Abstract
Tetragonal BiFeO3 films with the thickness of 30 nm were grown epitaxially on (001) oriented LaAlO3 substrate by using pulsed laser deposition (PLD). The transverse photovoltaic effects were studied as a function of the sample directions in-plane as well as the angle between the linearly polarized light and the plane of the sample along X and Y directions. The absorption onset and the direct band gap are ~2.25 and ~2.52 eV, respectively. The photocurrent depends not only on the sample directions in-plane but also on the angle between the linearly polarized light and the plane of the sample along X and Y directions. The results indicate that the bulk photovoltaic effect together with the depolarization field was ascribed to this phenomenon. Detailed analysis presents that the polarization direction is along [110] direction and this depolarization field induced photocurrent is equal to ~3.53 μA/cm2. The BPV induced photocurrent can be approximate described as Jx ≈ 2.23cos(2θ), such an angular dependence of photocurrent is produced as a consequence of asymmetric microscopic processes of carriers such as excitation and recombination.
Keywords
- transverse
- photovoltaic effect
- depolarization
- tetragonal BiFeO3
- photocurrent
1. Introduction
Driven by the energy crisis all over the world, more and more researchers have begun to investigate a broad spectrum of candidate materials for thin-film photovoltaic cells as a renewable energy production [1, 2, 3]. Among them, ferroelectric photovoltaic effect has been received considerable attention in the past few years because of its potential application in optoelectronics, information storage and energy conversion [4, 5]. However, different mechanisms have been proposed to explain experimental observations in the literature, such as the depolarization field effect [6, 7, 8], interface effect [9], domain theories [10], or spin polarization [11]. It is worth mentioning that bulk photovoltaic effect (BPVE) is another primary mechanism, which was discovered in noncentrosymmetric ferroelectrics several decades ago. It is often suggested that different from p-n junction based systems, BPVE does not require an asymmetric interface, especially its photovoltage is not limited by the band gap of the material, which can reach 103 V/cm or more and it is called anomalous photovoltaic effect [12, 13, 14]. All of the various ferroelectric materials, BiFeO3 (BFO) is of particular interest because of its robust ferroelectricity, room temperature coexistence of ferroelectric and antiferromagnetic orders and the possible magnetoelectric couple effect. More important, the band gap of BFO (~2.7 eV) is smaller than many other ferroelectric materials (more than 3 eV), making it become a more suitable candidate materials for the next generational thin-film photovoltaic cells. Apart from the fundamental research on its ferroelectric properties, the photovoltaic effect of BFO has been reported in ceramics, nanowires, single crystals and highly oriented films [15, 16, 17]. However, in all of these studies, BFO is a rhombohedrally distorted perovskite [18] belonging to the
2. Experimental process
Tetragonal phase BiFeO3 (T-BFO) films of ~30 nm thickness were fabricated epitaxially by pulsed laser deposition (PLD) technique on (001)-oriented LaAlO3. In this experimental process, KrF excimer laser with the wavelength of 248 nm was used for deposition, the deposition frequency is 3 Hz with an energy of about 240 mJ. The substrate was kept at 650°C with 11 Pa of oxygen atmosphere. In the course of deposition, the substrate holder was still rotated with the speed of 360°/min so that the thickness variation of the film can be reduced and the uniform composition of the film can be obtained as much as possible. Followed the deposition, oxygen stoichiometric T-BFO films were in situ annealed in 500 Pa oxygen pressure and cooled slowly down at 5°C/min to room temperatures to avoid the effect of deficient oxygen. Structural characterization was performed using X-ray diffraction (XRD), using M/s Bruker make D8-Discover system. Room temperature transmittance and reflectance spectra were collected by using a Perkin-Elmer Lambda-900 spectrometer (with the energy of 0.41–6.53 eV). For the conductive characteristics measurements, platinum electrodes of 200 μm length and interelectrode distances of 20 μm were fabricated by sputter deposition using conventional photolithography and lift-off technique. The photoelectric effect was measured by illuminating the gap between the electrodes with a
3. Results and discussion
Figure 1(a) shows the results of the X-ray diffraction
As shown schematically in Figure 1(c), the edge of the Pt electrode was aligned along the [100] and [110] directions of the substrate, so as to ensure the electric field directions. The applied electric field
The transmission spectrum of the samples were displayed in Figure 2(a), the direct band gap is extracted by a linear extrapolation of an (
To measure the photovoltaic effect of T-BFO films, we illuminated the gap areas between the top Pt electrodes, unwanted light illumination on the surfaces was avoided by covering with black tape. In the process of measuring the photovoltaic effect, the central electrode was connected with the negative side of source meter (Keithley 6517) and the outer electrodes were linked to the positive one. The photocurrent density
In order to elucidate the crystallographic direction and polarization dependence of photocurrent, we measured the PV current by changing the angle between the [100] direction and the direction of current flow (
Therefore, our results mentioned above cannot be explained by the depolarization field effect simply. Such a crystallographic direction dependence can be described in the framework of the bulk photovoltaic (BPV) theory, where the photocurrent is produced as a consequence of asymmetric microscopic processes such as excitation and recombination of photon induced electrons and holes [25, 26]. According to this theory, the dependence of the photocurrent on the polarization orientation of incident light can be expressed by a bulk photovoltaic tensor. The BPV effect has been studied extensively, it is assumed that the photocurrent in non-centrosymmetric ferroelectrics materials depends on the orientation of the crystal with respect to the projections of the electric field of the linearly polarized light onto the plane of the sample along
where
In order to prove the existence of the BPV in our films, we measured the photo current for T-BFO films by changing the angle between the plane of the linearly polarized light and the direction of current flow (
For the above four directions, the results can be fitted very well to a cosine function. The fitting function is for the four directions as follows:
Therefore, the photovoltaic effect along [-110] and [1-10] directions are both perpendicular to [110] direction, which is a product of BPV, yet it is a total effects combined depolarization and BPV effects when the photocurrent measured parallel to [110] and [-1-10] directions. Based on above analysis, depolarization field induced photocurrent is equal to the constant current
4. Conclusions
Tetragonal BiFeO3 films with the thickness of 30 nm were grown epitaxially on (001) oriented LaAlO3 substrate by using pulsed laser deposition (PLD) and the photovoltaic effect of tetragonal BiFeO3 along different crystallographic direction with in plane symmetric electrodes was investigated, the absorption onset and the direct band gap are ~2.25 and ~2.52 eV, respectively. The photocurrent exhibits definitive angular and direction dependency, indicating obvious bulk photovoltaic effect and depolarization field effect. The photocurrent depends not only on the sample directions in-plane but also on the angle between the linearly polarized light and the plane of the sample along
Acknowledgments
This work is supported by the National Natural Science Foundation of China (51402031), the Program for Innovation Teams in University of Chongqing, China (Grant no. CXTDX201601032) and the Natural Science Foundation Project of Chongqing (CSTC2015jcyjA50015).
References
- 1.
Ginley D, Green MA, Collins R. Solar energy conversion toward 1 terawatt. MRS Bulletin. 2008; 33 :355 - 2.
Gur I, Fromer NA, Geier ML, Alivisatos AP. Air-stable all-inorganic nanocrystal solar cells processed from solution. Science. 2005; 310 :462 - 3.
O’Regan B, Grätzel M. Optical electrochemistry I: Steady-state spectroscopy of conduction-band electrons in a metal oxide semiconductor electrode. Nature London. 1991; 353 :737 - 4.
Fridkin VM, Popov BN. Anomalous photovoltaic effect in ferroelectrics. Soviet Physics Uspekhi. 1978; 21 :981 - 5.
Glass AM, von, der Linde D, Negran TJ. High‐voltage bulk photovoltaic effect and the photorefractive process in LiNbO3. Applied Physics Letters. 1974; 25 :233 - 6.
Glass AM, von, der Linde D, Auston DH, Negran TJ. Excited state polarization, bulk photovoltaic effect and the photorefractive effect in electrically polarized media. Journal of Electronic Materials. 1975; 4 :915 - 7.
Brody PS, Crowne F. Mechanism for the high voltage photovoltaic effect in ceramic ferroelectrics. Journal of Electronic Materials. 1975; 4 :955 - 8.
Nelson J. The Physics of Solar Cells. London: Imperial College Press; 2003 - 9.
Gregg BA. Excitonic solar cells. The Journal of Physical Chemistry B. 2003; 107 :4688 - 10.
Yang SY, Seidel J, Byrnes SJ, Shafer P, Yang C-H, Rossell MD, Yu P, Chu Y-H, Scott JF, Ager JW, III LWM, Ramesh R. Above-bandgap voltages from ferroelectric photovoltaic devices. Nature Nanotechnology. 2010; 5 :143-147 - 11.
Ganichev SD, Prettl W. Spin photocurrents in quantum wells. Journal of Physics: Condensed Matter. 2003; 15 :R935 - 12.
Basu SR, Martin LW, Chu Y-H, Gajek M, Ramesh R, Rai RC, Xu X, Musfeldt JL. Photoconductivity in BiFeO3 thin films. Applied Physics Letters. 2008; 92 :091905 - 13.
Gao RL, Yang HW, Sun JR, Zhao YG, Shen BG. Oxygen vacancies induced switchable and nonswitchable photovoltaic effects in Ag/Bi0.9La0.1FeO3/La0.7Sr0.3MnO3 sandwiched capacitors. Applied Physics Letters. 2014; 104 :031906 - 14.
Wang J, Neaton JB, Zheng H, Nagarajan V, Ogale SB, Liu B, Viehland D, Vaithyanathan V, Schlom DG, Waghmare UV, Spaldin NA, Rabe KM, Wuttig M, Ramesh R. Epitaxial BiFeOmultiferroic thin film heterostructures. Science. 2003; 299 :1719 - 15.
Catalan G, Scott JF. Physics and Applications of Bismuth Ferrite. Advanced Materials. 2009; 21 :2463 - 16.
Li JF, Wang JL, Wutiig M, Ramesh R, Wang NG, Ruette B, Pyatakov AP, Zvezdin AK, Viehland D. Dramatically enhanced polarization in (001), (101), and (111) BiFeO3 thin films due to epitiaxial-induced transitions. Applied Physics Letters. 2004; 84 :5261 - 17.
Gao RL, Chen YS, Sun JR, Zhao YG, Li JB, Shen BG. Complex transport behavior accompanying domain switching in La0.1Bi0.9FeO3 sandwiched capacitors. Applied Physics Letters. 2012; 101 :152901 - 18.
Fischer P, Polomska M, Sosnowska I, Szymanskig M. Temperature dependence of the crystal and magnetic structures of BiFeO3. Journal of Physics C. 1980; 13 :1931 - 19.
Tütüncü HM, Srivastava GP. Electronic structure and lattice dynamical properties of different tetragonal phases of BiFeO3. Physical Review B. 2008; 78 :235209 - 20.
Fridkin VM. Bulk photovoltaic effect in noncentrosymmetric crystals. Crystallography Reports. 2001; 46 :654 - 21.
Ederer C, Spaldin NA. Effect of epitaxial strain on the spontaneous polarization of thin film ferroelectrics. Physical Review Letters. 2005; 95 (25):257601 - 22.
Cheng CJ, Lu CL, Chen ZH, You L, Chen L, Wang J, Wu T. Thickness-dependent magnetism and spin-glass behaviors in compressively strained BiFeO3 thin films. Applied Physics Letters. 2011; 98 (24):242502 - 23.
Zhang JX, He Q, Trassin M, Luo W, Yi D, Rossell MD, Yu P, You L, Wang CH, Kuo CY, Heron JT, Hu Z, Zeches RJ, Lin HJ, Tanaka A, Chen CT, Tjeng LH, Chu YH, Ramesh R. Microscopic origin of the giant ferroelectric polarization in tetragonal-like BiFeO3. Physical Review Letters. 2011; 107 (14):147602 - 24.
Chen ZH, Zou X, Ren W, You L, Huang CW, Yang YR, Yang P, Wang JL. Study of strain effect on in-plane polarization in epitaxial BiFeO3 thin films using planar electrodes. Physical Review B. 2012; 86 :235125 - 25.
Chen P, Podraza NJ, Xu XS, Melville A, Vlahos E, Gopalan V, Ramesh R, Schlom DG, Musfeldt JL. Optical properties of quasi-tetragonal BiFeO3 thin films. Applied Physics Letters. 2010; 96 :131907 - 26.
Sturman BI, Fridkin VM. The Photovoltaic and Photorefractive Effects in Noncentrosymmetric Materials. Gordon and Breach Science; 1992 - 27.
Ji W, Yao K, Liang YC. Evidence of bulk photovoltaic effect and large tensor coefficient in ferroelectric BiFeO3 thin films. Physical Review B. 2011; 84 :094115 - 28.
Festl HG, Hertel P, Kratzig E, von Baltz R. Investigations of the Photovoltaic Tensor in Doped LiNbO3. Physica Status Solidi (B). 1982; 113 :157-164