SrRuO3 heterostructure sputtered at low temperature

Journal of Crystal Growth 316 (2011) 71–74

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Epitaxial SrRuO3/BiFeO3/SrRuO3 heterostructure sputtered at low temperature Q.X. Zhao a,n, J.K. Ma a, D.Y. Wei a, K.M. Wang a, X.H. Li a, X.Y. Zhang b, B.T. Liu a a b

College of Physics Science and Technology, Hebei University, Hebei 071002, China State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Hebei 066004, China

a r t i c l e in f o

abstract

Article history: Received 5 October 2010 Received in revised form 24 November 2010 Accepted 10 December 2010 Communicated by A. Ohtomo Available online 17 December 2010

SrRuO3(SRO)/BiFeO3(BFO)/SrRuO3 heterostructure is fabricated on the (0 0 1) SrTiO3 substrate by magnetron sputtering, in which BFO film is prepared at a low temperature of 525 1C. The structural and physical properties of the SRO/BFO/SRO heterostructure are investigated. It is found that the whole SRO/BFO/SRO heterostructure is epitaxially grown on SrTiO3 substrate with (0 0 1) orientation. The remanent polarization and coercive field of a typical SRO/BFO/SRO capacitor, measured at 300 kV/cm, are 38.5 mC/cm2 and 120 kV/cm, respectively. The capacitor shows little fatigue up to 1010 switching cycles, and relatively small leakage current density of 7.5  10  4 A/cm2 at 120 kV/cm. Moreover, the SRO/BFO/ SRO heterostructure exhibits a weak ferromagnetic behavior with a saturation magnetization of 9.3 emu/ cm3 and a coercive field of 338 Oe. & 2010 Elsevier B.V. All rights reserved.

Keywords: A3. Magnetron sputtering A3. Low temperature growth B1. Epitaxial BiFeO3 film B2. Ferroelectricity

1. Introduction Multiferroic materials, such as BiFeO3 (BFO), YMnO3, and TbMnO3, have attracted considerable interest due to the simultaneous existence of ferroelectricity, ferromagnetism (or antiferromagnetism), and ferroelasticity. Moreover, the coupling effects, e.g. ferroelectricity and ferromagnetism, make the multiferroic materials to be widely studied for a number of important potential applications, such as data storage, spintronics, sensors, actuators, and microelectromechanical systems, which are compatible with the semiconductor technology [1]. Among these multiferroic materials, BFO is an ideal candidate of multiferroic materials, which can be used at room temperature [2–5] due to its high Curie temperature of TC ¼1103 K and Neel temperature of TN ¼643 K corresponding to ferroelectric and magnetic phase transition, respectively. BFO in film form has intrigued great interests, and various methods, such as pulsed laser deposition (PLD) [4,6], sol–gel synthesis [7], and rf-sputtering [8,9] have been used to prepare BFO films. Wang et al. [4] prepared (0 0 1) oriented epitaxial SrRuO3 (SRO)/BFO/SRO capacitor heterostructure on SrTiO3 (STO) substrate and found that the crystal structure of the film is monoclinic in contrast to bulk, which is rhombohedral. The films display a room-temperature spontaneous polarization (50–60 mC/cm2) almost an order of magnitude higher than that of the bulk (6.1 mC/cm2). The observed enhancement was

n

Corresponding author. Tel./fax: + 86 312 5977033. E-mail address: [email protected] (Q.X. Zhao).

0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.12.024

corroborated by first-principles calculations and found to originate from a high sensitivity of the polarization to small changes in lattice parameters [9]. Very good structural and physical properties of Mn doped BFO (BFMO) film have been reported using the commercial BFMO precursors, in which BFMO films were annealed at 550 1C for 10 min in nitrogen atmosphere [10–12]. Although magnetron sputtering is one of the most important methods to prepare high quality and large-sized sample, only few results are reported on epitaxial BFO film with good ferroelectric properties, which may be attributed to the relative complexity of the preparation. In addition, BFO film prepared by magnetron sputtering may lower the growth temperature due to the lower deposition rate compared with those of the other methods, such as pulsed laser deposition method. The low temperature growth of the film may further lower the interface interdiffusion or reaction [13]. By optimizing the deposition parameters, we can obtain epitaxial BFO films on SRO/STO at a temperature of 525 1C, much lower than 600–700 1C for the growth of most epitaxial BFO films [8,9,14]. In this paper, we report the structural and physical properties of the low temperature prepared epitaxial SRO/BFO/SRO heterostructure grown on STO substrate using rf magnetron sputtering.

2. Experimental procedure (0 0 1) STO substrate was transferred into the sputtering chamber right after ultrasonically rinsed in acetone, 2-propanol, then dried with high-purity N2. 100 nm-thick SRO film was deposited on the STO

STO(002)

SRO/BFO(002)

STO(001)

substrate at 600 1C using rf magnetron sputtering at a power of 70 W in a 3 Pa mixture of Ar and O2 with a ratio of Ar:O2 ¼3:1. The base pressure was maintained at 2  10  4 Pa, and the target-substrate distance is 5 cm. 800 nm-thick BFO film was then deposited on the SRO/STO by sputtering from a Bi1.1FeO3 ceramic target, in which the 10% excess bismuth is to compensate for the Bi volatilization during subsequent thermal treatment. The deposition temperature was fixed at 525 1C, while the pressure was set to 1 Pa with an Ar:O2 ratio of 3:1. After deposition, the BFO film was in situ cooled at 3 1C/min to 360 1C in an oxygen atmosphere of 80 kPa, and then cooled to room temperature at 5 1C/min after an interval of 1 h. A Pt (100 nm)/SRO (100 nm) integrated top electrode was sputtered sequentially on the BFO/SRO/ STO heterostructure through a shadow mask to produce a circular pads of 7.85  10  5 cm2 for electrical properties measurement. At last, the whole Pt/SRO/BFO/SRO/STO heterostructure was annealed at 500 1C for 1 min by a rapid annealing furnace in order to anneal the top SRO electrode. The phase and crystallinity of the sample was characterized by X-ray diffraction (XRD) with Cu Ka radiation. The electrical properties of the SRO/BFO/SRO capacitors were measured at room temperature using a precision LC unit from Radiant Technologies. Leakage behavior and magnetic properties of the BFO film were characterized using a Keithley 2601 I–V system and a vibrating sample magnetometer (VSM), respectively.

SRO/BFO(001)

Q.X. Zhao et al. / Journal of Crystal Growth 316 (2011) 71–74

Intensity (a.u.)

72

Kβ

30

20

40 2 (deg)

50

60

270

360

Fig. 1(a) shows the XRD y 2y scan of a BFO/SRO/STO heterostructure. Only (0 0 l) peaks of BFO and SRO film can be found without any observable secondary phases, demonstrating that both BFO and SRO are highly (0 0 1) oriented along the normal to the substrate. To further study if SRO and BFO are epitaxial growth on STO substrate, the in-plane orientation was investigated by performing XRD f scan. As observed from the spectra of Fig. 1(b), using the pseudocubic (1 1 0) reflection of the BFO, four symmetric sharp peaks originating from BFO can be observed, indicating that the BFO film has good in-plane orientation. The ferroelectric hysteresis loop of the SRO/BFO/SRO capacitor was examined at a frequency of 2.5 kHz by a ferroelectric tester. Fig. 2(a) plots a typical hysteresis loop of the polarization vs electric field (P–E) for the SRO/BFO/SRO capacitor measured at 300 kV/cm. A saturated ferroelectric hysteresis loop with remanent polarization (2Pr) of 77 mC/cm2 and the average coercive field (Ec) of 123 kV/cm can be observed. The 2Pr of 77 mC/cm2 is quite higher than those of epitaxial BFO thin films [14,15]. It is found that Ec of  86 kV/cm at negative field is different from that of 160 kV/cm at positive field, which can be attributed to the different thermal history of the interfaces of BFO/SRO top and BFO/SRO bottom electrode. The average Ec value of our sample is lower than 260 kV/cm of rf sputtering-grown epitaxial BFO thin film [9]. In order to further characterize the switching properties of the capacitor, the relations of the switchable polarization (DP¼P*P^) and coercive field (Ec) of SRO/BFO/SRO capacitor with applied electric field (E) are illustrated in Fig. 2(b). P* represents the switching polarization between two pulses with the opposite polarity, and P^ represents the nonswitching polarization between two pulses with the same polarity. We can see that both the switchable polarization and coercive field were saturated at electric fields higher than 230 kV/cm. The switchable polarization and coercive field of the capacitor at the electric field of 300 kV/cm are 79 mC/cm2 and 121 kV/cm, respectively. The fatigue behavior of the SRO/BFO/SRO capacitor was caused by applying bipolar pulses with a frequency of 100 kHz at an electric field of 300 kV/cm. As shown in Fig. 3(a), no obvious degradation of polarization can be found up to 1010 switching cycles. The inset of Fig. 3(a) shows the hysteresis loops of the BFO capacitor, measured before and after fatigue measurements. Small differences can be found

Intensity (a.u.)

3. Results and discussion

0

180  (deg)

90

Fig. 1. (a) X-ray diffraction y  2y scan curve and (b) f scan of the BFO (1 1 0) plane for the BFO/SRO/STO heterostructure.

from the loops, implying that the SRO/BFO/SRO capacitor has very good fatigue resistance. Fig. 3(b) shows the swithchable polarization (DP) of the SRO/BFO/SRO capacitor as a function of retention time up to 104 s. The test capacitor was applied with a write pulse of  300 kV/cm and a read pulse of 240 kV/cm for the measurement. The DP remains essentially constant for retention time up to 104 s, indicating that the SRO/BFO/SRO capacitor has good retention characteristics. Fig. 3(c) shows the pulse width dependence of DP measured at 180 kV/cm. The polarization demonstrated weak pulse width dependence, showing behavior very similar to those films prepared by the metal organic chemical vapor deposition (MOCVD) method [16]. Fig. 4 presents the characteristics of the leakage current density vs electric field (J–E) of the SRO/BFO/SRO capacitor measured at room temperature. At the applied field of 120 kV/cm, J is around 7.5  10  4 A/cm2, comparable to the previous reports [17]. The inset of Fig. 4 shows the relation of ln(J) vs E1/2 of the SRO/BFO/SRO capacitor at positive bias, which describes the characteristic of the interface-limited Schottky emission. The leakage current density is given by "

1 JS ¼ AT exp  kB T kB T 2

f



q3 U

4pe0 er d

1=2 # ð1Þ

Q.X. Zhao et al. / Journal of Crystal Growth 316 (2011) 71–74

73

60 100

60

0 -20

0

Polarization (μC/cm2)

50

20 ΔP (μC/cm2)

Polarization (μC/cm2)

40

-50

BF AF

30 0 -30 -60

-40

-300

-60

0 150 -150 Electric field (kV/cm)

300

-100

-300

-150 0 150 Electric field (kV/cm)

300 10-1

101

103

105

107

109

1011

Cycles

200

100

150

0

100

50

50

-50

25 ΔP (μC/cm2)

50

Εc (kV/cm)

ΔP (µC/cm2)

75

ΔP

0

-ΔP

-25

-100 100

0 150

200

250

-50

300

Electric field (kV/cm) -75 Fig. 2. (a) Ferroelectric hysteresis loop of the SRO/BFO/SRO capacitor measured at 300 kV/cm and (b) the switchable polarization and coercive field (Ec) of the capacitor as a function of applied electric field.

100

101

102 Retention time (s)

103

104

150

100

50 ΔP (μC/cm2)

where A is the Richardson constant, f is the height of the Schottky barrier, q is the electron charge, er is the dielectric constant of the film, e0 is the permittivity of free space, and d is the sample thickness. A good straight line fitting to the data can be observed as shown in the inset of Fig. 4. Based on the slope of the line we can obtain a dielelctric constant of  6.1, which is very close to the dielectric constant of BFO (er ¼6.25). This indicates that the Schottky emission mainly account for the leakage current mechanism of the SRO/BFO/SRO capacitor. This is different from the Poole–Frenkel emission of the SRO/BFO/SRO capacitor grown on DyScO3 substrate [18], which may be attributed to the difference in film microstructure. The magnetic hysteresis (M–H) loop was measured using a vibrating-sample magnetometer. Fig. 5 presents a room-temperature magnetization-field (M–H) curve of the SRO/BFO/SRO heterostructure measured at 20 kOe along the out-of-plane direction. It can be found that the BFO film exhibits weak ferromagnetism with a saturated magnetization (Ms) of 9.3 emu/cm3 and a coercivity (Hc) of 338 Oe. Several reasons have been suggested for the weak ferromagnetic behavior of the BFO thin film, including the existence of Fe2 + , epitaxial strain, and change in canting angle of Fe3 + cations [4,19,20]. The exact origin for the weak ferromagnetism in BFO thin film is an issue for further debate [21]. Since X-ray photoelectron spectroscopy did

0

-50

-100

-150 10-5

10-4

10-3

10-2

10-1

Pulse width (s) Fig. 3. (a) The switchable polarization of the SRO/BFO/SRO capacitor as a function of switching cycle. The inset presents the ferroelectric hysteresis loops before (BF) and after (AF) cycling the fatigue measurement, (b) the switchable polarization of the SRO/BFO/SRO capacitor as a function of retention measurement time, and (c) switchable polarization as a function of pulse width.

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Q.X. Zhao et al. / Journal of Crystal Growth 316 (2011) 71–74

4. Conclusion

10-3

In summary, we have grown high quality epitaxial BFO thin film on the (0 0 1) STO substrate with a SRO buffer layer at 525 1C using magnetron sputtering. It is found that the SRO/BFO/SRO capacitor possesses not only good ferroelectric switching properties, such as fatigue resistance and retention, but also a weak magnetic property. The leakage current density of the SRO/BFO/SRO capacitor satisfies the interface-limited Schottky emission behavior.

10-5 -3

10-6

ln (J) (A/cm2)

Current density (A/cm2)

10-4

10-7

-4 -5 -6

Acknowledgements

-7

10-8

0

2

4 E

1/2

6

8

10

12

1/2

(kV/cm)

10-9 -120

-80

-40

40 0 80 Electric field (kV/cm)

120

160

Fig. 4. The leakage current density vs electric field of the SRO/BFO/SRO capacitor for both negative and positive biases. The inset shows the ln(J) vs E1/2 fitting of the leakage current density at positive bias, indicating the interface-limited Schottky emission behavior.

Magnetization (emu/cm3)

10

5

0

-5

-10 -20

-10

0 10 Magnetic field (kOe)

20

Fig. 5. A typical magnetic hysteresis loop of the SRO/BFO/SRO heterostructure measured at 20 kOe.

not support the existence of Fe2 + , we believe that the weak ferromagnetism of the BFO film mainly arises from the increase in canting angles [22].

This work was supported by the NSFC (60876055, 11074063), the NSF of Hebei Province (E2009000207, E2008000620, 08B010), the SRFDP (20091301110002), and the Key Basic Research Program of Hebei Provincial Applied Basic Research Plan (10963525D).

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