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Some reasons for the formation of grain boundaries and melt inclusions in growing large BBO crystals by TSSG technique
ARTICLE IN PRESS
Journal of Crystal Growth 297 (2006) 259–263 www.elsevier.com/locate/jcrysgro
Some reasons for the formation of grain boundaries and melt inclusions in growing large BBO crystals by TSSG technique E.G. Tsvetkov Institute of Geology & Mineralogy SB of the RAS, Novosibirsk 630058, Russia Received 19 January 2006; received in revised form 5 July 2006; accepted 22 September 2006 Communicated by V. Fratello Available online 22 November 2006
Abstract Data reported in this work are the result of the first crystallomorphological analysis of the formation of most typical structural defects in growing large BBO crystals by the top seeded solution growth (TSSG) technique combined with pulling. These defects are grain boundaries with varying scales of disordered angle orientations and melt inclusions. It is these defects that to a great degree restrict the yield of suitable material of high-optical quality from grown BBO boules. We believe that the main reason for the formation of these defects is the incoherent joining of different sectors of independent growth, which form the crystal volume, and the specific features of crystallization at the interface of regeneration type. The results presented in this paper may be useful in improving technologies for the production of large BBO single crystals by correcting growth conditions, and optimizing the schemes of cutting them to produce high-quality nonlinear optical (NLO) elements. r 2006 Elsevier B.V. All rights reserved. PACS: 42.70.Mp; 61.72; 81.10.Aj; 81.10.Dn Keywords: A1. Crystal morphology; A1. Defects; A2. Growth from high temperature solutions; B1. Borates; B2. Nonlinear optic materials
1. Introduction Nowadays the readily reproducible technology for growth of large BBO (low-temperature barium metaborate, b-BaB2O4) single crystals, one of the most efficient and widely used nonlinear optical (NLO) materials, is still being developed. In paper [1] one can find some of the references covering this subject. The task of the designers of this technology is to increase both the yield of high-quality material and the sizes of BBO crystals. This is necessary for the production of NLO elements of all the required sizes and orientations. The solution of this kind of problem is complicated by the necessity to use a high-temperature solution (hereafter ‘‘solution’’) for growing BBO crystals in their thermodynamic stability field (i.e. below 925 1C [2]) and, hence, by the well-known specific problems of crystal growth from such a crystallization medium. Tel./fax: +7 383 3337607.
E-mail address: [email protected]. 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.09.053
One of the components for successful solution of the problem is revealing the relationship between most typical structural defects with the preset crystal growth conditions. BBO crystals grown from solutions always have growth and post-growth defects of various types, which were described elsewhere [3–11]. Some of our previous works discuss general problems of BBO crystallization from solutions, formation of structural defects, and improvement of the procedure for growing BBO crystals [3–7,12]. The aim of this work is to analyze further the reasons for the formation of major defects during the growth of BBO single crystals, namely grain boundaries with disordered angle orientation of different scales and melt inclusions. 2. Experimental procedure The starting materials for preparing solutions with initial compositions (0.46–0.45)BaO (0.44–0.45)B2O3 0.1Na2O were barium and sodium carbonates and boric acid of high-purity grade. The advantages of H3BO3 over
ARTICLE IN PRESS 260
E.G. Tsvetkov / Journal of Crystal Growth 297 (2006) 259–263
Fig. 1. Scheme of usual positioning of the rhombohedra faces on the upper side of Z-axis BBO crystal with a quasihexagonal or rounded cross section (a); typical view of the core zone of the Z-axis BBO crystal, formed by the entrapped melt inclusions, in the X-plate of longitudinal section (b).
the glass-like chunks of B2O3, and Na2O over NaF, were reported by us in Ref. [7]. The procedure of the solution synthesis was described in Ref. [4]. To grow BBO single crystals, we used platinum crucibles with dimensions + (100–120) (120–140) mm which were 2/3 filled with solution. In some cases, platinum cover-disks with a central window up to 85 mm in diameter were used to modify the temperature gradients in the region of the melt near to the surface. The equipment (furnaces, thermocontrollers, etc.) and the typical conditions of BBO crystal growth by the TSSG technique combined with pulling (typical temperature gradients, cooling rates of solutions, seed rotation and pulling rates, etc.) were presented in our previous works [7,12]. Only Z-oriented seeds were used for growing BBO single crystals in this study. Varying the temperature distribution in the melt volume and near the crystallization front (by placement of the crucible relative to the heater, using thermal liners and furnace cover plates of varying construction, etc.), we grew crystals whose configuration of the growth interface was close to flat or convex. A standard growth scenario for these crystals anticipates pulling of slowly rotating seed at a rate of 0.5–0.7 mm/day. The seed was grown out to 65–75 mm in diameter. The boule was pulled with a constant cross section and the crystal was decanted after reaching the length of 20 mm along the Z-axis. After decantation these crystals were cooled to room temperature over a period of about 20 h. Grown BBO crystals were subjected to a thorough morphological analysis in the ‘‘as-grown’’ state. Then they were oriented and cut, optically treated, and, in some cases, selectively chemically etched [3] on some planes. Structural defects were examined by shadow and interferencepolarization optical methods using red or green laser beams [3] and using optical microscopy of varying magnification.
3. Formal analysis and experimental results It is known that the main defects restricting the yield of suitable BBO material of high-optical quality are melt inclusions and grain boundaries of various scales of disorientation. Fig. 1a demonstrates the scheme of usual positioning of faces on the upper side of Z-axis BBO crystal relative to the crystallographic axes. Typical core zone of grown BBO crystal (beneath the seed) in the thick X-plate1 (Fig. 1a) of a longitudinal section, where mass entrapments of melt inclusions may take place, is shown in Fig. 1b. Dimensions of this zone may vary to a considerable degree, reaching half the diameter of a crystal boule. Besides this core, grown crystals often demonstrate platelike zones of entrapped inclusions forming a ‘‘three-ray star’’ with h1 1 2¯ 0i orientation of the rays [11] or a ‘‘six-ray star’’. In Fig. 2 one can see typical low-angle boundaries in the same X-plate (a) and a pair of grain boundaries isolating a wedge-shaped growth sector with some misorientation of their structure (b) relative to the host crystal matrix. In Ref. [13] this type of sectors was interpreted as twins with the different crystallographic polarity. Independence of these defects formation from the purity of starting materials, accuracy of temperature control, and from a crystal growth rate allowed us to suppose a specific growth mechanism as a basic reason for the occurrence of these defects. Our propositions concerning this subject are as follows. As is known, almost all BBO crystals designed for production of NLO elements are pulled along Z-axis. It is a fact that in the BBO crystal structure there is no (0 0 0 1) plane with a great reticular density of atomic groupings (Fig. 3) and, therefore, no real BBO crystal with {0 0 0 1} faces. Potential faces on the growth interface of Z-axis BBO crystals are the rhombohedral f0 1 1¯ 4g and f1 0 1¯ 2g faces (Fig. 3). However, as the growth of large BBO single 1
Plate that is perpendicular to the X-axis.
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Fig. 2. Typical view of the low-angle boundaries in X-plates visualized in the interference-polarization optical scheme (a) and a pair of grain boundaries isolating a wedge-shaped sector of independent growth (b).
Fig. 3. The crystal structure of BBO: projection on the ð1 1 2¯ 0Þ plane of unit cell.
crystals requires high-gradient thermal fields in melt media [5,7,12], the conditions for the formation of these faces as a macroscopic flatness at the crystallization front are not realized. In this case any non-singular interface of Z-axis BBO crystal is the surface for regeneration growth of the above rhombohedra face sectors, i.e. it will have significant atomic roughness compared to the singular faces f0 1 1¯ 4g and f1 0 1¯ 2g. The maximum roughness occurs in the zone of the potential apex of the rhombohedra and edges of these conjugate and rotated rhombohedra (Fig. 4a) forming a six-ray star in the (0 0 0 1) cross section (Fig. 4b). Apparently, in these zones, whose atomic scale relief have a most rough structure and promote adsorption of microportions of impurity-rich solution, inclusions are more easily trapped by growing BBO crystals. Such adsorption is also stimulated by the surface electric potential because of extraordinary nonsaturation of interatomic bonds. This can be inferred from the postgrowth examination of crystals. The scheme in Fig. 4a also explains how the core zone of inclusion entrapment expands when the crystallization interface flattens or becomes concave. The increased concentration of any impurities in solution ahead of this zone is related to the character of free-convective flows and the impossibility of
stimulating forced convection (by speeding up crystal rotation) under high-gradient thermal conditions of BBO crystal growth [12]. The apexes and edges of crystals are the most actively growing parts, sites of continuous or predominant generation of any growth form (layers, hillocks, etc.). This results from the substantially lower reticular density of atomic groupings in corresponding sites of the interface, significant nonsaturation of their close and distant bonds responsible for their active interaction with the components of the neighboring crystallization medium. This is clearly evident from the quite specific localization of solution portions on the BBO crystal growth surface solidified after decantation (Fig. 5). This localization of permanent growth sources of Z-axis BBO crystals and predominant directions of movement of newly generated growth forms on their interface lead to a more or less definite arrangement of zones where their fronts join (Fig. 6a). The atomic-incoherent joining is responsible for the formation of edge dislocation walls (misfit dislocations), which are, in fact, low-angle boundaries (Fig. 2) penetrating most of the volume of the crystals. The dislocation character of these boundaries is clearly supported by selective etching of corresponding crystal cuts [3]. It can be inferred from the relative position of the joining zones that in longitudinal fragments of BBO crystals (similar to that shown in Fig. 1b) these low-angle boundaries will be visualized mainly in their center, while in the peripheral parts of fragments they will be either not observable or seen only after the examined sample is appreciably rotated (Fig. 6b). Moreover, some of these boundaries can be observed only in the cross section perpendicular to the plane of the examined fragment. It is important to take account of this feature when testing Z-axis BBO crystals for manufacturing of NLO elements. Most likely, it is the above feature of BBO crystal growth that is responsible for the occurrence (in reentering angles) of independent growth forms, e.g. crystal nuclei with a different orientation (relative to host boule) leading to the formation of blocky growth sectors of various scales (Fig. 2b). Most likely, the formation of the previously observed growth twins of type II with the contact plane of ð1 1 2¯ 0Þ orientation in Z-axis BBO crystals [13] proceeds in
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Fig. 4. Formal scheme of microstructuring of different sites at the growth interface of Z-axis BBO crystals, related to their atomic roughness (a); localization of potential edges of f0 1 1¯ 4g and f1 0 1¯ 2g rhombohedra at the growth interface and corresponding zones of predominant entrapment of inclusions in BBO boule with quasihexagonal or rounded cross section (b).
Fig. 5. Zones of regular localization of solution portions on the growth interface of Z-axis BBO crystals solidified on their decantation.
crystal, that allowed them to be identified with edge shapes of f0 1 1¯ 4g and f1 0 1¯ 2g rhombohedra. Hence, in this case a six-ray system of melt inclusions and low-angle boundaries normally formed on the Z-axis of BBO crystals and the yield of optical quality material from the grown boules was rather small. If the thermal conditions of crystal growth were favorable for the formation of a convex interface with the angle of its generatrix to the (0 0 0 1) plane of 101 or more, the growth sources of the f1 0 1¯ 2g sectors were dominated and a three-ray system of melt inclusions and low-angle boundaries typically formed in the boules. Hence, the yield of material suitable for production of fairly large NLO elements without grain or twin boundaries increased. And, finally in the crystals with a growth interface angle of 25–301 the effect of independent growth sources on the interface was significantly suppressed; the growth interface seemed uniformly convex, and the yield of high-quality material was maximal. 4. Summary
this way2. Normally, this scenario of crystal formation is possible at the initial stage of growth from a seed, provided that the solution is highly supersaturated. To reveal the nature of the above defect formation in BBO crystals resulting from the configuration of the growth interface, we studied a number of boules grown under conditions of different temperature distributions in the crystallization zone, i.e. in the zone of the solution near to the surface and the space above the melt. First of all we performed morphological analysis of decanted crystals with quasihexagonal or rounded cross sections and generally flat growth interfaces. The interfaces in these crystals appeared to have a smooth convexoconcave form (Fig. 7). The radially directed zones of convexity formed a six-ray star at the interface and had the orientation strictly related to the external faceting of 2 We have not found twins of type I with the ð0 1 1¯ 0Þ contact plane reported in Ref. [13] in our BBO crystals.
As a summary to the crystallomorphological analysis of the formation of the most typical structural defects in growing large BBO crystals by TSSG technique, we give the following conclusions.
The main reason for the entrapment and specific localization of melt inclusions in BBO crystals is their sectorial growth at a regeneration type interface or, in other words, significant atomic roughness of the growth surface in the zones of potential edges and apex of rhombohedral faces. The formation of the low-angle boundaries in BBO crystals, as well as ð1 1 2¯ 0Þ twin boundaries, is related to the atomic-incoherent joining of fronts of independent growth forms on the interfaces of boules when pulling them along the Z-axis. Competitive formation of growth sectors of rhombohedra f0 1 1¯ 4g and f1 0 1¯ 2g might lead to a complicated but quite regular system of preferential
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Fig. 6. Directions of predominant movement of independent growth forms of rhombohedra faces f0 1 1¯ 4g (as an example) and regular arrangement of the zones where their fronts join at the interface of Z-axis BBO crystals with a triangle-like cross section (a); probable localization of low-angle boundaries in Z-axis BBO crystals with a quasihexagonal or rounded cross section, the central-axial longitudinal plates included (b).
cooperation in implementation of this work. Crystals grown by them were used as a subject for studying the reasons for the most typical formation of defects. The author is grateful to E.G. Serbulenko and D.A. Nagorsky for the technical assistance in carrying the investigation, and also to Dr. A.M. Yurkin for his support. The work was financially supported by Grant no. 155 from the Integration Project of the SB of the RAS and Grant no. 04-05-64438 from the Russian Foundation for Basic Research.
References
Fig. 7. Generally flat growth interface of decanted Z-axis BBO single crystal with its smoothed convexo-concave relief.
localization of these boundaries, and boundaries with a greater angle off-orientation. The growth of BBO crystals with a convex (to 25–301) interface promotes the least number of melt inclusions and low-angle boundaries and the maximum yield of material suitable for manufacture of large high-quality NLO elements.
Acknowledgments We express our thanks to technologists G.G. Khranenko, V.A. Silaev, and A.A. Ostrovskii for their active
[1] W. Chen, A. Jiang, G. Wang, J. Crystal Growth 256 (2003) 383. [2] K.H. Hubner, Neues Jahrb. Mineral. Monatsh. 111 (1969) 325. [3] E.G. Tsvetkov, G.G. Khranenko, E.G. Samoilova et al., in: Procedings of the IV International Conference ‘‘Crystals: Growth, Properties, Real Structure, Application’’, vol. 2 (Alexandrov, VNIISIMS, 1999), p. 45–56 (in Russian). [4] E.G. Tsvetkov, A.A. Tomilenko, G.G. Khranenko, et al., Crystallogr. Rep. 45 (2000) 698. [5] V.I. Tyurikov, E.G. Tsvetkov, V.D. Antsygin, et al., Opt. Mater. 14 (2000) 313. [6] E.G. Tsvetkov, G.G. Khranenko, J. Crystal Growth 275 (1–2) (2005) e675. [7] E.G. Tsvetkov, G.G. Khranenko, V.P. Solntsev, J. Crystal Growth 275 (1–2) (2005) e2123. [8] L.K. Cheng, W. Bosenberg, C.L. Tang, J. Crystal Growth 89 (1988) 553. [9] R.S. Feigelson, R.J. Raymakers, R.K. Route, et al., J. Crystal Growth 97 (1989) 352. [10] Q.L. Zhao, J. Appl. Cryst. 28 (1995) 267. [11] M. Roth, D. Perlov, J. Crystal Growth 169 (1996) 734. [12] E.G. Tsvetkov, A.M. Yurkin, Crystallogr. Rep. 43 (1998) 1080. [13] B.G. Wang, Z.P. Lu, E.W. Shi, et al., Cryst. Res. Technol. 33 (1998) 929.
Journal of Crystal Growth 297 (2006) 259–263 www.elsevier.com/locate/jcrysgro
Some reasons for the formation of grain boundaries and melt inclusions in growing large BBO crystals by TSSG technique E.G. Tsvetkov Institute of Geology & Mineralogy SB of the RAS, Novosibirsk 630058, Russia Received 19 January 2006; received in revised form 5 July 2006; accepted 22 September 2006 Communicated by V. Fratello Available online 22 November 2006
Abstract Data reported in this work are the result of the first crystallomorphological analysis of the formation of most typical structural defects in growing large BBO crystals by the top seeded solution growth (TSSG) technique combined with pulling. These defects are grain boundaries with varying scales of disordered angle orientations and melt inclusions. It is these defects that to a great degree restrict the yield of suitable material of high-optical quality from grown BBO boules. We believe that the main reason for the formation of these defects is the incoherent joining of different sectors of independent growth, which form the crystal volume, and the specific features of crystallization at the interface of regeneration type. The results presented in this paper may be useful in improving technologies for the production of large BBO single crystals by correcting growth conditions, and optimizing the schemes of cutting them to produce high-quality nonlinear optical (NLO) elements. r 2006 Elsevier B.V. All rights reserved. PACS: 42.70.Mp; 61.72; 81.10.Aj; 81.10.Dn Keywords: A1. Crystal morphology; A1. Defects; A2. Growth from high temperature solutions; B1. Borates; B2. Nonlinear optic materials
1. Introduction Nowadays the readily reproducible technology for growth of large BBO (low-temperature barium metaborate, b-BaB2O4) single crystals, one of the most efficient and widely used nonlinear optical (NLO) materials, is still being developed. In paper [1] one can find some of the references covering this subject. The task of the designers of this technology is to increase both the yield of high-quality material and the sizes of BBO crystals. This is necessary for the production of NLO elements of all the required sizes and orientations. The solution of this kind of problem is complicated by the necessity to use a high-temperature solution (hereafter ‘‘solution’’) for growing BBO crystals in their thermodynamic stability field (i.e. below 925 1C [2]) and, hence, by the well-known specific problems of crystal growth from such a crystallization medium. Tel./fax: +7 383 3337607.
E-mail address: [email protected]. 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.09.053
One of the components for successful solution of the problem is revealing the relationship between most typical structural defects with the preset crystal growth conditions. BBO crystals grown from solutions always have growth and post-growth defects of various types, which were described elsewhere [3–11]. Some of our previous works discuss general problems of BBO crystallization from solutions, formation of structural defects, and improvement of the procedure for growing BBO crystals [3–7,12]. The aim of this work is to analyze further the reasons for the formation of major defects during the growth of BBO single crystals, namely grain boundaries with disordered angle orientation of different scales and melt inclusions. 2. Experimental procedure The starting materials for preparing solutions with initial compositions (0.46–0.45)BaO (0.44–0.45)B2O3 0.1Na2O were barium and sodium carbonates and boric acid of high-purity grade. The advantages of H3BO3 over
ARTICLE IN PRESS 260
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Fig. 1. Scheme of usual positioning of the rhombohedra faces on the upper side of Z-axis BBO crystal with a quasihexagonal or rounded cross section (a); typical view of the core zone of the Z-axis BBO crystal, formed by the entrapped melt inclusions, in the X-plate of longitudinal section (b).
the glass-like chunks of B2O3, and Na2O over NaF, were reported by us in Ref. [7]. The procedure of the solution synthesis was described in Ref. [4]. To grow BBO single crystals, we used platinum crucibles with dimensions + (100–120) (120–140) mm which were 2/3 filled with solution. In some cases, platinum cover-disks with a central window up to 85 mm in diameter were used to modify the temperature gradients in the region of the melt near to the surface. The equipment (furnaces, thermocontrollers, etc.) and the typical conditions of BBO crystal growth by the TSSG technique combined with pulling (typical temperature gradients, cooling rates of solutions, seed rotation and pulling rates, etc.) were presented in our previous works [7,12]. Only Z-oriented seeds were used for growing BBO single crystals in this study. Varying the temperature distribution in the melt volume and near the crystallization front (by placement of the crucible relative to the heater, using thermal liners and furnace cover plates of varying construction, etc.), we grew crystals whose configuration of the growth interface was close to flat or convex. A standard growth scenario for these crystals anticipates pulling of slowly rotating seed at a rate of 0.5–0.7 mm/day. The seed was grown out to 65–75 mm in diameter. The boule was pulled with a constant cross section and the crystal was decanted after reaching the length of 20 mm along the Z-axis. After decantation these crystals were cooled to room temperature over a period of about 20 h. Grown BBO crystals were subjected to a thorough morphological analysis in the ‘‘as-grown’’ state. Then they were oriented and cut, optically treated, and, in some cases, selectively chemically etched [3] on some planes. Structural defects were examined by shadow and interferencepolarization optical methods using red or green laser beams [3] and using optical microscopy of varying magnification.
3. Formal analysis and experimental results It is known that the main defects restricting the yield of suitable BBO material of high-optical quality are melt inclusions and grain boundaries of various scales of disorientation. Fig. 1a demonstrates the scheme of usual positioning of faces on the upper side of Z-axis BBO crystal relative to the crystallographic axes. Typical core zone of grown BBO crystal (beneath the seed) in the thick X-plate1 (Fig. 1a) of a longitudinal section, where mass entrapments of melt inclusions may take place, is shown in Fig. 1b. Dimensions of this zone may vary to a considerable degree, reaching half the diameter of a crystal boule. Besides this core, grown crystals often demonstrate platelike zones of entrapped inclusions forming a ‘‘three-ray star’’ with h1 1 2¯ 0i orientation of the rays [11] or a ‘‘six-ray star’’. In Fig. 2 one can see typical low-angle boundaries in the same X-plate (a) and a pair of grain boundaries isolating a wedge-shaped growth sector with some misorientation of their structure (b) relative to the host crystal matrix. In Ref. [13] this type of sectors was interpreted as twins with the different crystallographic polarity. Independence of these defects formation from the purity of starting materials, accuracy of temperature control, and from a crystal growth rate allowed us to suppose a specific growth mechanism as a basic reason for the occurrence of these defects. Our propositions concerning this subject are as follows. As is known, almost all BBO crystals designed for production of NLO elements are pulled along Z-axis. It is a fact that in the BBO crystal structure there is no (0 0 0 1) plane with a great reticular density of atomic groupings (Fig. 3) and, therefore, no real BBO crystal with {0 0 0 1} faces. Potential faces on the growth interface of Z-axis BBO crystals are the rhombohedral f0 1 1¯ 4g and f1 0 1¯ 2g faces (Fig. 3). However, as the growth of large BBO single 1
Plate that is perpendicular to the X-axis.
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261
Fig. 2. Typical view of the low-angle boundaries in X-plates visualized in the interference-polarization optical scheme (a) and a pair of grain boundaries isolating a wedge-shaped sector of independent growth (b).
Fig. 3. The crystal structure of BBO: projection on the ð1 1 2¯ 0Þ plane of unit cell.
crystals requires high-gradient thermal fields in melt media [5,7,12], the conditions for the formation of these faces as a macroscopic flatness at the crystallization front are not realized. In this case any non-singular interface of Z-axis BBO crystal is the surface for regeneration growth of the above rhombohedra face sectors, i.e. it will have significant atomic roughness compared to the singular faces f0 1 1¯ 4g and f1 0 1¯ 2g. The maximum roughness occurs in the zone of the potential apex of the rhombohedra and edges of these conjugate and rotated rhombohedra (Fig. 4a) forming a six-ray star in the (0 0 0 1) cross section (Fig. 4b). Apparently, in these zones, whose atomic scale relief have a most rough structure and promote adsorption of microportions of impurity-rich solution, inclusions are more easily trapped by growing BBO crystals. Such adsorption is also stimulated by the surface electric potential because of extraordinary nonsaturation of interatomic bonds. This can be inferred from the postgrowth examination of crystals. The scheme in Fig. 4a also explains how the core zone of inclusion entrapment expands when the crystallization interface flattens or becomes concave. The increased concentration of any impurities in solution ahead of this zone is related to the character of free-convective flows and the impossibility of
stimulating forced convection (by speeding up crystal rotation) under high-gradient thermal conditions of BBO crystal growth [12]. The apexes and edges of crystals are the most actively growing parts, sites of continuous or predominant generation of any growth form (layers, hillocks, etc.). This results from the substantially lower reticular density of atomic groupings in corresponding sites of the interface, significant nonsaturation of their close and distant bonds responsible for their active interaction with the components of the neighboring crystallization medium. This is clearly evident from the quite specific localization of solution portions on the BBO crystal growth surface solidified after decantation (Fig. 5). This localization of permanent growth sources of Z-axis BBO crystals and predominant directions of movement of newly generated growth forms on their interface lead to a more or less definite arrangement of zones where their fronts join (Fig. 6a). The atomic-incoherent joining is responsible for the formation of edge dislocation walls (misfit dislocations), which are, in fact, low-angle boundaries (Fig. 2) penetrating most of the volume of the crystals. The dislocation character of these boundaries is clearly supported by selective etching of corresponding crystal cuts [3]. It can be inferred from the relative position of the joining zones that in longitudinal fragments of BBO crystals (similar to that shown in Fig. 1b) these low-angle boundaries will be visualized mainly in their center, while in the peripheral parts of fragments they will be either not observable or seen only after the examined sample is appreciably rotated (Fig. 6b). Moreover, some of these boundaries can be observed only in the cross section perpendicular to the plane of the examined fragment. It is important to take account of this feature when testing Z-axis BBO crystals for manufacturing of NLO elements. Most likely, it is the above feature of BBO crystal growth that is responsible for the occurrence (in reentering angles) of independent growth forms, e.g. crystal nuclei with a different orientation (relative to host boule) leading to the formation of blocky growth sectors of various scales (Fig. 2b). Most likely, the formation of the previously observed growth twins of type II with the contact plane of ð1 1 2¯ 0Þ orientation in Z-axis BBO crystals [13] proceeds in
ARTICLE IN PRESS 262
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Fig. 4. Formal scheme of microstructuring of different sites at the growth interface of Z-axis BBO crystals, related to their atomic roughness (a); localization of potential edges of f0 1 1¯ 4g and f1 0 1¯ 2g rhombohedra at the growth interface and corresponding zones of predominant entrapment of inclusions in BBO boule with quasihexagonal or rounded cross section (b).
Fig. 5. Zones of regular localization of solution portions on the growth interface of Z-axis BBO crystals solidified on their decantation.
crystal, that allowed them to be identified with edge shapes of f0 1 1¯ 4g and f1 0 1¯ 2g rhombohedra. Hence, in this case a six-ray system of melt inclusions and low-angle boundaries normally formed on the Z-axis of BBO crystals and the yield of optical quality material from the grown boules was rather small. If the thermal conditions of crystal growth were favorable for the formation of a convex interface with the angle of its generatrix to the (0 0 0 1) plane of 101 or more, the growth sources of the f1 0 1¯ 2g sectors were dominated and a three-ray system of melt inclusions and low-angle boundaries typically formed in the boules. Hence, the yield of material suitable for production of fairly large NLO elements without grain or twin boundaries increased. And, finally in the crystals with a growth interface angle of 25–301 the effect of independent growth sources on the interface was significantly suppressed; the growth interface seemed uniformly convex, and the yield of high-quality material was maximal. 4. Summary
this way2. Normally, this scenario of crystal formation is possible at the initial stage of growth from a seed, provided that the solution is highly supersaturated. To reveal the nature of the above defect formation in BBO crystals resulting from the configuration of the growth interface, we studied a number of boules grown under conditions of different temperature distributions in the crystallization zone, i.e. in the zone of the solution near to the surface and the space above the melt. First of all we performed morphological analysis of decanted crystals with quasihexagonal or rounded cross sections and generally flat growth interfaces. The interfaces in these crystals appeared to have a smooth convexoconcave form (Fig. 7). The radially directed zones of convexity formed a six-ray star at the interface and had the orientation strictly related to the external faceting of 2 We have not found twins of type I with the ð0 1 1¯ 0Þ contact plane reported in Ref. [13] in our BBO crystals.
As a summary to the crystallomorphological analysis of the formation of the most typical structural defects in growing large BBO crystals by TSSG technique, we give the following conclusions.
The main reason for the entrapment and specific localization of melt inclusions in BBO crystals is their sectorial growth at a regeneration type interface or, in other words, significant atomic roughness of the growth surface in the zones of potential edges and apex of rhombohedral faces. The formation of the low-angle boundaries in BBO crystals, as well as ð1 1 2¯ 0Þ twin boundaries, is related to the atomic-incoherent joining of fronts of independent growth forms on the interfaces of boules when pulling them along the Z-axis. Competitive formation of growth sectors of rhombohedra f0 1 1¯ 4g and f1 0 1¯ 2g might lead to a complicated but quite regular system of preferential
ARTICLE IN PRESS E.G. Tsvetkov / Journal of Crystal Growth 297 (2006) 259–263
263
Fig. 6. Directions of predominant movement of independent growth forms of rhombohedra faces f0 1 1¯ 4g (as an example) and regular arrangement of the zones where their fronts join at the interface of Z-axis BBO crystals with a triangle-like cross section (a); probable localization of low-angle boundaries in Z-axis BBO crystals with a quasihexagonal or rounded cross section, the central-axial longitudinal plates included (b).
cooperation in implementation of this work. Crystals grown by them were used as a subject for studying the reasons for the most typical formation of defects. The author is grateful to E.G. Serbulenko and D.A. Nagorsky for the technical assistance in carrying the investigation, and also to Dr. A.M. Yurkin for his support. The work was financially supported by Grant no. 155 from the Integration Project of the SB of the RAS and Grant no. 04-05-64438 from the Russian Foundation for Basic Research.
References
Fig. 7. Generally flat growth interface of decanted Z-axis BBO single crystal with its smoothed convexo-concave relief.
localization of these boundaries, and boundaries with a greater angle off-orientation. The growth of BBO crystals with a convex (to 25–301) interface promotes the least number of melt inclusions and low-angle boundaries and the maximum yield of material suitable for manufacture of large high-quality NLO elements.
Acknowledgments We express our thanks to technologists G.G. Khranenko, V.A. Silaev, and A.A. Ostrovskii for their active
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