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Characterization of electronic structure and defect states of thin epitaxialBiFeO3 films by UV-visible absorption and cathodoluminescencespectroscopies

A. J. Hauser,1 J. Zhang,1,2,3 L. Mier,2 R. A. Ricciardo,2 P. M. Woodward,2 T. L. Gustafson,2

L. J. Brillson,1,3 and F. Y. Yang1,a1Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA

2Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA

3Department of Electrical and Computer Engineering, The Ohio State University,Columbus, Ohio 43210, USA

Received 21 March 2008; accepted 14 May 2008; published online 2 June 2008

UV-visible absorption and cathodoluminescence spectra of phase-pure epitaxial BiFeO3 thin filmsgrown on SrTiO3001 substrates by ultrahigh vacuum sputtering reveal a bandgap of 2.692.73 eVfor highly strained 70 nm thick BiFeO3 films. This bandgap value agrees with theoreticalcalculations and recent experimental results of epitaxial BiFeO3 films, demonstrating only minimalbandgap change with lattice distortion. Both absorption and cathodoluminescence spectra showdefect transitions at 2.20 and 2.45 eV, of which the latter can be attributed to defect states due tooxygen vacancies. 2008 American Institute of Physics. DOI: 10.1063/1.2939101

BiFeO3 has received considerable interest in the pastseveral years due to the simultaneous ferroelectric and mag-netic properties in epitaxial BiFeO3 thin films.

14 The multi-functionality of multiferroics, including BiFeO3, with poten-tial coupling between ferroelectric and magnetic orderparameters, can lead to application in magnetoelectricdevices.1 Thus, an experimental investigation of the elec-tronic structure, especially the bandgap and defect states ofepitaxial BiFeO3 films, is of particular importance for bothscientific understanding and potential technological applica-tions. There have been several experimental measurementsof the bandgap of BiFeO3 using UV-visible absorption spec-troscopy and ellipsometry on polycrystalline BiFeO3 films,

nanowires, nanotubes, and bulk single crystals with reportedbandgap of2.5 eV,58 and recently on epitaxial BiFeO3films grown by pulsed-laser deposition PLD and molecularbeam epitaxy with measured bandgap between 2.66 and2.81 eV.911 However, a detailed study of defect states ofepitaxial BiFeO3 thin films is still needed to understandthe electronic structure. In this letter, we report the charac-terization of bandgap as well as defect state features of epi-taxial, phase-pure BiFeO3 films as a function of growthtemperature.

Epitaxial BiFeO3 thin films were grown on SrTiO3001substrates by radio-frequency rf sputtering using an ultra-high vacuum magnetron sputtering system with a base

vacuum of 5

1010

Torr. The target was synthesizedthrough multistep chemical processing and sintering andverified to be phase-pure stoichiometric BiFeO3 by x-ray dif-fraction XRD. Previous reports on the growth of BiFeO3epitaxial films used standard off-axis geometry and targetswith excess Bi2O3.

12 We investigated both on-axis and off-axis growths with a chimney-shaped stainless steel shieldbetween the substrate and the target. XRD revealed that bothgeometries produced phase-pure BiFeO3 epitaxial filmswhile the on-axis growth gave a higher deposition rate. Allsamples in this article were grown by on-axis sputtering. A

mixture of argon and oxygen Ar:O2 =10: 3, total pressure of250 mTorr was used for depositions at substrate tempera-tures TS of 500, 550, and 600 C. The deposition rate cali-brated at TS =500 C was 0.15 /s. All films were grown for80 min, resulting in film thicknesses of 72 nm for TS =500and 550 C, and 64 nm for TS =600 C. Atomic force mi-croscopy measurements gave a surface roughness of4 .

XRD confirms the epitaxial relationship of all theBiFeO3 films with the SrTiO3001 substrates. Figures 1aand 1c show the /2scan and three-dimensional rockingcurve, respectively, for the BiFeO3 film grown at 600 C.For comparison, Fig. 1b shows the /2scans of aSrTiO3001 substrate and the plastic holder for XRD mea-

surements. Figure 1 demonstrates that the BiFeO3 film isphase pure with only the 00n peaks. The broad feature inFig. 1a near 2=30 is due to the plastic sample holder forXRD measurements lower curve in Fig. 1b. The rockingcurve in Fig. 1c gives a full width at half maximum of0.4, indicating high uniformity for the 64 nm thick film.XRD spectra of the BiFeO3 films grown at TS =500 and550 C are virtually identical to those shown in Fig. 1. Theout-of-plane lattice parameter c of all three BiFeO3 filmsobtained from the /2scans using Braggs law is 4.068 ,in excellent agreement with previous report of a 60 nmthick BiFeO3 film c4.06 made by sputtering

12

and slightly larger than those for 70 nm thick BiFeO3films c4.02 4.04 made by PLD.2,13 Bulk BiFeO3 hasa rhombohedral crystal structure with a=5.6343 and=59.348 and is often indexed by a pseudocubic latticewith a =3.96 .14 In general, epitaxial BiFeO3 films grownon SrTiO3001 have in-plane compressive strain a=3.905 for SrTiO3, and consequently a considerable lat-tice distortion with a large c / a ratio between 4.068 /3.96and 4.068/3.905 in either a tetragonal or monoclinicstructure.2,15 Magnetization measurements of the BiFeO3films were carried out using a LakeShore vibrating samplemagnetometer VSM with a sensitivity of 1106 emu. Noferromagnetic hysteresis was detected in our BiFeO3 films.

aElectronic mail: fyyang@mps.ohio-state.edu.

APPLIED PHYSICS LETTERS 92, 222901 2008

0003-6951/2008/9222/222901/3/$23.00 2008 American Institute of Physics92, 222901-1Downloaded 19 Nov 2010 to 143.248.16.135. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissio

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The bandgap of the BiFeO3 films was first measured byUV-visible absorption, as shown in Fig. 2a for threeBiFeO3 films grown at 500, 550, and 600 C. To determinethe bandgap, we plot E2 versus Ein Fig. 2b for all threefilms, where and Eare the absorption coefficient and pho-ton energy, respectively. A good linear fit above the bandgapindicates direct gap of BiFeO3.

10 The linear extrapolation ofE2 to 0 gives bandgap of 2.703, 2.687, and 2.697 eV forfilms grown at TS =500, 550, 600 C, respectively. In addi-

tion, we plot E0.5 versus Ein Fig. 2b for the film grownat TS =600 C. The E0.5 versus Eplot does not show twoclear slopes expected for an indirect gap material.16 We notethat a simple linear extrapolation of the versus Eplots inFig. 2a gives an inaccurate bandgap of 2.5 eV.

Our measured bandgap values are in good agreementwith the predicted bandgap of 2.8 eV for BiFeO3 using ascreened exchange method.17 In addition, for all threesamples, there is an absorption tail extending down to2.2 eV. The d/dEversus Eplot in the inset of Fig. 2aalso reveals another peak at 2.46 eV, similar to the absorp-tion spectrum observed at low temperature for epitaxialBiFeO3 films.

9 Both of these two features correspond to sub-

bandgap transitions that are also detected in the cathodolu-minescence CL spectra as discussed below. They are likely

due to the absorption from defect states within the bandgapof the BiFeO3 films.

We corroborated the bandgap values by nanoscale-depth-resolved CL spectroscopy. Figures 3a and 3b showCL spectra taken with electron beam energies Ebeam 1 and2 keV, respectively, for three BiFeO3 films with TS =500,550, and 600 C. Monte Carlo simulation of the depth-resolved CL emission profile in BiFeO3 indicates that atEbeam=1 and 2 keV the CL emission is almost exclusivelyfrom the top 30 and 65 nm, respectively, of the films. Thus,

FIG. 1. /2XRD scans in logarithmic scale ofa a BiFeO3 film grown at

600 C and b A SrTiO3001 substrate top and a plastic sample holderfor XRD measurements bottom. c Three-dimensional /2rockingcurve scan for the BiFeO3 film in a.

FIG. 2. a UV-visible absorption spectra for BiFeO3 films grown at tem-peratures of 500, 550, and 600 C. The curves for TS =550 and 600 C areshifted vertically for clarity. The inset shows d/dEbetween 2.2 and2.65 eV with an absorption peak at 2.46 eV. b Plots ofE2 vs EforTS =550, 600, and 500 C from top down and E

0.5 vs Efor TS=600 C. The linear extrapolation ofE2 to 0 gives a bandgap of 2.703,2.687, and 2.697 eV for TS = 500, 550, and 600 C, respectively.

FIG. 3. CL spectra of BiFeO3 films with electron beam energy ofa 1 keVand b 2 keV.

222901-2 Hauser et al. Appl. Phys. Lett. 92, 222901 2008

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Fig. 3 reflects only the electronic structure of the films with-out contribution from the substrates. The most prominentfeature in the CL spectra is the peak around 2.45 eV for allsamples, corresponding to the sub-bandgap emission ob-served in the inset of Fig. 2a. The shoulder at 2.73 eV inthe CL spectra is due to the band edge emission, in closeagreement with the bandgap determined by UV-visible ab-sorption in Fig. 2b. The onset of absorption seen in the

UV-visible spectra Fig. 2a is also evidenced here at2.20 eV as a shoulder on the left side of the central peak.We notice that despite the large tetragonal or mono-

clinic distortion of our epitaxial BiFeO3 films, the measuredbandgap of 2.692.73 eV by UV-visible absorption and CLis very close to the reported bandgap of 2.66 2.81 eV givenby UV-visible absorption measurements for epitaxial BiFeO3films of various thickness grown by PLD on SrTiO3 sub-strates with different orientations.911 It appears that thebandgap of BiFeO3 does not change significantly due to lat-tice distortion.

One possible mechanism for the feature at 2.45 eV in theinset of Fig. 2a and Fig. 3 is the defect states due to oxygenvacancies. In stoichiometric BiFeO3, the bandgap is betweenthe O 2p and Fe 3dlevels. The oxygen vacancies lower theadjacent Fe 3dlevels, resulting in sub-bandgap defect states.Initial theoretical calculation suggests that defect states dueto oxygen vacancies lead to transitions at 0.3 eV below thebandgap, in excellent agreement with the 2.45 eV peak0.28 eV below the bandgap of 2.73 eV observed in our CLmeasurements.18 However, the density of defects due to oxy-gen vacancies should be low because oxygen vacancies leadto Fe2+ ions which enhances ferromagnetism in BiFeO3 Ref.19, while no ferromagnetism was detected in the BiFeO 3films by VSM measurements.

The origin of the 2.2 eV peak which is considerably lessintense than the 2.45 eV peak in the CL measurements can-

not be assigned unambiguously. We note that a similar fea-ture was observed by UV-visible absorption in many studieson different forms of BiFeO3.

57,911 Another concern forBiFeO3 is Bi vacancies since both anion and cation vacan-cies can be found in perovskites. However, the formation ofBi1xFeO3 in our films is negligible for several reasons. Firstof all, consistent with the bulk phase diagram, the phaseBi2Fe4O9 is always observed whenever the Bi /Fe ratio fallsbelow one.8 For this reason, we used 50% excess Bi2O3 tocompensate the high volatility of Bi during the target synthe-sis which involved high temperature sintering at 800 C. Af-ter the sintering, the remaining excess Bi was removed bynitric acid which did not attack BiFeO3. Using this proce-

dure, we obtained stoichiometric BiFeO3 target without anydetectable impurity phases from XRD. It is important to notethat we do not see any Bi2Fe4O9 impurity in our films. Sec-ondly, different phases in epitaxial films in this caseBi1xFeO3 are unlikely since such a phase would be a mixedvalence compound between Fe3+ and Fe4+. This is a ratherhigh oxidation state for Fe, which can only be stabilizedunder quite oxidizing conditions. Furthermore, although it is

difficult to know for sure, such a phase is likely to be ametal, similar to SrFeO3 and SrFeO2.88 which are both me-tallic because of the Fe3+ /Fe4+ mixed valency. We see nosignature of a metal in our epitaxial films. Hence it is un-likely that Bi vacancies are present.

In conclusion, we have grown phase-pure epitaxialBiFeO3 films on SrTiO3001 substrates using on-axis ultra-high vacuum sputtering. UV-visible absorption measured a

bandgap of 2.70 eV, and sub-bandgap defect states at 2.2 and2.46 eV for all films. CL measurements confirmed bandgapand clearly identified the two subgap optical transitions. Thefeature at 2.46 eV is likely due to oxygen vacancies in theBiFeO3 films. CL has unique capability to identify defectstates with high sensitivity in epitaxial BiFeO3 films, whichis important in the application of this promising material.

L.J.B. and J.Z. acknowledge support from Office ofNaval Research Grant No. N00014-07-1-0470 Colin Wood.

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