24P003-O
Polarization-induced Photovoltaic Effectin Er-doped BiFeO3 Ferroelectric Films
Er添加BiFeO3強誘電体薄膜における分極誘起光起電力効果
Yohei Ukai 1, Shuhei Yamazaki 2, Yukihiro Nomura 1, Takeshi Kawae 2 and Akiharu Morimoto 2
鵜飼洋平 1, 山崎修平 2，野村幸寛 1，川江 健 2, 森本章治 2
1

Graduate School of Natural Science & Technology, Kanazawa University
2
College of Science & Engineering, Kanazawa University
Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan
金沢大院自然 1, 金沢大理工 2 〒920-1192 石川県金沢市角間町

Ferroelectric hysteresis behavior was observed in Er-doped BiFe0.97Mn0.03O3 capacitor using Au top andSrRuO3 bottom electrodes. A photovoltaic effect was observed in the current density vs. voltage
measurement under a light irradiation onto the sample. This photovoltaic effect is ascribed to theferroelectric polarizations.

1. Introduction
Novel solar cells are studied for achievement of alow cost and a high conversion efficiency while Sisolar cell is the major one. For example, thephotovoltaic effect caused by the polarization
reversal is different from well-known photovoltaiceffect, because the photo-generated carriers inferroelectric materials are not separated by thebuilt-in field of the conventional p-n junction.
However, the photovoltaic conversion efficiencies
in ferroelectric capacitors are limited by the small
current density (around 10-6mA/cm2) and the large
band gap energy (around 3.5 eV). Therefore,
recently, polarization-induced photovoltaic effect inBiFeO3 (BFO) has attracted much attention[1,2]due to a huge remnant polarization and a small bandgap (Eg = 2.67eV) of BFO among ferroelectricperovskite materials. The latter feature suggests ahigh absorption coefficient in visible light region,
leading to a promising material for solar cellapplication.
So far, we tried to dope Er atoms, emitting lightin the optical communication band, into
ferroelectric LiNbO3 and BFO films for developinga novel optical switches using the piezoelectriceffect. However, the electrical conductivity of BFO
films varied depending on the Er content.
Therefore, this result led us to polarization-induced
photovoltaic effect of Er-doped BFO films forobtaining a synergistic effect of the semiconducting
feature and the ferroelectric feature in Er-doped
BFO films. In addition, it is attracting our attention
that the photoluminescence (PL) spectrum and thePL intensity vary with the local crystallographic
symmetry of Er in various host materials[3],leading to clarifying the local environment of Er

atoms in oxides .
In the present study, we fabricated Er-doped BFO
thin film using the pulsed laser deposition (PLD)
method. As a result, we have observed aphotovoltaic effect depending on the ferroelectricpolarization reversal in Er-doped BFO films.
2. Experimental
The conductivity of non-doped BFO films is
known to be governed by deviation of the Bi
content from stoichiometry and the valencefluctuation of Fe ion. We reported a leakage current
reduction, or a conductivity reduction, bysubstituting the Bi and Fe sites by rare-earth andtransition metals, respectively[4].
Here, wesubstituted Fe site with Mn atom to obtain a low
conductivity sample as a starting host material. Eratoms are incorporated into the host material,
resulting in BiEr0.01Fe0.97Mn0.03O3 (Er:BFM) thinfilm. For sample preparation, firstly, the surface ofNb(0.05wt.%)-doped SrTiO3(001) (Nb:STO) wastreated with buffered hydrofluoric acid solution.
Then, Er:BFM thin film of 170nm thickness wasdeposited on it by PLD using bottom electrode of
100nm thick SrRuO3 (SRO) film. Finally, usingvacuum evaporation method Au top electrodes weredeposited on it. The X-ray diffraction (XRD), theferroelectricand
theelectricaltransport
measurements were performed. For characterizationof the photovoltaic property, Ar+ laser (488nm inthe wave length) was employed with an irradiation
intensity around 1-3 W/cm2.
3. Results and Discussion
XRD result showed no Bi deficiencies and (001)preferential orientation of Er:BFM on thethe Er incorporation scheme. The future PL
measurement for these materials will give us
valuable knowledge in materials design forimprovement of the photovoltaic effect because Er
atoms can be used as probes for knowing the localenvironment.
8Current Density (mA/cm )SRO-coated Nb:STO substrate. The electric fielddependences of the electric polarization (P-Eproperty) for various maximum applied fields at afrequency of 10 kHz were shown in Fig.1. The
hysteresis caused by ferroelectricity was observed,indicating the existence of the polarization reversal
although rounded P-E hysteresis curves due to someleakage current can be seen.

2100
500-50-100@R.T., 10kHz

420-2-4-3-2-1

01

23Voltage (V)
0250

500

750 1000

Electric Field (kV/cm)

Fig.1 Polarization vs. electric field (P-E) hysteresis loops
with measurement frequency of 10 kHz at R.T.

Figures 2 shows the photo- and dark current
densities vs. applied voltage (J-V) characteristics.
Figure 3 shows the expanded one of Fig.2. The
voltage was applied to the top electrode against thebottom one. The J-V measurements under theirradiation were done with both the sweep
directions, that is, the forward voltage sweep fromminus to plus and the reverse one from plus tominus. As shown in Fig.2 asymmetric pattern wasobserved in dark, indicating a rectification behavior.
Under the irradiation the short-circuit current
density Jsc and the open-circuit voltage Voc are+0.04 mA/cm2 and -0.07 V, respectively, for theforward voltage sweep, and are -0.15 mA/cm2 and+0.24 V, respectively, for the reverse voltage sweep.
The hysteresis behavior in the J-V curve of thepresent Er:BFM film under the illumination is
ascribed to a photovoltaic effect caused by thepolarization reversal as proposed by Yang et al. fornon-doped BFO film[2]. It should be noted thatthe polarization-dependent photovoltaic effect does
not necessarily require the conventional built-in
potential, which is essential for the separation ofphoto-generated carriers in the normal p-n or
Schottky junctions.
So far, conductivity changes with the Er content
have been observed in our previous experiments.
The small short-circuit current density in thepresent ferroelectric sample is expected to turn tolarge one by optimizing the Er content or improving

Fig.2 Photo- and dark current density vs. voltage (J-V)
characteristics
0.3
0.2

2-150
-1000 -750 -500 -250

Current Density (mA/cm )2Polarization (µC/cm )150dark
photo- forward voltage sweep
photo- reverse voltage sweep

60.1
0.0
-0.1

dark
photo- forward voltage sweep
photo- reverse voltage sweep

-0.2
-0.3
-0.1

0.0

0.1

0.2

0.3

Voltage (V)Fig.3 Expanded view of the current density vs. voltage
curves in Fig.3 near zero bias field (V=0)

4. Summary
Ferroelectric hysteresis behavior originating fromthe polarization reversal was observed in Er:BFM
capacitor with Au top and SRO bottom electrodes.
A photovoltaic effect was also observed in thecurrent density vs. voltage measurement under alight irradiation. This photovoltaic effect is ascribedto the ferroelectric polarization.
References[1] S. Y. Yang et al.: Appl. Phys. Lett. 95 (2009)
062909.
[2] S. Y. Yang et al.: Nature Nanotech. 5 (2010) 143.
[3] M. Kumeda et. al.: Mat. Res. Soc. Symp.
Proc. 910 (2007) 131.
[4] T. Kawae et al.: Jpn. J. Appl. Phys. 47 (2008) 7586.