The investigation of (Bi,La)(Ga,Fe)O3–PbTiO3 thin film prepared by PLD technique

Materials Science and Engineering B 177 (2012) 140–144

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The investigation of (Bi,La)(Ga,Fe)O3 –PbTiO3 thin film prepared by PLD technique Shengwen Yu ∗ , Dongmei Sun, Wufeng Yang, Jinrong Cheng School of Materials Science and Engineering, Shanghai University, China

a r t i c l e

i n f o

Article history: Received 10 May 2011 Received in revised form 9 October 2011 Accepted 31 October 2011 Available online 15 November 2011 Keywords: BiFeO3 Films Ferroelectric properties Electrical properties

a b s t r a c t In this report, (Bi,La)(Ga,Fe)O3 –PbTiO3 (BLGF–PT) thin film was prepared on platinum coated Si wafer by pulsed laser deposition (PLD) method. BLGF–PT ceramic with morphotropic phase boundary (MPB) composition was used as the PLD target. The spot of Bi2 Fe4 O9 impurity phase in X-ray diffraction profile implies a composition deviation in the BLGF–PT thin film from that of target. However, the MPB feature of co-existence of rhombohedral and tetragonal phases remains remarkable in the film. The characterization results of the leakage current and ferroelectric hysteresis loop indicate a desirable insulation in this BLGF–PT thin film. Polarization response by positive-up-negative-down pulse measurement and retention property are further carried out at room temperature to check the intrinsic ferroelectric performance. Moreover, high Curie temperature of 465 ◦ C has been discovered in this BLGF–PT thin film, which may promote the application scope of the thin film. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Bismuth ferrite (BiFeO3 , BFO) is a unique multiferroic material with both Curie and Neel temperatures well above room temperature [1–3]. Massive efforts have been undertaken to explore the potential application of BFO on multi-manipulated devices, such as magneto-electronics [2,3], spintronics [4], photonics [5], and magnonics [6]. In view of the present Si-based manufacturing industry, the integration of BFO thin film on Si wafer is a naturally following consideration. To judge the electrical performance and the compatibility with Si wafer, platinum (Pt) electrode and Pt coated Si wafer are conventionally adopted in the ferroelectric thin films by constructing a capacitor configuration of metal–ferroelectric film–metal structure [7]. Although Pt offers good electrical contact and high temperature stability [8], interface reaction or inter-diffusion between the film and electrodes is inevitable and deterioration is subsequently induced [9]. It is well known that the leakage matter is a long standing problem hovering over the BFO material which is usually ascribed to the small amount of Fe2+ varied from Fe3+ and oxygen vacancies [10,11]. This notorious issue becomes more serious in the BFO thin film system because of the additional amount of defects and vacancies imported in the film fabrication process. To improve the insulation in BFO thin film system, various approaches have been tried to suppress the formation of defects

∗ Corresponding author at: School of Materials Science and Engineering, Shanghai University, 149 YanChang road, Box 32, Shanghai 200072, PR China. Tel.: +86 21 56332704; fax: +86 21 56332694. E-mail address: [email protected] (S. Yu). 0921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2011.10.019

and oxygen vacancies, including depositing epitaxial film on single crystal substrates [1,12–15], doping with aliovalent ions [16–20], introducing conductive oxide buffer layer [9,21,22] or forming solid solution with other perovskite oxide [23–25]. Among these attempts, deliberately depositing the BFO thin films on single crystal substrates indeed have shown excellent results, which leads to very high product cost in return. Our previous study reveals that bismuth ferrite–lead titanate (BiFeO3 –PbTiO3 , BFO–PT) solid solution is a very promising BFO based material with greatly improved insulation and enhanced ferroelectricity [25–27]. When co-substituted with La and Ga, the electrical resistivity of the BFO–PT ceramic ((Bi,La)(Ga,Fe)O3 –PbTiO3 , BLGF–PT), especially that bearing the composition at morphotropic phase boundary (MPB) can reach up to 1013  cm at room temperature [25,27]. This suggests that depositing this BLGF–PT thin film directly on commercial Pt coated Si wafer is a worthwhile try. Pulsed laser deposition (PLD) technique is known for the precise composition reproduction of the ceramic target and employed a lot in the synthesis of BFO-based thin films [1,13,19]. Therefore BLGF–PT thin film is going to be prepared by PLD method and investigated in this work. The electrical properties and the Curie temperature will be studied to understand the possible application within this thin film. 2. Experimental method BLGF–PT thin film was fabricated on Pt(1 1 1)/TiO2 /SiO2 /Si(1 0 0) substrate by PLD technique (KrF excimer,  = 248 nm). 0.6(Bi0.9 La0.1 )(Ga0.05 Fe0.95 )O3 –0.4PbTiO3 ceramic with MPB composition was sintered by solid-state reaction following the same process in Ref. [27] and used as the target in this work. In the light of previous work [28], the thin film was deposited under

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oxygen environment of 50 mTorr with the substrate temperature kept at 600 ◦ C and the laser energy density was set at 5 J/cm2 . The as-deposited thin film was further in situ annealed at 700 ◦ C under oxygen atmosphere of 200 mTorr for 60 min to obtain better crystallization. The thickness of the final BLGF–PT thin film is around 250 nm which was determined by TENCOR profilometer (Alphastep 500). Pt top electrodes were directly sputtered on the surface of the annealed thin film through a shadow mask with a diameter of 0.2 mm. The phase structure of the thin film was checked by X-ray diffraction (XRD) with a D/MAX-RC diffractometer using CuK␣ radiation (40 kV/100 mA,  = 0.154 nm) with step of 4◦ /min. The leakage current density of the BLGF–PT thin film was detected with Keithley 4200 to give a general insulation view. To investigate the potential application of the BLGF–PT thin film on ferroelectric devices, hysteresis loop, polarization response by positive-upnegative-down (PUND) pulse measurement and retention property were carried out by using Ferroelectric Material Test System (Precision Premier II, Radiant Technology, USA). Finally, the temperature dependent dielectric property of the thin film was measured by an impedance analyzer HP 4192A to find out the Curie temperature. All the measurements were performed at room temperature except the temperature dependent dielectric spectrum. 3. Results and discussions 3.1. XRD analysis Fig. 1 is the XRD patterns of the prepared BLGF–PT thin film. It shows that the BLGF–PT thin film is polycrystalline in perovskite structure. Like the typical feature of the BLGF–PT target with MPB composition [25], both rhombohedral and tetragonal phases are also simultaneously observed in the thin film. Symbols such as (1 1 0)R and (1 1 0)T in Fig. 1 represent rhombohedral and tetragonal phases, respectively. Very weak peaks of impurity phase are detected, which is identified as Bi2 Fe4 O9 . The presence of Bi2 Fe4 O9 phase can be attributed to the loss of Bi during the deposition and in situ annealing process because Bi element is highly volatile under low pressure at high temperature which causes unavoidable stoichiometry fluctuation in most film deposition techniques. This suggests that the composition of the BLGF–PT thin film may have differed from that of the target and a rapid thermal annealing process would be more appropriate for the thin film fabrication. It is recorded that (1 − x)BLGF–xPT with La-20 at.%, Ga-5 at.% and x = 0.43, (1 − x)BLF–xPT with La-10 at.% and x = 0.43

Si(400)

Bi2Fe4O9

Pt(111)

(110)R

BLGF-PT

141

[34], (1 − x)BLGF–xPT with La-10 at.%, Ga-5 at.% and x = 0.4 are all in the vicinity of MPB [27]. With this understanding, the concept of MPB is rather a composition region with multi-structure phases co-existed than a definite composition boundary line. Therefore the co-existence of rhombohedral and tetragonal phases in the BLGF–PT thin film appears very reasonable if the composition of the thin film merely deviates slightly to less Bi content while still fluctuates within the MPB region. 3.2. Electrical behaviour investigation 3.2.1. Leakage examination The J–E curve at room temperature of the BLGF–PT thin film is plotted in Fig. 2. It gives that the leakage current densities of the thin film are 2.6 × 10−6 , 8.6 × 10−5 and 7.0 × 10−4 A/cm2 at 100, 400 and 741 kV/cm, respectively. These values are much lower than those in our previous research on BFO–PT thin film [29] and the BFO–PT thin films in other group [30], which indicates improved insulation in this BLGF–PT thin film. Meanwhile, the leakage data is also better than that in the Ti-doped BFO thin film with 200 nm thickness prepared on SrRuO3 buffered SrTiO3 substrate by PLD [19] and can be comparable with the value in the BFO thin film of 200 nm carefully prepared on LaNiO3 buffered Pt/Si wafer under low temperature [21]. 3.2.2. Ferroelectric hysteresis loop survey Although the leakage result of the BLGF–PT thin film looks desirable, ferroelectric natures should be characterized to survey the application on microelectronic devices, such as ferroelectric random access memory (FRAM). Fig. 3(a) and (b) give the room temperature ferroelectric loops performed under diverse frequency and applied field, respectively. The hysteresis P–E loops measured above 100 Hz show typical ferroelectric characteristics and almost remain the same shape. Interestingly, hysteresis loop can still be obtained in the BLGF–PT thin film under a low measuring frequency of 10 Hz with applied voltage of 20 V (∼800 kV/cm) (shown in Fig. 3(a)). So far as we know, most well-shaped hysteresis loops of BFO-based thin films obtained at room temperature are usually measured under relative high frequency such as 1 kHz. Few BFO-based thin film is reported exhibiting typical P–E loop when measuring under lower than 100 Hz because the leakage effect usually zooms in under lower measuring frequency. When increasing the frequency up to 1 kHz, high voltage of 39 V can be applied to the BLGF–PT thin film, which is corresponding to an applied field high of 1560 kV/cm (shown in Fig. 3(b)). To present a clear view, the relationships of 2Pr –E and 2Ec –E of the BLGF–PT thin film at 1 kHz have been concluded in Fig. 3(c). The values of remnant polarization (2Pr ) and coercive field (2Ec ) of the film under 1560 kV/cm are 74 ␮C/cm2 -2

-3

10

(220)R

(211)R

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2θ (degree) Fig. 1. XRD –2 scan patterns of BLGF–PT thin film. Symbols such as (1 1 0)R and (1 1 0)T represent rhombohedral and tetragonal phases, respectively.

10

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Applied Electric Field,kV/cm Fig. 2. The leakage current density of the BLGF–PT thin film with thickness of ∼250 nm.

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b 75 50 25

10 Hz @20V 100 Hz 1K Hz 10K Hz

Polarization (μC/cm2)

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Applied Electric Field,kV/cm

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-800

0

E, kV/cm Fig. 3. (a) and (b) give the P–E hysteresis loops of the BLGF–PT thin film under diverse frequency and applied field, respectively. (c) The relationships of 2Pr –E and 2Ec –E of the BLGF–PT thin film at 1 kHz.

and 550 kV/cm, respectively. Both the shape of the P–E loops and the uptrend of 2Pr –E, 2Ec –E curves illustrate that the BLGF–PT thin film is still away from ferroelectric saturation even under a high applied voltage of 39 V at 1 kHz. This infers that the BLGF–PT thin film may endure much higher applied voltage. While our previous BFO–PT thin films with 200–300 nm thickness can hardly endure 39 V. In another word, the leakage problem in this BLGF–PT thin film seems indeed ameliorated. 3.2.3. PUND pulse inspection and retention test The positive-up-negative-down (PUND) pulse measurement was executed to further scrutinize the intrinsic ferroelectric performance within the BLGF–PT thin film. Unlike the continuous measuring wave used in P–E hysteresis measurement, the P, U, N, and D pulses were applied individual by individual after a period of delay time, which made the polarization response in PUND measurement much less affected by the leakage and nonlinear dielectric factors and resulted in a relaxed P value (P ≈ 2Pr ) [10,31]. Fig. 4(a) is the remnant polarization values of BLGF–PT thin film extracted from both hysteresis loops and PUND measurements. The measuring frequency for P–E loop is 1 kHz. And the pulse width and pulse delay time for PUND measurement are 1 ms and 1000 ms, respectively. It is conspicuous that the value of Pr in PUND is absolutely relaxed and almost half of that in P–E measurement. The explicit relaxation effect of Pr in PUND reflects the existence of nonferroelectric components in the BLGF–PT thin film, which could be the defects, vacancies or interfaces in the thin film capacitor system. No evidence of saturation of the polarization Pr with applied electric field is observed in Fig. 4(a) in either P–E or PUND measurement. Referring to Fig. 3(c), the applied electric field of 800 kV/cm in Fig. 4(a) is far not enough to gain polarization saturation for this BLGF–PT thin film. However, non-ferroelectric components could exert restraint on the switching polarization in the PUND measurement and lead to response failure when increasing the applied

voltage. In this work, the switching polarization refused to respond when the applied voltage was over 22 V (1220 kV/cm). Therefore, 15 V (600 kV/cm) was selected as the secure applied voltage to inspect the pulse width dependence of switching polarization P (shown in Fig. 4(b)), which is about 1.67Ec of the BLGF–PT thin film. It is seen that the switching polarization P increases with the applied pulse width and the slope is about 6.9 ␮m/cm2 per decade of pulse width. The opposite-state retention characteristic of the BLGF–PT thin film was further examined under 15 V and recorded in Fig. 4(c). The inset in Fig. 4(c) demonstrates the normalized reduction of P in retention. The reduction of P within 30,000 s at room temperature is about 37.2%. Although, the retention loss in this work is larger than those in the SRO buffered BLFO thin film [10] and BFO thin film on BPO electrodes [22], it is better than other PZT ferroelectric thin films with retention loss greater than 40% after 104 s [32]. Besides the defects induced by the Bi2 Fe4 O9 impurity phase, the interfaces between the Pt electrodes and the BLGF–PT thin film are also abundant of defects and vacancies [21,31]. These may result in charge redistribution and incomplete compensation for the polarization charge and free charge in the electrodes when taking switching polarization performance and cause retention loss ultimately [10,22]. 3.3. Curie temperature study The above knowledge of the leakage issue and ferroelectric performance reveal that applicable integration of the BLGF–PT thin film on Si wafer for devices is possible, however further improvement is mandatory. Meanwhile, another practical issue in the BLGF–PT thin film is the working temperature, which normally depends on the Curie temperature (Tc ). It is known that phase transition at the Curie temperature in a ferroelectric material usually leads to strong dielectric anomalies [33]. Therefore it is convenient

S. Yu et al. / Materials Science and Engineering B 177 (2012) 140–144

b

Polarization [ΔP] (μC/cm2)

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Time (s) Fig. 4. (a) The remnant polarization values of BLGF–PT thin film extracted from both hysteresis loop and PUND measurements. The measuring frequency for P–E loop is 1 kHz. The pulse width and pulse delay time for PUND measurement are 1 ms and 1000 ms, respectively. (b) The pulse width dependence of switching polarization in the BLGF–PT thin film with the applied electric field of 600 kV/cm (1.67Ec ) and measuring pulse delay time of 1000 ms. (c) The retention characteristic of the BLGF–PT thin film under 15 V. The inset in (c) is the normalized retention.

to discover the Tc of the BLGF–PT thin film by scanning the temperature dependent dielectric spectrum. Fig. 5 is the temperature dependent capacitance and dielectric loss of the BLGF–PT thin film which was measured at 1 MHz. With raising the temperature from room temperature to 600 ◦ C, a capacitance peak around 465 ◦ C is apparently observed, while the loss of the thin film nearly maintains stable before 465 ◦ C and then climbs sharply. This indicates that the Curie temperature of the thin film is probably ∼465 ◦ C. According to Ref. [25], we know that the Curie temperature of the BLGF–PT ceramic target is 386 ◦ C. It infers that the Tc of the BLGF–PT thin film has shifted 80 ◦ C higher to that of ceramic target. There are several possible reasons. Defects and interfaces in the BLGF–PT

1000

@1M Hz

o

~465 C

8

800

thin film definitely introduce extra inhomogeneous domains and display pinning effect during the measurement, which causes the broadness of permittivity peak and shift of Tc [33]. However, the higher shift of Tc in the thin film can be mainly due to the loss of Bi and slightly composition deviation to less Bi content as already addressed in the XRD analysis. It has been clearly pointed out that the BLGF–PT system presents V-shaped boundary in the Tc –x boundary line, where x is the content of PbTiO3 [25]. And the Tc of 0.6(Bi0.9 La0.1 )(Ga0.05 Fe0.95 )O3 –0.4PbTiO3 is tangibly evident in the lowest 386 ◦ C as shown in the phase diagram in Ref. [25]. Any slight composition deviation will lead to higher Tc . For example, the Tc of 0.58(Bi0.9 La0.1 )(Ga0.05 Fe0.95 )O3 –0.42PbTiO3 is about ∼450 ◦ C [25]. It is also reported that the Tc of 0.57(Bi0.9 La0.1 )FeO3 –0.43PbTiO3 ceramic is 462 ◦ C [34]. Hence, it is justifiable to conclude that the Curie temperature of the BLGF–PT thin film in this work is around 465 ◦ C, which is 80 ◦ C higher than the corresponding target. And the slight composition deviation in the BLGF–PT thin film should take the major responsibility.

400

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tanδ

Capacitance,pF

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Temperature,oC Fig. 5. The temperature dependent capacitance and dielectric loss of the BLGF–PT thin film carried out at 1 MHz.

4. Conclusions BLGF–PT thin film was prepared by PLD method with a 0.6(Bi0.9 La0.1 )(Ga0.05 Fe0.95 )O3 –0.4PbTiO3 ceramic target which is in the MPB composition. The weak sign of Bi2 Fe4 O9 phase indicates the composition of the BLGF–PT thin film deviates slightly from that of target. However, the co-existence of rhombohedral and tetragonal phases in the BLGF–PT thin film is prominent. The results of leakage current and ferroelectric hysteresis loop measurements imply the insulation of this BLGF–PT thin film is improved which enables the thin film to endure much higher applied voltage. The polarization response inspection by PUND measurement and retention loss of 37.2% within 30,000 s show an acceptable intrinsic

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ferroelectric performance in the BLGF–PT thin film. More significantly, the Curie temperature of the BLGF–PT thin film is ∼465 ◦ C, higher than that of the corresponding ceramic target. As a result, although further improvement is necessary, this BLGF–PT thin film possesses advantages in direct integration with Si wafer and is eligible in the application for devices demanding for high endurance of applied field and temperature. Acknowledgements This work was supported by Shanghai Special Foundation of Nanotechnology under Grant No. 1052nm07300, Natural Science Foundation of Shanghai under Grant No. 08ZR1407700, Shanghai Rising Star Program under Grant No. 08QH14008, Shanghai Education Development Foundation under grant No. 08SG41, Shanghai Leading Academic Disciplines (S30107) and National Nature Science Foundation of China under Grant No. 50872080. References [1] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 1719–1722. [2] N.A. Spaldin, M. Fiebig, Science 309 (2005) 391–392. [3] S.H. Baek, H.W. Jang, C.M. Folkman, Y.L. Li, B. Winchester, J.X. Zhang, Q. He, Y.H. Chu, C.T. Nelson, M.S. Rzchowski, X.Q. Pan, R. Ramesh, L.Q. Chen, C.B. Eom, Nat. Mater. 9 (2010) 309–314. [4] H. Béa, M. Gajek, M. Bibes, A. Barthélémy, J. Phys. Condens. Matter 20 (2008) 434221. [5] T. Choi, S. Lee, Y.J. Choi, V. Kiryukhin, S.-W. Cheong, Science 324 (2009) 63– 66. [6] P. Rovillain, R. de Sousa, Y. Gallais, A. Sacuto, M.A. Méasson, D. Colson, A. Forget, M. Bibes, A. Barthélémy, M. Cazayous, Nat. Mater. 9 (2010) 975–979.

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