Influence of rare-earth elements doping on structure and optical properties of BiFeO3 thin films fabricated by pulsed laser deposition

Applied Surface Science 307 (2014) 543–547

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Influence of rare-earth elements doping on structure and optical properties of BiFeO3 thin films fabricated by pulsed laser deposition Jian Liu a , Hongmei Deng b , Huiyi Cao a , Xuezhen Zhai a , Jiahua Tao a , Lin Sun a , Pingxiong Yang a,∗ , Junhao Chu a a Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronics, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China b Instrumental Analysis and Research Center, Institute of Materials, Shanghai University, 99 Shangda Road, Shanghai 200444, China

a r t i c l e

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Article history: Received 16 December 2013 Received in revised form 12 March 2014 Accepted 9 April 2014 Available online 18 April 2014 Keywords: BiFeO3 Eu doping Thin films Pulsed laser deposition Microstructure Optical properties

a b s t r a c t The Bi1−x Eux FeO3 (BEFOx , x = 0, 0.03, 0.05, 0.07, 0.1) films were grown on LaNiO3 coating Si substrates by pulsed laser deposition. X-ray diffraction patterns indicate that the films exhibit a (1 0 0)-highly oriented pseudocubic perovskite crystal structure. Scanning electron microscopy exhibits that the number of island-like structures decreases with increasing Eu dopant. The position of the A1 -1 mode of the films in the Raman spectra shifts to higher wavenumber with increasing x. With increasing the amount of Eu, the refractive index increases and the extinction coefficient decreases. In addition, the band gap of BEFOx films decreases with increasing Eu dopant. © 2014 Elsevier B.V. All rights reserved.

1. Introduction As a member of multiferroics, BiFeO3 (BFO) has attracted more and more attention since their interesting physical properties [1–3] and potential applications [4–9] which are introduced in recent years. BFO is the only single phase multiferroic material at room temperature, which possesses a high Curie temperature and antiferromagnetic Neel temperature (TC ∼810 ◦ C, TN ∼380 ◦ C) [10]. The BFO has a rhombohedral distorted perovskite structure with the space group of R3c, and a relatively low crystallization temperature [11]. Recently, BFO films have been reported for its photovoltaic effect [12–14], which makes them potential material for photonic devices. Thus, it is necessary to study the optical properties of BFO films. The A-site (Bi-site) substituting by using rare-earth ions have some advantages to improve the multiferroic properties of the BFO material, such as eliminating the impurity phase and the secondary phase [15,16], reducing the leakage current [17,18] and decreasing the optical band gap [19]. However, the effects of rare-earth element dopants on the optical properties of Bi1−x Eux FeO3 films have not yet been reported.

∗ Corresponding author. Tel.: +86 21 54345157; fax: +86 21 54345119. E-mail addresses: [email protected], [email protected] (P. Yang). http://dx.doi.org/10.1016/j.apsusc.2014.04.071 0169-4332/© 2014 Elsevier B.V. All rights reserved.

In this paper, the Bi1−x Eux FeO3 (x = 0, 0.03, 0.05, 0.07 and 0.1) thin films were grown on LaNiO3 (LNO) coating Si (1 0 0) substrates by pulsed laser deposition (PLD). And the effects of Eu-doping on the film’s microstructures, surface morphologies, and optical properties have been investigated. Especially, the optical constants and optical band gap of the BEFOx films are analyzed by spectroscopic ellipsometry (SE). 2. Experimental The BEFOx (x = 0, 0.03, 0.05, 0.07, 0.1) thin films were grown on LNO/Si (1 0 0) substrates by PLD method. At first, the five ceramic targets corresponding to the different Eu composition were prepared by the conventional solid state reaction method. Appropriate amounts of high purity Bi2 O3 (with 5% mol excess), Eu2 O3 and Fe2 O3 powders (99.9%, Sinopharm Chemical Reagent Co. Ltd., China) were mixed and grinded together in absolute alcohol media for 8 h. Then the uniform-mixed powders were pressed into disks and sintered for 2 h at 810 ◦ C. In order to obtain the good quality films, the parameters among the deposition were optimized. The key parameters used to grow the BEFOx films by PLD method are summarized in Table 1. The crystalline structure and morphology of the films were investigated by X-ray diffraction (XRD, Bruker D8 Advance) and

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J. Liu et al. / Applied Surface Science 307 (2014) 543–547

KrF 248 nm 1.6 J/cm2 10 Hz Bi1−x Eux FeO3 (x = 0, 0.03, 0.05, 0.07, 0.1) LNO/Si (1 0 0) 6 cm 700 ◦ C 3 × 10−4 Pa 10 Pa 30 min

scanning electron microscopy (SEM, Philips XL30FEG), respectively. The spectroscopic ellipsometry (SE) spectra were fitted based on the Lorentz oscillator mode, and optical thin film software, namely SC630UVN (Shanghai Sanco Instrument Co. Ltd.), was used to analyze ellipsometric data. All these measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows the XRD patterns of BEFOx (x = 0, 0.03, 0.05, 0.07 and 0.10) ceramic targets and thin films on LNO/Si (1 0 0) substrates. In Fig. 1(a), the XRD patterns of the BEFOx ceramic targets characterize a rhombohedrally distorted perovskite structure, and no secondary phases are observed. As shown in Fig. 1(b), the XRD patterns indicate that the films exhibit a (1 0 0)-highly oriented pseudocubic perovskite crystal structure similar to the LaNiO3 buffer layer, and no impurity phases are observed. The peak (1 0 0) of BEFOx films (the inset of Fig. 1(b)) suggests a shift in the peak position towards higher 2 value with the increasing x, which resulted from a structural distortion of the BFO lattice due to the substitution of Eu ˚ is smaller than that of Bi3+ because the radius of Eu3+ (∼0.95 A) ˚ similar to the report of [6]. (∼1.03 A), The tolerance factor (t) can be expressed as (RA + RO ) , t= √ 2(RB + RO ) where RA , RB , and RO are the ionic radii of the A, B, and O sites. For EuFeO3 , the tolerance factor t is 0.92. Thus, with the increasing

(b)

Intensity (arb. unit)

(116) (122) (214) (300)

(006) (202)

0.10

(024)

Intensity (arb. unit)

(102)

(104) (110)

(a) Bi1-xEuxFeO3 ceramic targets

0.07 0.05

Bi1-xEuxFeO3 films

22

23

2 (degree)

24

BFO(200) LNO(200)

Values

Laser source Wavelength Energy density Repetition rate Target Substrate Distance between target and substrate Substrate temperature Base pressure Oxygen pressure Film growth time

Intensity (arb.unit)

Parameters

of Eu concentration, the BiFeO3 –EuFeO3 solution will favour the orthoferrite (Pnma) rather than rhombohedral (R3c) structure [20]. The (0 0 2) peak for the Eu-free BiFeO3 films is almost absent due to the low relative intensity. The doublet structure of the (0 0 2) peak for Bi0.93 Eu0.07 FeO3 film might suggest a pseudo-tetragonal phase [21]. And the highly oriented (h00) growth could be verified by XRD scans of the BFO (2 0 2) and LNO (2 0 2) planes [22]. Fig. 2 shows the surface and cross-sectional SEM images of BEFOx (x = 0, 0.03, 0.05, 0.07, 0.10) thin films. As shown in Fig. 2(a) and (b), a number of island-like structures can be observed from the surface of the BiFeO3 and Bi0.97 Eu0.03 FeO3 films, and the grain size ranges from 120 to 200 nm. Compared with pure BiFeO3 , the number of island-like structures decreases for BEFOx (x = 0.05, 0.07, 0.10) films. This result suggests that proper Eu doping in BiFeO3 films could restrain the formation of island-like structures, it is related to the decrease of the grain size. The thickness of the BEFOx films can be calculated from the cross-sectional micrographs to be 324–419 nm. Fig. 3 shows the Raman spectra of BEFOx films. In order to obtain each Raman active mode, the measured spectra of the BEFOx films were fitted, and the fitted curves were decomposed into individual Lorentzian components. As shown in Fig. 3(b), there are eight modes observed in Bi0.95 Eu0.05 FeO3 films. According to previous work [10], three peaks at 137, 216, and 490 cm−1 are A1 modes, while the peaks at 76, 270, 517, 698 cm−1 are E modes. An intense peak observed at 400 cm−1 originates from the LaNiO3 substrate [23], as marked with the asterisk (*). The peak position of the A1 -1 mode (137 cm−1 ) of the BEFOx films is shown in the inset of Fig. 3(b). It suggests that the peak position of the A1 -1 mode shifts to higher wavenumber with increasing the amount of Eu, which could be expressed by (146.8 + 1.59x) cm−1 . The A1 -1 mode originates from the Bi O covalent bond and the mass of Eu3+ is smaller than that of Bi3+ . Thus, substitution of Bi3+ ions by Eu3+ produced stronger covalent bond, which resulted in the blue shift of the A1 -1 mode peak position. The optical constants of the BEFOx films are derived from spectroscopic ellipsometry measurements at 400–1200 nm wavelength range. Experimental spectroscopic ellipsometry spectra are fitted based on the Lorentz oscillator model [24–26], and optical thin film software, namely SC630UVN (Shanghai Sanco Instrument Co. Ltd.), was used to analyze ellipsometric data.

BFO(100) LNO(100)

Table 1 Optimized parameters for Bi1−x Eux FeO3 films grown by pulsed laser deposition.

0.10 0.07 0.05

0.03

0.03

x=0 20

30

x=0 40

2 (degree)

50

60

20

30

40

50

60

2 (degree)

Fig. 1. (a) XRD patterns of Bi1−x Eux FeO3 ceramic targets and films on LaNiO3 /Si (1 0 0) structures. The inset of (b) shows the magnified image of (1 0 0) peak for 2 range of 22–24◦ .

J. Liu et al. / Applied Surface Science 307 (2014) 543–547

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Fig. 2. Surface and cross-sectional SEM images of Bi1−x Eux FeO3 films (a) x = 0, (b) x = 0.03, (c) x = 0.05, (d) x = 0.07 and (e) x = 0.10.

The Lorentz oscillator model can be expressed as [24] ε(E) = ε1 (∞) +



Ai Ei2 − E 2 − ii E

where ε(∞) refers to the dielectric constant at very large photon energies, Ai is the amplitude of the ith oscillator with the unit of (eV)2 ,  i is the damping factor of the ith oscillator with the unit of eV, and Ei is the resonant energy with the unit of eV.

J. Liu et al. / Applied Surface Science 307 (2014) 543–547

(a)

(b)

Bi0.95Eu0.05FeO3

E

A -1 Wavenumber (cm

-1

Bi1-xEuxFeO3

)

546

Intensity (arb. unit)

0.10

155 150 0 2 4 6 8 10 Eu composition x (%)

1

A1

160

0.07

LaNiO3

A1 E

E

0.03

E

A1

x=0 800

600

400

200

-1 Wavenumber (cm )

200

400

600

800

Fig. 3. Measured Raman scattering spectra of Bi1−x Eux FeO3 (x = 0, 0.03, 0.05, 0.07, 0.10) films. Besides, their fitted spectra (thick solid line) and the decomposed active modes (green solid lines) are obtained. The inset of Fig. 3 (b) shows the variation of A1 -1 active mode position of Bi1−x Eux FeO3 films.

The optical constants including the refractive index and extinction coefficient are given by

B0 E+C0

i

E 2 −Bi E+Ci

where Ai , Bi , and Ci are some parameters

related to electron transition, n(∞) is the refractive index when photon energy E → ∞, and Eg is the energy band gap. The LaNiO3 substrate is present in the optical model. The optical constants, refractive index n and extinction coefficient k of the BEFOx films are displayed in Fig. 4. Obviously, for pure BFO film, a peak of the refractive index of 2.25 ± 0.04 appears at 2.80 eV, which is believed to correspond to the direct band gap energy [26]. The refractive index n as a function of the Eu dopant at photon energy of 2 eV is shown in the inset of Fig. 4 (a). The refractive index for Eu-doped BFO films is larger than that of the pure BFO film. The extinction coefficient is close to zero at the range of 1–2 eV. And it increases sharply from 2.7 to 3.0 eV, which suggests strong photon absorption. The variation of extinction coefficient k as a function of the Eu dopant at photon energy of 2 eV is shown in the inset of Fig. 4(b), which indicates that the extinction coefficient k decreases with the concentration of Eu doping. The (˛E)2 versus h for BEFOx films is plotted in Fig. 5. The optical band gap (Eg ) is determined by extrapolating straight line of (␣E)2 verse photon energy curve to the intercept on horizontal photon energy axis. The optical band gap of BFO is 2.77 eV, which is close to the value obtained from refraction index shown in Fig. 4 (a), and that of other four BEFOx (x = 0.03, 0.05, 0.07, 0.10) films are 2.73 eV, 2.71 eV, 2.68 eV and 2.57 eV, respectively. The band gap of BEFOx films vs x is shown in inset of Fig. 5. Obviously, the band gap decreases with the increasing Eu dopant, which can be expressed by (2.78–0.018x) eV. These results indicate that Eu doping can reduce the optical band gap of BFO films fabricated on LNO/Si (1 0 0) substrates by PLD method. In the fundamental absorption edges region, the absorption is due to the transition from the bottom of conduction band to the top of valence band. Substitution of Eu in BFO thin film may cause an increase in density of states in the valence band.

Fitted Curve

n

Bi1-xEuxFeO3

2.7

3.2

2.4

(a)

Photon energy at 2 eV

0

2 4 6 8 10 Eu composition x (%)

2.8

2.4 2.0 0.04 Fitted Curve

0.03

k

n(E) = n(∞) +



(E − Eg )2

Refractive index (n )

Ai E 2 − Bi E + Ci

3.0



E x tin c tio n c o e ffic ie n t (k)

k(E) =



3.6

1.5 0.02

Photon energy at 2 eV

0

2 4 6 8 10 Eu composition x (%)

0.10 0.07 0.05 0.03 x=0

(b)

1.0

0.5

0.0 1.0

1.5

2.0

2.5

3.0

Energy (eV) Fig. 4. The refractive index n and extinction coefficient k for the Bi1−x Eux FeO3 (x = 0, 0.03, 0.05, 0.07, 0.10) films. Offset of 0.2 for k data one by one. The inset (a) and inset (b) show the refractive index n and extinction coefficient k at photon energy of 2 eV vs. Eu composition, respectively.

J. Liu et al. / Applied Surface Science 307 (2014) 543–547

6

2.8

This work was supported by the National Natural Science Foundation of China (60990312 and 61076060), Science and Technology Commission of Shanghai Municipality (10JC1404600). References

Eg (eV)

2.7

4

2

10

2

-2

(αE) (10 eV cm )

8

Acknowledgments

x=0 0.03 0.05 0.07 0.10

Bi1-xEuxFeO3

547

2.6

2

0 1.6

0 2 4 6 8 10 Eu composition x (%)

2.0

2.4

2.8

Energy (eV) Fig. 5. (˛E)2 plotted as function of photon energy E for the Bi1−x Eux FeO3 (x = 0, 0.03, 0.05, 0.07, 0.10) films. The inset shows the variation of optical band gap of Bi1−x Eux FeO3 films.

Besides, the Eu ion may also create localized states in the band gap [10]. This will lead a shift in the absorption edge towards lower photon energy. Thus, the substitution of Eu in BFO films causes a decrease in the optical band gap. 4. Conclusions The Bi1−x Eux FeO3 (BEFOx , x = 0, 0.03, 0.05, 0.07, 0.1) films were grown on the LNO/Si (1 0 0) substrates by PLD method. And the effects of Eu composition on the structures, morphologies and optical properties were investigated. The XRD patterns confirm the dominant orientation of the BEFOx films is (1 0 0). SEM images exhibits that the number of island-like structures decreases with increasing Eu dopant. Three A1 and four E modes are observed in Raman spectra. The BEFOx films have a great absorption in the photon energy range of 2.7–3.0 eV, and the refractive index and extinction coefficient is 2.25 and 0.04 at 2 eV for BFO film. In addition, the band gap of the BEFOx films decreases with the increasing Eu dopant.

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