Structural investigation of bulk and thin film PLZT using X-ray absorption spectroscopy

Structural investigation of bulk and thin film PLZT using X-ray absorption spectroscopy

22 Thin Solid Films, 240 (1994) 22-27 Structural investigation of bulk and thin film PLZT using X-ray absorption spectroscopy R. B. Greegor, F. W. L...

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Thin Solid Films, 240 (1994) 22-27

Structural investigation of bulk and thin film PLZT using X-ray absorption spectroscopy R. B. Greegor, F. W. L y t l e The Boeing Company, MS 8Y-28, Seattle, WA 98124 (USA)

A . Y. W u Department of Electrical Engineering, National Taiwan Ocean University, Taiwan (Taiwan)

(Received April 30, 1993; accepted November 3, 1993)

Abstract Thin film electro-optic and non-linear optical materials are of interest for applications in high-speed integrated optical

devices. Materials of the system Pbl_x/iooLax/loo(Zry/looTil-y/~OO)l-x/40oO3or PLZT x/y/(lO0-y) are attractive since they can be integrated into Si and GaAs substrates using suitable deposition techniques. In this investigation we examine the structural properties of r.f. magnetron sputter-deposited PLZT using X-ray absorption near-edge spectroscol~y (XANES). For XANES analysis, four samples were selected: (1) a hig~alyoriented PLZT 28/0/100 film of ~4500 A deposited on SiO2; (2) a highly oriented PLZT 28/0/100 film of ~4500 A deposited on a 2 ~tm SiO2 buffer layer over a Si(100) substrate; (3) a highly oriented PLZT 28/0/100 film of ~4500 ~ deposited on AI203 (1102); and (4) a commercial ceramic wafer of PLZT 9•65/35. The XANES experiments were performed at the Stanford Synchrotron Radiation Laboratory (SSRL) using electron yield and fluorescence techniques. Data was taken at the Ti K-edge (4966.4 eV) and compared to reference spectra. Of the reference spectra, the Ti K-edge spectra of the PLZT most closely resemble perovskite (SrTiO3). The surface and bulk thin film are similar and all the 28/0/100 spectra resemble the spectra of 9/65/35, indicating similar cubic perovskite structures for these materials.

1. Introduction

Pbl_x/looLax/loo(Zry/looTil_y/lOO)l_x/4OO03 or

PLZT

x/y/(lO0-y) thin film has large electro-optic (EO) or non-linear optical (NLO) effects as compared with the corresponding bulk material [1-8]. These effects make it a possible candidate for use in integrated electrooptical devices, such as waveguides [9-11], modulators [4, 9, 11], switches [9-12], shutters [4, 5, 8], frequency doublers [2, 3] and electrical or optical memory devices [4, 13]. In such applications the PLZT is sputterdeposited onto a Si or GaAs base which has been overlaid with a SiO2 buffer layer [8]. The PLZT film deposited by sputtering has a cubic perovskite structure with the [100], [ 110] or [ 111] direction normal to the plane of the substrate [8, 14]. For films deposited by the solution coating method the grains are randomly distributed with no preferred orientation [4]. The electrooptic effects demonstrated by PLZT vary depending on the composition, thermal history and applied voltages [4]. Field-induced birefringence and field-induced secondary harmonic generation are thought to be related to (a) the field-induced anisotropic orientation of ferroelectric domains and (b) a contribution of the nonlinear susceptibilities of the material, which contain electronic and ionic contributions [5-7]. The suscepti-

0040-6090/94/$7.00 SSDI 0040-6090(93)02976-K

bilities of the material have been modelled using an anharmonic oscillator which represents the motion of the octahedrally coordinated cation (e.g. Ti) in the perovskite structure. When applied external fields are large, the centrosymmetry of the films may be broken due to field-induced polarization, polarization switching or induced phase changes. The fundamental building blocks of the oxygen octahedra or the lattice structure of the materials are affected and high-order EO effects appear [ 5 - 7]. In this paper, measurements of the Ti K X-ray absorption near-edge structure (XANES) are made. XANES is very sensitive to the structural environment in which the absorbing atom is located. Slight changes in the symmetry, bond lengths and species of the coordinating atoms have significant effects on the measured absorption spectra. The XANES was measured for thin film PLZT on differing substrates and bulk ceramic material.

2. Experimental details Thin film PLZT of the system Pbl_x/looLax/lOO x/y/(lO0- y) were prepared using a radio-frequency magnetron sputtering (Zry/10oTil-y/lO0)l- x/40003 or PLZT

© 1994 - - Elsevier Sequoia. All rights reserved

R. B. Greegor et aL / Investigation of P L Z T using X-ray absorption spectroscopy

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28 Fig. I. X-raypowderdiffractionscans of PLZTmaterials.(A) Highly oriented PLZT28/0[100 filmon fusedsilicaSiO2substrate; (B) highly oriented PLZT 28/0/100 filmon SiO2(2 ~tmthick)/Si(100) substrate; (C) highlyoriented PLZT 28/0/100 film on AI203 (1102) substrate; and (D) PLZT 9/65/35 commercialwafer. All the films are oriented with the [100] direction normal to the surface of the substrate. The scale is intensity(arbitrary unit) vs. 20. Note that the peak height-tobackground ratio is very large in all films.

technique incorporating compound ceramic targets [8]. For the investigation four samples were used: (1) a highly oriented PLZT 28/0/100 film of about 4500/~ deposited on commercially fused 1.6 mm thick SiO2; (2) a highly oriented PLZT 28/0/100 film of about 4500/~ deposited on a 2 ~tm SiO2 buffer layer over a Si(100) substrate; (3) a highly oriented PLZT 28/0/100 film of about 4500/~ deposited on A1203 (1102); and (4) a commercial hot-pressed, optically polished PLZT 9•65• 35 ceramic wafer manufactured by Motorola Inc. Figure 1 shows the powder X-ray diffraction scans for these four samples. Note that in all the PLZT thin film samples the [ 100] orientation of the film is perpendicular to the substrate surface and only the (100) and (200) peaks of the perovskite structure appear in the X-ray diffraction spectra ((300) and (400) peaks also appeared but are not shown in Fig. 1). Also, the height of the (100) peak of all the films of ~4500/~ thickness is actually higher than the height of the (110) peak in the commercial PLZT 9•65/35 wafer and the peak-to-background ratio in the film is much larger than in the commercial wafer [8]. Figure 2 shows the layer structure for a PLZT waveguide modulator using the above materials and deposi-

23

,TO

PLZT

sio2 Si or GaAs

Fig. 2. The layerstructurefor a PLZTthinfilmwaveguidemodulator. tion techniques. The SiO2 is used as a buffer layer, since its index of refraction (n = 1.47) is less than the PLZT film (n = 2.5) [9]. A 1000/1, ITO (indium tin oxide, n = 1.8) superstrate film serves as a capping layer and contact electrode [9]. As can be seen, the set of samples selected for this investigation permits an in-depth examination of the effect of the PLZT and substrate interaction and comparison to bulk ceramic material of a different composition, i.e. '28/0/100 vs. 9•65•35. In the future, experiments are planned to examine the PLZT and superstrate or electrode interaction due to ageing, fatigue and applied electric field. For the XANES, several reference spectra were used (all the crystal structural information of the materials relevant to our experiments can be found in ref. 15): (1) 5 Ixm Ti foil; (2) powdered TiO2 (with the anatase, rutile and brookite crystal structure); (3) powdered Ti203; (4) powdered NiTiO3; and (5) powdered SrTiO3. For XANES experiments, all the powdered samples were ground to a fine mesh of 400, suspended in duco cement diluted with acetone and spread in a thin layer of ,~25 lma thickness on aluminium foil. The Ti foil was used to determine the K-edge position (4966.4 eV) of Ti metal. The remaining oxides were aligned with respect to the metal edge. Anatase, rutile and brookite show the sensitivity of the XANES technique to slight changes in the coordination geometry about the absorption site. Anatase is tetragonal with each Ti atom having six oxygen neighbours, ranging in distance from 1.91/~ to 1.95/~, while rutile (having the cassiterite form, also tetragonal) has T i - O bond lengths ranging from 1.95 A to 1.98 A. Brookite is orthorhombic with each Ti atom having near neighbour oxygen bond lengths from 1.87/~ to 2.04 A. In rutile there is one, and anatase two, short oxygen-to-oxygen separations at 2.43 ,~. Brookite also has close anion-anion contact with oxygen-oxygen separations varying upwards from 2.49/~. Ti203, is rhombohedral, having Ti in a higher oxidation state ( 3 + ) with octahedrally coordinated oxygen distances from 2.01 ,~ to 2.08/~.

24

R. B. Greegor et al. / Investigation of P L Z T using X-ray absorption spectroscopy

This structure can be considered as a slightly distorted hexagonal close-packing of oxygen ions, with the small Ti ions lying in some of the interstices. The arrangement is illustrated by ilmenite (FeTiO3) if its two types of metallic atoms are taken as identical. The NiTiO3 standard used in this study also has the ilmenite structure. SrTiO3 has the cubic perovskite structure with Ti coordinated by six oxygen atoms in the faces of the cube and Sr at the corners. The XANES experiments on the above samples were performed at the Stanford Synchrotron Radiation Laboratory (SSRL). The experimental techniques used at such X-ray sources and the data analysis methods subsequently employed have been discussed in detail elsewhere [16-19]. At SSRL, beam line IV-1 (a wiggler side station) was used while the storage ring was being run in a dedicated mode with the electron beam energy at 3 GeV and a stored beam current of 30-80 mA. A Si(220) double crystal monochromator detuned approximately 50% to reduce beam harmonics was used to select the desired X-ray energy in the vicinity of the Ti K-edge at 4966.4 eV. The monochromator slits were adjusted to 1 mm, which provided a resolution of approximately 0.3 eV at the Ti K-edge. Scans were made from 100eV below the Ti K-edge energy to 350 eV above the Ti K-edge energy. At the K-edge the monochromator was moved in steps of approximately 0.25 eV and above the edge ( +20 eV) the step size was adjusted to produce a step of 1-2 eV or a step less than or equal to 0.06/~-i. Both fluorescence and total electron yield data were collected. The fluorescence yield data were taken in the standard manner with the sample at a 45 ° angle to the incident X-rays. The total electron yield data were collected simultaneously with fluorescence data. The simultaneous collection of fluorescence and total electron yield data allows a comparison of surface ( ,~ top 500 A) and bulk signals originating from different depths within the sample.

3. Results and discussion

The XANES for the anatase, rutile and brookite polymorphs of TiO2 are shown in Fig. 3. In this figure the zero of energy is taken at the K-edge for the Ti metal, which is at 4966.4 eV. In all of these samples the Ti is surrounded by six oxygen near neighbours. Note the sensitivity of the spectra to the changes in the bond length and coordination geometry, as described previously. The features from approximately 0 to 12 eV are ls-o 3d bound state transitions, which are dipole-forbidden but quadrupole (very weak transition) allowed. If the centre of inversion at the Ti site is distorted, then the dipole transition becomes allowed and becomes stronger for more distorted octahedra. The features at

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the top of the absorption rise are due to ls ~ 4p transitions and multiple scattering effects, which are followed at higher energy by the oscillations due to the extended X-ray absorption fine structure (EXAFS); This region begins at 30-50 eV above the K absorption edge and is not examined in this investigation. In Fig. 4 the XANES of Ti203, NiTiO3 and SrTiO3 are compared with thin film PLZT (28/0/100) on an Si substrate with an SiO2 buffer layer. Note that the SrTiO3 XANES exhibits the closest resemblance to the XANES of the PLZT film. As discussed above, SrTiO3 has a cubic or pseudocubic perovskite structure with the Ti-O, -Sr, -Ti distances at 1.95 A, 3.38 ~ and 3.91 A respectively and a unit cell dimension of 3.91 [15]. We expect the T i - O separation in the thin film PLZT to be comparable ( +__0.1 A) and the coordination number to be 6. In four-fold coordination the ls--. 3d feature would be much more intense than that observed for the XANES of the thin film PLZT. For PLZT having the perovskite structure we expect cubic closest packing of oxygen anions and the Pb and La cations. In this arrangement the Zr and Ti cations occupy the

R. B. Greegor et al. / Investigation of P L Z T using X-ray absorption spectroscopy

25

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Fig. 4. Ti K-edge XANES of (A) thin film PLZT 28/0[100 on SiO2/Si, (B) SrTiO3, (C) NiTiO3 and (D) Ti203.

octahedral interstices in the close-packed array while the Pb and La are in sites coordinated by 12 oxygens. The match of the XANES of SrTiO3 and the thin film PLZT is not outstanding because the reference sample was polycrystalline and the PLZT is highly oriented in the direction perpendicular to the substrate and randomly oriented in the substrate plane. Since synchrotron radiation is highly polarized in the plane of the storage ring, bonds along the polarization direction are enhanced, whereas for the polycrystalline reference all bonds are equally sampled by the X-radiation. Also, the second neighbours about Ti in PLZT are obviously different from those in the SrTiO3 reference sample. The Ti203 and NiTiO3 spectra are quite different from the PLZT film but resemble each other except in the 1s ~ 3d region, where NiTiO3 shows a more intense transition indicative of higher distortion in the Ti-O octahedral cage. This similarity is not totally surprising since NiTiO 3 and Ti203 can both be represented by an h.c.p, array of oxygen atoms with the metal cations in some of the interstices. Figure 5 shows the XANES for the various PLZT samples considered in this investigation. It should be

0.0 10.0 20,0 30.0 40.0 50.0 80,0 ENERGY (eV)

Fig. 5. Ti K-edge XANES of PLZT materials (all taken in fluorescent yield, except A which was taken in electron yield): (A) 28/0/100 PLZT on SiO2, (B) 28/0/100 PLZT on SiO2, (C)28/0/100 PLZT on A1203 (1102), (D)28/0/100 PLZT on SiO2/Si(100) and (E) 9/65/35

commerc~l wafer. noted that the features from ~ 5 0 - 6 0 eV above the Ti K absorption edge are due to artifacts from the Si(220) monochromator used and are not part of the absorption features or the EXAFS. All of the spectra in Fig. 5 were collected in the fluorescence mode except for the 28/0/100 PLZT thin film on SiO2 which was examined in both the fluorescence and electron yield modes. Note that the data for the electron yield is noisier than for the fluorescence data. This is due to charge build-up and subsequent discharging in the insulator. However, the major features of the e-yield and fluorescence data are identical. This indicates that the structure in the top ~500/~ is the same as in the lower ~4000/~, since e-yield is sensitive to the surface, and fluorescent yield samples to depths many times the film thickness. The 1/e depth sampled by the electron yield technique is given by d(A) = 0.011/pE 3/2, where p is the density in g cm -3 and E is the Auger energy in eV [20]. If p = 7.55gcm -3 and E ( K L L ) = 3881 eV at the Ti Kedge, then the 1/e sampling depth for electron yield is

26

R. B. Greegor et al. / Investigation of PLZT using X-ray absorption spectroscopy

,~352/~. It should also be noted that the edge positions, as determined by the peak in the derivative of the XANES, do not vary among the P L Z T samples, indicating that the process variations do not affect the valence of the Ti. The edge position was the same as for SrTiO3, in which the Ti has a 4 + formal oxidation state. Comparison of the fluorescence data for the 28/0/100 P L Z T on SIO2, A1203 and SiO2/Si shows that these spectra are essentially identical. The rhombohedral structure of alumina or the tetrahedrally coordinated Si in silica have no apparent effect on the resulting structure of the P L Z T overlayer. Further comparison of all the 28/0/100 data and the 9/65/35 P L Z T shows only minor differences. The bulk 9/65/35 sample also has the cubic perovskite structure or slightly distorted rhombohedral perovskite structure [21], whereas 28/0/ 100 P L Z T has a cubic perovskite structure. The ls ~ 3d transition for the 9/65/35 sample is more intense, which indicates a higher degree of disorder in the T i - O octahedral cage than in the thin film 28/0/100 PLZT. This increased disorder in the T i - O coordination favours a slightly distorted rhombohedral perovskite structure for the 9/65/35 P L Z T sample. Bulk ceramic 28/0/100 P L Z T is paraelectric and has a cubic perovskite crystal structure. However, thin film 28/0/100 possesses the largest EO coefficient among (x/O/lO0) series [6]. Comparison of the An vs. E curves of 28/0/100 with normal and Pb-deficient compositions shows that the normal composition has a considerably larger field-induced birefringence (FIB) [3]. It has been suggested that this birefringence is due to a variety of effects, including distortions in the T i - O octahedral building blocks of the material [7]. From the investigation conducted here it should be possible to make a direct structural measurement of these distortions, since XANES is very sensitive to such effects.

probably indicates that the structure favours a slightly distorted rhombohedral perovskite arrangement as compared with the cubic-like perovskite structure for the other samples. The edge positions, as determined by the peaks in the derivatives of the XANES, are the same for all the P L Z T samples indicating that the oxidation state of the Ti is not altered by the process variations. The edge position was the same as for SrTiO3, in which the Ti has a 4 + formal oxidation state. Of the reference spectra used, (TiO2 (anatase, rutile, brookite), Ti203, NiTiO3 (ilmenite structure) and SrTiO3 (perovskite structure)), the P L Z T thin film and bulk ceramic exhibited absorption spectra most similar to SrTiO3. In SrTiO3 the T i - O bond length is 1.95/~, the T i - S r separation is 3.38 A and the T i - T i distance is 3.91/~ [13]. The structure is cubic or pseudocubic with a unit cell dimension of 3.905 ,~ at room temperature. In the thin film P L Z T we expect similar ( 0 . 1 ,~) T i - O distances with a coordination number of 6. For P L Z T having the perovskite-like structure, a cubic closest packing of oxygen anions and the Pb/La cations should be observed. In this arrangement the Zr and Ti cations occupy the octahedral interstices in the close-packed array, while the Pb and La are in sites coordinated by 12 oxygens. The presence of Ti in four-fold coordination would be indicated by a ls ~ 3d transition of much greater magnitude than that observed, along with a shortening of the T i - O bond length by ,,~0.2/~. The application of an external electric field may distort the symmetry of the T i - O octahedron, which would result in increased magnitude of the ls ~ 3d feature. This work demonstrates that the XANES technique is very sensitive to distortions in the T i - O octahedral building blocks of these materials and should be a useful diagnostic for such structural changes.

Acknowledgments 4. Conclusions The XANES of four P L Z T samples have been measured and compared with standard reference spectra for which the structures are known. Films ~4500 A, thick of 28/0/100 P L Z T on SiO2, A1203 and SiO2/Si all have similar spectra, indicating that the substrate or substrate buffer layer does not influence the bulk properties of the thin film overlayer. Also, the top ~ 500 ,~ surface layer of 28/0/100 P L Z T on an SiO2 substrate is the same as the underlying bulk thin film. All of these spectra were similar to the spectra of depoled and annealed bulk ceramic 9/65/35 PLZT, which has essentially the cubic or slightly distorted rhombohedral perovskite structure [21]. The ls ~ 3d feature in the spectra for the 9/65/35 sample is more intense than for the other samples, which

The work of R B G and F W L was supported in part by DOE grant DE-FG06-84ER45121. AYW was supported by A F O S R and DARPA. We wish to thank SSRL for excellent beam time and experimental assistance.

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