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J. Phys. D: Appl. Phys. 47 (2014) 075502 V S Puli et al

illumination; they possess a switchable photocurrent withrespect to the direction of the ferroelectric polarization. Aswitchable photocurrent strongly affected by the interfaceswith electrodes was observed by Choi et al [7] in a BFObulk single crystal. It was also reported that the PV currentin some BFO thin-lm samples cannot be switched at all and

is entirely attributed to the interface depletion layer betweenthe BFO and electrode. Choi et al [7] presented that diode-like rectication follows the polarization-dependent interfaceband bending of BFO single-crystal slabs that are electricallyswitchable, that is, V oc and I sc are possessed by the remanentpolarization directions. Yang et al recently demonstrated thatthe PV effect in BFO lms that originated from the 71 and/or109 domain walls owing to asymmetry in polarization [ 5].Some results in the reports from Choi et al [7] and Kundyset al [8] showed an angular dependence of the photocurrent onthe light polarization direction, although this has never beeninterpreted as any bulk PV effect. In BFO based PV devicesboth the metallic and conducting oxide top electrodes havebeen used, however, the oxide electrodes have shown betterPV responses in ferroelectric PV devices. A recent studyconrmed that narrow band gapBiFeO 3 ferroelectric thin lmswith Sn doped In 2O3 (ITO) top electrodes played a key rolein achieving larger PV properties and is about 25 times higherthan Au top electrode BFO lms [ 1, 9]. It was also reportedthat the PV current in some BFO thin-lm samples cannotbe switched at all and is entirely attributed to the interfacedepletion layer between the BFO and electrode [ 10].

Improved PV voltage and photocurrent densities wereobtained when transparent indium tin oxide (ITO) wasused as top electrode instead of gold electrode in

bilayered (Na 0.5Bi0.5)1 x Ba x TiO 3 (NBTBT)/BiFeO 3 (BFO)heterostructured lms [ 11 ]. Epitaxial multiferroic BiFeO 3 / La 0.7Sr0.3MnO 3 (BFO/LSMO) heterostructured lms grownon LaAlO 3 substrate under illumination showed signicantlyincreased photocurrent, indicating that the depolarizationelectric eld separates the photo-generated charge carriers[12]. Additionally, when the ferroelectric polarization of theBFO layer is switched, the switched short-circuit photocurrentand open-circuit voltagewerealsoobservedsuggestingthat thePV effect is due to the contribution from both the ferroelectricpolarization and the electrode/lm interface [ 12]. SurfacePV properties were observed in BFO microspheres which

were synthesized by a facile hydrothermal method, which isrelated to both the ferroelectric polarization and the Schottkybarrier [ 13].

Very recently Guo et al [14], also reported that, reversiblyswitchable diode, PV effect was observed in Sr substitutedBFO polycrystalline thin lms (with high oxygen vacancyconcentrations), whereas it is hard detect such an effect in thecase of pristine BFO thin lms (with lower oxygen vacancies)until the polarization ipping occurs. Co-substitution(A-site and B-site) in perovskite BFO lattice has beenattempted by various researchers, to further improve theelectric and magnetic properties. In general, co-substitutionwill suppress oxygen vacancies and reduce conductivity.Co-doping or substitution in BFO, substantially improveferroelectric (saturation polarization ( P s) 70 C cm

2 )

and dielectric properties without losing fundamental structuraland ferroelectric properties of BiFeO 3 and also improveferromagnetic properties [ 15, 16]. Secondary phases weresignicantly suppressed in Pr and Sc co-substituted BFOceramics, when compared to pure BFO [ 17]. PFM resultsalso corroborated lower leakage current behaviour and

enhanced magnetic properties in Pr and cobalt co-substitutedBFO (BPFCO) thin lms [ 18]. Enhanced magnetizationand ferroelectric properties were observed in co-substitutedBPFCO nanoparticles [ 19]. Cube-on-cube epitaxially grownsingle-phase Nb-doped BiFeO 3 (BFNO) lms on SRO/STOsubstrate, have shown that the piezoelectricity property isincreased with varying the orientation from (0 0 1) to (1 1 1),and PFM phase images have shown ferroelectric polarizationpoints downward due to self-poling effect [ 20].

In our previous studies we report, well-dened magnetichysteresis behaviour with high magnetic moment for transitionmetal modied BSFCO ceramics and the origin of the strongferromagnetic property co-substituted BFO ceramics may bedue to the presence of rare earth and transition metal ions at thelattice sites of BFO at room temperature [ 21]. Yang et al [5],in their experimental studies on BFO, showed that the specicarrangement of magnetic domains of a ferroelectric materiallead to a new PV charge separation mechanismwhichproducesphoto voltages that are higher than band gaps of the materials.Interestingly, common ferromagnetic transition metals such asFe, Ni and Co possess the characteristic Stoner energy bandgap 1 eV, which can allow absorption and visible radiationfrom the Sun [ 22, 23]. These ferromagnetic transition metalsconvert the observed sun radiation to electrical energy whichwill lead to PV effect [ 22, 23].

Numerous studies are available on the structural, dielec-tric, multiferroic properties of pure BFO and co-substitutedBFO bulk ceramics and thin lms and few reports are madeon PV behaviour of pure BFO. However, there are no reportsavailable on PV behaviour of co-substituted BFO thin lmsto the best of our knowledge. In this paper we reportPV behaviour for Sm and Co substituted BiFeO 3 thin lmsgrown by pulse laser deposition (PLD). Our results showthat Bi 0.9Sm 0.1Fe 0.95 Co 0.05 O3 (BSFCO) might be an alterna-tive material of choice for PV energy applications. Addition-ally, we also conrm ferroelectric behaviour in BSFCO thinlms usingpiezoresponse forcemicroscopy (PFM), whichwasfound to be superior to pure BFO. Results of these studies arepresented in thin paper.

2. Experimental procedure

Ceramic target of Bi 0.9Sm 0.1Fe 0.95 Co 0.05 O3 (BSFCO) wassynthesized by a solgel synthesis, as described in ourpreviousreport [ 21]. Polycrystalline multiferroic BSFCO thin lmsof thickness from 300360 nm were grown by pulsed laserdeposition on Pt/TiO 2 /SiO 2 /Si substrate at 700750

C, underan oxygen partial pressure of 100 mTorr, annealed for 30 min,and were subsequently cooled to room temperature in 300Torroxygen partial pressure. An excimer laser (KrF, = 248nm)with a laser energy density of 25 J cm 2 , pulse repetition rateof 10 Hz with a substrate target distance of > 5 cm, was used

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Figure 1. (a ) X-ray diffraction patterns of Bi 0.9Sm 0.1Fe 0.95 Co 0.05 O3(BSFCO) thin lms; ( b) surface topography (AFM) image of Bi0.9Sm 0.1Fe 0.95 Co 0.05 O3 (BSFCO) thin lms.

for lm growth. Test structures were fabricated by sputteringPt to create top electrodes with 200 m diameter at roomtemperature.

3. Results and discussion

XRD pattern of the Sm and Co co-doped BFO (BSFCO)thin lms grown on (1 1 1)Pt/TiO 2 /SiO 2 /(1 0 0)Si substrateis shown in gure 1(a ). The BSFCO lms prepared on(1 1 1)Pt/TiO 2 /SiO 2 /(1 0 0)Si substrate exhibited diffractionpeaks such as BFO(0 1 2), (10 4, (1 1 0), (1 1 2) and (2 2 4),

which indicated that the lms consisted of randomly-orientedpeaks corresponding to perovskite rhombohedral structurewith space group R3c (JSPDS le No. 86-1518). Thestrong and sharp diffraction peaks suggest that BSFCO lmsare polycrystalline. A small amount of impurity phasecorresponding to orthorhombic Bi 2Fe4O9 (Fe-rich phase)was found to exist along with the desired major phase of BSFCO. The Bi 2Fe 4O9 phase in BFO ceramics and thin lmshas already been reported and corresponds to paramagneticimpurity and is marked with asterisks (gure 1(a )). Theresult is consistent with literature which reported that undopedand rare-earth doping cannot eliminate the impurity phase inBFO [ 21, 24, 25]. Impurity phases (Bi 36 Fe 24 O57 , Bi25 FeO 40and Bi 2Fe 4O9 ) are obvious because of its chemical kinetics of formation in meta-stable BFO [ 26, 27]. However Yan et al ,

reported that undoped, La doped BFO and La and Coco-substituted BFO thin lms exhibited pure phase [ 28, 29].

Figure 1(b) shows the surface topography image of theco-doped thin lm acquired using standard AFM technique.The lm presents granular microstructure with root-mean-square (rms) roughness being 5.7 nm. It is clear that lm

was quite smooth to carry out the piezoresponse measurementand avoid any surface- generated artefacts. Moreover, withSm 3+ ion at Bi 3+ -site in BFO, we expect a good signal in PFMimage as compared to that of pristine BFO lm [ 17, 18]. Thisis because the bond strength of rare-earth ion with oxygenion is much stronger than that with bismuth and oxygenions. Therefore, less volatilization of oxygen is expected inSm-doped BFO lm leading to the more insulating lm whichis helpful in performing the PFM measurement (BFO lms areconducting due to oxygen vacancies, which leads to the highleakage currents and early breakdown). Such behaviour wehave recently reported in Pr and Co co-doped BFO [ 18, 30],and expect the same phenomenon to occur for the presentcase. The other reason for higher PFM signals in the presentcase is the smaller size of Sm 3+ ion as compared to Bi 3+ ion.The smaller ion presence may also cause the change in bondlengths and the electron re-distribution which should help inhigher piezoactivity [ 31]. The idea behind using the PFMtechniquewas to conrmtheswitchable polarizationand hencethe local ferroelectric/piezoelectric nature of the synthesizedlm. This is the advantage of PFM technique that it can beused to investigate the single or few grain(s), whereas thebulk technique acquires average polarization on an ensembleof grains. The other advantage is the direct visualization of theferroelectric domains, which is not possible by any of the bulk

techniques for polarization measurement.In general, it is difcult to measure conventional (macro-

scopic) ferroelectric P E loops of BFO based multiferroicmaterials, due to its highly leakage current behaviour andits interference signicantly giving articially large polariza-tions while hampering intrinsic polarization switching [ 32].PFM technique helps to characterize the materials with lowerresistivity (higher conductivity), which hinders macroscopicpolarization measurement. Ferroelectricity/piezoelectricity inthese BSFCO lms was conrmed using PFM. Piezoresponseimages were obtained using a conducting AFM probe andapplying a local electric eld bias across conductive AFM

tip as top electrode and Pt/SiO 2 /TiO 2 /Si substrate as bottomelectrode of the lm with the bottom electrode grounded.Further, the polarization switching was carried out by

writing two square patterns of bigger area 5 5 m2 and thecentral 3 3 m2 area inside OPP-PFM images of 8 8 m2

area by applying poling (writing) on the lms surface with 9 V and +9 V dc voltages, respectively.

Figure 2(a ) shows the square patterns written withthe application of 9 V bias voltage. It can be clearlynoticed that the polarization changes its direction with thecorresponding change in bias voltage (seen as a change inphase). The local hysteresis loop further ascertains theferroelectric nature of the lm (gure 2(b)). The out-of-plane (OP) piezoresponse amplitude and phase images areshown in gure 2. The phase PFM images clearly show the

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Figure 2. (a ) PFM phase image and conrms the switchability of ferroelectric polarization under 9 V bias voltage of Bi0.9Sm 0.1Fe 0.95 Co 0.05 O3 (BSFCO) thin lm; ( b) local piezoelectricphase hysteresis loop of Bi 0.9Sm 0.1Fe 0.95 Co 0.05 O3 (BSFCO) thinlms.

strong piezoelectric response and oppositely written squareregions establishing that BSFCO lms support ferroelectricity.Present Sm and cobalt co-substitutedBiFeO 3 lms have shownclear contrasts with ferroelectric switching behaviour in PFMamplitude and phase images, which might be attributed toferroelectricity (piezoelectricity) in the lms. Pr and cobalt

co-substituted BiFeO 3 (BPFCO) thin lms have not shownany PFM contrast, which might be attributed to deteriorationof ferro/piezoelectricity in the BPFCO thin lms owing tostrong electronegativity (1.13 on the Pauling scale) of Pr 3+

substitution at Bi 3+ siteinBFOlattice[ 18]. Clear PFMcontrastwas observed (gure 2(a )) in the case of Sm 3+ which may beattributed to its stronger electronegativity (1.17 on the Paulingscale). These results indicate that co-substitution of SmCoin BFO is helpful in improving ferroelectric/piezoelectricproperties. However, it may not be the case for othersubstitutions [ 18].

Local ferroelectric switching characteristics at nanoscalelevel is investigated using switching spectroscopy PFM(SS-PFM) technique, which capitulates spatially resolvedmaps of imprint voltage, coercive bias saturation, switchable

response, and nucleation bias [ 33]. The SS-PFM phasehysteresis loops are shown in gure 2(b). PFM hysteresisloops were obtained at xed location on the top electrode asa function of switching pulse amplitude (pulse duration was25 ms) superimposed on ac modulation bias with amplitudeof 0.81V pp (peak to peak) at 25 kHz. The phase change in

PFM loops is about 180

, which indicates complete domainswitching in the BSFCO lm. The local electromechanicalhysteresis loop implied very little asymmetry in ferroelectricphase hysteresis loops. Negative ferroelectric coercive forcewas predicted to be larger than the positive coercive force,which nally might be attributed to imprint effect in material.However, both upward and downward polarizations arecompletely stable with about a phase angle of 70 and 85 ,respectively. Imprint effect in thepresent lmsmightbe causedby strain gradients generated in substrate bending switchespolarization in polycrystalline ferroelectric lms [ 34]. Thus,after attesting the local ferroelectric behaviour of the lm, wecarried out the PV measurement on the lm.

Figure 3(a ) shows a P E hysteresis loop at 4 kHzfrequency using the positive-up negative-down (PUND)method at 10 V, which uses, successively, the two positiveand two negative electrical pulses, to directly measureelectrical P E hysteresis loops, the loops clearly show theexistence of a ferroelectric hysteresis and this measurementshow the existence of a strong diode-like behaviour,characterized by a maximum saturation at 3 C cm 2 , atpositive directional voltages; a similar type of behaviour wasalso reported by Yang et al , for PV ferroelectric BFO lms [ 1].

The PV characteristics of BSFCO lm was studied in thesandwich electrode conguration. Figure 3(a ) shows current

densityvoltage ( J V ) curves of BSCFO thin lms withand without illumination in sandwich electrode conguration(metalinsulatormetal (MIM)) for the polarization up stateby applying a positive voltage pulse. MIM congurationis good for low voltage poling process, to avoid electricalbreakdown. As can be seen, the J V curves taken in both theconditions of dark and with illumination revealed reversiblediode-like behaviour, however, in the case of illumination, theJ V curve exhibited higher current density both in reverseand forward bias compared to that of the dark J V curves.The BSFCO thin-lm capacitors showed clear PV responses(e.g., photocurrent at zero bias) with and without illumination,

as shown in inset of the gure 3(b), in sandwich electrodeconguration for the polarization up state by applying apositive voltage pulse. The observed short-circuit currentdensity( J sc ) is 0.051 A cm

2 and theopen-circuit voltage(V oc ) is 0.9V under illumination. The high V oc for theBSFCO lm observed in our case with illumination might bedue to signicant piezoelectric response in the as-depositedlm, indicating it is self-polarized and the observed PV effectoriginatedfromthe ferroelectric polarization [ 6]. The resultingopen-circuit voltage V oc of the junction solar cell is about0.9V, and this may be attributed to the recombination of electrons at anode and the reduced junction resistance. Thehigher open-circuit voltage in the present BSFCO might bedue to increase in the number of domain walls and this is thereason for domain wall PV effect in BFO based ferroelectrics

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Figure 3. (a ) a Polarization-electric eld hysteresis loops at 4kHzfrequency using the positive-up negative-down (PUND) method at 10 V reveal diode-like behaviour in positive direction, ( b) the dark

and illuminated current densityvoltage ( J V ) curves in sandwichelectrode conguration for the polarization up state by applying apositive voltage pulse, ( c) the dark and illuminated J V curves insandwich electrode conguration for the polarization down state byapplying a negative voltage pulse for Bi 0.9Sm 0.1Fe 0.95 Co 0.05 O3(BSCFO) thin lms.

[5]. Figure 3(c) shows the dark and illuminated J V curveswhere the measurement was done on diagonal electrodes insandwich electrode conguration for the polarization downstate by applying a negative voltage pulse. The illuminatedJ V curves for the BSFCO lms for diagonal electrodes havealso shown appreciable PV response. The short-circuit currentdensity ( J sc ) for the polarization down state by applying anegative voltage pulse was 0.0002 A cm 2 and the open-

Figure 4. Variation of current as a function of time upon turning thelight on and off for Bi 0.9Sm 0.1Fe 0.95 Co 0.05 O3 (BSCFO) thin lms.

circuit voltage ( V oc ) 0.5V when illuminated. The short-circuit current passed through the sample is large, when thepolarization and the current direction is down by applying anegative voltage pulse, the diode forward direction is from topto bottom and along the polarization direction. This type of switchable nature of PV effect in present BSFCO lms is dueto spontaneous polarization and also to the presence of low-energy band gap ( 1 eV) transition metal (Co) substation atFe in the BSFCO lattice [ 22, 23, 35].

Commercial silicon based solar cells have more powerconversion efciencies (PCEs) than BFO based ferroelectricsolar cells; poor PCE in ferroelectrics is due to theirintrinsically low bulk conductivity of ferroelectric domains[36]. Increase in bulk conductivity is not feasible due to

highly leaky ferroelectric domains, which cannot withstandstrong electrical polarizations [ 36]. PV voltages can be turnedon and off and their polarity can be switched by applyingexternal electric eld in ferroelectric PVs, which in turncontrols ferroelectric domain structures and their PV outputis controlled [ 5]. The switching (on and off state of lightsource) behaviour of the BSFCO lm was studied in sandwichelectrode conguration. For this study the incident light withintensity of 1kWm 2 wasexposed periodically with periodsof 300s and the photocurrent response was measured as afunction of time for a xed bias voltage of 6 V. The obtainedcurve is shown in gure 4. As can be seen, the photocurrentshows a rapid increase followed by a slow increase withoutreaching saturation upon exposing to light. When the light isturned off, the photocurrent decreases rapidly rst and thendecreases slowly to reach its original state. Photocurrentincreases from 0.0023 to 0.027 A cm 2 , when light wasexposed. However, it was observed that the maximumphoto-current increased gradually with increasing number of exposure periods. The increase in saturation current mightbe due to heating of BSFCO, which in turn decreases theresistance of the capacitor.

4. Conclusions

In summary, good-quality transition metal modied BiFeO 3polycrystalline thin lms have been prepared on Pt/TiO 2 /

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