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Conventional and synchrotron radiation back reflection topography of GdCa4O(BO3)3 crystals
Journal of Alloys and Compounds 382 (2004) 153–159
Conventional and synchrotron radiation back reflection topography of GdCa4 O(BO3)3 crystals M. Lefeld-Sosnowska a,∗ , E. Olszy´nska a,b , W. Wierzchowski b , K. Wieteska c , W. Graeff d , A. Paj˛aczkowska b , A. Kłos b a
Institute of Experimental Physics, University of Warsaw, Ho˙za 69, 00-681 Warsaw, Poland b Institute of Electronic Materials Technology, Wólczyñska 133, 01-919 Warsaw, Poland c Institute of Atomic Energy, 05-400 Otwock, Swierk, ´ Poland d HASYLAB at DESY, Notkestr. 85, D-22603 Hamburg, Germany
Received 15 September 2003; received in revised form 29 February 2004; accepted 5 March 2004
Abstract The investigations of GdCa4 O(BO3 )3 single crystals grown by the Czochralski technique were performed by conventional and synchrotron radiation (SR) monochromatic wave topography mainly in back reflection geometry. It was found that back reflection topography is effectively able to reveal the dislocation structure even in thin samples. Different distribution of defects was observed in samples cut off from the top, middle and end part of the “as-grown” crystal. SR monochromatic wave topographs taken for two flanks of the rocking curve show the characteristic inversion of diffraction contrast. Both monochromatic wave and projection Lang topographs did not reveal any segregation fringes proving high homogeneity of the chemical composition of the examined crystal. © 2004 Elsevier B.V. All rights reserved. Keywords: Insulators; Crystal growth; Dislocations; X-ray diffraction; Synchrotron radiation
1. Introduction Highly efficient visible and UV solid-state laser sources are of great importance for many applications. The emission in the range of visible light can be realised by frequency doubling of diode pumped IR solid-state lasers, using non-linear optical crystals. For this reason in the last decade one can observe a growing interest in non-linear optical materials. The most promising way for the realisation a visible laser light source is the self-frequency conversion (self-frequency doubling SFD or sum-frequency missing SFM) of solid-state lasers, in which both laser action and non-linear conversion is realised in one crystal. Among many crystals used for SFD purposes ([1,2,3] and references cited herein), the family of rare earth calcium oxyborates ReCa4 O(BO3 )3 (with Re = Ln3+ , Nd3+ , Sm3+ , Gd3+ , Er3+ , Y3+ ) have been developed, and their crystal structure determined as monoclinic with the point group Cm [3,4]. Especially great interest is devoted to GdCa4 O(BO3 )3 (GdCOB) crystals owing to their ∗
Corresponding author. Tel.: +48-22-5532172; fax: +48-22-6294229. E-mail address: [email protected] (M. Lefeld-Sosnowska).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.003
good optical parameters and the possibility of doping by Nd3+ , Yb3+ or Er3+ laser active ions. Moreover, they can be grown by the Czochralski technique [4–6]. The crystals are non-hygroscopic, easy to polish and of high hardness. In order to achieve a high efficiency of second harmonic generation, crystals of high structural quality are needed. Crystal lattice defects are the source of lattice strains, which change the crystal optical properties, mainly the refractive indices. The aim of the paper was the investigation of extended crystal lattice defects in as-grown GdCOB crystals. Conventional X-ray projection topography and synchrotron radiation monochromatic wave topography were used. The back reflection geometry of diffraction, especially effective for thick crystals, was mainly employed.
2. Experimental GdCa4 O(BO3 )3 (GdCOB) single crystals were grown in the Institute of Electronic Materials Technology in Warsaw, by means of the Czochralski technique [7] in nitrogen at-
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mosphere with a velocity of 1.1 mm/h. The growth direction was [0 1 0]. The samples in the form of plane parallel plates, cut off perpendicularly to the crystal growth axis b, were mechanically and chemically polished to a thickness: 300 m (samples A1 and C), 470 m (samples B and A2). They were cut off from the top (A), middle (B) and end (C) parts of the grown crystals. Conventional X-ray projection Lang topographs were taken using radiation from a laboratory source: in transmission using Mo K␣1 and back reflection geometry using Mo K␣1 and Cu K␣1 radiation. Topographs were recorded on Ilford L4 nuclear plates with an emulsion thickness of 50 m. The presented pictures correspond to the registered ones on the plate: the black contrast corresponds to the increased diffracted beam intensity. The back reflection projection topographs were taken in symmetrical and asymmetrical reflections for several crystallographic zones. The monochromatic wave double-crystal topographs were recorded using synchrotron radiation at the monochromatic beam Station E2 at HASYLAB (DESY, Hamburg). A highly monochromatic with a wavelength of 0.115 nm was obtained using a piezoelectrically stabilised double-crystal monochromator with successive (5 1 1) and (3 3 3) reflections from Si crystal. The topographs were taken for (0 8 0) symmetric reflection.
3. Results The conventional back reflection topographs obtained with Mo K␣1 radiation provided a good visibility of dislocations, comparable or better than in the transmission topographs. Contrasts in topographs obtained with Cu K␣1 radiation were much lower, but these results are also valuable as they can exclude a possibility of existing of narrow elongated volume defects, formed by local dissolving of crystal during growth (named “solute trails”). They can be very narrow and easily confused with dislocations. A comparison of projection back reflection topograph taken for sample A1 ((2 14 0) reflection of MoK␣1 radiation) with transmission one taken for (4 0 0) reflection is given in Fig. 1a and b. In these topographs long straight contrasts most probably due to dislocations are seen. The line directions are perpendicular to the [1 0 0] direction and most probably they are elongated in [1¯ 0 2] direction. The observed length of the dislocation images can be well explained, assuming that they are located in the (0 1 0) plane perpendicularly to the growth axis b, while the accuracy of crystal plane cutting is about 10 . The central part (the “core”) is covered with the dislocations. In some region the directions of these dislocations are disturbed, either owing to strains, or by interactions with another defects (see Fig. 2a and b).
Fig. 1. X-ray projection topographs of sample A1 (d = 300 m), Mo K␣1 radiation: (a) (2 14 0) asymmetric back reflection; (b) (4 0 0) symmetric transmission reflection.
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Fig. 2. Parts of the topograph of sample A1 (Fig. 1a): (a) fragment I; (b) fragment II.
Fig. 3. X-ray projection back reflection topographs of sample B (d = 470 m), Mo K␣1 radiation: (a) reflection (0 8 0); (b) reflection (0 16 0); (c) reflection (2 14 0).
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¯ (c) [1¯ 0 2] – Fig. 4. X-ray projection topographs of sample C (d = 300 m), Mo K␣1 radiation, back (0 16 0) reflection: (a) [1 0 0]; (b) [5¯ 0 2]; crystallographic directions in the diffraction plane; (d) back (2 14 0) reflection; (e) transmission: (4¯ 0 0) reflection; (d, e) azimuth as in (a).
M. Lefeld-Sosnowska et al. / Journal of Alloys and Compounds 382 (2004) 153–159
The back reflection topographs of sample B cut off from the middle part of the “as-grown” crystal do not exhibit the contrast connected with the “core” (Fig. 3a–c). The topographs were taken in symmetric reflections (0 8 0) (Fig. 3a), (0 16 0) (Fig. 3b) and asymmetric one (2 14 0), in order to show the reflection of the highest resolution. The penetration depth for these reflections for Mo K␣1 radiation is 21.2, 42.4 and 10.25 m respectively. The highest resolution was obtained for (2 14 0) reflection. In the case of this reflection the penetration depth is the lowest, so the very thin surface layer is imaged in the topograph. Moreover low glancing angle of primary beam causes low shortening of the image. In these topographs long, straight contrasts are seen. In some places close to the crystal edges instead of long straight dislocations, one can find dense and not well resolved contrasts. They are probably connected with dislocations inclined to the crystal surface, appearing with high density. In these places the high density of etch pits was observed [8]. The back reflection topographs obtained for Mo K␣1 radiation for sample C (cut off from the end part of the as-grown crystal) are shown in Fig. 4a–d. Fig. 4a–c was obtained for the symmetrical (0 16 0) reflection, but for different crystallographic direction lying in the diffraction plane (differ¯ and ent azimuth). In Fig. 4a–c the directions [1 0 0], [5¯ 0 2] [1¯ 0 2] are in the diffraction plane respectively. In the topograph in Fig. 4a the line contrasts are perpendicular to
157
the direction [1 0 0]. In Fig. 4b the straight dislocation lines changed their direction but are well seen. However, in Fig. 4c the line directions are horizontal, but their contrast is very weak. In the topograph (Fig. 4d), taken using (2 14 0) back reflection for the same crystal azimuth as in Fig. 4a, the distribution of dislocations with better resolution as in (0 16 0) reflection is observed. The central part of the sample is free from long straight dislocations, but the strains connected with the boundary of the “core” gives weak contrast. On the contrary, this contrast is very well seen in Fig. 4e showing the transmission topograph for the same azimuth as in Fig. 4a. Comparing the topographs in Figs. 1a, 3c, and 4d one can see the difference in dislocation distribution. Moreover, the contrast corresponding to the “core” edge is seen only in topographs of samples A and C – cut off from the crystal top and end part respectively. In a case of the samples with the thickness 470 m it was possible to obtain the SR monochromatic wave images from large area of the sample, corresponding to a particular slope of the rocking curve flanks. Contrary to that, thin samples were usually significantly bent and only a narrow stripe was reproduced. For each azimuth of the sample three topographs were registered, corresponding to the two flanks of the rocking curve, and to its maximum (Fig. 5a–c). A representative monochromatic wave topographs obtained for sample A2 are shown in Fig. 5a–d. The image of the defect structure in
Fig. 5. SR plane wave topographs of sample A2 (d = 470 m), λ = 0,115 nm, (0 8 0) reflection, SR beam direction [1¯ 0 0]: (a) low-angle rocking curve (RC) flank; (b) RC maximum; (c) high-angle RC flank; (d) SR beam direction [1¯ 0 2], high-angle RC flank.
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¯ (a) low-angle RC flank; (b) Fig. 6. SR plane wave topographs of sample B (d = 470 m), λ = 0,115 nm, (0 8 0) reflection, SR beam direction [2 0 1]: ¯ reflection (0 16 0). RC maximum; (c) high-angle RC flank; (d) projection topograph of sample B, beam direction [2 0 1],
the SR back reflection topographs was to some extent similar to the contrast in the projection reflection topographs but the dislocation images produces complex black–white contrasts, corresponding to the details of the dislocation strain field. In particular we may notice that the dislocations out cropping on the surface produce characteristic black–white rosettes, similar to those firstly described by Bonse and Kappler [9]. It may be followed in Fig. 5a–c. In the monochromatic wave SR topographs shown in Fig. 5 we observed similar diffraction contrast changes of long straight dislocations for different azimuth as in the case of the previously discussed back reflection projection Lang topographs. Especially for the azimuth corresponding to the [1¯ 0 2] crystallographic direction in the diffraction plane (Fig. 5d), the long straight contrasts practically vanish (compare Fig. 4c). Horizontal black lines contrasts in Fig. 5d correspond to residual strains connected with dislocations, which are imaged in Fig. 5a and b as vertical lines. The explanation to that comes from the fact that the topographic methods are sensitive only to the component of misorientation in the plane of diffraction. It seems to be much probable that the effective deformation of long straight dislocations consists mainly in lattice bending around the dislocation line. This situation may in particular occur in the case of dominating edge component of the Burgers vector and a relatively low Bragg angle reducing the contribution of the interplanar spacing change.
Comparing the topographs for two flanks of the rocking curve, the characteristic inversion of the contrast from “black” to “white” can be seen. Moreover, the curvature of the crystal lattice due to the long-range strains causes the high or weak diffracted beam intensity on the topographs, taken for opposite sides of the rocking curve (Fig. 6a and b). Both monochromatic wave and projection topographs did not reveal any segregation fringes proving high homogeneity of the chemical composition of the examined crystal.
4. Conclusions The GdCa4 O(BO3 )3 single crystals grown by Czochralski technique were investigated by conventional projection back reflection and transmission X-ray topography and by SR monochromatic wave reflection topography. The present experiment indicated the possibility of revealing of dislocation structure with the use of back reflection topography. That eliminates the problem of preparation of thin samples. The dislocation lines, running most probably in [1¯ 0 2] direction and long-range strains at the boundary of the crystal “core” were detected by means of projection topographs. Different dislocation distribution in the sample cut off from the top, middle and the end of the crystal was found. The characteristic diffraction contrast inversion taken at two
M. Lefeld-Sosnowska et al. / Journal of Alloys and Compounds 382 (2004) 153–159
flanks of the rocking curve on the SR monochromatic wave was observed. The differences in diffraction contrast of dislocations, observed in symmetrical reflections taken for different azimuths of the sample were detected by projection and SR monochromatic wave topography as well, indicating the domination of the edge component of the Burgers vector. Both monochromatic wave and projection Lang topographs did not reveal any segregation fringes proving high chemical composition homogeneity of the examined crystal.
Acknowledgements The technical assistance of Jerzy Bondziul is much appreciated. The work was partly supported by State Committee for Scientific Research under the project no. T11 B05422.
159
References [1] A. Brenier, J. Lumin. 91 (2000) 121–132. [2] J. Wang, Z. Shao, J. Wei, X. Hu, Y. Liu, B. Gong, G. Li, J. Lu, M. Guo, M. Jiang, Prog. Cryst. Growth Charact. Mater. 40 (2000) 17–31. [3] D. Vivien, G. Aka, A. Kahn-Harari, A. Aron, F. Mougel, J.-M. Benitez, B. Ferrand, R. Klein, G. Kugel, N. Le Nain, M. Jacquet, J. Cryst. Growth 237–239 (2002) 621. [4] T.N. Khamaganova, V.K. Trunow, B.F. Dzhurinskii, Russ. J. Inorg. Chem. 36 (1994) 484. [5] R. Norrestam, M. Nygren, J.O. Bovin, Chem. Mater. 4 (1992) 737. [6] Z. Shujun, Z. Jiguo, Ch. Zhenxiang, Zh. Guangyong, H. Jianru, Ch. Huanchu, J. Cryst. Growth 203 (1999) 168. [7] A. Paj˛aczkowska, A. Kłos, B. Hilczer, N. Menguy, A. Novosselov, Cryst. Growth Des. 1 (2001) 363. [8] M. Lefeld-Sosnowska, E. Olszyñska, A. Paj˛aczkowska, A. Kłos, J. Cryst. Growth 262 (2004) 388–394. [9] V. Bonse, E. Kappler, Z. Naturforsch. 13a (1958) 348.
Conventional and synchrotron radiation back reflection topography of GdCa4 O(BO3)3 crystals M. Lefeld-Sosnowska a,∗ , E. Olszy´nska a,b , W. Wierzchowski b , K. Wieteska c , W. Graeff d , A. Paj˛aczkowska b , A. Kłos b a
Institute of Experimental Physics, University of Warsaw, Ho˙za 69, 00-681 Warsaw, Poland b Institute of Electronic Materials Technology, Wólczyñska 133, 01-919 Warsaw, Poland c Institute of Atomic Energy, 05-400 Otwock, Swierk, ´ Poland d HASYLAB at DESY, Notkestr. 85, D-22603 Hamburg, Germany
Received 15 September 2003; received in revised form 29 February 2004; accepted 5 March 2004
Abstract The investigations of GdCa4 O(BO3 )3 single crystals grown by the Czochralski technique were performed by conventional and synchrotron radiation (SR) monochromatic wave topography mainly in back reflection geometry. It was found that back reflection topography is effectively able to reveal the dislocation structure even in thin samples. Different distribution of defects was observed in samples cut off from the top, middle and end part of the “as-grown” crystal. SR monochromatic wave topographs taken for two flanks of the rocking curve show the characteristic inversion of diffraction contrast. Both monochromatic wave and projection Lang topographs did not reveal any segregation fringes proving high homogeneity of the chemical composition of the examined crystal. © 2004 Elsevier B.V. All rights reserved. Keywords: Insulators; Crystal growth; Dislocations; X-ray diffraction; Synchrotron radiation
1. Introduction Highly efficient visible and UV solid-state laser sources are of great importance for many applications. The emission in the range of visible light can be realised by frequency doubling of diode pumped IR solid-state lasers, using non-linear optical crystals. For this reason in the last decade one can observe a growing interest in non-linear optical materials. The most promising way for the realisation a visible laser light source is the self-frequency conversion (self-frequency doubling SFD or sum-frequency missing SFM) of solid-state lasers, in which both laser action and non-linear conversion is realised in one crystal. Among many crystals used for SFD purposes ([1,2,3] and references cited herein), the family of rare earth calcium oxyborates ReCa4 O(BO3 )3 (with Re = Ln3+ , Nd3+ , Sm3+ , Gd3+ , Er3+ , Y3+ ) have been developed, and their crystal structure determined as monoclinic with the point group Cm [3,4]. Especially great interest is devoted to GdCa4 O(BO3 )3 (GdCOB) crystals owing to their ∗
Corresponding author. Tel.: +48-22-5532172; fax: +48-22-6294229. E-mail address: [email protected] (M. Lefeld-Sosnowska).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.003
good optical parameters and the possibility of doping by Nd3+ , Yb3+ or Er3+ laser active ions. Moreover, they can be grown by the Czochralski technique [4–6]. The crystals are non-hygroscopic, easy to polish and of high hardness. In order to achieve a high efficiency of second harmonic generation, crystals of high structural quality are needed. Crystal lattice defects are the source of lattice strains, which change the crystal optical properties, mainly the refractive indices. The aim of the paper was the investigation of extended crystal lattice defects in as-grown GdCOB crystals. Conventional X-ray projection topography and synchrotron radiation monochromatic wave topography were used. The back reflection geometry of diffraction, especially effective for thick crystals, was mainly employed.
2. Experimental GdCa4 O(BO3 )3 (GdCOB) single crystals were grown in the Institute of Electronic Materials Technology in Warsaw, by means of the Czochralski technique [7] in nitrogen at-
154
M. Lefeld-Sosnowska et al. / Journal of Alloys and Compounds 382 (2004) 153–159
mosphere with a velocity of 1.1 mm/h. The growth direction was [0 1 0]. The samples in the form of plane parallel plates, cut off perpendicularly to the crystal growth axis b, were mechanically and chemically polished to a thickness: 300 m (samples A1 and C), 470 m (samples B and A2). They were cut off from the top (A), middle (B) and end (C) parts of the grown crystals. Conventional X-ray projection Lang topographs were taken using radiation from a laboratory source: in transmission using Mo K␣1 and back reflection geometry using Mo K␣1 and Cu K␣1 radiation. Topographs were recorded on Ilford L4 nuclear plates with an emulsion thickness of 50 m. The presented pictures correspond to the registered ones on the plate: the black contrast corresponds to the increased diffracted beam intensity. The back reflection projection topographs were taken in symmetrical and asymmetrical reflections for several crystallographic zones. The monochromatic wave double-crystal topographs were recorded using synchrotron radiation at the monochromatic beam Station E2 at HASYLAB (DESY, Hamburg). A highly monochromatic with a wavelength of 0.115 nm was obtained using a piezoelectrically stabilised double-crystal monochromator with successive (5 1 1) and (3 3 3) reflections from Si crystal. The topographs were taken for (0 8 0) symmetric reflection.
3. Results The conventional back reflection topographs obtained with Mo K␣1 radiation provided a good visibility of dislocations, comparable or better than in the transmission topographs. Contrasts in topographs obtained with Cu K␣1 radiation were much lower, but these results are also valuable as they can exclude a possibility of existing of narrow elongated volume defects, formed by local dissolving of crystal during growth (named “solute trails”). They can be very narrow and easily confused with dislocations. A comparison of projection back reflection topograph taken for sample A1 ((2 14 0) reflection of MoK␣1 radiation) with transmission one taken for (4 0 0) reflection is given in Fig. 1a and b. In these topographs long straight contrasts most probably due to dislocations are seen. The line directions are perpendicular to the [1 0 0] direction and most probably they are elongated in [1¯ 0 2] direction. The observed length of the dislocation images can be well explained, assuming that they are located in the (0 1 0) plane perpendicularly to the growth axis b, while the accuracy of crystal plane cutting is about 10 . The central part (the “core”) is covered with the dislocations. In some region the directions of these dislocations are disturbed, either owing to strains, or by interactions with another defects (see Fig. 2a and b).
Fig. 1. X-ray projection topographs of sample A1 (d = 300 m), Mo K␣1 radiation: (a) (2 14 0) asymmetric back reflection; (b) (4 0 0) symmetric transmission reflection.
M. Lefeld-Sosnowska et al. / Journal of Alloys and Compounds 382 (2004) 153–159
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Fig. 2. Parts of the topograph of sample A1 (Fig. 1a): (a) fragment I; (b) fragment II.
Fig. 3. X-ray projection back reflection topographs of sample B (d = 470 m), Mo K␣1 radiation: (a) reflection (0 8 0); (b) reflection (0 16 0); (c) reflection (2 14 0).
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¯ (c) [1¯ 0 2] – Fig. 4. X-ray projection topographs of sample C (d = 300 m), Mo K␣1 radiation, back (0 16 0) reflection: (a) [1 0 0]; (b) [5¯ 0 2]; crystallographic directions in the diffraction plane; (d) back (2 14 0) reflection; (e) transmission: (4¯ 0 0) reflection; (d, e) azimuth as in (a).
M. Lefeld-Sosnowska et al. / Journal of Alloys and Compounds 382 (2004) 153–159
The back reflection topographs of sample B cut off from the middle part of the “as-grown” crystal do not exhibit the contrast connected with the “core” (Fig. 3a–c). The topographs were taken in symmetric reflections (0 8 0) (Fig. 3a), (0 16 0) (Fig. 3b) and asymmetric one (2 14 0), in order to show the reflection of the highest resolution. The penetration depth for these reflections for Mo K␣1 radiation is 21.2, 42.4 and 10.25 m respectively. The highest resolution was obtained for (2 14 0) reflection. In the case of this reflection the penetration depth is the lowest, so the very thin surface layer is imaged in the topograph. Moreover low glancing angle of primary beam causes low shortening of the image. In these topographs long, straight contrasts are seen. In some places close to the crystal edges instead of long straight dislocations, one can find dense and not well resolved contrasts. They are probably connected with dislocations inclined to the crystal surface, appearing with high density. In these places the high density of etch pits was observed [8]. The back reflection topographs obtained for Mo K␣1 radiation for sample C (cut off from the end part of the as-grown crystal) are shown in Fig. 4a–d. Fig. 4a–c was obtained for the symmetrical (0 16 0) reflection, but for different crystallographic direction lying in the diffraction plane (differ¯ and ent azimuth). In Fig. 4a–c the directions [1 0 0], [5¯ 0 2] [1¯ 0 2] are in the diffraction plane respectively. In the topograph in Fig. 4a the line contrasts are perpendicular to
157
the direction [1 0 0]. In Fig. 4b the straight dislocation lines changed their direction but are well seen. However, in Fig. 4c the line directions are horizontal, but their contrast is very weak. In the topograph (Fig. 4d), taken using (2 14 0) back reflection for the same crystal azimuth as in Fig. 4a, the distribution of dislocations with better resolution as in (0 16 0) reflection is observed. The central part of the sample is free from long straight dislocations, but the strains connected with the boundary of the “core” gives weak contrast. On the contrary, this contrast is very well seen in Fig. 4e showing the transmission topograph for the same azimuth as in Fig. 4a. Comparing the topographs in Figs. 1a, 3c, and 4d one can see the difference in dislocation distribution. Moreover, the contrast corresponding to the “core” edge is seen only in topographs of samples A and C – cut off from the crystal top and end part respectively. In a case of the samples with the thickness 470 m it was possible to obtain the SR monochromatic wave images from large area of the sample, corresponding to a particular slope of the rocking curve flanks. Contrary to that, thin samples were usually significantly bent and only a narrow stripe was reproduced. For each azimuth of the sample three topographs were registered, corresponding to the two flanks of the rocking curve, and to its maximum (Fig. 5a–c). A representative monochromatic wave topographs obtained for sample A2 are shown in Fig. 5a–d. The image of the defect structure in
Fig. 5. SR plane wave topographs of sample A2 (d = 470 m), λ = 0,115 nm, (0 8 0) reflection, SR beam direction [1¯ 0 0]: (a) low-angle rocking curve (RC) flank; (b) RC maximum; (c) high-angle RC flank; (d) SR beam direction [1¯ 0 2], high-angle RC flank.
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¯ (a) low-angle RC flank; (b) Fig. 6. SR plane wave topographs of sample B (d = 470 m), λ = 0,115 nm, (0 8 0) reflection, SR beam direction [2 0 1]: ¯ reflection (0 16 0). RC maximum; (c) high-angle RC flank; (d) projection topograph of sample B, beam direction [2 0 1],
the SR back reflection topographs was to some extent similar to the contrast in the projection reflection topographs but the dislocation images produces complex black–white contrasts, corresponding to the details of the dislocation strain field. In particular we may notice that the dislocations out cropping on the surface produce characteristic black–white rosettes, similar to those firstly described by Bonse and Kappler [9]. It may be followed in Fig. 5a–c. In the monochromatic wave SR topographs shown in Fig. 5 we observed similar diffraction contrast changes of long straight dislocations for different azimuth as in the case of the previously discussed back reflection projection Lang topographs. Especially for the azimuth corresponding to the [1¯ 0 2] crystallographic direction in the diffraction plane (Fig. 5d), the long straight contrasts practically vanish (compare Fig. 4c). Horizontal black lines contrasts in Fig. 5d correspond to residual strains connected with dislocations, which are imaged in Fig. 5a and b as vertical lines. The explanation to that comes from the fact that the topographic methods are sensitive only to the component of misorientation in the plane of diffraction. It seems to be much probable that the effective deformation of long straight dislocations consists mainly in lattice bending around the dislocation line. This situation may in particular occur in the case of dominating edge component of the Burgers vector and a relatively low Bragg angle reducing the contribution of the interplanar spacing change.
Comparing the topographs for two flanks of the rocking curve, the characteristic inversion of the contrast from “black” to “white” can be seen. Moreover, the curvature of the crystal lattice due to the long-range strains causes the high or weak diffracted beam intensity on the topographs, taken for opposite sides of the rocking curve (Fig. 6a and b). Both monochromatic wave and projection topographs did not reveal any segregation fringes proving high homogeneity of the chemical composition of the examined crystal.
4. Conclusions The GdCa4 O(BO3 )3 single crystals grown by Czochralski technique were investigated by conventional projection back reflection and transmission X-ray topography and by SR monochromatic wave reflection topography. The present experiment indicated the possibility of revealing of dislocation structure with the use of back reflection topography. That eliminates the problem of preparation of thin samples. The dislocation lines, running most probably in [1¯ 0 2] direction and long-range strains at the boundary of the crystal “core” were detected by means of projection topographs. Different dislocation distribution in the sample cut off from the top, middle and the end of the crystal was found. The characteristic diffraction contrast inversion taken at two
M. Lefeld-Sosnowska et al. / Journal of Alloys and Compounds 382 (2004) 153–159
flanks of the rocking curve on the SR monochromatic wave was observed. The differences in diffraction contrast of dislocations, observed in symmetrical reflections taken for different azimuths of the sample were detected by projection and SR monochromatic wave topography as well, indicating the domination of the edge component of the Burgers vector. Both monochromatic wave and projection Lang topographs did not reveal any segregation fringes proving high chemical composition homogeneity of the examined crystal.
Acknowledgements The technical assistance of Jerzy Bondziul is much appreciated. The work was partly supported by State Committee for Scientific Research under the project no. T11 B05422.
159
References [1] A. Brenier, J. Lumin. 91 (2000) 121–132. [2] J. Wang, Z. Shao, J. Wei, X. Hu, Y. Liu, B. Gong, G. Li, J. Lu, M. Guo, M. Jiang, Prog. Cryst. Growth Charact. Mater. 40 (2000) 17–31. [3] D. Vivien, G. Aka, A. Kahn-Harari, A. Aron, F. Mougel, J.-M. Benitez, B. Ferrand, R. Klein, G. Kugel, N. Le Nain, M. Jacquet, J. Cryst. Growth 237–239 (2002) 621. [4] T.N. Khamaganova, V.K. Trunow, B.F. Dzhurinskii, Russ. J. Inorg. Chem. 36 (1994) 484. [5] R. Norrestam, M. Nygren, J.O. Bovin, Chem. Mater. 4 (1992) 737. [6] Z. Shujun, Z. Jiguo, Ch. Zhenxiang, Zh. Guangyong, H. Jianru, Ch. Huanchu, J. Cryst. Growth 203 (1999) 168. [7] A. Paj˛aczkowska, A. Kłos, B. Hilczer, N. Menguy, A. Novosselov, Cryst. Growth Des. 1 (2001) 363. [8] M. Lefeld-Sosnowska, E. Olszyñska, A. Paj˛aczkowska, A. Kłos, J. Cryst. Growth 262 (2004) 388–394. [9] V. Bonse, E. Kappler, Z. Naturforsch. 13a (1958) 348.