflection, indicating a ‘‘cube-on-cube’’ epitaxial growth on STO. The FWHM of the peaks is around 21, indicating good epitaxy. To demonstrate the essential features of the HLPE process, Fig. 4 (right top insert) shows a cross-sectional SEM view of a 350 nm-thick BFO film grown on seeded STO, under a layer of flux 100 nm thickness. The film was grown at 795 1C, pO2 ¼ 8 Pa, for 300 s (growth rate 1.2 nm/ s). The image clearly shows that growth occurred under the flux layer. Generally, the film growth rate increases with supersaturation. The growth rate (up to 2 nm/s) is enhanced compared to standard PLD because the sticking coefficient increases when deposition is onto a liquid
surface. This is to be expected since a liquid surface can be considered to be ideally rough [30]. In addition, the mobility of the constituent species in BFO is more rapid in the dense liquid than in the vapour phase. A wide range of deposition rates and corresponding values of s have been investigated for values of substrate temperature ranging from 805 1C down to 700 1C. The conditions have resulted in growth rates of 0.5–2 nm s1. Hence, it is clear that the increase in s leads to a much increased growth rate and better epitaxy. To confirm the multiferroic properties of the BFO thin films, the RT magnetization as a function of magnetic field and ferroelectric polarization as a function of electric field were measured. Fig. 6 shows two magnetic hysteresis loops measured on two 350 nm-thick BFO films deposited under 0.8 and 26 Pa oxygen pressures. The loops were measured at 293 K with
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M. Li et al. / Journal of Crystal Growth 293 (2006) 128–135
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(003) BFO (003) STO
10.5
(002) BFO
0 10.0
(001) STO
(001) BFO
1x10
50 2θ (degree)
60
70
Fig. 4. XRD pattern of BiFeO3 thin film grown under optimal conditions, left insert shows the rocking curve of the film and right top inset shows the cross-sectional FE SEM image.
Fig. 5. XRD pole figure scan of the (1 2 0) reflection of the HLPE BFO film.
Fig. 6. Out-of-plane magnetization–field (M–H) curves of the 350 nmthick BFO films deposited at oxygen pressures of 0.8 Pa (red curve) and 26 Pa (blue curve).
the field applied perpendicular to the substrate plane. The films show a saturated, weak ferromagnetic response at RT. The saturation magnetization increased from 5.1 emu/ cm3 for the film deposited at 26 Pa to 26.5 emu/cm3 for the film deposited at 0.8 Pa. For the sample deposited at 0.8 Pa, there was some tetragonal, nonmagnetic b-Fe2O3 impurity present as shown by XRD. However, we cannot completely exclude the existence of some additional g-Fe2O3 in the residual flux layer surface. g-Fe2O3 is ferrimagnet with a saturation magnetization, Ms, of 450 emu/cm3 [31]. For the sample deposited at 26 Pa, the Ms was 5.1 emu/cm3, which is calculated to be 0.03 mB/Fe. In this sample there is a small amount of Bi2O3 in the residual flux, but no Fe2O3 as
detected by XRD. Therefore, the magnetic contribution is presumed to come from the BFO. The weak net moment of 5.1 emu/cm3 is comparable to the value of bulk BFO, and is consistent with the findings of Wang et al. [2] BFO has a G-type [32] or even more complicated [33,34] antiferromagnetic structure with weak ferromagnetic ordering due to spin canting in the unit cell [35]. Magnetic interactions between Fe2+ and Fe3+ could also produce weak ferromagnetism in BFO [36]. It might be expected that Fe2+ ions would be present in the films because they were grown in the presence of a melt at a higher temperature (and hence a more reducing environment) than by standard PLD growth. Indeed, considerable
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Fig. 7. Ferroelectric polarization–electric (P–E) hysteresis loops of a typical HLPE BFO film grown on (0 0 1) SRO/STO, measured at 1.25 kHz at room temperature.
amounts of Fe2+ have been found in other ferrite single crystals grown from high-temperature melts [37]. However, since the Ms value are similar to samples grown by standard methods, we can conclude that either any possible additional Fe2+ in the HLPE film does not yield enhanced ferromagnetism or there is simply no additional Fe2+ in the HLPE films. The conductive SRO layer was used as the bottom electrode and gold electrodes were evaporated onto the top of the film. RT polarization–electric field (P–E) hysteresis loops of a sample grown at 795 1C, p(O2) ¼ 8 Pa by HLPE is shown in Fig. 7, measured at frequencies of 1.25 kHz on 250 mm diameter capacitor structures. The range of applied fields was 70 to 270 MV/m, which indicates that high voltages are required to obtain saturated P–E curves. Under the 270 MV/m applied field, the spontaneous polarization (Ps), remanent polarization (Pr), and coercive field (Ec) are 42.2, 0.9 mC/cm2, and 90 MV/m, respectively. The remanent polarization of 2Pr ¼ 1.8 mC/cm2 is similar to that (3.5–6.1 mC/cm2) of single-crystal BFO [38], but it is much smaller than the highest reported values in some thin films [2,7]. As we have shown, the sample grown under the optimal (in terms of epitaxy and phase purity) HLPE conditions has an unconsumed surface flux layer with the presence of Bi2O3 in this layer. With the improvement of the process to consume the flux layer or to remove it completely, it is anticipated that the saturation polarization values will increase significantly. There is also no inherent limitation to the growth of much thicker (3 mm) films by HLPE and
the thickness limitation is worth exploring further once the ferroelectric properties are optimized. 4. Conclusions Epitaxial BiFeO3 (BFO) thin films have been grown on single-crystal substrates of SrTiO3 (STO) by a novel hybrid liquid-phase epitaxy (HLPE) method. During film growth, a thin liquid flux layer was fed with appropriate components to maintain a constant supersaturation necessary for the solid–liquid growth. The HLPE films grown on (0 0 1) STO were single phase, highly textured and highly oriented along the c-axis of the pseudo-cubic cell with an in-plane texture of 21 and an out-of-plane texture of 0.371. The growth rate of HLPE films is enhanced considerably compared to standard PLD film (i.e. up 2 nm/s compared to 0.1 nm/s) because the sticking coefficient increases when deposition is onto a liquid surface, and because there is a high supersaturation which is maintained during growth. HLPE provides a potential method for depositing thick (41 mm) BFO films for practical applications in MEMS and actuators. Further work is now required to reduce or eliminate the residual surface flux layer, which likely interferes with the polarization measurements. Acknowledgements The support for Dr. Meicheng Li by a UK Royal Society China Fellowship is gratefully acknowledged.
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