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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 3, Iss. 2 — Feb. 1, 2013
  • pp: 126–142
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A micro-Raman spectroscopic investigation of He+-irradiation damage in LiNbO3

Hsu-Cheng Huang, Jerry I. Dadap, Ophir Gaathon, Irving P. Herman, Richard M. Osgood, Jr., Sasha Bakhru, and Hassaram Bakhru  »View Author Affiliations


Optical Materials Express, Vol. 3, Issue 2, pp. 126-142 (2013)
http://dx.doi.org/10.1364/OME.3.000126


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Abstract

Imaging micro-Raman spectroscopy is used to investigate the materials physics of radiation damage in congruent LiNbO3 as a result of high-energy (~MeV) He+ irradiation. This study uses a scanning confocal microscope for high-resolution three-dimensional micro-Raman imaging along with reflection optical microscopy (OM), and scanning electron microscopy (SEM). The tight optical excitation beam in the Raman system allows spatial mapping of the Raman spectra both laterally and normal to the irradiation axis with ≤1 μm resolution. Point defects and compositional changes after irradiation and surface deformation including blistering and microstress are observed in the stopping region. We demonstrate that the probed area of the damaged region is effectively “expanded” by a beveled geometry, formed through off-angle polishing of a crystal facet; this technique enables higher-resolution probing of the ion-induced changes in the Raman spectra and imaging of dislocation line defects that are otherwise inaccessible by conventional probing (depth and edge scan). Two-dimensional (2D) Raman imaging is also used to determine the defect uniformity across an irradiated sample and to examine the damage on a sample with patterned implantation. The effects of different He+ doses and energies, together with post-irradiation treatments such as annealing, are also discussed.

© 2013 OSA

1. Introduction

Increasingly, complex oxide crystals and epitaxial thin films, with their remarkable physical properties [1

1. L. Arizmendi, “Photonic applications of lithium niobate crystals,” Phys. Status Solidi A 201(2), 253–283 (2004). [CrossRef]

], are of interest both for studies in basic condensed matter physics and for advanced microdevices such as high-performance acoustic and photonic applications. These oxide crystals, including, for example, lithium niobate (LiNbO3), strontium titanate and its alloys, yttrium iron or aluminum garnet, and lanthanum aluminate, exhibit a wide variety of functionalities. In addition, the fabrication of devices from these crystals often involves their irradiation by energetic particles, including light ions. For example, He ion implantation is an important step in optical-waveguide [2

2. J. Rams, J. Olivares, P. J. Chandler, and P. D. Townsend, “Mode gaps in the refractive index properties of low-dose ion-implanted LiNbO3 waveguides,” J. Appl. Phys. 87(7), 3199–3202 (2000). [CrossRef]

] fabrication and thin-film exfoliation methods [3

3. M. Levy, R. M. Osgood, R. Liu, L. E. Cross, G. S. Cargill, A. Kumar, and H. Bakhru, “Fabrication of single-crystal lithium niobate films by crystal ion slicing,” Appl. Phys. Lett. 73(16), 2293–2295 (1998). [CrossRef]

]. Exposure to ion bombardment is also a consideration in the performance of practical applications of devices, such as surface acoustical wave (SAW) and photonic devices, in extreme environments. Because of this widespread interest, it is crucial to achieve a clear and specific understanding of the chemical and structural response of complex oxides to ion irradiation.

Damage due to high-energy-ion exposure can be analyzed using well-developed ion-beam-probing instrumentation. This includes Rutherford backscattering spectrometry (RBS) [4

4. A. Kling, M. F. da Silva, J. C. Soares, P. F. P. Fichtner, L. Amaral, and F. Zawislak, “Defect evolution and characterization in He-implanted LiNbO3,” Nucl. Instrum. Meth. B 175–177(0), 394–397 (2001). [CrossRef]

], nuclear reaction analysis (NRA) [5

5. R. M. Roth, D. Djukic, Y. S. Lee, R. M. Osgood, S. Bakhru, B. Laulicht, K. Dunn, H. Bakhru, L. Wu, and M. Huang, “Compositional and structural changes in LiNbO3 following deep He+ ion implantation for film exfoliation,” Appl. Phys. Lett. 89(11), 112906 (2006). [CrossRef]

], and particle induced X-ray emission (PIXE) [6

6. T. Volk and M. Wohlecke, Lithium Niobate: Defects, Photorefraction and Ferroelectric Switching (Springer-Verlag, Berlin, Heidelberg, 2008).

]. These methods have been used extensively and successfully for material characterization of radiation damage, including that in complex oxides; however, these methods require extensive use of accelerator-based-beam-line systems and in-vacuum environment for experimentation. Confocal micro-Raman spectroscopy can provide an alternate approach to probing energetic ion damage and material changes; it provides a direct approach for sampling over the set of crystal vibrational normal modes and thus is sensitive to crystallinity and composition. This technique is laboratory based and can be operated at ambient atmospheric conditions with different sample configurations. In addition, the use of a tightly focused optical excitation beam, with computer control and data taking, allows mapping of the Raman spectra both laterally and normal to the irradiation axis with 1-micrometer spatial resolution. Its utility has been reported for determining the processing-induced materials properties of several complex oxides [7

7. J. E. Spanier, M. Levy, I. P. Herman, R. M. Osgood, and A. S. Bhalla, “Single-crystal, mesoscopic films of lead zinc niobate-lead titanate: Formation and micro-Raman analysis,” Appl. Phys. Lett. 79(10), 1510–1512 (2001). [CrossRef]

9

9. S. Banerjee, D.-I. Kim, R. D. Robinson, I. P. Herman, Y. Mao, and S. S. Wong, “Observation of Fano asymmetry in Raman spectra of SrTiO3 and CaxSr1-xTiO3 perovskite nanocubes,” Appl. Phys. Lett. 89(22), 223130 (2006). [CrossRef]

] including studying the influences of stresses and stoichiometry in lead- and barium-based ABO3 perovskites [10

10. P. S. Dobal and R. S. Katiyar, “Studies on ferroelectric perovskites and Bi-layered compounds using micro-Raman spectroscopy,” J. Raman Spectrosc. 33(6), 405–423 (2002). [CrossRef]

] and other materials such as diamond [11

11. D. N. Jamieson, S. Prawer, K. W. Nugent, and S. P. Dooley, “Cross-sectional Raman microscopy of MeV implanted diamond,” Nucl. Instrum. Meth. B 106(1–4), 641–645 (1995). [CrossRef]

13

13. I. De Wolf, “Micro-Raman spectroscopy to study local mechanical stress in silicon integrated circuits,” Semicond. Sci. Technol. 11(2), 139–154 (1996). [CrossRef]

]. In addition, Kostritskii and Moretti [14

14. S. M. Kostritskii and P. Moretti, “Micro-Raman study of defect structure and phonon spectrum of He-implanted LiNbO3 waveguides,” Phys. Status Solidi C 1(11), 3126–3129 (2004). [CrossRef]

] have done pioneering work on ion damage in He-implanted LiNbO3 and identified important damage effects in this material. However, despite these insights important questions remain.

In this paper, we use imaging micro-Raman spectroscopy measurements to readily analyze and characterize materials properties of an important complex oxide, i.e. LiNbO3, after high-energy-ion irradiation by He+. Thus, we utilize this method to measure local compositional changes as well the depth and depth distribution of the implanted ions. In addition, we determine the spatial resolution, crystallinity, and compositional information gathered based on three different and complementary incident optical beam/sample geometries for the previously irradiated crystal, viz depth profiling into the top surface of the crystal, depth profiling based on scanning laterally along a crystal edge facet, and higher-spatial-resolution depth scanning using a small-angle-beveled edge. We also use 2D imaging based on computer-controlled scanning and data processing to image the stopping region and spatial variations of damage adjacent to patterned regions, via the intensities of allowed and forbidden modes. The modal intensities at the boundary of irradiation and the implantation uniformity are also discussed. The utility of Raman scattering to optimize post-irradiation processes, such as annealing, is also shown.

2. Experimental

Congruent Z-cut LiNbO3 wafers (Crystal Technology) were diced into 1 cm2-area samples and irradiated by He+ ions along the crystalline Z-axis with doses ranging from 1012 to 5 × 1016 cm−2 at 1.2 - 3.8 MeV energies. During the irradiation, samples were tilted at 7° from the Z axis to prevent channeling and the irradiating beam was raster scanned to achieve a uniform dose. During irradiation, the samples were water cooled to avoid overheating. Patterned irradiation was accomplished by placing a 0.5 mm-thick metal sheet with circular openings (~500 μm in diameter) on the top surface of the sample. The mask was attached to the sample at the mask periphery with silver paste with no hard contact in the center. In addition, after the irradiation, selected samples (not patterned) were annealed in a furnace at temperatures ranging from 250 - 600°C under laboratory-ambient pressure conditions. The annealing temperature was carefully adjusted to temperatures less than 800°C such that the loss of Li and oxygen were to a large extent minimized [15

15. B.-U. Chen and A. C. Pastor, “Elimination of Li2O out-diffusion waveguide in LiNbO3 and LiTaO3,” Appl. Phys. Lett. 30(11), 570–571 (1977). [CrossRef]

]. In addition to micro-Raman spectroscopy, which is the central technique for this work, the irradiated samples were investigated with optical microscopy (OM) and scanning electron microscopy (SEM).

For Raman probing, a diode-laser-based laser light source, 532 nm wavelength and 2.7 mW power, was used as the excitation source, along with a computer controlled X-Y-Z stage. The beam was focused by a 100 × microscope objective, NA = 0.85 using a confocal pinhole alignment to a spot size of 1 μm. The experiments were performed in a backscattering geometry with three different optical configurations; these are shown in Fig. 1
Fig. 1 Schematic of the three experimental orientations for our micro-Raman probing beam: (a) depth scan from the surface into the bulk along the Z-axis, (b) edge scan along the X-axis and (c) bevel scan along the polished side (beveled plane).
.

The first configuration was direct, top-down probing (see Fig. 1(a)), with the incident light parallel to the Z-axis and the focus scanned along the Z-axis (depth scan); in this configuration the selection rules allowed backscattering of the E(TO) (152, 236, 263, 322, 365, 432, and 581 cm−1) and A1(LO) (274, 331, 432 and 875 cm−1) phonon modes. Note that lithium niobate is in the space group R3c and has a distorted Perovskite-type structure. This structure should yield 13 phonon peaks for our backscattering orientation. However, for our experiments only 10 peaks are apparent, an effect explained earlier [16

16. J. G. Scott, S. Mailis, C. L. Sones, and R. W. Eason, “A Raman study of single-crystal congruent lithium niobate following electric-field repoling,” Appl. Phys., A Mater. Sci. Process. 79(3), 691–696 (2004). [CrossRef]

] as due to nonstoichiometric intrinsic defects in congruent LiNbO3. The Rayleigh length was estimated to be ~2 μm. Two-dimensional imaging was also performed with this optical configuration to study the degree of uniformity of the irradiation and, on samples with the patterned implantation to examine the transition from the irradiated to unirradiated regions. Using this patterned sample, the beam was scanned over a square region containing unmasked circular regions that were irradiated. Different step resolutions were used to probe the sample, including a 640 μm × 640 μm array using with 5-μm steps, a 16 μm × 16 μm with 0.4 or 0.2 μm steps.

The second optical configuration probed (see Fig. 1(b)) along the X-axis and into an edge facet with the light perpendicular to the Z-axis and scanned along the Z-axis (edge scan); for this orientation, backscattering of the E(TO) and A1(TO) (254, 276, 332 and 631 cm−1) phonon modes is allowed. Finally the third configuration (see Fig. 1(c)) allowed interrogation of the beveled sample edges (bevel scan). In this case, the two edges of the implanted samples were carefully angle-lapped to an optical finish so as to be aligned 5° from the Z-axis (XY plane); because of the small angle of the beveling, the polarization selection rules in this case were essentially the same as for the first configuration. The sample was polished until the defect region was fully exposed at the surface. Scanning was then carried out with the polished planes oriented parallel to the stage. The sketches in Fig. 1 show clearly the three scanning directions in our experiments. Scanning was done in increments of 0.2 μm until the beam focus crossed to >2-3 μm beyond the heavily damaged region.

Note that for the depth scan, i.e. Fig. 1(a), an optical correction factor must be applied to obtain the true depth. In particular, in our measurements, we directly measured the movement of the stage and then applied a correction factor due to refractive-index alteration of the beam focus in the crystal in order to measure the actual depth inside the sample. For LiNbO3, the refractive index at 532 nm, as given by Sellmeier equation [17

17. K. K. Wong, ed., Properties of Lithium Niobate (INSPEC, The Institution of Electrical Engineers, London, UK, 2002).

], of LiNbO3 is ~2.3, which means that, by a Gaussian ray-transfer matrix calculation, the actual probed depth was ~2.3 times larger than the measured position of the Raman microscope Z-stage.

3. Results and discussion

Our experiments presented below used micro-Raman spectroscopy following irradiation of crystalline LiNbO3 with ~MeV He+ ions to probe the location of structural and chemical changes. Our presentation is organized so as to examine the resulting material changes using each of the three scanning orientations discussed above and to consider each approach in a separate section.

3.1. Depth-dependent changes in irradiated samples (depth scan)

Figure 2(a)
Fig. 2 Comparison of the Raman spectra made on unirradiated (“virgin”) with depth scan, irradiated (“after implantation”), and post-irradiation annealed (“after annealing”) samples: (a) at the depth of the ion stopping region and (b) at the surface. Furnace annealing was carried out at 250°C for 30 min. In (a), a shoulder is apparent in spectra between 600 cm−1 and 750 cm−1.
shows the marked changes in the LiNbO3 Raman spectra using the configuration of Fig. 1(a), after irradiation over a range of Raman shifts from 100 to 1000 cm−1. The spectra are taken for a virgin sample, an irradiated surface, and an annealed sample; all were made with the probe focal point positioned ~10 µm beneath the surface, i.e. at the plane of the ion stopping range. The irradiation was carried out with 3.8 MeV He+ ions at a dose of 5 × 1016 cm−2. In addition, unless otherwise specified, all annealing was done at 250°C for 30 min following irradiation. Figure 2(b) shows measurements made under identical conditions, but with the focal spot positioned so that the crystal surface was sampled.

Prior studies in our group and others have shown that low-temperature annealing of an irradiated sample can alter the effects of radiation damage. Specifically, we have shown [20

20. A. Ofan, O. Gaathon, L. Zhang, K. Evans-Lutterodt, S. Bakhru, H. Bakhru, Y. Zhu, D. Welch, and R. M. Osgood, “Twinning and dislocation pileups in heavily implanted LiNbO3,” Phys. Rev. B 83(6), 064104 (2011). [CrossRef]

,25

25. A. Ofan, L. Zhang, O. Gaathon, S. Bakhru, H. Bakhru, Y. Zhu, D. Welch, and R. M. Osgood, “Spherical solid He nanometer bubbles in an anisotropic complex oxide,” Phys. Rev. B 82(10), 104113 (2010). [CrossRef]

] that during He+ irradiation, the high-density of point defects and He atoms inserted into the crystal lattice lead to the formation of clustering of defects, including, at high dose, He bubble formation. In fact, our earlier experiments showed that there was a threshold Dt for which the concentration of the generated vacancy-interstitial pairs are high enough to form a defect network. In addition, these results indicated that the He inclusions become mobile at elevated temperatures. In addition, TEM imaging studies show that at higher temperature (350°C), the inclusions aggregate together to form larger defects. Avrahami et al. [26

26. E. Zolotoyabko, Y. Avrahami, W. Sauer, T. H. Metzger, and J. Peisl, “Strain profiles in He-implanted waveguide layers of LiNbO3 crystals,” Mater. Lett. 27(1–2), 17–20 (1996). [CrossRef]

,27

27. Y. Avrahami and E. Zolotoyabko, “Structural modifications in He-implanted waveguide layers of LiNbO3,” Nucl. Instrum. Meth. B 120(1–4), 84–87 (1996). [CrossRef]

] used high-resolution X-ray diffraction to reveal that He-ion irradiation results in lattice swelling. Thus for a dose of ~1016 cm−2, the profile of induced strain as a function of depth was found to approach a step-like shape with the strain increasing abruptly at the ion range; in addition, annealing at temperatures above 200°C was found to lead to partial recovery of the crystal lattice.

We have used micro-Raman spectroscopy to find a dependence on dose and annealing conditions, which is consistent with the above experiments of annealing in irradiated crystals. In particular, after annealing of an irradiated crystal, our Raman microprobe was used to study the 631 cm−1-mode signal versus dose and annealing temperature (see Figs. 6(a)
Fig. 6 (a) The intensity of the 631 cm−1 feature with depth scanning vs. irradiation dose of 1.5 MeV He+ ions. A signal is seen only when the dose is >1016 cm−2. (b) Effect of annealing for 30 minutes at different temperatures on the peak intensity of 631 cm−1 following irradiation at 3.8 MeV to a total dose of 5 × 1016 cm−2. This change is due to annealing-induced recovery of damage. The inset shows an Arrhenius plot of the peak intensity irradiated sample relative to that for the virgin sample for T 3 250°C, which gives an activation energy of 0.32 ± 0.07 eV.
and 6(b)); the duration of the annealing process was 30 min for all the experiments. Our experiments showed that signals from the broadband shoulder are seen only when the dose is >1016 cm−2 (Fig. 6(a)). The existence of this threshold dose is attributed to a nonlinear dependence on the local concentration of ion-induced defects. Insight into this threshold behavior can be obtained via earlier work by Schrempel et al. [28

28. F. Schrempel, T. Gischkat, H. Hartung, E.-B. Kley, and W. Wesch, “Ion beam enhanced etching of LiNbO3,” Nucl. Instrum. Meth. B 250(1–2), 164–168 (2006). [CrossRef]

]. In these experiments Rutherford backscattering spectrometry (RBS) at 1.4 MeV He+-ions was used to show that for doses greater than ~1016 cm−2, the relative defect concentration increased abruptly. This increase was explained by the formation of heavily damaged defect clusters. Note TRIM simulation shows that a 1.5 MeV He+ irradiation to a dose of 1016 cm−2 causes a total vacancy concentration of 1022 cm−3, or ~0.25 dpa (displacement per atom), thus indicating a very high degree of damage in the implantation area at this ion fluence.

In addition, 2D Raman mapping was utilized to image the damage distribution in a sample with patterned irradiation. To carry out patterning, a 0.5 mm-thick metal sheet with circular-grid openings was placed on the sample top surface so that only the circle regions (with diameters of ~500 μm) were exposed to the He+ irradiation (3.8 MeV energy and 5 × 1016 cm−2 dose). Figure 8(a)
Fig. 8 2D Raman images showing the intensity variation of two Raman modes after irradiation of a masked sample. The patterning of the ion beam (3.8 MeV He+, 5 × 1016 cm−2 dose) utilized a shadow mask consisting of a metal circular grid affixed to the sample. The optical image (a) labels the regions being with or without irradiation (regions I and II, respectively). The inset box indicates the region where the Raman imaging was carried out. In (b) and (c), the Raman maps were analyzed using an allowed (875 cm−1) and forbidden mode (631 cm−1). In the irradiated regions the signals of active modes decrease while the forbidden modes are “turned on”, respectively, due to irradiation-induced crystal disorder. In (d) and (e), a finer and smaller scan was performed and the results show that the patterned implantation process was uniform.
shows an optical image of the irradiation pattern, with the Roman numerals (I) and (II) indicating the regions that are irradiated and not irradiated, respectively. Figures 8(b) and 8(c) are examples of 2D Raman image maps of the allowed (875 cm−1) and forbidden (631 cm−1) modes, taken at the depth of the stopping region, i.e. ~10 μm. As discussed previously, the crystallinity of LiNbO3 is damaged in the irradiated regions such that there is an intensity loss for active modes and a gain for the normally forbidden modes. In the unirradiated, masked regions, the crystal quality remains unaffected and thus only the active modes dominate. This behavior results in the high-contrast maps shown in the figures.

A second set of 2D images of the irradiated regions, denoted by (I) and having even finer resolution over a smaller scan distance, were taken at several locations to examine the uniformity of the implantation process. These results show high uniformity of radiation damage based upon the measured standard deviation of the signal intensity. For example, Figs. 8(d) and 8(e) show a set of typical Raman images from a square of 10 μm × 10 μm with a 0.4- μm-step resolution, which were collected using the 875 cm−1 active and the 631 cm−1 normally forbidden modes. It is clear that when the scan area is away from the implantation boundary, the signals from irradiated regions are uniform, with a standard deviation in the normalized intensity of ~0.010 for the 875 cm−1 mode and ~0.015 for the 631 cm−1mode, respectively. These results indicate that the patterned implantation process is in general uniform.

Figure 8 also shows that the shapes of the Raman images conform well to the overall irradiated pattern. Our Raman probe can also be used to examine the details of the boundary region at the edge of the masked regions. A higher resolution area mapping of a 16 μm × 16 μm square region was carried out. The Raman imaging using the 875 cm−1 and 631 cm−1 modes is displayed in Figs. 9(a)
Fig. 9 Fine spatial resolution of Raman mapping of a defect region in a patterned sample. Panels (a) and (b) show 2D scans of the active 875 cm−1 and normally forbidden 631 cm−1 modes, with scan step of 0.4 μm, respectively, while (c) is a line scan across the boundary, as indicated by the arrows in (a) and (b), with a scan step of 0.2 μm. From (c), it is clear that the signals of the two modes decrease/increase within a specific width of the boundary, denoted by (III) in (a) and (b), and stay uniform outside the transition regions, marked by (I) and (II). The data have been normalized with respect to their maximum values.
and 9(b); (c) is a line scan across the edge of the mask, as indicated by the arrows in (a) and (b). Region (I) is irradiated while region (II) is masked. In addition, we notice that in the region with smaller higher resolution, a transition region is apparent near the boundary, as denoted by (III). We attribute this transition region to the masking procedure for this patterned irradiation. The lack of full adhesion of the mask to the surface results in a gradation in the irradiation damage in the boundary region. In addition, in this region He+ scattering from the mask edge also contributes to a ~4% increase in the radius of the irradiated region compared to the radius of the mask opening.

3.2. Data obtained from probing across the edge facet (edge scan)

Radiation damage was also probed via scanning across an end facet, using the geometry shown in Fig. 1(b). A different set of selection rules applies for this optical configuration as compared with those used in the depth-scanning configuration. Figure 10
Fig. 10 Raman spectra obtained from an edge scan at three depths from the surface: in the near surface, at the ion stopping region, and deep in the bulk, where there is a negligible effect of irradiation (shown in the optical image to the right). The sample was irradiated by 3.8 MeV He+ with a dose of 5 × 1016 cm−2. Note the intensities of the active modes drop and the appearance of a shoulder in the 800 to 900 cm−1 region.
shows representative Raman spectra from the near-surface region, at the position of the ion stopping region and at a point deep within the virgin region of the crystal; the locations are specified in the optical image to the right of the spectra. Examining first the A1(TO4) 631 cm−1 mode, its peak intensity weakens and broadens as the scan probes close to the region of expected irradiation damage. Note that this mode shows some evidence of a red shift as the beam is scanned closer to the heavily damaged region; this shift is tentatively attributed to strain. From 800 to 900 cm−1, additional spectral features emerge when the beam interrogates the sample close to the stopping range. These features appear to originate from irradiation-induced activation of the A1(LO) mode at 875 cm−1, which is forbidden for the perfect crystal in this beam configuration. A contribution from the E(LO) forbidden mode at 880 cm−1 is also possible, although its effective Raman scattering cross section is much less than that of the A1(LO) 875 cm−1 mode [30

30. M. M. Sushchinskiy, ed., Inelastic Light Scattering in Crystals (Nova Science Publishers, 1987), Vol. 180, p. 81.

]. The observation of this band was previously briefly noted by Kostritskii et al. [14

14. S. M. Kostritskii and P. Moretti, “Micro-Raman study of defect structure and phonon spectrum of He-implanted LiNbO3 waveguides,” Phys. Status Solidi C 1(11), 3126–3129 (2004). [CrossRef]

].

The change in the Raman signal with distance along the scan direction is seen in Fig. 11
Fig. 11 A plot of signal versus distance using scanned distance from the edge of the top surface for three modes. The sample was irradiated by 3.8 MeV, 5 × 1016 cm−2 He+ doses. For all three curves the maxima or minima of the intensities (relative to those in the virgin sample) occur at the position of the ion range, which is a depth of ~10 μm (shown in dashed line).
. In particular, the plot shows the peak intensity of the irradiated sample minus that of the unirradiated sample for the E(TO1) 152 cm−1 and A1(TO4) 631 cm−1 active modes and the A1(LO) 875 cm−1 forbidden mode versus distance. It is seen that irradiation results in the rise of the previously forbidden modes and a decrease in the signal of the active mode, with the maximum/minima of this effect occurring at the stopping range (~10 μm).

It is useful to compare the data obtained using this geometry with the results obtained by depth profiling from the top surface. For our data measured when scanning across the end facet, the spectral widths are narrower, due to the narrow width of the stopping region, i.e. ~0.5 μm, and the higher spatial resolution possible in this case. This higher spatial resolution is possible given the 0.5-1 μm spot size of laser as compared with the ~2 μm Rayleigh length, which determines spatial resolution for scanning along the Z direction. In Fig. 11, it is clear that the profiles have much longer “tails” toward the surface than is predicted from the negative skew [31

31. J. F. Ziegler, ed., Handbook of Ion Implantation Technology (Elsevier Science Publishers B.V. Netherlands, 1992), p. 13.

] (i.e. the most probable depth of He+ ions is greater than the mean depth) obtained from SRIM simulation. Thus for the data in Fig. 11, with irradiation conditions 3.8 MeV and 5 × 1016 cm−2 He+ dose, the calculated full width of the ion distribution is ~1 μm. Our data, however, show an effective width of ~3 μm due to this tail. Jamieson et al. [11

11. D. N. Jamieson, S. Prawer, K. W. Nugent, and S. P. Dooley, “Cross-sectional Raman microscopy of MeV implanted diamond,” Nucl. Instrum. Meth. B 106(1–4), 641–645 (1995). [CrossRef]

] also observed this same long-tail phenomenon in diamond. The origin of this apparent extension of the damaged region toward the surface that is far greater than that predicted from SRIM is unclear at present. Irradiation-induced strain could lead to this effect and measurements of shifts in the Raman spectra indicate that some strain is present. But the fact that the normally allowed mode (631cm−1) is strongly quenched would suggest that more than strain is present.

3.3. Increasing resolution by edge beveling (bevel scan)

Prior research on ion-irradiated LiNbO3 has shown that line defects are obtained after ion irradiation [20

20. A. Ofan, O. Gaathon, L. Zhang, K. Evans-Lutterodt, S. Bakhru, H. Bakhru, Y. Zhu, D. Welch, and R. M. Osgood, “Twinning and dislocation pileups in heavily implanted LiNbO3,” Phys. Rev. B 83(6), 064104 (2011). [CrossRef]

]. This research has shown that in addition to line defects, meandering line defects connecting the straight ones are also observed after mild thermal stressing. These line defects are distributed within a planar layer in the straggle range. In our experiments, these defects are readily seen in our thinned stopping regions. In the immediate region of the sample used to obtain Fig. 13(a), no obvious line defects were apparent; although they were observed in optical microscopy of the neighboring regions; see the inset image in Fig. 13(a). The stripe, emphasized by added dashed lines in the image, indicates the “spread-out” damaged region (~10 μm) on the beveled plane, while the red arrow line is the probe-beam scan direction. The average spacing of the curved lines is ~5 μm, as marked by the black arrows in the inset optical image. Again, note that the location and the direction of the scan were carefully chosen such that they did not cross obvious dislocation lines. In Fig. 13(a), besides the broad peak from the damaged region, there are four additional spatial peaks marked by green arrows. These peaks are most likely due to the buried meandering lines, as the spacing between the rightmost and leftmost two peaks corresponds well to the separation seen in the optical visible lines (i.e. ~5 μm). Thus, the arrows in the figure indicate where these meandering line defects are located. Notice that use of the Raman probe allows ready detection of defect lines, which are otherwise unobservable by visible light microscopy. Also due to the shallow-angle beveled edge, the spatial resolution and the structure of the line defects is enlarged.

One potential difficulty with the off-cut angle-polishing approach is that in a heavily irradiated sample there is possibly distorted edge surface region. The distortion in this narrow region is due to the blistering and micro-cracks, which are known at high irradiation doses. Thus Primak [33

33. W. Primak, “Expansion, crazing and exfoliation of lithium niobate on ion bombardment and comparison results for sapphire,” J. Appl. Phys. 43(12), 4927–4933 (1972). [CrossRef]

] pointed out that at a dose of ~1016 cm−2, surface deformation and destruction were observed on the stopping plane when using lower energy ions than in our case, i.e. 140 keV vs 3.8 MeV. In our experiments, when the dose is above 1 × 1016 cm−2, exfoliation occurs on the polished plane, while surface quality of the XY plane remains as seen in optical microscopy, see panels (a) and (c) in Fig. 14
Fig. 14 Optical imaging showing the effect of annealing at 250°C: (a), (b) are top views (XY plane [22]) while (c), (d) are planes of beveled region. Figures (a) and (c) were taken before annealing, while (b) and (d) were taken after annealing.
. Since this damage concentration is high, surface crazing readily appears in the stopping range upon polishing; this effect is seen clearly in (c). Presumably due to the stress in the damaged region, heating of the irradiated crystal can lead to additional damage. Thus panels (b) and (d) show that after further annealing additional micro-domains become apparent (see [22

22. A. Ridah, P. Bourson, M. D. Fontana, and G. Malovichko, “The composition dependence of the Raman spectrum and new assignment of the phonons in LiNbO3,” J. Phys. Condens. Matter 9(44), 9687–9693 (1997). [CrossRef]

]) in the top view (XY plane) of panel (b) and in addition, the incipient exfoliation in panel (d) has progressed noticeably.

4. Conclusions

In conclusion, we have used micro-Raman spectroscopy to diagnose and image damage in oxide crystals following high-energy ion irradiation and have demonstrated that it is a powerful and versatile approach. In particular, we have used vibrational Raman scattering on He+ irradiated LiNbO3 samples to observe the effects of local damage in both allowed and forbidden modes, local Li-atom depletion, and the spatial distribution of damage. We show, for example, there is a threshold dosage, for which the lattice distortion is apparent and that annealing is important for recovering the sample crystallinity.

Furthermore, we have found that complementary information can be obtained from three Raman micro-probe configurations and preparation methods, due to their different geometries, spatial resolution, and polarization sensitivities. Depth-dependent damage data can be obtained most readily and in a nondestructive manner by scanning along the Z-axis (depth scan). While this scan direction gives qualitative and useful information, it requires no additional sample preparation, and is very suitable for in situ or real time analysis of the degree of crystallinity during processing, it has limited spatial resolution. Scanning along a polished or cleaved edge is an alternate approach (edge scan), but, while it permits probing different modes, the narrow spatial dimensions on the edge compared to the laser-spot size limits spatial resolution. Scanning on the beveled plane (bevel scan) provides the best spatial resolution and information regarding dislocation line defects and surface morphology of the irradiated sample, but the sample preparation process is, of course, somewhat more complex and care must be taken to guard against changes in morphology in the thinned region of the sample edge during polishing. Finally, we have shown the utility of 2D imaging in probing the resolution of a patterned, irradiated sample and examining the uniformity of irradiation-induced effects and damage profiles and insight into the nature of the ion-induced local degradation of the crystal.

Acknowledgments

The authors gratefully acknowledge useful comments and discussions with Prof. Aron Pinczuk and Dr. YuMeng You and with Profs. John Kymissis and Dirk Englund. This work was supported by the Department of the Defense, Defense Threat Reduction Agency (DTRA) under HDTRA1-11-1-0022.

References and links

1.

L. Arizmendi, “Photonic applications of lithium niobate crystals,” Phys. Status Solidi A 201(2), 253–283 (2004). [CrossRef]

2.

J. Rams, J. Olivares, P. J. Chandler, and P. D. Townsend, “Mode gaps in the refractive index properties of low-dose ion-implanted LiNbO3 waveguides,” J. Appl. Phys. 87(7), 3199–3202 (2000). [CrossRef]

3.

M. Levy, R. M. Osgood, R. Liu, L. E. Cross, G. S. Cargill, A. Kumar, and H. Bakhru, “Fabrication of single-crystal lithium niobate films by crystal ion slicing,” Appl. Phys. Lett. 73(16), 2293–2295 (1998). [CrossRef]

4.

A. Kling, M. F. da Silva, J. C. Soares, P. F. P. Fichtner, L. Amaral, and F. Zawislak, “Defect evolution and characterization in He-implanted LiNbO3,” Nucl. Instrum. Meth. B 175–177(0), 394–397 (2001). [CrossRef]

5.

R. M. Roth, D. Djukic, Y. S. Lee, R. M. Osgood, S. Bakhru, B. Laulicht, K. Dunn, H. Bakhru, L. Wu, and M. Huang, “Compositional and structural changes in LiNbO3 following deep He+ ion implantation for film exfoliation,” Appl. Phys. Lett. 89(11), 112906 (2006). [CrossRef]

6.

T. Volk and M. Wohlecke, Lithium Niobate: Defects, Photorefraction and Ferroelectric Switching (Springer-Verlag, Berlin, Heidelberg, 2008).

7.

J. E. Spanier, M. Levy, I. P. Herman, R. M. Osgood, and A. S. Bhalla, “Single-crystal, mesoscopic films of lead zinc niobate-lead titanate: Formation and micro-Raman analysis,” Appl. Phys. Lett. 79(10), 1510–1512 (2001). [CrossRef]

8.

J. E. Spanier, R. Robinson, F. Zhang, S.-W. Chan, and I. P. Herman, “Size-dependent properties of CeO2-y nanoparticles as studied by Raman scattering,” Phys. Rev. B 64(24), 245407 (2001). [CrossRef]

9.

S. Banerjee, D.-I. Kim, R. D. Robinson, I. P. Herman, Y. Mao, and S. S. Wong, “Observation of Fano asymmetry in Raman spectra of SrTiO3 and CaxSr1-xTiO3 perovskite nanocubes,” Appl. Phys. Lett. 89(22), 223130 (2006). [CrossRef]

10.

P. S. Dobal and R. S. Katiyar, “Studies on ferroelectric perovskites and Bi-layered compounds using micro-Raman spectroscopy,” J. Raman Spectrosc. 33(6), 405–423 (2002). [CrossRef]

11.

D. N. Jamieson, S. Prawer, K. W. Nugent, and S. P. Dooley, “Cross-sectional Raman microscopy of MeV implanted diamond,” Nucl. Instrum. Meth. B 106(1–4), 641–645 (1995). [CrossRef]

12.

A. A. Balandin, S. Ghosh, W. Z. Bao, I. Calizo, D. Teweldebrhan, F. Miao, and C. N. Lau, “Superior thermal conductivity of single-layer graphene,” Nano Lett. 8(3), 902–907 (2008). [CrossRef] [PubMed]

13.

I. De Wolf, “Micro-Raman spectroscopy to study local mechanical stress in silicon integrated circuits,” Semicond. Sci. Technol. 11(2), 139–154 (1996). [CrossRef]

14.

S. M. Kostritskii and P. Moretti, “Micro-Raman study of defect structure and phonon spectrum of He-implanted LiNbO3 waveguides,” Phys. Status Solidi C 1(11), 3126–3129 (2004). [CrossRef]

15.

B.-U. Chen and A. C. Pastor, “Elimination of Li2O out-diffusion waveguide in LiNbO3 and LiTaO3,” Appl. Phys. Lett. 30(11), 570–571 (1977). [CrossRef]

16.

J. G. Scott, S. Mailis, C. L. Sones, and R. W. Eason, “A Raman study of single-crystal congruent lithium niobate following electric-field repoling,” Appl. Phys., A Mater. Sci. Process. 79(3), 691–696 (2004). [CrossRef]

17.

K. K. Wong, ed., Properties of Lithium Niobate (INSPEC, The Institution of Electrical Engineers, London, UK, 2002).

18.

G. R. Paz-Pujalt and D. D. Tuschel, “Depth profiling of proton exchanged LiNbO3 waveguides by micro-Raman spectroscopy,” Appl. Phys. Lett. 62(26), 3411–3413 (1993). [CrossRef]

19.

A. Ofan, O. Gaathon, L. Vanamurthy, S. Bakhru, H. Bakhru, K. Evans-Lutterodt, and R. M. Osgood, “Origin of highly spatially selective etching in deeply implanted complex oxides,” Appl. Phys. Lett. 93(18), 181906 (2008). [CrossRef]

20.

A. Ofan, O. Gaathon, L. Zhang, K. Evans-Lutterodt, S. Bakhru, H. Bakhru, Y. Zhu, D. Welch, and R. M. Osgood, “Twinning and dislocation pileups in heavily implanted LiNbO3,” Phys. Rev. B 83(6), 064104 (2011). [CrossRef]

21.

J. Ziegler, 2008, http://www.srim.org.

22.

A. Ridah, P. Bourson, M. D. Fontana, and G. Malovichko, “The composition dependence of the Raman spectrum and new assignment of the phonons in LiNbO3,” J. Phys. Condens. Matter 9(44), 9687–9693 (1997). [CrossRef]

23.

U. Schlarb, S. Klauer, M. Wesselmann, K. Betzler, and M. Wöhlecke, “Determination of the Li/Nb ratio in lithium niobate by means of birefringence and Raman measurements,” Appl. Phys., A Solids Surf. 56(4), 311–315 (1993). [CrossRef]

24.

P. Galinetto, M. Marinone, D. Grando, G. Samoggia, F. Caccavale, A. Morbiato, and M. Musolino, “Micro-Raman analysis on LiNbO3 substrates and surfaces: compositional homogeneity and effects of etching and polishing processes on structural properties,” Opt. Lasers Eng. 45(3), 380–384 (2007). [CrossRef]

25.

A. Ofan, L. Zhang, O. Gaathon, S. Bakhru, H. Bakhru, Y. Zhu, D. Welch, and R. M. Osgood, “Spherical solid He nanometer bubbles in an anisotropic complex oxide,” Phys. Rev. B 82(10), 104113 (2010). [CrossRef]

26.

E. Zolotoyabko, Y. Avrahami, W. Sauer, T. H. Metzger, and J. Peisl, “Strain profiles in He-implanted waveguide layers of LiNbO3 crystals,” Mater. Lett. 27(1–2), 17–20 (1996). [CrossRef]

27.

Y. Avrahami and E. Zolotoyabko, “Structural modifications in He-implanted waveguide layers of LiNbO3,” Nucl. Instrum. Meth. B 120(1–4), 84–87 (1996). [CrossRef]

28.

F. Schrempel, T. Gischkat, H. Hartung, E.-B. Kley, and W. Wesch, “Ion beam enhanced etching of LiNbO3,” Nucl. Instrum. Meth. B 250(1–2), 164–168 (2006). [CrossRef]

29.

Y. Kong, J. Xu, X. Chen, C. Zhang, W. Zhang, and G. Zhang, “Ilmenite-like stacking defect in nonstoichiometric lithium niobate crystals investigated by Raman scattering spectra,” J. Appl. Phys. 87(9), 4410–4414 (2000). [CrossRef]

30.

M. M. Sushchinskiy, ed., Inelastic Light Scattering in Crystals (Nova Science Publishers, 1987), Vol. 180, p. 81.

31.

J. F. Ziegler, ed., Handbook of Ion Implantation Technology (Elsevier Science Publishers B.V. Netherlands, 1992), p. 13.

32.

R. Srnanek, R. Kinder, B. Sciana, D. Radziewicz, D. S. McPhail, S. D. Littlewood, and I. Novotny, “Determination of doping profiles on bevelled GaAs structures by Raman spectroscopy,” Appl. Surf. Sci. 177(1–2), 139–145 (2001). [CrossRef]

33.

W. Primak, “Expansion, crazing and exfoliation of lithium niobate on ion bombardment and comparison results for sapphire,” J. Appl. Phys. 43(12), 4927–4933 (1972). [CrossRef]

OCIS Codes
(130.3730) Integrated optics : Lithium niobate
(160.4670) Materials : Optical materials
(300.6450) Spectroscopy : Spectroscopy, Raman
(310.3840) Thin films : Materials and process characterization

ToC Category:
Thin Films

History
Original Manuscript: November 26, 2012
Revised Manuscript: December 14, 2012
Manuscript Accepted: December 15, 2012
Published: December 21, 2012

Citation
Hsu-Cheng Huang, Jerry I. Dadap, Ophir Gaathon, Irving P. Herman, Richard M. Osgood, Sasha Bakhru, and Hassaram Bakhru, "A micro-Raman spectroscopic investigation of He+-irradiation damage in LiNbO3," Opt. Mater. Express 3, 126-142 (2013)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-3-2-126


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References

  1. L. Arizmendi, “Photonic applications of lithium niobate crystals,” Phys. Status Solidi A201(2), 253–283 (2004). [CrossRef]
  2. J. Rams, J. Olivares, P. J. Chandler, and P. D. Townsend, “Mode gaps in the refractive index properties of low-dose ion-implanted LiNbO3 waveguides,” J. Appl. Phys.87(7), 3199–3202 (2000). [CrossRef]
  3. M. Levy, R. M. Osgood, R. Liu, L. E. Cross, G. S. Cargill, A. Kumar, and H. Bakhru, “Fabrication of single-crystal lithium niobate films by crystal ion slicing,” Appl. Phys. Lett.73(16), 2293–2295 (1998). [CrossRef]
  4. A. Kling, M. F. da Silva, J. C. Soares, P. F. P. Fichtner, L. Amaral, and F. Zawislak, “Defect evolution and characterization in He-implanted LiNbO3,” Nucl. Instrum. Meth. B175–177(0), 394–397 (2001). [CrossRef]
  5. R. M. Roth, D. Djukic, Y. S. Lee, R. M. Osgood, S. Bakhru, B. Laulicht, K. Dunn, H. Bakhru, L. Wu, and M. Huang, “Compositional and structural changes in LiNbO3 following deep He+ ion implantation for film exfoliation,” Appl. Phys. Lett.89(11), 112906 (2006). [CrossRef]
  6. T. Volk and M. Wohlecke, Lithium Niobate: Defects, Photorefraction and Ferroelectric Switching (Springer-Verlag, Berlin, Heidelberg, 2008).
  7. J. E. Spanier, M. Levy, I. P. Herman, R. M. Osgood, and A. S. Bhalla, “Single-crystal, mesoscopic films of lead zinc niobate-lead titanate: Formation and micro-Raman analysis,” Appl. Phys. Lett.79(10), 1510–1512 (2001). [CrossRef]
  8. J. E. Spanier, R. Robinson, F. Zhang, S.-W. Chan, and I. P. Herman, “Size-dependent properties of CeO2-y nanoparticles as studied by Raman scattering,” Phys. Rev. B64(24), 245407 (2001). [CrossRef]
  9. S. Banerjee, D.-I. Kim, R. D. Robinson, I. P. Herman, Y. Mao, and S. S. Wong, “Observation of Fano asymmetry in Raman spectra of SrTiO3 and CaxSr1-xTiO3 perovskite nanocubes,” Appl. Phys. Lett.89(22), 223130 (2006). [CrossRef]
  10. P. S. Dobal and R. S. Katiyar, “Studies on ferroelectric perovskites and Bi-layered compounds using micro-Raman spectroscopy,” J. Raman Spectrosc.33(6), 405–423 (2002). [CrossRef]
  11. D. N. Jamieson, S. Prawer, K. W. Nugent, and S. P. Dooley, “Cross-sectional Raman microscopy of MeV implanted diamond,” Nucl. Instrum. Meth. B106(1–4), 641–645 (1995). [CrossRef]
  12. A. A. Balandin, S. Ghosh, W. Z. Bao, I. Calizo, D. Teweldebrhan, F. Miao, and C. N. Lau, “Superior thermal conductivity of single-layer graphene,” Nano Lett.8(3), 902–907 (2008). [CrossRef] [PubMed]
  13. I. De Wolf, “Micro-Raman spectroscopy to study local mechanical stress in silicon integrated circuits,” Semicond. Sci. Technol.11(2), 139–154 (1996). [CrossRef]
  14. S. M. Kostritskii and P. Moretti, “Micro-Raman study of defect structure and phonon spectrum of He-implanted LiNbO3 waveguides,” Phys. Status Solidi C1(11), 3126–3129 (2004). [CrossRef]
  15. B.-U. Chen and A. C. Pastor, “Elimination of Li2O out-diffusion waveguide in LiNbO3 and LiTaO3,” Appl. Phys. Lett.30(11), 570–571 (1977). [CrossRef]
  16. J. G. Scott, S. Mailis, C. L. Sones, and R. W. Eason, “A Raman study of single-crystal congruent lithium niobate following electric-field repoling,” Appl. Phys., A Mater. Sci. Process.79(3), 691–696 (2004). [CrossRef]
  17. K. K. Wong, ed., Properties of Lithium Niobate (INSPEC, The Institution of Electrical Engineers, London, UK, 2002).
  18. G. R. Paz-Pujalt and D. D. Tuschel, “Depth profiling of proton exchanged LiNbO3 waveguides by micro-Raman spectroscopy,” Appl. Phys. Lett.62(26), 3411–3413 (1993). [CrossRef]
  19. A. Ofan, O. Gaathon, L. Vanamurthy, S. Bakhru, H. Bakhru, K. Evans-Lutterodt, and R. M. Osgood, “Origin of highly spatially selective etching in deeply implanted complex oxides,” Appl. Phys. Lett.93(18), 181906 (2008). [CrossRef]
  20. A. Ofan, O. Gaathon, L. Zhang, K. Evans-Lutterodt, S. Bakhru, H. Bakhru, Y. Zhu, D. Welch, and R. M. Osgood, “Twinning and dislocation pileups in heavily implanted LiNbO3,” Phys. Rev. B83(6), 064104 (2011). [CrossRef]
  21. J. Ziegler, 2008, http://www.srim.org .
  22. A. Ridah, P. Bourson, M. D. Fontana, and G. Malovichko, “The composition dependence of the Raman spectrum and new assignment of the phonons in LiNbO3,” J. Phys. Condens. Matter9(44), 9687–9693 (1997). [CrossRef]
  23. U. Schlarb, S. Klauer, M. Wesselmann, K. Betzler, and M. Wöhlecke, “Determination of the Li/Nb ratio in lithium niobate by means of birefringence and Raman measurements,” Appl. Phys., A Solids Surf.56(4), 311–315 (1993). [CrossRef]
  24. P. Galinetto, M. Marinone, D. Grando, G. Samoggia, F. Caccavale, A. Morbiato, and M. Musolino, “Micro-Raman analysis on LiNbO3 substrates and surfaces: compositional homogeneity and effects of etching and polishing processes on structural properties,” Opt. Lasers Eng.45(3), 380–384 (2007). [CrossRef]
  25. A. Ofan, L. Zhang, O. Gaathon, S. Bakhru, H. Bakhru, Y. Zhu, D. Welch, and R. M. Osgood, “Spherical solid He nanometer bubbles in an anisotropic complex oxide,” Phys. Rev. B82(10), 104113 (2010). [CrossRef]
  26. E. Zolotoyabko, Y. Avrahami, W. Sauer, T. H. Metzger, and J. Peisl, “Strain profiles in He-implanted waveguide layers of LiNbO3 crystals,” Mater. Lett.27(1–2), 17–20 (1996). [CrossRef]
  27. Y. Avrahami and E. Zolotoyabko, “Structural modifications in He-implanted waveguide layers of LiNbO3,” Nucl. Instrum. Meth. B120(1–4), 84–87 (1996). [CrossRef]
  28. F. Schrempel, T. Gischkat, H. Hartung, E.-B. Kley, and W. Wesch, “Ion beam enhanced etching of LiNbO3,” Nucl. Instrum. Meth. B250(1–2), 164–168 (2006). [CrossRef]
  29. Y. Kong, J. Xu, X. Chen, C. Zhang, W. Zhang, and G. Zhang, “Ilmenite-like stacking defect in nonstoichiometric lithium niobate crystals investigated by Raman scattering spectra,” J. Appl. Phys.87(9), 4410–4414 (2000). [CrossRef]
  30. M. M. Sushchinskiy, ed., Inelastic Light Scattering in Crystals (Nova Science Publishers, 1987), Vol. 180, p. 81.
  31. J. F. Ziegler, ed., Handbook of Ion Implantation Technology (Elsevier Science Publishers B.V. Netherlands, 1992), p. 13.
  32. R. Srnanek, R. Kinder, B. Sciana, D. Radziewicz, D. S. McPhail, S. D. Littlewood, and I. Novotny, “Determination of doping profiles on bevelled GaAs structures by Raman spectroscopy,” Appl. Surf. Sci.177(1–2), 139–145 (2001). [CrossRef]
  33. W. Primak, “Expansion, crazing and exfoliation of lithium niobate on ion bombardment and comparison results for sapphire,” J. Appl. Phys.43(12), 4927–4933 (1972). [CrossRef]

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