Structural, morphological and optical properties of BiFe0.99Cr0.01O3 thin films

Accepted Manuscript Structural, morphological and optical properties of BiFe0.99Cr0.01O3 thin films Shaan Ameer, Kajal Jindal, Savita Sharma, Pradip K. Jha, Monika Tomar, Vinay Gupta PII:

S0042-207X(18)31441-6

DOI:

10.1016/j.vacuum.2018.09.051

Reference:

VAC 8264

To appear in:

Vacuum

Received Date: 31 July 2018 Revised Date:

21 September 2018

Accepted Date: 25 September 2018

Please cite this article as: Ameer S, Jindal K, Sharma S, Jha PK, Tomar M, Gupta V, Structural, morphological and optical properties of BiFe0.99Cr0.01O3 thin films, Vacuum (2018), doi: https:// doi.org/10.1016/j.vacuum.2018.09.051. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Structural, morphological and optical properties of BiFe0.99Cr0.01O3 thin films Shaan Ameer1, Kajal Jindal1, Savita Sharma1, Pradip K. Jha2, Monika Tomar3, Vinay Gupta1,

1

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*

Department of Physics and Astrophysics, University of Delhi, New Delhi-110007. 2

Department of Physics, Miranda House, University of Delhi, New Delhi-110007.

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3

Department of Physics, DDU College, University of Delhi, New Delhi-110078.

*

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Corresponding email: [email protected]

Abstract

In this paper, the linear optical response of Cr doped BiFeO3 (BFCO) using first principles calculations based on the density functional theory (DFT) is reported. The electronic and

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magnetic properties of BFCO are studied. Cr doping is found to result in an increase in magnetization in BFO. To corroborate the theoretical understanding in BFCO, Cr doped BFO thin film is also deposited via a multistep spin coating technique. The structural and optical

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theory.

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characterization of the thin film are performed and the obtained results are compared with the

Keywords: Cr doped BFO, linear optical response, DFT, thin films, structural and morphological properties

ACCEPTED MANUSCRIPT 1. Introduction BiFeO3 (BFO) is a well known multiferroic material which displays ferroelectricity and ferromagnetism simultaneously at room temperature [1,2]. It crystallizes in various perovskite structures among which R3c phase is the most studied phase, since, in R3c phase

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both ferroelectric and ferromagnetic phase coexists [3]. Here, 6s2 lone pair of Bi derives the electric polarization, whereas, the partially filled d orbitals of Fe are responsible for the magnetization [4]. The Curie temperature which corresponds to maximum temperature upto

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which BFO is ferroelectric is reported to be about 1100 K, whereas, the Neel temperature is

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about 640 K [5]. In its pure form, there is a ferromagnetic coupling among the spins of Fe lying in the (111) plane of the rhombohedral structure, whereas, there is an antiferromagnetic (AFM) ordering in the consecutive (111) planes. Thus, the spins are oriented in a G-type antiferromagnetic (G-AFM) order [3]. In addition to G-AFM, a non-collinear spin cycloidal order with the spatial period of ~ 62 Å is also superimposed in BFO [6]. Many workers have

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demonstrated the breaking of spin cycloidal ordering due to incorporation of dopants as the origin of ferromagnetism in BFO.

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BFO has been doped with various B-site dopants which includes transition elements like Sc [7], Ti [8], Mn [9], Ni [10], Zn [11], Cu [12], V [13] etc. where optimal doping with these

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elements have been observed to significantly enhance the electrical, ferroelectric, ferromagnetic and optical properties of BFO. Because of the similar ionic radii of Cr3+ and Fe3+ [14], Cr is an attractive dopant in perovskite BFO. Cr doped BFO has been studied experimentally in the past by various workers [15-17]. Kim et al., 2006 reported enhancement of ferroelectric properties and decrease in leakage current on doping with Cr at B-site in BiFeO3 [18,19]. Sui et al. (2018) reported reduction of leakage current in BFO by 2 orders of magnitude on doping with Cr [20]. Moreover, the magnetization has been observed to increase significantly by Cr doping in BFO [21]. The photovoltaic behavior in

ACCEPTED MANUSCRIPT BiFeO3/BiCrO3 bilayer is reported to be quite improved as compared to pure BFO and BiCrO3 [22,23]. Nie et. al. (2016) attributed improvement in the photovoltaic response of BiFeO3/BiCrO3 to the band gap narrowing in the bilayer system [22]. As the detailed optical properties of BFCO may influence the photovoltaic response, thus, it is imperative to

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investigate the properties of BFCO by first principles. An effort has been made by Deng et al., (2012) [24] to investigate the optical properties of BFCO in the near IR-visible region by using ellipsometry technique. The detailed theoretical studies of optical properties of Cr

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doped BFO (BFCO) have not been reported in literature and needs attention. In this paper, an attempt has been made to study the optical properties of BFCO by first principles based on

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the density functional theory (DFT), and to validate the results in the light of experimental studies based on Cr doped BFO thin film formed via a multistep spin coating technique.

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2. Materials and Methods

Stoichiometric quantities of Bi (NO3)3.5H2O (5 mol% extra to compensate to the volatility of Bi), Cr (NO3)3.9H2O and Fe (NO3)3.9H2O were dissolved in 2-methoxyethanol to make 0.1

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M solution of 1% Cr doped BFO. The salts of Bi, Fe and Cr were taken to be 99.99 % pure on trace metal basis and were procured from Sigma-Aldrich. Acetic acid (2 vol%) was added

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in the resulting mixture while the mixture being magnetically stirred. The solution was constantly stirred for 4 hours. The obtained solution of 0.1 M BiFe0.99Cr0.01O3 was spin coated at 2300 rpm for 25 seconds to obtain Cr doped BFO thin film on the Corning glass substrate (Corning 709). The layer so obtained was heated at 250 оC for 2 minutes and was subsequently cooled before the deposition of the next layer. The desired thickness of ~ 200 nm was attained after deposition of four such layers. The obtained thin film of BiFe0.99Cr0.01O3 was subsequently annealed at 525 оC for 1 hour in the presence of N2 gas.

ACCEPTED MANUSCRIPT Thickness of BFCO thin film was measured using Dektak thickness profiler (Veeco 150). To characterize the structural phase of BFCO thin film, X-Ray diffractometer (Rigaku) was used. The optical transmittance measurement was carried out using the Perkin Elmer (Lambda 35) spectrometer. Field Emission Scanning electron microscopy (FESEM) [Zeiss, GeminiSEM

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500] and Atomic Force microscopy (AFM) [WiTec, Combined Confocal Raman AFM System] was used to perform the surface morphological studies of the sample.

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3. Computational Details

Spin polarized DFT based calculations for BFCO were performed using the projector

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augmented wave method as implemented in the Vienna ab initio simulation package (VASP). The generalized gradient approximation was used for the approximation of the exchange and correlation functional in DFT. To account for the localization of the d-orbitals of the transition metals (Fe and Cr), local spin density approximation (LSDA)+ U functional was

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used (U=4.0 eV for both Fe and Cr) where U is the Hubbard parameter. The energy cut-off in the plane wave basis set was set to be 400 eV. The structure of the Cr doped BFO is relaxed by using conjugate gradient algorithm until the Hellman-Feynman forces between the ions

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becomes lesser than 0.02 eV/Å. The energy tolerance for the self-consistent field (scf) convergence was set to 1.0 × 10-6 eV. Monkhorst-Pack grid of size (4×4×4) was used to

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choose the k-points in the first Brillouin zone for scf calculations, whereas, the grid of size (9×9×9) was used for the calculations of the density of states and band structure. 4. Results and Discussion 4.1.Computational Results The relaxed geometry of Cr doped BFO is obtained. Cr doping is not found to significantly distort the structure of BFO and the lattice parameters are found to be a= 5.56 Å, c= 13.74 Å for the relaxed R3c structure of BFCO. The obtained values of lattice parameters of BFCO

ACCEPTED MANUSCRIPT are found to be close to that reported in literature for BFO [25]. The schematic diagram of the relaxed R3c structure is shown in Figure 1. Figure 2 shows the calculated spin polarized total density of states of Cr doped BFO. The Fermi level is set at zero of the energy scale. It may be observed that no states are present between the valence band maxima and conduction band

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minima. The difference between the valence band maxima and conduction band minima which corresponds to optical band gap of BFCO is found to be slightly above 2 eV (Figure 2). It is important to highlight that G-type antiferromagnetic ordering was imposed on BFCO

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before relaxation [26] as shown in Figure 3. Cr doping results in the onset of magnetization in BFO as is evident in the density of states calculations where asymmetric spin states are

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observed. The magnetization of Cr doped BFO is obtained to be 1.9941

/ (unit cell) which

is equivalent to 49.42 emu/cm3. The high value of magnetization on Cr doping in BFO suggests that Cr doping is a promising approach to induce magnetism in BFO. The results are interesting as BFO itself is antiferromagnetic or weakly ferromagnetic in nature. Cr

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incorporation in BFO leads to partial cancellation of spins, thereby, producing net magnetization.

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To calculate the linear optical properties of Cr doped BFO, only the single particle excitations are considered. Fermi golden rule is applied to obtain the imaginary part of dielectric function

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involving interband transitions [27]: =

where

2

, ,

|〈

| . |

〉|

(

−

− ) (1)

is the electric polarization vector, and ! denote the valence and conduction bands

respectively,

is the permittivity of vacuum and

is the volume of the unit cell. The real

part of the dielectric function is obtained by Kramers-Kronig transformation which is given by

" (#)

- '()(*)

= 1 + % &.

' ) +,)

/0 [28]. It is important to point out that in this formulation, the

ACCEPTED MANUSCRIPT excitonic and local field effects are ignored. The values of the real and imaginary part of dielectric constant are plotted as a function of energy as shown in Figure 4. The physical quantities like refractive index (n) and extinction coefficient (κ) are obtained as: n + iκ = 4

"

+5

[28]. The calculated values of n and κ for photon energy up to 20 eV are shown in

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Figure 5. The large-wavelength (~ 1000 nm) refractive index as calculated in the present formalism is ~ 2.7 (Figure 5(a)) which is slightly higher than that determined experimentally (~ 2.2) by Deng et al. (2012) [24] for BFCO using spectrometric ellipsometry technique. The

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maximum value of calculated refractive index obtained in the present work for BFCO is ~ 3.72 at photon energy of ~ 2.63 eV and is very close to the maximum value of n ~ 3.65 at

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photon energy of ~ 2.89 eV for pure BFO as reported by Kumar et al. (2008) [29]. Importantly, the variation of calculated refractive index as a function of photon energy (Figure 5(a)) is quite similar to that is experimentally reported [29] in the range of energy from 0.7 eV to 6.0 eV (Inset Figure 5(a)). The extinction coefficient which measures the

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attenuation of the light inside the material is about 0 for the energy of photons below the band gap. For higher photonic energies, the extinction coefficient increases reaching its maximum at 6.2 eV with subsequent maxima at 3.2 eV and 4.4 eV which may correspond to the

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resonant transition among the levels (Figure 5(b)). It is important to note that the calculated

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extinction coefficient (κ) of BFCO in the present work is ~ 1 (Figure 5(b)) for photon energy of around 3 eV. The result is quite close to the experimentally reported result (κ ~ 1 for wavelength ~ 400 nm) by Deng et al. (2012) [24]. It is quite interesting to note that the peaks in value of calculated extinction coefficient in the energy range from 0.7 to 5 eV (Figure 5(b)) matches closely to that obtained using ellipsometry technique for BFO [29]. The absorption coefficient (α) as explained by Beer-Lambert-Bouguer law is plotted over a wide range of incident light (Figure 6). It is interesting to note the absorption edge around photon energy near to the band gap energy ~ 2.0 eV (Figure 6). Since, BFCO has a

ACCEPTED MANUSCRIPT band gap in the visible region, it is an attractive candidate for photovoltaic applications. The electronic structure calculations reveal the band gap of ~ 2 eV for Cr doped BFO. 4.2. Structural and Morphological Properties

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X-ray diffraction (XRD) pattern of the obtained BFCO thin film has been shown in the Figure 7(a). The peaks in the XRD of BiFe0.99Cr0.01O3 thin film are indexed to the R3c phase [30] on a logarithmic scale. It is important to observe that no extra peak corresponding to

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secondary phases is obtained in the XRD spectra of BFCO thin film. Thus, the obtained BFCO thin film is found to crystallize well in the R3c phase (Figure 7(a)). Le Bail fitting was

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performed to obtain the lattice parameters corresponding to R3c phase of the obtained thin films. The values of lattice parameters are obtained to be a = 5.56 Å and c = 13.76 Å. The tetragonality ratio (c/a) is found to be 2.47. The obtained values of lattice parameters of BFCO thin film are close to those obtained by relaxing geometry of BFCO based on ab initio

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studies as discussed earlier. Figure 7 (b) shows the AFM image of BFCO thin film. The evenly distributed grains (~50-200 nm) along with a rough morphology is observed for BFCO thin film. The rms roughness of BFCO thin film as obtained using AFM image

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(Figure 7(b)) is found to be about 26 nm. The high roughness can be attributed to the deposition of thin film via multistep spin-coating technique [31]. FESEM image is also

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shown in Figure 7(c) where the size of crystallites is found to lie in a range of ~50-100 nm. It is interesting to note that some larger grains (~ 200 nm) are visible in AFM plot, since in AFM, coalescence of grains may be observed. The grain boundaries are known to play an important role in governing the electrical behavior and optical scattering in a material [32]. 4.3. Optical Properties Figure 8 shows the optical transmittance curve of 1% Cr doped BFO thin films over the range of UV-Vis-NIR region of electromagnetic spectrum. It is worthwhile to note that the

ACCEPTED MANUSCRIPT transmittance of BFCO is quite low even in the low photon energy region. This can be attributed to the rough surface and nanocrystalline nature of the deposited BFCO thin film. Since, the presence of grain boundaries leads to the scattering of incident light, the thin film exhibits around 60% transmittance for the wavelength of 700 nm. The absorption coefficient 789

"

:;<<

"..

=>(?(%)) [33]. The optical

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(α) is obtained for each wavelength using the relation: α =

direct band gap of the thin film can be obtained using the plot between (hc/λ) and (hc/λ)n known as Tauc plot, as shown in Figure 8 (inset). The value of exponent n = 0.5, 2 and 3 is

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related to the indirect allowed, direct and indirect forbidden optical transitions respectively

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[34]. The linear region in the plot of (αhc/λ)2 versus (hc/λ) which corresponds to direct transition is fitted with the straight line which is extrapolated to the (hc/λ) axis (Figure 8(inset)). The intercept of the straight line on (hc/λ) axis gives the value of the band gap of the thin film which is obtained to be 2.48 eV. It is important to note that the theoretically calculated value of band gap energy (slightly above 2 eV) is much smaller than that obtained

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experimentally. There are many factors responsible for apparent difference between the experimentally obtained band gap (2.48 eV) and the calculated band gap (~2 eV); the main

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reason is the under-estimation of band gap energies by GGA-PBE approximation [35]. Also, the CSD deposited thin film is nanocrystalline and granular in nature which leads to quantum

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confinement effect. The quantum confinement effect blue-shifts the band gap of a material with respect to the bulk [36]. 5. Conclusion

The linear optical response of Cr doped BFO is calculated using the first principles study based on DFT. Moreover, on using the LSDA+U functional for taking into account the strong localization and correlation of the d-electrons of Cr and Fe with U = 4.0 eV for both Cr and Fe gives the band gap value of Cr doped BFO ~ 2 eV. Single phase BiFe0.99Cr0.01O3 thin film

ACCEPTED MANUSCRIPT is also fabricated using the multistep spin coating technique. The structural parameter of the fabricated R3c phase of BiFe0.99Cr0.01O3 thin film are a = 5.56 Å, c = 13.76 Å. The value of optical band gap for the deposited BFCO thin film is obtained to be 2.48 eV. As is revealed in electronic structure calculations, Cr doping in BFO leads to appearance of magnetization in

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an otherwise G-type anti-ferromagnetically ordered BFO. Since, BFO is a well-known multiferroic material, Cr doping in BFO makes it more suitable for spintronics device applications.

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Acknowledgements

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Authors are thankful to Department of Science and Technology (DST), India. One of the authors (SA) is thankful to CSIR, India for the research fellowship. References

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ferromagnetism. J. Appl. Phys. 120 (2016) 135305. Figure captions

Figure 1. Relaxed structure of R3c phase of Cr doped BFO.

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Figure 2. Calculated spin polarized total density of states of the Cr doped BFO. Figure 3. G-type antiferromagnetic ordering in BFCO.

energy.

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Figure 4. Calculated (a) real and (b) imaginary part of dielectric constant versus photon

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Figure 5. Calculated (a) refractive index (n) and (b) extinction coefficient (κ) of Cr doped BFO versus photon energy. Figure 6. Absorption coefficient (α) of Cr doped BFO versus photon energy. Figure 7. (a) XRD pattern, (b) AFM graph, and (c) FESEM image of BiFe0.99Cr0.01O3 thin film. Figure 8. Transmittance curve of ~ 200 nm thick BFCO thin film. The inset shows the Tauc plot for a direct transition and the corresponding optical band gap energy.

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First-principles calculations based on density functional theory are performed to calculate the structural, electronic, magnetic and optical properties of Cr doped BFO thin films The real and imaginary parts of dielectric constant, refractive index, and extinction coefficient of Cr doped BFO is also calculated The Cr doped BFO thin film is fabricated and its structural, morphological and optical characterization is performed The experimental findings are corroborated with the theoretical results obtained using first principles calculations