Structure Deformation in GdCOB Single Crystals Grown by the Czochralski Method Andrzej Klos,*,† Jarosław Z. Domagala,‡ Andrzej Bajor,† and Anna Pajaczkowska† Institute of Electronic Materials Technology, 01-919 Warszawa, Poland, and Institute of Physics, Polish Academy of Sciences, 02-608 Warszawa, Poland
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 9 3253–3256
ReceiVed December 13, 2007; ReVised Manuscript ReceiVed May 19, 2008
ABSTRACT: Gadolinium calcium oxide borate GdCa4O(BO3)3 melts congruently, and single crystals with diameters of up to 35 mm and lengths of up to 70 mm were grown by the Czochralski method. Deformation of the crystal structure was observed. X-ray topography and conoscopic and polariscopic investigations revealed the core situated in the central part of the crystal, which was not visible with the naked eye. The relative (b/bav) difference in the lattice parameter b in the core region compared with the neighboring crystal areas was evaluated to be on the order of 9.6 × 10-4. Outside of the core region the mean value of the lattice parameter is constant, while fluctuations in spatial distribution of the Bragg angle were discovered. The effect of growth instabilities causes a certain deformation of the growth plane (interface). However, it did not disturb optical properties, and was quite a good crystalline area to yield a considerable second harmonic (532 nm) power in laser experiments. Introduction Self-frequency doubling (SFD) materials seem to be very attractive for generating wavelengths in a broadband spectrum from UV to the IR. These materials also can be used for color displays, high density optical data storage, laser printing, biotechnology, eye-safe generation, and other applications.1–3 Currently, the most interesting SFD properties have been reported for YAl3(BO3)4 (YAB), GdCa4O(BO3)3 (GdCOB), and YCa4O(BO3)3 (YCOB) crystals doped with Nd3+ and Yb3+ ions.4–6 The reason for this is that the planar borate groups (BO3)3- in these crystals contain a conjugated π-orbital system that is favorable for second order susceptibilities, although the absorption edges of the planar groups lie within the narrow wavelength range of 190-200 nm.7 GdCOB crystallizes in the space group Cm, the monoclinic and noncentrosymmetric structure. The unit cell constants are a ) 0.8104, b ) 1.603, c ) 0.355 nm and β ) 101.25°.8 For many years it is well-known that one of the crucial factors governing the efficiency of the second and higher harmonics is the optical homogeneity of crystals. A considerable spread distribution of the principal refractive indices in the sample area will degrade this efficiency, and can result in inhomogeneous distribution of the intensity of the harmonic. Therefore, it is very important to grow good quality optical crystals. GdCOB melts congruently, and, therefore, it is possible to grow high-quality large single crystals by the Czochralski method. However, some experiments (spectroscopic, polariscopic, conoscopic, X-ray) performed in our laboratory revealed a core that was situated in the center of crystals, while outside the core several other defects like strains, striations, and dislocations, could be also observed. This core was not visible with the naked eye but was evidenced in X-ray topographs.9 However, sometimes it is not so easy to determine, especially in the presence of striations, whether an X-ray topograph shows a core boundary, or rather a striation itself (Figure 1).16 In this paper it was evidenced that the core region is quite suitable for the generation of the second harmonic. However, * To whom correspondence should be addressed. E-mail: Andrzej.Klos@ itme.edu.pl. † Institute of Electronic Materials Technology. ‡ Institute of Physics, Polish Academy of Sciences.
Figure 1. Typical X-ray transmission topograph of GdCOB wafer cut perpendicularly to the growth direction. The topograph shows striations and the core boundary which looks like a striation itself. Fortunately, the core is also clearly visible in the polarized, convergent beam of light (optical conoscope). In the core itself no other striations are visible.
since its discovery, the nature of this core, as well as some of its physical properties, remained uncovered. Therefore, the aim of this paper was to perform more detailed studies on the nature of the core region in GdCOB crystals. Apart from conoscopic, polariscopic, and X-ray projection topography, we have especially used rocking curve imaging (RCI) techniques based on high-resolution diffraction methods. Detailed results of our investigations which are presented here have shown a certain increase in the lattice parameters, as well as in the value of natural birefringence in the core region. Experimental Procedures Crystal Growth. The crystals were grown by the Czochralski method in nitrogen atmosphere in the Cyberstar Oxypuller 03-05 device using an iridium crucible (either of 60 or 100 mm diameter) and iridium afterheater. The GdCOB compound was prepared from the stoichio-
10.1021/cg7012232 CCC: $40.75 2008 American Chemical Society Published on Web 07/23/2008
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Figure 2. Single crystal of GdCa4O(BO3)3. Figure 5. Map of the Bragg angle along GdCOB crystal diameter, (0 5 0) reflection.
Figure 3. Core in the tail part of GdCOB crystal evidenced by the optical conoscope (He-Ne laser).
Figure 6. Map of the ω-X-ray beam incident angle, along the GdCOB crystal diameter.
Figure 4. Core evidenced by spectropolarimetric measurements in GdCOB sample cut parallel to the growth direction. metric composition of high purity oxides of Gd2O3, B2O3, and CaCO3. Prior to pulling of the crystals, the powders were synthesized by the solid-state reaction at 1200 °C for 24 h. The crystals were usually grown using an automated, computer-monitored system, based on controlling two crucial parameters, namely, their weight and diameter. However, sometimes, the automated system was switched off, and the control parameters were inserted by the operator from the computer’s keyboard. The pulling and rotation rates were adjusted between 0.8-1.2 mm/h and 10-40 rpm, respectively. Temperature gradient was adjusted to 10 °C/mm just above the surface of the melt. Crystals up to 70 mm in length and 35 mm in diameter were grown in the b () direction (Figure 2). The crystals which were obtained using the weight-and-diameter control system have shown periodical, small fluctuations in diameter. The crystals were either colorless (undoped) or had a specific color
Figure 7. Intensity of the recorded (diffracted) beam for fixed values of ω and θ in region A. associated with the doping ions. In neither case one could observe macroscopic defects or the core itself with the naked eye in our crystals. Experimental Methods. Optical conoscopic images were obtained using the convergent beam of light, for example, refs 10 and 11 from the He-Ne laser. Birefringence dispersion parameters were calculated from the spectropolarimetric measurements. This technique and adequate formulas have been described in details in ref 12. High-resolution X-ray RCI technique13 was used for mapping the lattice deformation in and outside the core region. Measurements were made using a Philips X’Pert diffractometer with parabolic X-ray mirror. Ge 4-fold monochromator, two orthogonal slits (the size of the vertical
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one can be changed up to 20 µm), 3-fold Ge analyzer as well as CuKR1 radiation source were used in these experiments. Second harmonic (SH - 532 nm) was generated using an YAG:Nd (1064 nm) pulsed laser. We did not try to achieve high conversion efficiency in this experiment. Therefore, the laser beam (4 mm in diameter) was not focused onto the samples with an additional lens, which is typical in other experiments. However, instead of achieving efficiencies on the order of tens of a percent, the conversion efficiency in our experiment was roughly only 1%. Fortunately, however, we could thus omit parasitic effects, like heating of the sample local volume by the laser beam. This local heating would greatly disturb our experiment, and could falsify the measurement itself, that is, one could be mistaken about whether the SH power in the core is smaller or greater than in the neighboring regions. The excessive heat changes the principal refractive indices, and even a small change has a major influence on the phase matching angle (PMA), for example, ref 14. Samples were oriented according to the PMA calculated for GdCOB in ref 4.
Results and Discussion From Figure 2 it is evident that the crystals were grown with a convex front of crystallization. This can be explained by large viscosity of the melt, and, therefore, a small value of the Reynolds number. The convex front of crystallization, associated with faceting, is one of the primary reasons for formation of similar core, for example, in Y3Al5O12 (YAG) single crystals.15 The core is always well seen in conoscopic images (e.g., Figure 3). The convergent conoscopic beam was parallel to the crystallographic direction. In this figure one can see an abrupt change in the value of natural birefringence at the core’s boundary, evidenced by a change in the density of the isochromatic fringes. Outside the core, the isochromatics become “thicker” and are separated by a larger distance from each other. The core spreads along the axis of the crystal. This can be evidenced for example by spectropolarimetric measurements shown in Figure 4. In this figure, unlike the others presented in this paper, the birefringence dispersion parameter is obtained for sample cut parallel to the growth direction. The core region, associated with changes of the birefringence, is easily visible in the central part of the sample. However, from this figure it is also evident that the core is not straight along the entire crystal axis. The nature of this core was also investigated by high resolution X-ray techniques. That allows us to obtain quantitative information on crystallographic misorientations and lattice quality on large areas of the sample associated with high spatial resolution. By this method one could obtain maps of diffraction parameters along the diameter of -oriented samples. In Figure 5 an exemplary map of the twice Bragg angle 2θ (vs distance from the center of the sample - x) is shown. Darker regions correspond to larger intensities of the diffracted beam recorded by the detector. One can see that in the central part of the sample, that is, for -2 mm < x < 2 mm, this dependence is a (quasi)parabolic function with minimum located at x ) -0.5 mm. This means that in the central part of the sample the lattice parameter b reaches a maximum, while at the core boundary it reaches a minimum. From the Bragg law, one can obtain adequate values for the parameter b: 16.02812 Å at the center and 16.02710 Å at the core boundary. By differentiating the Bragg formula one can obtain deformation of the lattice parameter b:
∆b 1 )∆θ b tan(θ)
(1)
Considering here that the average value of b, that is, bav ) 16.027, an increase in the ∆b/bav, estimated to be equal to
Figure 8. Spatial maps of Bragg and ω angles along the crystal diameter, (0 16 0) reflection.
approximately 9.6 × 10-4, can be seen in the core itself compared with the neighboring crystal areas. This can be explained by stoichiometry variations in the core region, since small differences in the ratio of Gd and Ca were also observed.16 One can conclude that the reason for this effect might be supercooling of the melt. Changes in the lattice parameter can be the reason for lattice plane deformation. This can be seen on the RCI map in Figure 6 (ω is angle of the beam incident on the sample), where the rocking curve in triple axis mode was collected point by point along the x-axis. Three different regions correspond very well to these shown in Figure 5. In the regions marked as A and C, the values of the angle ω are constant, while in region B the angle ω increases linearly with x. This means that in the core region the reflected planes are convex. The radius of curvature R can be calculated from the formula (standard software X’Pert Epitaxy and Smoothing v. 4.0):
R ) ∆x · 180 ⁄ (∆ω*π)
(2)
Using adequate data in Figure 6, for ∆x ) 3.3 mm one can calculate that the value of ∆ω ) 0.0068° in the region B, and, hence, from the above formula R equals 27.8 m. Besides, apart of the changes in ω themselves, in Figure 6 one can see periodic fluctuations in the spatial distribution of the ω angle out of the core region. This is even more pronounced in Figure 7 in which the graph of intensity of the recorded beam in the ω mode in region A is shown. These fluctuations correspond to rings which are observed in the X-ray Lang projection topography, and the period of which is about 0.5 mm. The reason for such fluctuations might be waving of the (010) plane. This can be also observed on spatial maps of the Bragg and ω angles in Figure 8. One can see that maximum and minimum values in the both maps perfectly match. Moreover, the Bragg angles maximum lie on the straight line, and it means that the mean value of the unit cell parameter b is constant outside the core region. In Figure 9 one can see spread distribution of the SH power along diameters of 15 GdCOB samples cut at a certain (PMA)
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which they are constantly observed. A high value of the Prandtl number, resulting from high viscosity of the melt, means that the heat transport in the melt is strongly influenced by mass flows in natural and forced convection. Any disorder of these flows change conditions of crystallization, and this can lead to perturbation of the growth process, including formation of striations and the core itself. It was also found that the core in GdCOB crystals did not disturb optical properties, and was quite a good crystalline area to yield a considerable second harmonic (532 nm) power in laser experiments. Figure 9. SH power (in arbitrary units) displayed for the left, center (the core), and right parts of samples cut from GdCOB crystals doped with either Nd or Yb ions. Figure marked with the dotted line ( · · · ) is for the sample with the largest SH power in the core, that marked with the dashed line (---) is for the sample with the lowest SH power in the core, while that marked with the solid line (s) is an average calculated for 15 measured samples, respectively.
angle to the growth direction. The crystals differ in Nd or Yb concentration. No considerable difference in SH power between the core and the neighboring regions was observed. Summary and Conclusions By conoscopic and X-ray experiments it was discovered that many GdCOB crystals are grown with a core along their pulling axes. An increase in the lattice parameter b compared with its average value bav in the core region compared with the neighboring crystal areas was evaluated to be on the order of 9.6 × 10-4. This is equivalent to a certain strain in the crystal lattice, as well as to a certain curvature at the (010) plane. Outside of the core region the mean value of the lattice parameter is constant, while fluctuations in spatial distribution of the Bragg angle were discovered. This can be attributed to segregation of crystal components during the growth process. Investigation of several crystals has shown that instabilities of the growth process at the central part of crystals is a typical case here, while crystals growing without the core should be treated as an unique case, and a matter of luck or coincidence. Changes in crystal sizes, crucible diameter, and other parameters of the growth process had no practical influence on crystal quality. The effect of growth instabilities causes a certain deformation of the growth plane (interface); however, facets are not observed in these crystals in contrast to the YAGs in
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