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Growth temperature dependent dielectric properties of BiFeO3 thin films deposited on silica glass substrates
Thin Solid Films 520 (2012) 4470–4474
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Growth temperature dependent dielectric properties of BiFeO3 thin films deposited on silica glass substrates Taimur Ahmed ⁎, Andrei Vorobiev, Spartak Gevorgian Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
a r t i c l e
i n f o
Article history: Received 31 January 2011 Received in revised form 14 February 2012 Accepted 23 February 2012 Available online 3 March 2012 Keywords: Bismuth ferrite Thin films Amorphous silica glass substrate Growth temperature Secondary phases Permittivity Ex-situ annealing
a b s t r a c t We have studied the dependence of dielectric properties on the deposition temperature of BiFeO3 thin films grown by the pulsed laser deposition technique. Thin films have been grown onto amorphous silica glass substrates with pre-patterned Au in-plane capacitor structures. It is shown that on the amorphous glass substrate, BiFeO3 films with a near-bulk permittivity of 26 and coercive field of 80 kV/cm may be grown at a deposition temperature of about 600 °C and 1 Pa oxygen pressure. Low permittivity and higher coercive field of the films grown at the temperatures below and above 600 °C are associated with an increased amount of secondary phases. It is also shown that the deposition of BiFeO3 at low temperature (i.e. 500 °C) and post deposition ex-situ annealing at elevated temperature (700 °C) increases the permittivity of a film. The applied bias and time dependence of capacitance of the films deposited at 700 °C and ex-situ annealed films are explained by the de-pinning of the ferroelectric domain-walls. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Perovskite ABO3 oxides are of great interest in many applications due to their multifunctional (semiconducting, electrochromic, magnetoresistive, dielectric, multiferroic etc.) properties [1]. Specifically, the ionic conductivity and presence of oxygen vacancies make perovskite oxides useful for different sensing devices. For gas sensitivity, an oxide material should contain at least one readily reducible ion together with one that is readily oxidizable [2]. In recent years, multiferroic BiFeO3 (BFO) has been the focus of extensive study due to its potential use in nonvolatile ferroelectric or magnetoelectric memories, sensors, terahertz devices and photonic devices [3,4]. BFO, both in bulk and nanoparticle form, is suitable for gas sensing because it contains two metals (Bi and Fe) that provide two relatively easily accessible oxidation states [2]. Moreover, BFO is characterized by high thermal and chemical stability in comparison with simple metal oxides [5]. In this work, BFO thin films are deposited by pulsed laser deposition (PLD) and their growth temperature dependent dielectric properties are measured to study the feasibility of fabricating a disposable and inexpensive device that could be later used in gas sensing applications. The focus of our studies is the optimization of permittivity because permittivity is assumed to be the potential sensing parameter in BFO thin film gas sensors. Amorphous silica
⁎ Corresponding author. E-mail address: [email protected] (T. Ahmed). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.02.082
glass (SiO2) is used as a substrate because it is cheaper than the single crystal substrates (like SrTiO3, DyScO3, sapphire, etc.) and SiO2 can also withstand high deposition temperatures during the PLD process. The challenge of growing BFO films on amorphous SiO2 substrates is addressed by A. Vorobiev et al. [6] where buried coplanar Au electrodes are used as nucleation centers. Following the same configuration as in [6], we pattern Au electrodes as interdigitated capacitors (IDC) and grow BFO films on top of the electrodes. This configuration – featuring buried interdigital electrodes instead of parallel plate configuration with electrodes on top of the BFO film – facilitates the growth of BFO films on the amorphous substrates and also provides a larger active area for the gas sensing, thereby increasing the device sensitivity. It has already been reported that, on single crystal substrates, single-phase BFO thin films with limited thicknesses (25–240 nm) can only be grown in a narrow range of deposition temperature (550–600 °C) and pressure (around 1 Pa) [7–9]. It is not obvious whether these deposition conditions are also applicable for the growth of BFO films on the amorphous SiO2 substrates. The present work seeks to address this uncertainty, by growing a number of BFO films at different temperatures while keeping the oxygen pressure constant at 1 Pa. 2. Experimental details Bottom Au (500 nm)/TiO2 (50 nm) IDC electrodes are deposited by DC magnetron sputtering on amorphous SiO2 substrates and subsequently patterned by photolithography and ion beam milling.
T. Ahmed et al. / Thin Solid Films 520 (2012) 4470–4474
BFO thin films with a thickness of 500 nm are grown on top of the electrodes by PLD using a Bi1,1FeO3 ceramic target with excess of Bi to compensate for high Bi re-evaporation. A KrF excimer laser (λ = 248 nm and τ = 30 ns) source with an energy density of 1.5 J/ cm 2 is used to ablate the ceramic target. The distance between the SiO2 substrate and the ceramic target is kept fixed at 6 cm in the deposition chamber. The growth temperature is held constant in the range of 500–750 °C. After the deposition, the films were cooled down to room temperature under 50 kPa oxygen pressure. Films grown at 500 °C are also ex-situ annealed at 700 °C for 2 h in 2 L/min oxygen flow. Top contact pads of Au (500 nm)/Ti (50 nm) are deposited by e-beam evaporation. Top contact pads are capacitively connected with the bottom contact pads of the buried IDC electrodes, through the BFO film. A schematic illustration of the BFO film's test structure, with the in-plane buried IDC electrodes, is shown in Fig. 1(a). The dielectric properties of the deposited films have been characterized at 1 MHz for the permittivity measurements and 30 MHz for hysteresis measurements, both by using a PH 4285A LCR-meter. The X-ray diffraction (XRD) spectra of the BFO films were obtained using a Philips X'pert SW 3040 diffractometer equipped with a Cu Kα point radiation source, a MRD lens, a thin collimator and a Ni filter. Farnell's model has been used for the calculation of apparent permittivity [10] using a thickness of 500 nm, measured for the film grown at 600 °C. 3. Result and discussion Micrographs in Fig. 1(b)–(g) show BFO thin films grown over buried IDC electrodes at deposition temperature ranging between 500 and 750 °C. As it can be seen from the micrographs, the film in the electrodes' gap changes color with the growth temperature. It is assumed that the film grown in the gap most likely consists of a mixture of phases including stoichiometric BFO and secondary
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phases either enriched by Bi at lower growth temperatures (following the target composition) or enriched by Fe at higher growth temperatures (due to Bi re-evaporation). We assume that the certain color of an area in the gap, distinguishable in the optical images of Fig. 1, is defined by the refractive index of the phase dominating in that area. It can also be seen from the images in Fig. 1 that the growth of BFO phases is initiated at the edges of buried Au IDC electrodes and the phases develop into the gap with increasing growth temperature. As the growth temperature increases to a certain level the gap is filled by film areas with almost uniform color which indicates the predomination of a certain phase. It is well known that at higher growth temperatures Bi reevaporates and leads to the growth of Fe-enriched secondary phases along with the BFO stoichiometric phase [8,9]. One way to overcome this issue is to deposit BFO thin films at low temperatures followed by post deposition ex-situ annealing at elevated temperatures. To prove this approach, the BFO film grown at 500 °C has been ex-situ annealed. Fig. 2 shows the micrograph of BFO film grown at 500 °C followed by ex-situ annealing at 700 °C for 2 h in oxygen flow. As can be seen, the annealed film reveals color patterns (in the gap) different from the as-deposited film (Fig. 1b) and the patterns are comparable with those of the film deposited at approximately 600 °C (Fig. 1d). It is assumed that the post-deposition annealing results in the re-crystallization of the BFO phase, an increase of grain size and decrease in the defect density i.e., improvement of the crystallinity which is also confirmed by atomic force microscope images, not shown here. This post-deposition annealing results in the change of the apparent refractive index of the film and hence changes the color in comparison to the as-deposited film. Fig. 3 illustrates the effect of deposition temperature and postdeposition annealing on the permittivity (εBFO) of BFO thin films at zero DC bias. H. Bea et al. [8] demonstrated the growth of single phase BFO films with thicknesses less than 100 nm at 1 Pa deposition
Fig. 1. A 3D schematic of the coplanar device, with indication of a larger active area intended for gas sensing applications (a). Optical micrographs of BFO films grown over Au IDC electrodes at different growth temperatures (b) 500 °C (c) 550 °C (d) 600 °C (e) 650 °C (f) 700 °C (g) 750 °C and under 1 Pa constant pressure.
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Fig. 2. Optical micrographs of BFO film grown at 500 °C and ex-situ annealed at 700 °C under a constant oxygen flow.
pressure. In our case, the films are five times thicker. So we assume the presence of Bi- and Fe-enriched secondary phases, with different amounts, in the whole range of our growth temperature i.e. 500–750 °C. As seen from Fig. 3, the maximum permittivity (εBFO = 26) in our film grown at 600 °C is slightly less than that of the bulk ceramic BFO (εBFO = 35) [11]. This can be explained by the presence of small amount of secondary phases along with the dominant stoichiometric BFO phase. At lower growth temperatures (b600 °C) the BFO films most likely have an excess of Bi-containing secondary phases (like Bi2O3) due to the excess of Bi in the ceramic BFO target [9]. This partly explains the lower permittivity (16 b εBFO b 26) of the films because the permittivity of the Bi2O3 phase calculated from the refractive index should be approximately 5 [12]. At higher growth temperatures (>600 °C) Bi re-evaporates and the films are depleted of Bi. As a result these films contain Fe-enriched secondary phases like Bi2Fe4O9, α-Fe2O3 and γ-Fe2O3 [9]. It is likely that as the temperature increases, the amount of the Fe-enriched secondary phases increases, resulting in decreased permittivity due to the lower permittivity of the Fe-enriched phases. In accordance with Ref. [13] permittivity of the α-Fe2O3 phase should be approximately 6. Shinde et al. [14] reported the permittivity of Fe2O3 thin films as about 2. Similarly, we assume that the film grown at 750 °C contains a higher amount of Fe2O3 secondary phases than the stoichiometric BFO phase which results in the lowest measured permittivity (~2). The permittivity of the film deposited at 500 °C followed by ex-situ annealing at 700 °C is also shown in Fig. 3. As suggested above, the post-deposition annealing increases the size of the BFO grains
Fig. 3. Dependence of apparent permittivity (εBFO) on film growth temperature and post-deposition annealing, measured at 1 MHz and room temperature.
Fig. 4. XRD patterns of the three as-deposited BFO films grown over the un-patterned Au bottom layer at 500 °C, 600 °C and 700 °C deposition temperatures.
resulting in a decrease of the relative amount of the nonstoichiometric grain boundaries with presumably lower permittivity. Consequently the permittivity of the annealed film increases in comparison with that of the as-deposited film. The XRD analysis confirms the earlier assumption that the change in the BFO films' phase composition with growth temperature is governed by the Bi re-evaporation. Fig. 4 shows the XRD spectra of the BFO films deposited on a non-patterned Au layer at 500, 600 and 700 °C. The films grown at 500 and 600 °C reveal reflections from the stoichiometric BFO(001) and BFO(110) families of planes. The BFO(111) reflection is probably masked by the strong Au(111) peak. The reflections from Bi2O3, Bi2Fe4O9 and Fe3O4 secondary phases are also registered. The BFO film grown at 500 °C reveals relatively higher intensities of the reflections from the Bi2O3 phase in comparison with that of the BFO and Fe-enriched phases. This indicates that the excess of Bi in the entire film's composition is due to the excess of Bi in the ceramic target (Bi1,1FeO3). The XRD spectra of the BFO film grown at 700 °C reveals no peaks associated with the Bi2O3 phase, indicating the absence of Bi-enriched secondary phases. Also the intensities of
T. Ahmed et al. / Thin Solid Films 520 (2012) 4470–4474
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Fig. 5. C(V) response of BFO films grown at (a) 500 °C (b) 600 °C (c) 700 °C and (d) ex-situ annealed at 700 °C film, grown at 500 °C.
the reflections from the stoichiometric BFO phase are drastically reduced in comparison with that of the Fe-enriched phases. This can be explained by more intensive Bi re-evaporation from the condensation surface at the higher growth temperatures. Fig. 5 shows the dependence of the BFO film's capacitance on the applied electric field (C(V) response). The films grown at 500 °C (Fig. 5a) and 600 °C (Fig. 5b) reveal typical ferroelectric response. As explained earlier, the film deposited at 500 °C contains a higher amount of non-ferroelectric Bi-enriched secondary phases (Bi2O3). This results in some distortion of the C(V) response (Fig. 5a) and in a higher effective coercive field (150 kV/cm), since a part of the applied field drops on the non-ferroelectric secondary phases. Some distortion in ferroelectric loops of the film grown at 600 °C can be explained by the presence of secondary phases (though in small amounts) and by charge defects, mainly oxygen vacancies. The film grown at 600 °C shows a lower coercive field of 80 kV/cm when compared to that of the bulk ceramic counterpart [15]. The ferroelectric domain structure and polarization dynamics in BFO thin films have been reported recently [16,17]. We assume domain-wall movement in our as-deposited and ex-situ annealed at 700 °C films—grown at 700 °C and 500 °C respectively. The domainwall movement results in the responses shown in Fig. 5(c) and (d) where measured capacitances are affected by both the applied fields and the domain-wall movement during the time required for the
field change. The films grown at 500 and 600 °C reveal reversible polarization (Fig. 5a–b) whereas rather strong time dependence observed in both of the films grown at 700 °C (Fig. 5c) and ex-situ annealed at 700 °C (Fig. 5d) is associated with the de-pinning of the domain-walls. Due to the lower density of defects in these films the de-pinning phenomenon contributes to the measured dielectric response. As reported in Ref. [16], the post-deposition annealing of BFO films results in the de-pinning of domains due to the sublimation of the Bi2O3 secondary phase associated with the pinning centers. One can thus also assume the de-pinning in our annealed film is enhanced in comparison with the as-grown film due to the reduction of the Bi2O3 secondary phase after the post deposition annealing. The C(V) response with and without the presence of a 0.2 T magnetic field, applied normal to the direction of applied electric field, has also been measured at 30 V DC bias (not shown here). Maximum magnetic field tuning of 0.21% was measured in the films grown at 600 °C which is consistent with the BFO films grown on amorphous silica glass substrate, as reported in Ref. [6]. To confirm the assumption of the domain-wall de-pinning, the time dependence of capacitance (the C(t) response) was measured at 30 MHz and 50-V DC bias. Fig. 6 shows the C(t) dependence of the BFO films grown at 700 °C and of the film grown at 500 °C followed by ex-situ annealing at 700 °C. The increase in capacitances with poling time, in both films, can be explained as domain-wall de-
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decrease in permittivity at lower and higher (than 600 °C) growth temperatures is explained by the excess of Bi- and Fe-enriched secondary phases, respectively. Thus by using a Bi-enriched target, the growth temperature can be optimized to grow predominantly single-phase BFO thin films with permittivity close to the ceramic counterpart. With the reported target composition and deposition conditions in this paper, the samples prepared at 600 °C are expected to be suitable for the further study of the gas sensing properties of BFO thin films. Acknowledgment The authors would like to thank the Swedish Research Council for partial support of this work. References Fig. 6. C(t) dependence of the as-deposited film grown at 700 °C (circles) and the film grown at 500 °C followed by ex-situ annealing at 700 °C (squares) in oxygen flow.
pinning. As can be seen, the characteristic time of the capacitance changes is 10–20 min which is in agreement with the time of the voltage cycles of the C(V) measurements (Fig. 5). In the asdeposited film, decrease in capacitance after 10 min of poling and an anomaly at the 15th minute are observed. Both can be associated with either the film degradation or the redistribution of the domain-wall configuration due to the local field caused by the accumulation of space charge at the film–electrode interface. The dependence of capacitance on applied bias in the ex-situ annealed film (Fig. 5d) reveals some saturation. Thus one can conclude that, in comparison with as-deposited film, ex-situ annealed film contains higher concentration of defects acting as pinning centers. 4. Conclusions We have grown BFO thin films by PLD on amorphous SiO2 substrates containing pre-patterned Au in-plane capacitor structures and have studied the influence of growth temperature on the dielectric properties. The observed changes in the permittivity of the BFO films with growth temperature are associated with the changes in the BFO films' phase composition. The XRD analysis indicates that the stoichiometric BFO phase dominates at about 600 °C growth temperature and results in maximum permittivity (εBFO = 26). The
[1] V. Lantto, S. Saukko, N.N. Toan, L.F. Reyes, C.G. Granqvist, J. Electroceram. 13 (2004) 721. [2] A.S. Poghossian, H.V. Abovian, P.B. Avakian, S.H. Mkrtchian, V.M. Haroutunian, Sens. Actuat. B 4 (1991) 545. [3] G. Catalan, J.F. Scott, Adv. Mater. 21 (2009) 2463. [4] Y.W. Li, J.Q. Xue, J.L. Sun, X.J. Meng, Z.M. Huang, J.H. Chu, L.H. Ding, W.F. Zhang, Appl. Phys. A 87 (2007) 125. [5] X.-L. Yu, Y. Wang, Y.-M. Hu, C.-B. Cao, H.L.-W. Chan, J. Am. Ceram. Soc. 92 (2009) 3105. [6] A. Vorobiev, N. Martirosyan, S.G. Eriksson, S. Gevorgian, IOP Conf. Ser.: Mater. Sci. Eng, 8, 2010. [7] S. Fujino, M. Murakami, S.-H. Lim, M. Wuttig, L.G. Salamanca-Riba, I. Takeuchi, Solid State Ionics 178 (2007) 1257. [8] H. Bea, M. Bibes, S. Fusil, K. Bouzehouane, E. Jacquet, K. Rode, P. Bencok, A. Barthelemy, Phys. Rev. B 74 (2006) 020101 (R). [9] H. Bea, M. Bibes, A. Barthelemy, K. Bouzehouane, E. Jacquet, A. Khodan, J.-P. Contour, S. Fusil, F. Wyczisk, A. Forget, D. Lebeugle, D. Colson, M. Viret, Appl. Phys. Lett. 87 (2005) 072508. [10] H.N. Al-Shareef, D. Dimos, M.V. Raymond, R.W. Schwartz, C.H. Mueller, J. Electroceram. 1 (1997) 145. [11] S. Kamba, D. Nuzhnyy, M. Savinov, J. Šebek, J. Petzelt, J. Prokleska, R. Haumont, J. Kreisel, Phys. Rev. B 75 (2007) 024403. [12] O. Rico-Fuentes, E. Sanchez-Aguilera, C. Velasquez, R. Ortega-Alvarado, J.C. Alonso, A. Ortiz, Thin Solid Films 478 (2005) 96. [13] K. Iwauchi, S. Yamamoto, Y. Bando, T. Hanai, N. Koizumi, T. Takada, S. Fukushima, Jpn. J. Appl. Phys. 10 (1971) 1513. [14] S.S. Shinde, R.A. Bansode, C.H. Bhosale, K.Y. Rajpure, J. Semicond. 32 (2011) 013001. [15] H.W. Jang, S.H. Baek, D. Ortiz, C.M. Folkman, R.R. Das, Y.H. Chu, P. Shafer, J.X. Zhang, S. Choudhury, V. Vaithyanathan, Y.B. Chen, D.A. Felker, M.D. Biegalski, M.S. Rzchowski, X.Q. Pan, D.G. Schlom, L.Q. Chen, R. Ramesh, C.B. Eom, Phys. Rev. Lett. 101 (2008) 107602. [16] T. Rojac, M. Kosec, B. Budic, N. Setter, D. Damjanovic, J. Appl. Phys. 108 (2010) 074107. [17] N. Balke, M. Gajek, A.K. Tagantsev, L.W. Martin, Y.-H. Chu, R. Ramesh, S.V. Kalinin, Adv. Funct. Mater. 20 (2010) 3466.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Growth temperature dependent dielectric properties of BiFeO3 thin films deposited on silica glass substrates Taimur Ahmed ⁎, Andrei Vorobiev, Spartak Gevorgian Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
a r t i c l e
i n f o
Article history: Received 31 January 2011 Received in revised form 14 February 2012 Accepted 23 February 2012 Available online 3 March 2012 Keywords: Bismuth ferrite Thin films Amorphous silica glass substrate Growth temperature Secondary phases Permittivity Ex-situ annealing
a b s t r a c t We have studied the dependence of dielectric properties on the deposition temperature of BiFeO3 thin films grown by the pulsed laser deposition technique. Thin films have been grown onto amorphous silica glass substrates with pre-patterned Au in-plane capacitor structures. It is shown that on the amorphous glass substrate, BiFeO3 films with a near-bulk permittivity of 26 and coercive field of 80 kV/cm may be grown at a deposition temperature of about 600 °C and 1 Pa oxygen pressure. Low permittivity and higher coercive field of the films grown at the temperatures below and above 600 °C are associated with an increased amount of secondary phases. It is also shown that the deposition of BiFeO3 at low temperature (i.e. 500 °C) and post deposition ex-situ annealing at elevated temperature (700 °C) increases the permittivity of a film. The applied bias and time dependence of capacitance of the films deposited at 700 °C and ex-situ annealed films are explained by the de-pinning of the ferroelectric domain-walls. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Perovskite ABO3 oxides are of great interest in many applications due to their multifunctional (semiconducting, electrochromic, magnetoresistive, dielectric, multiferroic etc.) properties [1]. Specifically, the ionic conductivity and presence of oxygen vacancies make perovskite oxides useful for different sensing devices. For gas sensitivity, an oxide material should contain at least one readily reducible ion together with one that is readily oxidizable [2]. In recent years, multiferroic BiFeO3 (BFO) has been the focus of extensive study due to its potential use in nonvolatile ferroelectric or magnetoelectric memories, sensors, terahertz devices and photonic devices [3,4]. BFO, both in bulk and nanoparticle form, is suitable for gas sensing because it contains two metals (Bi and Fe) that provide two relatively easily accessible oxidation states [2]. Moreover, BFO is characterized by high thermal and chemical stability in comparison with simple metal oxides [5]. In this work, BFO thin films are deposited by pulsed laser deposition (PLD) and their growth temperature dependent dielectric properties are measured to study the feasibility of fabricating a disposable and inexpensive device that could be later used in gas sensing applications. The focus of our studies is the optimization of permittivity because permittivity is assumed to be the potential sensing parameter in BFO thin film gas sensors. Amorphous silica
⁎ Corresponding author. E-mail address: [email protected] (T. Ahmed). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.02.082
glass (SiO2) is used as a substrate because it is cheaper than the single crystal substrates (like SrTiO3, DyScO3, sapphire, etc.) and SiO2 can also withstand high deposition temperatures during the PLD process. The challenge of growing BFO films on amorphous SiO2 substrates is addressed by A. Vorobiev et al. [6] where buried coplanar Au electrodes are used as nucleation centers. Following the same configuration as in [6], we pattern Au electrodes as interdigitated capacitors (IDC) and grow BFO films on top of the electrodes. This configuration – featuring buried interdigital electrodes instead of parallel plate configuration with electrodes on top of the BFO film – facilitates the growth of BFO films on the amorphous substrates and also provides a larger active area for the gas sensing, thereby increasing the device sensitivity. It has already been reported that, on single crystal substrates, single-phase BFO thin films with limited thicknesses (25–240 nm) can only be grown in a narrow range of deposition temperature (550–600 °C) and pressure (around 1 Pa) [7–9]. It is not obvious whether these deposition conditions are also applicable for the growth of BFO films on the amorphous SiO2 substrates. The present work seeks to address this uncertainty, by growing a number of BFO films at different temperatures while keeping the oxygen pressure constant at 1 Pa. 2. Experimental details Bottom Au (500 nm)/TiO2 (50 nm) IDC electrodes are deposited by DC magnetron sputtering on amorphous SiO2 substrates and subsequently patterned by photolithography and ion beam milling.
T. Ahmed et al. / Thin Solid Films 520 (2012) 4470–4474
BFO thin films with a thickness of 500 nm are grown on top of the electrodes by PLD using a Bi1,1FeO3 ceramic target with excess of Bi to compensate for high Bi re-evaporation. A KrF excimer laser (λ = 248 nm and τ = 30 ns) source with an energy density of 1.5 J/ cm 2 is used to ablate the ceramic target. The distance between the SiO2 substrate and the ceramic target is kept fixed at 6 cm in the deposition chamber. The growth temperature is held constant in the range of 500–750 °C. After the deposition, the films were cooled down to room temperature under 50 kPa oxygen pressure. Films grown at 500 °C are also ex-situ annealed at 700 °C for 2 h in 2 L/min oxygen flow. Top contact pads of Au (500 nm)/Ti (50 nm) are deposited by e-beam evaporation. Top contact pads are capacitively connected with the bottom contact pads of the buried IDC electrodes, through the BFO film. A schematic illustration of the BFO film's test structure, with the in-plane buried IDC electrodes, is shown in Fig. 1(a). The dielectric properties of the deposited films have been characterized at 1 MHz for the permittivity measurements and 30 MHz for hysteresis measurements, both by using a PH 4285A LCR-meter. The X-ray diffraction (XRD) spectra of the BFO films were obtained using a Philips X'pert SW 3040 diffractometer equipped with a Cu Kα point radiation source, a MRD lens, a thin collimator and a Ni filter. Farnell's model has been used for the calculation of apparent permittivity [10] using a thickness of 500 nm, measured for the film grown at 600 °C. 3. Result and discussion Micrographs in Fig. 1(b)–(g) show BFO thin films grown over buried IDC electrodes at deposition temperature ranging between 500 and 750 °C. As it can be seen from the micrographs, the film in the electrodes' gap changes color with the growth temperature. It is assumed that the film grown in the gap most likely consists of a mixture of phases including stoichiometric BFO and secondary
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phases either enriched by Bi at lower growth temperatures (following the target composition) or enriched by Fe at higher growth temperatures (due to Bi re-evaporation). We assume that the certain color of an area in the gap, distinguishable in the optical images of Fig. 1, is defined by the refractive index of the phase dominating in that area. It can also be seen from the images in Fig. 1 that the growth of BFO phases is initiated at the edges of buried Au IDC electrodes and the phases develop into the gap with increasing growth temperature. As the growth temperature increases to a certain level the gap is filled by film areas with almost uniform color which indicates the predomination of a certain phase. It is well known that at higher growth temperatures Bi reevaporates and leads to the growth of Fe-enriched secondary phases along with the BFO stoichiometric phase [8,9]. One way to overcome this issue is to deposit BFO thin films at low temperatures followed by post deposition ex-situ annealing at elevated temperatures. To prove this approach, the BFO film grown at 500 °C has been ex-situ annealed. Fig. 2 shows the micrograph of BFO film grown at 500 °C followed by ex-situ annealing at 700 °C for 2 h in oxygen flow. As can be seen, the annealed film reveals color patterns (in the gap) different from the as-deposited film (Fig. 1b) and the patterns are comparable with those of the film deposited at approximately 600 °C (Fig. 1d). It is assumed that the post-deposition annealing results in the re-crystallization of the BFO phase, an increase of grain size and decrease in the defect density i.e., improvement of the crystallinity which is also confirmed by atomic force microscope images, not shown here. This post-deposition annealing results in the change of the apparent refractive index of the film and hence changes the color in comparison to the as-deposited film. Fig. 3 illustrates the effect of deposition temperature and postdeposition annealing on the permittivity (εBFO) of BFO thin films at zero DC bias. H. Bea et al. [8] demonstrated the growth of single phase BFO films with thicknesses less than 100 nm at 1 Pa deposition
Fig. 1. A 3D schematic of the coplanar device, with indication of a larger active area intended for gas sensing applications (a). Optical micrographs of BFO films grown over Au IDC electrodes at different growth temperatures (b) 500 °C (c) 550 °C (d) 600 °C (e) 650 °C (f) 700 °C (g) 750 °C and under 1 Pa constant pressure.
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Fig. 2. Optical micrographs of BFO film grown at 500 °C and ex-situ annealed at 700 °C under a constant oxygen flow.
pressure. In our case, the films are five times thicker. So we assume the presence of Bi- and Fe-enriched secondary phases, with different amounts, in the whole range of our growth temperature i.e. 500–750 °C. As seen from Fig. 3, the maximum permittivity (εBFO = 26) in our film grown at 600 °C is slightly less than that of the bulk ceramic BFO (εBFO = 35) [11]. This can be explained by the presence of small amount of secondary phases along with the dominant stoichiometric BFO phase. At lower growth temperatures (b600 °C) the BFO films most likely have an excess of Bi-containing secondary phases (like Bi2O3) due to the excess of Bi in the ceramic BFO target [9]. This partly explains the lower permittivity (16 b εBFO b 26) of the films because the permittivity of the Bi2O3 phase calculated from the refractive index should be approximately 5 [12]. At higher growth temperatures (>600 °C) Bi re-evaporates and the films are depleted of Bi. As a result these films contain Fe-enriched secondary phases like Bi2Fe4O9, α-Fe2O3 and γ-Fe2O3 [9]. It is likely that as the temperature increases, the amount of the Fe-enriched secondary phases increases, resulting in decreased permittivity due to the lower permittivity of the Fe-enriched phases. In accordance with Ref. [13] permittivity of the α-Fe2O3 phase should be approximately 6. Shinde et al. [14] reported the permittivity of Fe2O3 thin films as about 2. Similarly, we assume that the film grown at 750 °C contains a higher amount of Fe2O3 secondary phases than the stoichiometric BFO phase which results in the lowest measured permittivity (~2). The permittivity of the film deposited at 500 °C followed by ex-situ annealing at 700 °C is also shown in Fig. 3. As suggested above, the post-deposition annealing increases the size of the BFO grains
Fig. 3. Dependence of apparent permittivity (εBFO) on film growth temperature and post-deposition annealing, measured at 1 MHz and room temperature.
Fig. 4. XRD patterns of the three as-deposited BFO films grown over the un-patterned Au bottom layer at 500 °C, 600 °C and 700 °C deposition temperatures.
resulting in a decrease of the relative amount of the nonstoichiometric grain boundaries with presumably lower permittivity. Consequently the permittivity of the annealed film increases in comparison with that of the as-deposited film. The XRD analysis confirms the earlier assumption that the change in the BFO films' phase composition with growth temperature is governed by the Bi re-evaporation. Fig. 4 shows the XRD spectra of the BFO films deposited on a non-patterned Au layer at 500, 600 and 700 °C. The films grown at 500 and 600 °C reveal reflections from the stoichiometric BFO(001) and BFO(110) families of planes. The BFO(111) reflection is probably masked by the strong Au(111) peak. The reflections from Bi2O3, Bi2Fe4O9 and Fe3O4 secondary phases are also registered. The BFO film grown at 500 °C reveals relatively higher intensities of the reflections from the Bi2O3 phase in comparison with that of the BFO and Fe-enriched phases. This indicates that the excess of Bi in the entire film's composition is due to the excess of Bi in the ceramic target (Bi1,1FeO3). The XRD spectra of the BFO film grown at 700 °C reveals no peaks associated with the Bi2O3 phase, indicating the absence of Bi-enriched secondary phases. Also the intensities of
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Fig. 5. C(V) response of BFO films grown at (a) 500 °C (b) 600 °C (c) 700 °C and (d) ex-situ annealed at 700 °C film, grown at 500 °C.
the reflections from the stoichiometric BFO phase are drastically reduced in comparison with that of the Fe-enriched phases. This can be explained by more intensive Bi re-evaporation from the condensation surface at the higher growth temperatures. Fig. 5 shows the dependence of the BFO film's capacitance on the applied electric field (C(V) response). The films grown at 500 °C (Fig. 5a) and 600 °C (Fig. 5b) reveal typical ferroelectric response. As explained earlier, the film deposited at 500 °C contains a higher amount of non-ferroelectric Bi-enriched secondary phases (Bi2O3). This results in some distortion of the C(V) response (Fig. 5a) and in a higher effective coercive field (150 kV/cm), since a part of the applied field drops on the non-ferroelectric secondary phases. Some distortion in ferroelectric loops of the film grown at 600 °C can be explained by the presence of secondary phases (though in small amounts) and by charge defects, mainly oxygen vacancies. The film grown at 600 °C shows a lower coercive field of 80 kV/cm when compared to that of the bulk ceramic counterpart [15]. The ferroelectric domain structure and polarization dynamics in BFO thin films have been reported recently [16,17]. We assume domain-wall movement in our as-deposited and ex-situ annealed at 700 °C films—grown at 700 °C and 500 °C respectively. The domainwall movement results in the responses shown in Fig. 5(c) and (d) where measured capacitances are affected by both the applied fields and the domain-wall movement during the time required for the
field change. The films grown at 500 and 600 °C reveal reversible polarization (Fig. 5a–b) whereas rather strong time dependence observed in both of the films grown at 700 °C (Fig. 5c) and ex-situ annealed at 700 °C (Fig. 5d) is associated with the de-pinning of the domain-walls. Due to the lower density of defects in these films the de-pinning phenomenon contributes to the measured dielectric response. As reported in Ref. [16], the post-deposition annealing of BFO films results in the de-pinning of domains due to the sublimation of the Bi2O3 secondary phase associated with the pinning centers. One can thus also assume the de-pinning in our annealed film is enhanced in comparison with the as-grown film due to the reduction of the Bi2O3 secondary phase after the post deposition annealing. The C(V) response with and without the presence of a 0.2 T magnetic field, applied normal to the direction of applied electric field, has also been measured at 30 V DC bias (not shown here). Maximum magnetic field tuning of 0.21% was measured in the films grown at 600 °C which is consistent with the BFO films grown on amorphous silica glass substrate, as reported in Ref. [6]. To confirm the assumption of the domain-wall de-pinning, the time dependence of capacitance (the C(t) response) was measured at 30 MHz and 50-V DC bias. Fig. 6 shows the C(t) dependence of the BFO films grown at 700 °C and of the film grown at 500 °C followed by ex-situ annealing at 700 °C. The increase in capacitances with poling time, in both films, can be explained as domain-wall de-
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decrease in permittivity at lower and higher (than 600 °C) growth temperatures is explained by the excess of Bi- and Fe-enriched secondary phases, respectively. Thus by using a Bi-enriched target, the growth temperature can be optimized to grow predominantly single-phase BFO thin films with permittivity close to the ceramic counterpart. With the reported target composition and deposition conditions in this paper, the samples prepared at 600 °C are expected to be suitable for the further study of the gas sensing properties of BFO thin films. Acknowledgment The authors would like to thank the Swedish Research Council for partial support of this work. References Fig. 6. C(t) dependence of the as-deposited film grown at 700 °C (circles) and the film grown at 500 °C followed by ex-situ annealing at 700 °C (squares) in oxygen flow.
pinning. As can be seen, the characteristic time of the capacitance changes is 10–20 min which is in agreement with the time of the voltage cycles of the C(V) measurements (Fig. 5). In the asdeposited film, decrease in capacitance after 10 min of poling and an anomaly at the 15th minute are observed. Both can be associated with either the film degradation or the redistribution of the domain-wall configuration due to the local field caused by the accumulation of space charge at the film–electrode interface. The dependence of capacitance on applied bias in the ex-situ annealed film (Fig. 5d) reveals some saturation. Thus one can conclude that, in comparison with as-deposited film, ex-situ annealed film contains higher concentration of defects acting as pinning centers. 4. Conclusions We have grown BFO thin films by PLD on amorphous SiO2 substrates containing pre-patterned Au in-plane capacitor structures and have studied the influence of growth temperature on the dielectric properties. The observed changes in the permittivity of the BFO films with growth temperature are associated with the changes in the BFO films' phase composition. The XRD analysis indicates that the stoichiometric BFO phase dominates at about 600 °C growth temperature and results in maximum permittivity (εBFO = 26). The
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