APPLIED PHYSICS LETTERS 87, 022903 共2005兲
Electron spin resonance investigation of oxygen-vacancy-related defects in BaTiO3 thin films V. V. Laguta, A. M. Slipenyuk, I. P. Bykov, and M. D. Glinchuk Institute for Problems of Material Science, NASc of Ukraine, 3 Krjijanovskogo Strasse, 03680 Kiev, Ukraine
M. Maglionea兲 and D. Michau Institute of Condensed Matter Chemistry of Bordeaux—UPR 9048 CNRS 87, Avenue Dr. Schweitzer F-33608 Pessac Cedex, France
J. Rosa and L. Jastrabik Institute of Physics, Academy of Science of Czech Republic, Na Slovance 2, 180 40 Prague 8, Czech Republic
共Received 30 November 2004; accepted 17 May 2005; published online 7 July 2005兲 The Ti3+ center, based on a regular Ti site perturbed by an oxygen vacancy 共VO兲, is identified by electron spin resonance 共ESR兲 in textured BaTiO3 films. The center shows tetragonal symmetry along cubic 具100典 axes with g-factors: g储 = 1.997, g⬜ = 1.904. The spectrum of this defect disappeared after the film annealing at 700 ° C in an O2 atmosphere. We describe the observed spectrum as Ti3+ – VO couple defects or F+ center, which have never been observed in bulk BaTiO3. ESR is thus a unique tool to identify oxygen-vacancy-related defects, which have a large effect on the performance of ferroelectric films. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1954900兴 The perovskite-type ferroelectrics have many important applications in the field of semiconductor devices. In particular, barium titanate 共BaTiO3兲-based thin films are one of the candidates as a dielectric in integrated high density capacitors.1 However, the main drawbacks of perovskitebased devices are fatigue, aging, and imprinting.2,3 It is believed that all these problems are related to oxygen vacancy defects, which are thought to be the most mobile and abundant in perovskite ferroelectrics.4 Such problems have been solved with respect to Bi-based layered thin films such as SBT and SBN, which are presently under production.5 Still, investigating oxygen-related point defects in perovskite thin films is of key relevance for their improvement, and this is the aim of the present report. Oxygen vacancies 共VO兲 represent donor-type defects that are able to bind one or two electrons, giving rise to singly charged 共F+ centers兲 or to neutral oxygen vacancies 共F0 center兲. Previous studies of such defects in BaTiO3 and other perovskite ferroelectrics, and first-principles calculations,6 have shown that, in the case of the F+ center, a trapped electron is located at a Ti3+eg 共3d3z2−r2兲 orbital forming a Ti3+ – VO paramagnetic complex. However, to date there is no direct experimental observation of these centers using the electron spin resonance 共ESR兲 technique. In the present letter we show that “pure” Ti3+ – VO defects do exist in BaTiO3 films. They are characterized by different g-factors than those in reduced BaTiO3 bulk crystals where more complex oxygen-vacancy defects occur.7 In the present report, the g-factor measured along the main axis of the center is very close to the g-factor of the free electron that corresponds to eg 共3d3z2−r2兲 ground state of the Ti3+ – VO, defect, in perfect agreement with theoretical predictions. a兲
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The BaTiO3 thin films were synthesized using standard radio frequency sputter deposition on amorphous SiO2 or 共100兲 Si substrates. On both types of substrates, the polycrystalline films with thickness between 340 and 4000 nm were oriented with one of the cubic axes along the direction perpendicular to the film surface 共关h00兴-oriented films兲 or at an angle of 45° to the film surface 共关hh0兴-oriented films兲. As already reported, this fiber texture stems from a bias that was applied to the substrates during the growth.8 All the other sputtering parameters were kept constant for all the films under consideration. ESR spectra were recorded at 9.22 GHz and at temperatures 5 – 40 K. The surface of the samples used were approximately 6 ⫻ 3 mm2. As an example, in Fig. 1 we present ESR spectra measured in the BaTiO3 films with different textures and for several thicknesses. Resonances denoted as Ti3+ were observed in all investigated films including those with a thickness of only 300 nm. The ESR signal intensity increases approximately proportionally to the thickness of the films. Annealing 1 h in oxygen atmosphere at T ⬇ 700 ° C completely suppresses these ESR spectra 共Fig. 1兲. The angular dependencies of the resonance fields were measured in the plane perpendicular to the surface of the film 共Fig. 2兲. They are typical for the axial 共tetragonal兲 symmetry of the paramagnetic center with the axial axis pointing along one of the 具100典 cubic directions of the BaTiO3 crystallite. The angular dependencies were fitted by axial symmetry spin Hamiltonian for particles with electron spin S = 1 / 2 and following g-factors: g储 = 1.997共1兲, g⬜ = 1.904共1兲. In the case of 共h00兲-oriented films, there are two clearly visible lines, which have maximum intensity at B 储 n. In the following, is the angle between B and the normal to the film surface n. When the magnetic field tilts from B 储 n, the ESR line of the centers with axial axis parallel to n shifts, respectively, from g储 共 = 0 ° 兲 to g⬜ 共 = 90° 兲 and vice versa. For other centers, for which axial axes lay in the plane of the
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Appl. Phys. Lett. 87, 022903 共2005兲
Laguta et al.
FIG. 2. Angular dependencies of Ti3+ resonance fields measured in the plane perpendicular to the film surface. Filled and open circles are experimental data for 共h00兲 and 共hh0兲 oriented films, respectively; smooth lines are the theoretical fit.
3+
FIG. 1. Ti ESR spectra in BaTiO3 polycrystalline films taken at T = 25 K. 共h00兲 and 共hh0兲 denote the orientation of one of the perovskite cubic axes of crystallites relative to the normal to film surface. The magnetic field was perpendicular to the film surface.
film, the angle between the magnetic field and axial axis is randomly distributed. However, there are always some centers for which this angle is equal or close to 90°. As in the case of powdered spectra, this gives rise to a sharp line with a resonance field corresponding to g⬜ because of the flat shape of the angular dependence at = 90°. No angular dependence of the ESR line positions were observed for magnetic field rotation in the plane of the film, in agreement with random distribution of 具100典 cubic axes in the film plane. For 共hh0兲-oriented films, two lines at B 储 n have been observed. One line corresponds to the centers with axial axis lying in the film plane 共 = 90° 兲, while the second line, two times higher in intensity, corresponds to the centers whose axial axes make an angle of 45° with the normal to the film surface. This line splits into two components when the magnetic field tilts from the n direction because axial axes become not equivalently oriented relative to B. There are several arguments to support that these observed resonances may be attributed to Ti3+ ions. First, the intensity of the spectrum is too high to be assigned to impurity ions. We estimated the concentration of paramagnetic ions as 1018 – 1019 cm−3, which is 2–3 orders higher than the concentration of any residual impurity usually present in the highest concentration in undoped BaTiO3 共Fe3+ or Cr3+兲. The spectra of Fe3+ and Cr3+ are not visible due to the very small volume of the investigated films 共10−5 – 10−4 cm3兲. Second, one of the g-factor values 共g⬜兲 is quite close to those of Ti3+ centers in BaTiO3 single crystals; g⬜ = 1.904.7 However, the g储 of the Ti3+ measured in the films, g储 = 1.997, differs remarkably from that in the bulk BaTiO3 single crystals, where g储 = 1.934. This fact suggests that here we are dealing with dz2 ground state of the Ti3+ ion because, as it has been predicted by crystal-field theory,9 only for this orbital 3d electron, g储 ⬇ 2.00. Moreover, there are rather clear indications that Ti3+ ions in BaTiO3 films contain in the nearest environment oxygen vacancies and, thus, the observed spectra belong to Ti3+ – VO centers 共singly ionized oxygen vacancy— F+ center兲. To confirm the existence of such coupled defects, the films were annealed at 700 ° C in oxygen and the ESR
line disappeared 共Fig. 1兲. In addition, previous Rutherford backscattering analysis10 provided some evidence of such high density of oxygen vacancies in as-grown films. In addition, theoretical calculations6 definitely and independently showed that the ground state wave function of F+ has a 共3z2 − r2兲 shape and a high density within the vacancy. For this ground state, crystal-field theory gives the following g-factors:
g储 = ge = 2.0023;
g⬜ = ge −
6 , E
where E is the energy distance to the upper t2g 共兩yz典 , 兩xz典兲 orbital and = 150 cm−1 is the spin-orbit coupling constant of the Ti3+ ion.9 The experimental g⬜ value corresponds to E = 9100 cm−1, which is reasonable for the BaTiO3 lattice. A question that can arise is why the Ti3+ – VO centers are visible only in BaTiO3 films but not in bulk crystals. This can be attributed to the specific conditions under which the thin films are processed. These films are generally synthesized and processed at much lower temperature than for bulk crystal synthesis 共⬃1300 ° C兲. In addition, the lattice parameter mismatch between the substrates 共Si, amorphous SiO2兲 and the films was not optimal in the present study. As a result, the films are highly polycrystalline, which usually results in charged defects concentrated on grain boundaries.11 We, however, have no direct evidence for such accumulation at present. To summarize, we have shown that ESR is a useful tool for probing oxygen-vacancy-related defects in ferroelectric thin films. We have evidenced Ti3+ – VO centers in BaTiO3 films that were not previously observed in bulk materials. Such unusual defects can result from the sputtering conditions that are far from equilibrium. The high density of oxygen vacancies may also be the driving source of the mean orientation of the films, which was shown to be tunable using a bias applied to the substrates during the film growth. We acknowledge financial support from the UkrainianFrench exchange program DNIPRO 05094TM and from the Conseil Régional d’Aquitaine in Bordeaux.
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