XPS study of photorefractive Sr0.61Ba0.39Nb2O6:Ce crystals

Solid State Communications, Vol. 98, No. 3, pp. 209-213, 1996 Copyright @ 1996 Published by Elsevicr Science Ltd Printed in Great Britain. All rights reserved 0038-1098/96 $12.00 + .OO

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xps STUDY

OF PHOTOREFRACTIVE

srO,6~&h)39Nb2o&e CRYSTALS

R. Niemann, K. Buse, R. Pankrath and M. Neumann Universitiit Osnabriick, Fachbereich Physik, D-49 069 Osnabriick, Germany (Received 11 October 1995; accepted 18 October 1995 by H. Eschrig)

X-ray photoelectron spectroscopy (XPS) measurements show that Ce occurs in photorefractive Srs,6iBae,3sNbzQ6 (SBN) CIJstalS mainly in the Valence State 3+. We give arguments that Ce3+j4+ is the photorefractive center in SBN:Ce with a typical concentration ratio CC~S+ /cw+ = 600. Quantitative: analysis of the XPS data yields a Ce distribution coefficient ofabout 0.6 + ‘0.3. 1. INTRODUCTION PHOTOREFRACTIVE Sri_,Ba,Nb& (SBN) crystals of congruently melting composition x = 0.39 are of special interest for many applications, e.g. volume holographic storage [l], self-pumped phase conjugation [2,3], photoassisted switching of ferroelectric domains [4] and fast recording with enhanced sensitivity at high light intensities [5]. Illumination of the crystals by two interfering light beams excites free charge carriers which move because of diffusion and drift in an electric field. A space charge field builds up which modulates the refractive index via the electroop tic effect; a volume phase hologram arises. Megumi et al. discovered 1977 that Ce doping improves the photorefractive sensitivity of SBN [6]. Recently a detailed spectroscopic study of absorption, photo and dark conductivity and light-induced absorption changes of SBN crystals with different contents of Ce was carried out [7]. These investigations show that probably the two-center model [8] and not the three-valence model [9] is valid for SBN:Ce at light intensities up to 100 kWmm2. A deep and a shallow photorefractive center are simultaneously present, each of them occuring in two different valence states. Further understanding of the photorefractive processes requires informations about valence state, distribution coefficient (ratio of Ce concentrations in crystal and melt) and site of the Ce ions. By optical absorption, micro-Raman measurements, photoluminescense and electron paramagnetic resonanl;e recently Giles et al. claimed to have observed Cc?+ in SBN:Ce in a nearly cubic environment [lo]. However, until now it is not clear whether Ce2+13+ or Ce:3+‘4+ are present and which valence state occurs with the highest concentration.

Doulliard et al. demonstrated at Cedoped YzO3 that X-ray photoemission spectroscopy (XPS) is a powerful tool for getting informations about valence states and environment of ions [ll]. Here we present a XPS study of SBN:Ce crystals and look especially for valence state and distribution coefficient of the Ce ions.

2. METHODS The Sro,6iBao,3$Ibz06:Ce (SBN:Ce) samples are grown in the crystal-growth laboratory of the physics department of the University of Osnabrtick by the Czochralski technique [7]. Doping with Ce is performed by adding CeO;! to the melt and crystals with 0.65 and 1.3 wt. o/ Ce in the melt are grown. The XPS measurements are carried out with a commercial spectrometer (Perkin-Elmer PHI 5600 ci) using Mg K, and monochromatized Al K, radiation. By a hemispherical analyser and a multichannel detector energy spectra of the emitted electrons are obtained and analysed with a spot diameter of 0.4 mm. The SBN single crystals are cleaved in vacuum at a base pressure of 3 x 10m9millibar and measurements are made at a base pressure of 3 x lo-i0 millibar. The spectra of Sr 3d, Ba 3d and Nb 3d were obtained using monochromatized Al K, radiation and corrected subtracting the inelastically scattered electrons using the Tougaard algorithm [I 21. The subtraction of a linear background is performed on the Ce 3d spectra. Because the MNN Auger peaks of Ba overlap with the Ce 3d peaks using Al K, excitation, the Ce 3d spectra are recorded with unmonochromatized Mg K, radiation. As the Ce concentration is very low, measurement times up to 70 hours are used. In order to avoid

209

PHOTOREFRACTIVE

210

Sr0.61Ba0.39Nb206:CeCRYSTALS

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Table 1. The Ce concentration in the melt cce,mclt, the Ce concentration in the crystal ccscrystal measured by XPS and the estimated distribution coefficient CQ~~&CC~,~I~ for different samples.

SBNCe

ce203

CeO,

\.x--(

Ce

lI(...I

880

,,.al.l‘l..(.l

890

900

910

920

minding Energy (eV) Fig. 1. XP spectrum of a Sro.6IBao.39Nb206 crystal doped with 1.3 wt. %I Ce in the melt and for comparison spectra of Ce203, CeO2 and Ce. contaminations the sample was repeatedly freshly fractured during this time.

Cerium Concentration cce.lWAt WkcryStal (wt. %I Ce) (wt. % Ce) 0.65 k 0.02 0.4kO.2 1.3OkO.05 0.7%0.2

Distribution Coefficient 0.6 2 0.3 0.5~0.2

final states can be observed: P, f’ and t0 [15]. However, the spectrum of SBNCe shows different 4f final states, too. Contributions from Q and f’ are observed, as it is characteristic for Ce3+. This can be seen in comparison to the spectrum of Ce203 [ll], serving as a reference of trivalent Ce. There is no evidence for a p final state contribution which would cause additional features at the lower binding energy site of the Ce 3d3,2 level and a separate peak at 9 18 eV. Such contribution would be characteristic for Ce4+, as it is clearly visible in the reference spectrum of CeOz, which is chosen as an example of tetravalent Ce. Because the signal to noise ratio is not very good, even after 72 hours of measuring time the presence of Ce4’ cannot be fully excluded: Ce appears to occur at least 90% in the trivalent state; the proportion of tetravalent Ce is smaller than 10 %. In addition, the multiplet splitting shows a ratio of the f’ to f2 valence states as it is observed in the reference spectra of Ce203, giving the clear hint, that the measured XP spectra cannot result from a superposition of two different t~valent Ce spectra, if they are separated more than 0.5 eV.

3. RESULTS 3.2. Quantitative Analysis of the Cerium Concentration 3.1. detection of Ce3’ Figure 1 shows the XP spectra of Ce 3d of the SBN crystal in comparison to the Ce 3d spectra of Ce203 [133,CeO;! [l l] and Ce metal [14]. The spectra of both SBN:Ce crystals are similar. Therefore the more intense spectrum of the sample doped with 1.3 wt. %) Ce in the melt is shown. Usually two single features resulting from Ce 3ds,z at lower and from Ce 3d3,2 at higher binding energy are expected, as it is shown in the spectrum of Ce metal. These features belong to the 4f’Sd’ 6s2 final states. A different situation is given for Ce203, caused by a charge transfer from the ligand, two different final states exist, f’ and L-IF. They result from the f’ ground state, belonging to trivalent Ce. This explains the appearence of two different features of the 3d3,2 and the 3d5,2 levels. The spectrum of CeO2 shows even more features. They result from a mixed valence ground state with contributions from trivalent and tetravalent Ce. Therefore three different

The Ce content of the crystals is estimated by comparison with the oxygen 1s signal, the most intense peak of the XP spectra. Table 1 summarizes the results. As mentioned above, the intensity of the Ce level is very low, resulting in a small signal to noise ratio. For this reason the uncertainty of the determined Ce concentrations is large. The distribution coefficient of Ce is defined as the ratio of the Ce ~n~ntrations in crystal and melt. 3.3. Core Level Spectra of the Crystal Compound Not only the Ce and oxygen signals were recorded, but also the XP spectra of the host material. After the subtraction of the Tougaard background, each spectrum was fitted using Doniach-SunjiC line profiles [ 161. The results are presented in figure 2, where the 3d core level XP spectra of strontium, barium and niobium are shown. It is obvious, that the spectra of bar-

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SriJ.61Ba0.39Nb206:CeCRYSTALS

211

tures of the Sr signal at higher binding energy.

4. DISCUSSION 4.1. IdentiJication of Ce3+14+as the Deep Photorefactive Center

130

132

136

134

138

Binding Energy (eV)

I

I

‘760

??O Binding

‘780

790

Energy

800

(eV)

Nb 3d

Two-beam coupling experiments yield that the photo conductivity of SBN:Ce is dominated by electrons [17]. In terms of electron conductivity the twocenter charge transport model can be described as follows: Two different centers are present, each of them occuring in two different valence states. One center is deep and the other is more shallow with respect to the conduction band edge. Excitation of electrons into the conduction band and recombination of electrons with these centers are possible. The concentrations of deep and shallow centers in SBN:Ce increase linearly with the Ce content up to 0.65 wt. % Ce in the melt [7]. Furthermore Ce doping yields a broad abso~tion band around 490 nm increasing linearly with the Ce content, too [7,10,18]. In the dark the shallow centers are empty - no electrons are captured - because of a large thermal generation rate [8]. Therefore only the deep traps filled with an electron can cause the absorption and most probably Ce is the deep center. The results of the XPS investigation show clearly that at least 90% of the Ce ions in SBN occur in the valence state 3+. Excitation of an electron into the conduction band creates Ce4+. Therefore the deep center can be identified as Ce3+14+. 4.2. Distribution Coejicient

205

200 Binding

Energy

210 (eV)

Fig. 2. XP spectra of Sr, Ba and Nb of a Sr0,6iBa(&Ib2Q6 crystal. The symbols represent measured values and the solid curves are fits (see text). ium and niobium can be fitted using single lines for both 3ds,2 and 3ds,2 levels. In contrast, the strontium spectra can only be fitted using four features, resulting from the superposition of two different strontium spectra with a shift of the binding energy of about 1.0 eV. For smaller exit angles the measurements become more surface sensitive and obviously influence the fea-

ofCerium

Quantitative analysis of the Ce XP spectrum yields a dist~bution coefficient of about 0.6 t 0.3 for the sample grown from a melt containing 0.65 wt. % Ce. Absorption and photo conducti~ty measu~ments show that the concentration of Ce in the crystal increases linearly with the Ce concentration in the melt up to 0.65 wt. O/ Ce in the melt [7]. Thus probably also for smaller Ce concentrations the distribution coefficient is about 0.6 + 0.3. The colour of the grown SBNCe crystals is homogeneous because the weight of the grown crystal amounts only 10 % of the weight of the melt. Thus the Ce concentration in the melt increases by less than 4 % during crystal growth. 4.3. Concentration Ratio of Ce3+ and Ce4’ The effective photorefractive trap density iV& is determined by the concentration c~4+ of the Ce4+ ions because the XPS measurements show ccea+ <<

212

PHOTOREFRACTIVE

Sr0,61Bao,39Nb2O&e CRYSTALS

coe3+. Ewbank et al. obtained from angular dependent beam-coupling measurements that SBN crystals doped with 0.1 wt. % CeOz have an effective trap density of Nee = 3 x 1G2 rnw3 1171.With a distribution coefficient of 0.6 we get an overall Ce concentration in the crystal of ca = 1.9 x 1O25rnB3. Using Ned = cce4+ and ccc = coe3+f CC,++we get cce3+/cce4+ = 600. This ratio is optimal for the photorefractive effect and yields the large photo conductivity and high sensitivity of this material [6,7]: The concentration c&4+ is just large enough to avoid pronounced space charge limiting effects and small enough to get a large photo conductivity and fast response of the crystal. In spite of these results further considerable improvement of the photorefractive performance of SBN:Ce, e.g. by thermal annealing or a speciai growth atmosphere, is not possible.

4.4. Secondary Centers Light-induced absorption effects in SBN:Ce crystals indicate that secondary photorefractive centers are present [5,19,20], but these centers are not identified yet. One might argue that Ce occurs on different crystal sites acting as a deep center on one site and as a shallow on another site. However, the XPS measurements show a single Ce spectrum. Only if the energy difference of Ce on different crystal sites is smaller than 0.5 eV or if the con~ntration of shallow centers is very small a single XP spectrum does not contradict Ce on two different sites. Both explanations are unlikely: There must be a considerable energy difference between deep and shallow traps for explanation of their different spectral and thermal behaviour. The shallow centers cause large absorption changes which require an appreciable concentration of secondary centers. Thus the single XP spectrum indicates that the secondary centers are probably not Ce on another crystal site.

4.5. Core Level Spectra Strontium atoms are located on two different crystal sites with different chemical su~oundings [Z1J. One may speculate, that this is the reason for the observed superposition of two Sr spectra. The dependence of the features of the Sr signal on the exit angle indicates that some SrO species contribute, which predominantly might exist at the surface.

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5. SUMMARY AND CONCLUSIONS XPS measurements with SBN:Ce crystals are carried out. Ce occurs mainly in the valence state 3+. We give arguments that Ce3+j4+ is the deep photorefractive center in SBN with a concentration ratio cce3+/cce4+ = 600. This ratio is optimal for the photorefractive effect and further considerable enhan~ment of the photorefractive pe~orman~ is not possible. The distribution coefficient of Ce, which is the ratio of the Ce concentrations in crystal and melt, is about 0.6 f 0.3 for Ce concentrations up to 0.65 wt. %I in the melt. The secondary photorefractive center is probably not Ce on another lattice site. A superposition of two XP Sr spectra with an appreciable dependence on the exit angle is observed.

Acknowledgement-Financial support of the Deutsche Forschungsgemeinschaft (SFB 225) is greatfully acknowledged.

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