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Mohammad Agha-Bolorizadeh

Kerman University of Technology Graduate

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Hamid Latifi

Shahid Beheshti University, Tehran, Iran

Mohammad Kazem Moravvej-Farshi

Tarbiat Modares University, Tehran, Iran

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University of Isfahan, Isfahan, Iran

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Shiraz University, Shiraz, Iran

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University of Insubria, Como, Italy

Masud Mansuripur

University of Arizona, AZ, USA

Jean Michel Nunzi

University of Angers, Angers, France

Gang Ding Peng

University of N.S.W., Sydney, Australia

Nasser N. Peyghambarian

University of Arizona, AZ, USA

Jawad A. Salehi

Sharif University of Technology, Tehran Iran

Surendra P. Singh

University of Arkansas, AR, USA

Muhammad Suhail Zubairy

Texas A & M University, TX, USA

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International Journal of Optics and Photonics (IJOP) Vol. 10, No. 1, Winter-Spring, 2016

31

Tunable Schottky Barrier in Photovoltaic BiFeO3 Based Ferroelectric Composite Thin Films

Seyed Mohammad Hosein Khalkhalia, Mohammad Mehdi Tehranchib c,

and Seyedeh Mehri Hamidic

aDepartment of Physics, University of Kharazmi, Tehran, Iran bDepartment of Physics, G.C., Shahid Beheshti University, Tehran, Iran.

cLaser and Plasma Research Institute, G.C., Shahid Beheshti University, Tehran, Iran

Corresponding Author Email: teranchi@sbu.ac.ir

Received: June 1, 2015, Revised: Dec. 9, 2015, Accepted: Jan. 3, 2016, Available online: Apr. 30, 2016.

ABSTRACT— We examine the photo-assisted polarization loop in a BiFeO3 thin film under UV light illumination. BiFeO3 thin film prepared by pulsed laser deposition method onto the BaTiO3 thin film and the polarization behavior has been measured under poling voltage. Our results show the engineered polarization due to controllable schottky barrier under inverse poling voltage. This control on schottky barrier height and then polarization of thin film can be opened the new insight in the ferroelectric devices. KEYWORDS: magneto-electric thin films, Ferroelectric polarization, Pulsed laser deposition, Schottky barrier.

I. INTRODUCTION Multiferroic composite multilayer based on Bismuth ferrite (BFO) thin film is a very promising structure with coexistence of electric and magnetic order [1]-[3]. These effects can be assisted by light illumination because the narrow band gap in BFO thin film [4]-[6]. This band gap in BFO structures lead to high spectral response located in the ultraviolet to visible region and a high conversion efficiency from the light to the electron-hole pairs which is separated by the internal electric field arising from the ferroelectric polarization of it. This high conversion efficiency leads to photo-assisted phenomena in multilayer composite which are arranged by the aid of the ferroelectric polarization (FEP) [7]-[9]. In these useful

composites, when we use metal electrode, we may have schottky barrier distortion in metallic electrode-ferroelectric interface [10], [11]. This schottky barrier height may be influenced on the effective voltage on the BFO layer and then on the FEP of this layer. Consequently reach to the ability of engineering and control of this barrier is needed in view of applications.

As mentioned above, to investigate the FEP and reach to the applicable multilayer structure based on BFO thin film, their FEP and influence of schottky barrier needs to be measured and controlled. Now according to our knowledge in the photovoltaic effect of BFO thin films, dependence of room temperature hysteresis loop of FEP and photo-assisted FEP of Au/BiFeO3/BaTiO3/Si/Al capacitor configuration on the height of photo-assisted schottky barrier (PA-SB) have been measured by leakage current compensation.

II. EXPERIMENTAL DETAILS Au /BiFeO3/ BaTiO3 heterostructure has been deposited onto Si (100) substrate using a 355 nm pulsed laser beam produced from third harmonic of Nd:YAG laser, which was focused on a target rotating at a frequency of 3 Hz at a distance of around 2 cm from the substrate. Typical laser parameters applied for deposition were as follows: pulse frequency of 10 Hz, laser fluence of 3 J/cm2. All depositions

S.M.H. Khalkhali et al. Tunable Schottky Barrier in Photovoltaic BiFeO3 Based Ferroelectric …

32

were carried out in a vacuum chamber evacuated down to 4×10-5 mbar and the oxygen ambient gas pressure was 120 mbar. The substrate was not intentionally heated and after deposition, each film was crystallized with rapid thermal annealing procedure at 600 °C for 60 minutes. Our two samples prepared by different thickness of BiFeO3 set to 170 and 240 nm under BaTiO3 thin film with thickness of 200 nm.

The crystalline phases of thin films were measured by means of x-ray diffraction (XRD). After that, we use the thermal evaporation method to prepare the Au contact at upper edge (on BFO layer) and Al contact at bottom edge of the sample (on Si) with thickness of 50 and 200 nm respectively. The schematic of sample is shown in figure 1.

Fig. 1: Schematic of the sample structure

In order to measure the hysteresis loop, we measured polarization current instead of measuring polarization charge. So we did series samples with a large resistance and applied a triangle voltage to the circuit. This voltage is proportional to the total current that flow in the circuit. This current include the polarization current, linear resistive and capacitive current. We measured total current in two frequencies for eliminating the resistive current and then the leakage current [12]. Then to get the polarization charge we had integrated the obtained polarization current. The PA-SB has been examined by different power of UV light illumination with wavelength set to 406 nm.

III. RESULTS AND DISCUSSION The XRD pattern of the sample shows that the BTO and BFO film are well crystallized in (110) plane and impurity peaks such as Bi2O3 or Bi2Fe4O9 do not appear [6] and we expected

that the polarization of it arranged normal to the (110) direction.

Fig. 2: room temperature P-E hysteresis loop for (a) sample 1 and (b) sample 2 with (Stars) and without (Circles) light illumination.

In order to investigate the ferroelectric properties of the bilayer films, measurements of ferroelectric hysteresis are performed with the two frequency measurement system. .Room temperature P–E hysteresis loops are measured with experimental measurement procedure to separate ferroelectric switching current and dielectric displacement current from the leakage current in leaky ferroelectric thin-film capacitor structures. This is performed with measurement at two adjacent frequencies. The result of this measurement is shown in Fig.2 for two samples with Au/ BiFeO3/ BaTiO3/Si/Al structure with different

International Journal of Optics and Photonics (IJOP) Vol. 10, No. 1, Winter-Spring, 2016

33

thickness of BFO layer set to 240 and 170 nm for sample 1 and sample 2 respectively.

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-0.003

-0.002

-0.001

0.000

0.001

0.002

Pola

riza

tion

Voltage (v)

No LED LED 1.13mW LED 1.56 mW

Fig. 3: (a) Room temperature P-E hysteresis loop with 1.13 and 1.56 mW (Stars and Squares) and without (Circles) light illumination, (b) saturation polarization versus LED intensity.

From Fig. 3, one can see the room temperature hysteresis loop of two samples with different thickness of BFO layer with and without light illumination. As shown in this figure, the sample has a capacitor and polarization current behavior. Consequently we observe that in these samples, the saturation polarization increases with illuminating with light and we have asymmetric manner in polarization between forward and backward bias. This can be attributed to the different schottky junction in these biases. The contact between Au electrode and BFO layer is a schottky contact and has different behavior in the forward and

backward biases. When this contact is illuminated, the schottky barrier is reduced [13], [14] and the voltage is applied to the bilayer structure. Increasing voltage on the structure result to increase polarization and saturate it.

In fact, we have enhanced saturation polarization after sufficient LED power as shown in Fig. 3 and when the sample illuminated with light, this change has been removed and the polarization of the sample drift to the saturation state. Accordingly, by decrease in the thickness of BFO layer in the sample 2, we have not saturation state in the inverse bias which is confirmed change in the resistance of the sample (Fig. 1(b)). When the sample illuminated with light, this change has been removed and the polarization of the sample drift to the saturation state.

It is clear that the photocurrent, both in full polarization up state and down state increase with the light illumination which is attributed to the improving electron hole pair generation. As shown in Figs. 1 and 2, the polarization state of the sample depends on the incidence light when we have poling voltage in the range from 2 V to -2 V. But in the inverse bias case, we have break point in the polarization state without any light illumination and after exposing the sample with light, one can see an efficient enhancement in the polarization of the sample due to decrease in the top schottky barrier height because electron-hole pair induced with light. In addition, it is observed that the light intensity can be influenced on the polarization state and the schottky barrier height of the sample (Fig. 2(b)). Dependence of the SB on the light illumination can be seen in sample 2 better than another one because the thinner BFO thin film which is ascribed to the height of the schottky barrier.

In fact, because the dependence of the schottky height in the Au/BFO interface on the boundary charge percentage, this useful PA-SB engineering can be enhanced the charge and also the voltage on the BFO layer and finally we can reach to the tunable schottky

(a)

Pola

riza

tion(

a.u.

)

S.M.H. Khalkhali et al. Tunable Schottky Barrier in Photovoltaic BiFeO3 Based Ferroelectric …

34

barrier with light and then the polarization rise from this layer.

IV. CONCLUSION In this paper, the Au/BFO/BTO heterostructures have been deposited onto Si substrate by pulsed laser deposition technique. This capacitor demonstrates obvious change in FEP under illumination of 406 nm after electric field polling. The break point in the P-E hysteresis loop of the samples indicates the appearance of the schottky barrier which is controlled and tuned by light illumination. In fact, we have tunable PA-SB engineering in these multilayer composites

REFERENCES [1] F. Zavaliche, S.Y. Yang, T. Zhao, Y.H. Chu,

M. P. Cruz, C. B. Eom, and R. Ramesh, “ Multiferroic BiFeO3 films: domain structure and polarization dynamics,” Phase Trans. Vol. 79, pp. 991-1017, 2006.

[2] S.N. Kallaev, R.G. Mitarov, Z.M. Omarov, G.G. Gadzhiev, and L.A. Reznichenko, “ Heat capacity of BiFeO3-based multiferroics,” Journal of Experimental and Theoret. Phys. Vol. 118, pp. 279-283, 2014.

[3] S. Pattanayak, R.N.P. Choudhary, and P.R. Das, “Effect of Praseodymium on Electrical Properties of BiFeO3 Multiferroic,” J. Electron. Mater. Vol. 43, pp. 470-478, 2014.

[4] T. Choi, Y.J. Choi, V. Kirykhin, and S.W. Cheong, “Switchable ferroelectric diode and photovoltaic effect in BiFeO3,” Science, Vol. 324, pp. 63-66, 2009.

[5] B. Kundys, M. Viret, D. Colson, and D.O. Kundys, “Light-induced size changes in BiFeO3 crystals,” Nature Mater. Vol. 9, pp. 803-805, 2010.

[6] S.M.H. Khalkhali, M.M. Tehranchi, and S.M. Hamidi, “Photo-magnetic assisted ferroelectric polarization in magneto-electric BiFeO3/BaTiO3 thin film,” J. Magn. Mag. Mater. Vol. 355, pp. 188-191, 2014.

[7] L.Y. Chen, J.C. Yang, C.W. Luo, K.H. Wu, J. Y. Lin, T.M. Uen, J.Y. Juang, Y.H. Chen, and T. Kobayashi, “Ultrafast photoinduced mechanical strain in epitaxial BiFeO3 thin

films,” Appl. Phys. Lett. Vol. 101, pp. 041902 (1-4), 2012.

[8] B. Liu, Z. Peng, J. Ma, J. Wang, Q. Zhao, and Y. Wang, Phys. Status. Solidi. A, “Enhanced photovoltaic effect of polycrystalline BiFeO3 film,” Vol. 210, pp. 819-822, 2013.

[9] C.M. Hung, M.D. Jiang, J. Anthoninappen, and C.S. Tu, “Photo-induced electric phenomena in antiferromagnetic BiFeO3 ceramics,” J. Appl. Phys. Vol. 113, pp. 17D905 (1-3), 2013.

[10] V. K. Yarmarkin, S.G. Shul’man, and V.V. Lemanov, “In the effect of spontaneous polarization on the height of the Schottky barrier at the metal-ferroelectric contact,” Ferroelectricity, Vol. 55, pp. 547-550, 2013.

[11] P. Juan, C. Lin, C. Liu, C. Chen, Y. Chang, and L. Yeh, “Temperature-dependent current conduction of metal-ferroelectric (BiFeO3)-insulator (ZrO2)-Silicon capacitors for nonvolatile memory applications,” Thin Solid Films, Vol. 539, pp. 360-364, 2013.

[12] R. Meyer, R. Waser, K. Prume, T. Schmitz, and S. Tiedke, “Dynamic leakage current compensation in ferroelectric thin-film capacitor structures,” Appl. Phys. Lett. Vol. 86, pp. 142907 (1-3), 2005.

[13] J. Liu, P. Fei, J. Song, X. Wang, Ch. Lao, R. Tummala, and Zh. L. Wang, “ Carrier Density and Schottky Barrier on the Performance of DC Nanogenerator,” Nano Lett. Vol. 8, pp. 328-332, 2008.

[14] L.C. Tran, F.M. Wemer, A.K.M. Newaz, and S.A. Solin, “Photo effects at the Schottky interface in extraordinary optoconductanc,” J. Appl. Phys. Vol. 114, pp. 153110 (1-4), 2013.

Seyed Mohammad Hosein Khalkhali, received the Ph. D. degree in physics from physics department of Shahid Beheshti University, Iran, in 2014. He has worked on

International Journal of Optics and Photonics (IJOP) Vol. 10, No. 1, Winter-Spring, 2016

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the ferroelectric and photovoltaic composite thin films.

Seyedeh Mehri Hamidi received the Ph.D. degree in photonics from Laser and Plasma Research institute, Shahid Beheshti University, Iran, in 2009. She is currently the director of magneto-plasmonic lab of Laser and Plasma research institute. She has worked on the research fields of magneto-plasmonic, nanophotonics, photonic and magnetophotonic crystals, Surface Plasmon Resonance, dielectric and magnetic waveguides and Pulsed laser deposition technique.

Mohammad Mehdi Tehranchi, received the Ph.D. degree in physics from Prokhorov General Physics Institute of the Russian Academy of Sciences (GPI RAS) in 1997. He is currently a professor of Physics and the director of magneto-photonic lab of Laser and Plasma research institute and Physics department of Shahid Beheshti University. He has worked on the research fields of magnetic materials (such as amorphous materials, multiferroic materials and magnetophotonic crystals) and magnetic effects (such as linear and nonlinear magneto-optical effects and Giant magnetoimpedance effects) which are utilized in magnetic sensors and nondestructive testing technology.

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