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A study of fracture and defects in single crystal YAG
ARTICLE IN PRESS
Journal of Crystal Growth 267 (2004) 502–509
A study of fracture and defects in single crystal YAG D.E. Eakins*, M. Held, M.G. Norton*, D.F. Bahr Center for Materials Research, School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920, USA Received 26 March 2003; accepted 2 April 2004
Communicated by L.F. Schneemeyer
Abstract Single crystals of Czochralski grown Nd:YAG have been observed to crack more frequently during growth as the bulk substitutional Nd concentration is increased. Examination of the fracture surfaces indicated that in some cases the cause of failure was microscopic voids acting as stress concentrators. In other cases, the origin of fracture could not be ascertained directly from observation of the fracture surfaces. High-resolution transmission electron microscope studies of samples taken from the core of boules containing 1.02% Nd suggested that spherical particles rich in Nd may act as sources of stress in these high-Nd-doped YAG crystals. The hydrostatic stress at the particle/matrix boundary has been determined from elasticity calculations to be 80 MPa. In boules containing 1.395% Nd, rod-shaped particles were observed throughout the crystal. From electron diffraction analysis these particles appeared to be regions of small misorientation, o1 . r 2004 Elsevier B.V. All rights reserved. PACS: 61.72.y; 81.10.Fq; 81.05.Je Keywords: A1. Characterization; A1. Defects; A1. Electron microscopy; A2. Czochralski method; B1. Oxides; B2. Laser materials
1. Introduction Neodymium-doped Y3Al5O12 (Nd:YAG) crystals are widely used as the pumping medium in solid-state lasers. The goal for commercial suppliers of Nd:YAG has been to grow highly doped, large-diameter crystals. Increasing the dopant *Corresponding author. Tel.: +1-509-335-4207; fax: +1509-335-8654. E-mail addresses: [email protected] (D.E. Eakins), [email protected] (M.G. Norton).
concentration results in a higher absorption coefficient, lower fluorescence lifetime, and greater overall laser efficiency [1]. However, raising the Nd concentration also increases the frequency of cracking during growth. Failure in this manner is unlike that which may occur when crystals are removed from the furnace. Commercial Nd:YAG is regularly provided with Nd concentrations ranging between 0 and 1.5 substitutional percent of yttrium sites; where from the molecular formula Y3x(Ndx)Al5O12, the substitutional percent Nd is given by x=3: For instance, 1.02 sub%
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.04.011
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Nd=0.153 at% Nd. Few crystals beyond 1.5 sub% Nd are available commercially. Nd:YAG may be grown by the Czochralski (Cz) process where crystal growth is obtained by pulling from a melt at high temperatures. There can be large temperature gradients throughout the crystal and melt resulting from crystal rotation, buoyancy, melt convection and low thermal conductivity [2,3]. Fracture of the boule may occur during cooling and is often assumed to be due to thermal expansion differences between the core and surface of the boule, and thermal shock on extraction from the furnace. Fracture during growth can occur as well, and may be detected by a sudden change in the growth rate. It may also be observed directly through a viewing port. If fracture occurs during growth then the process is halted, and the ruined boules used for defect characterization. There has been very little work reported in the literature concerning the cracking of Nd:YAG single crystals. The causes of cracking during and after production in similarly grown Nd-doped yttrium orthoaluminate (Nd:YAlO3) crystals have been investigated [4]. The factors contributing to cracking were identified as hoop stress, which develops due to the difference in cooling rates between the boule surface and core, surface defects such as iridium particles and decompositional grooving, and thermal shock during seeding. Cracking was minimized in these crystals by maintaining pull rates less than 3 mm/h. An appreciable amount of work has been done on growth defects in YAG [5–9]. Findings include the presence of solute trails resulting from unstable growth conditions, dislocation structures, Nd segregation near voids, and Ir inclusions resulting from incorporation of crucible material. Voids and Ir inclusions act as scatter centers in the crystal. Lowering the pull rate and increasing the speed of crystal rotation may remove these defects. The current study attempts to identify growth defects in Nd:YAG and determine which may be the cause of fracture during growth. Bulk samples were first examined with optical microscopy. The fracture surfaces were characterized using a scanning electron microscope (SEM) in an attempt to locate the origin of fracture. A
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transmission electron microscope (TEM) was used to determine the presence of defects and second phases. In order to determine which features may lead to fracture YAG samples with a range of Nd concentrations were examined.
2. Experimental procedure Single crystal boules of Nd:YAG grown by the Cz process were supplied by a commercial vendor. Typical growth conditions are pull rates of 0.4 mm/h, rotational rates of 20 rpm, and temperatures nearing 2100 K [2]. Unless specified otherwise, Nd concentrations are reported as substitutional percent of yttrium sites. The Nd concentrations in the unbroken boules were 0%, 1.02%, 1.395%, and 1.45% Nd. Since the dopant concentration varies along the boule length, concentration is determined by fitting to a dopant profile, and reported as an average. Additionally, several boules of 1.02% Nd that had fractured during growth were also provided. All samples supplied were produced under similar growth conditions. The fractured boules contained internal fracture surfaces that could be seen with the unaided eye. To expose the fracture surface, the boules were immersed in liquid nitrogen for approximately 60 s then placed in a warm water bath at approximately 80 C. The rapid change in temperature caused the fracture surfaces to extend and reach the free surface. The fractured pieces were collected, sputter coated with approximately 5 nm of gold, and examined using a JEOL 6400 SEM operated at 20 kV. The gold coating was necessary to reduce charging of the sample during electron irradiation. X-ray diffraction (XRD) was used to determine the effect of Nd concentration on the lattice parameter of YAG using the precise lattice parameter determination method [10]. Rods 4 mm in diameter by 20 mm in length of varying Nd concentration (0.0–1.45% Nd) were ground using an alumina mortar and pestle until they had a particle size less than 1 mm. XRD data were collected using a Siemens D-500 diffractometer in
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the 2y range 100–128 , with a dwell time of 2 s for each step size of 0.020 . Samples for TEM analysis were prepared from both unbroken and fractured 1.02% Nd crystals and from unbroken 1.395% Nd crystals. Samples were taken from both the core and the bulk regions of the boules. Discs 3 mm in diameter were cored from the desired areas using a diamondcoated coring bit. The specimens were then thinned by conventional bulk preparation methods. A chemical etch consisting of 1:1 H2SO4:H3PO4 at 180 C was used until perforation of the discs. The samples were allowed to soak in the acid mixture while the bath was heated to the desired temperature, similar to the procedure suggested by Gerber and Graf [11] using H3PO4. The samples were subsequently cleaned in an ion mill, and examined using a Philips CM200 and a JEOL JEM 2100 both operated at 200 kV. Because of fluorescence effects that occurred during electron irradiation, it was not possible to use energy dispersive spectroscopy to obtain quantitative chemical information from the samples. The influence of the emitted light results in degradation of the spectrum at low energies. Consequently, compositional information was obtained using wavelength dispersive spectroscopy (WDS) performed with a Cameca Camebax electron microprobe, which is relatively free of such spectral artifacts. The electron microprobe data were quantified using an undoped YAG crystal and pure Nd2O3 for standardization. Tests were conducted at an operating voltage of 15 kV.
breakdown samples revealed the presence of voids preceding the cracks near the core. Three types of voids were observed; spherical, comet shaped, and faceted. In contrast to voids found in YAG by Katsurayama et al. [8], WDS performed near the edge of faceted voids did not reveal any changes in composition. Results indicated a consistent Nd concentration of 0.975%. Several voids were also noticed in YAG crystals doped with 1.02% Nd that had not fractured during growth. This suggests that the presence of voids near the core does not necessarily result in fracture. Fig. 2 is a composite image of an entire fracture surface of YAG recorded by SEM. Closer
Fig. 1. Nd:YAG boule showing core breakdown.
3. Results and discussion Macroscopic examination of the Nd:YAG boules was initially done to document the appearance of crack planes, and to determine if macroscopic defects such as voids or inclusions were present. In several of the boules, a feature known as ‘‘core breakdown’’ could be observed. In such boules, fracture appears to begin near the core, extending into the bulk towards the melt (Fig. 1). The remaining boules that were examined contained fracture planes originating and ending at the crystal walls. Optical microscopy of the core
Fig. 2. Composite image of an entire fracture surface of Nd:YAG.
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examination of regions of the fracture surface showed the presence of river marks (Fig. 3). River marks on a fracture surface are the result of screw dislocations threading perpendicular to the plane of fracture [12]. The river marks were used to trace the crack path, ultimately determining the origin of failure. In some cases, voids were found at the crack origin as shown in Fig. 3. These voids can act as stress concentrators. It was also noticed that in several places, the crack path did not intersect the voids, but traveled past them. This observation suggests that fracture may also occur due to inhomogeneities in the crystal that are undetectable by SEM. TEM samples obtained from the bulk and core of 1.02% Nd boules revealed the presence of bend contours. Bend contours occur when the sample is bent and the diffracting planes are not parallel and do not all satisfy the Bragg condition. Bend contours were also visible in undoped YAG samples, suggesting that they are due to residual stresses introduced during crystal growth arising from the large temperature gradients and inhomogeneous cooling, rather than due to the addition of Nd. Although indirect evidence in the form of river marks in the fracture surface suggests the presence of dislocations, in all of the samples examined by TEM in this study no dislocations were found.
Fig. 3. SEM image of YAG fracture surface showing river marks indicated by arrows. Voids were also found on this fracture surface.
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Because of the apparent low dislocation density they are unlikely to contribute significantly to the overall stress state in the samples. In a study of Yb-doped YAG dislocation densities, measured by etch pits, were reported to decrease to nearly zero as samples were taken progressively further away from the seed [6]. Samples taken from the core region of the 1.02% Nd crystal contained a number of particles ranging from 0.5 to 4 mm in diameter (Fig. 4). In contrast, TEM samples from the bulk of these crystals appeared to be defect free. The darker contrast of the particles in Fig. 4 compared to the surrounding matrix indicates that they are either thicker or have a higher mass. Convergent beam electron diffraction (CBED) was used to approximate the thickness of the particle region compared to the matrix. The sample was tilted until a 2-beam condition was satisfied, and CBED patterns were captured from the matrix and particle. Only the largest particles, 4 mm in diameter, were examined. The method given by Williams and Carter [13] was used to determine an extinction distance for YAG of B150 nm. Hence the particles are nearly twice as thick as the surrounding matrix. The difference in thickness between particle and matrix is one
Fig. 4. Bright-field image showing particles in the core region.
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possible contrast mechanism between the two areas, though an explanation as to why these regions have different thickness (i.e., ion mill at different rates) cannot be obtained from the CBED patterns. High-resolution TEM was used to determine differences between the particles and the surrounding matrix. Good correlation was observed between the experimentally recorded images and the theoretical unit cell of the same scale and orientation. Moire! fringes spaced 2.44 nm apart were observed in the image of some of the particles as shown in Fig. 5. Moire! fringes are imaging artifacts caused in a TEM image by interference of the electron beam by layered lattices. They may arise because of differences in lattice spacing, misorientation, or a combination of both. Close examination of the moire! fringes in Fig. 5 shows that they result from the third case, i.e., there is both a difference in lattice spacing and orientation between the particle and matrix. The relationship between the fringe spacing, D; the interplanar spacings, d1 and d2 ; of the two lattices and the misorientation angle, b; are given by Eq. (1) [14]: d1 d2 D ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi: ð1Þ ðd1 d2 Þ2 þ d1 d2 b2
Fig. 5. Moir!e fringes observed in a high-resolution TEM image of a particle.
Along with satisfying Eq. (1), the interfering planes must also generate fringes that are aligned in the observed direction. Of all the low-index planes considered, only the (2 1 1) planes satisfy both these criteria. The interplanar spacings of particle ðd1 Þ and matrix ðd2 Þ were determined to be 0.49049 and 0.49037 nm, respectively, and the misorientation angle between the planes is 0.201 . At such a small angle of misorientation, observable splitting of the diffraction reflections due to rotation would not occur. The presence of moire! fringes was not observed in all of the images that were obtained for the many different particles examined in this study. The observation of moire! fringes implies that in the particle shown in Fig. 5 the confining stress of the matrix had been removed, possibly during sample preparation, thus allowing the particle to relax. In images of particles where moire! fringes were not observed, the particles remained under a stress imposed by the matrix. This stress results in equivalent lattice spacings for the particle and matrix and the appearance of bend contours associated with local strains and bending of the foil. The strain between the particle and matrix determined by analysis of the moire! fringe patterns can be used to determine indirectly the Nd concentration of the particle. This determination requires knowledge of the relationship between lattice strain and Nd concentration, which was established using the XRD precise lattice parameter method. Using the XRD data, a lattice parameter of 1.20115 nm was assigned to the matrix for a boule with the nominal bulk Nd concentration of 1.02% Nd. The amount of strain between the particle and matrix obtained by the moire! fringe analysis can then be used to calculate the lattice parameter of the particle, which was determined to be 1.20147 nm. This strain corresponds to a particle concentration of 2.77% Nd. Thus, rather than the particles being second phase precipitates they are actually localized regions of elevated Nd concentration. An analysis of the stress caused by such a region of strain in the crystal was performed. The following assumptions were made to simplify calculations: the particle is spherical, isotropic, and stress free. Again, because of the appearance
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of moire! fringes and the lack of bend contours at the particle boundary, it is assumed that the observed particle has been relieved of most of the stress due to removal of the confining matrix during sample preparation. Therefore, the stress needed to compress a spherical particle of observed diameter until the lattice parameter of the matrix is reached can be calculated. Using the strain determined from the moire! fringe analysis and the observed particle radius of 0.5 mm, the radius of the compressed particle was established. The initial and final particle radii were then used to determine the eigenstrains, e where e11 ¼ e22 ¼ e33 : From the equation for hydrostatic stress on a spherical-shaped inhomogeneity [15], a stress of 79.8 MPa at the particle/matrix boundary was calculated using a value of 115 GPa for the elastic constant of YAG and a value of 0.28 for the Poisson ratio. Since the particle is assumed to be symmetrical and isotropic, s11 ¼ s22 ¼ s33 : An additional calculation for the stress as a function of distance away from the particle was performed following the calculation for stress in a spherical pressure vessel of infinite thickness [16]. A plot of the radial stress as a function of distance from a particle of radius 0.49986 mm with an associated hydrostatic stress of 79.8 MPa is shown in Fig. 6. The stress caused by a particle 1 mm in diameter affects a large volume of the sample with respect to its radius. As shown earlier in Fig. 4, the individual particles can be quite close with respect to their stress distribution and it is expected that this may influence, and complicate, the true stress state of the boule. The actual stress state will be further complicated by the presence of differently sized particles. Particles as large as 4 mm in diameter have been observed, and will affect a greater volume of material than the particle considered. Thus, the presence of an irregular and complex state of stress due to randomly distributed, differently sized sources, in conjunction with hoop stresses arising from large thermal gradients, may be a cause of fracture. Fig. 7 is an image of particles observed in TEM samples prepared from boules containing 1.395% Nd. Such high dopant concentrations are extremely rare, and large boules cannot be formed. The rod-shaped particles, 25 mm in length and 5 mm in
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Fig. 6. Plot of stress as a function of distance from a particle 1 mm in diameter.
Fig. 7. Optical and TEM images of a rod-shaped particle (indicated by the arrow) in a 1.395% Nd:YAG specimen.
width, were observed throughout the bulk of the crystal as well as in the core. Unlike the spherical particles observed only in the core of 1.02% Nddoped boules, defects present in the bulk would reduce product yield and result in decreased laser
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efficiency if included in the laser rods. Electron diffraction patterns obtained from the rods and matrix were similar indicating that they are the same structure and phase. Comparing a diffraction pattern obtained from the matrix at the [1 1 1] zone axis, to another recorded from a rod at the same sample orientation, a shift in the intensity distribution of the reflections was observed. This implies a small angular misorientation between the particle and bulk material of less than 1 [17]. At present, the effect of the rods on the stress state of the bulk crystals and their role in fracture has not been determined.
4. Conclusion Several different types of defects have been found in Cz grown Nd-doped YAG single crystals. Voids of various shapes could easily be identified using light microscopy and they were also present on fracture surfaces examined using SEM. In some cases voids could be identified as a cause of fracture of the boule, while in others it was clear that fracture was initiated by other defects. The appearance of river marks on the fracture surface provided indirect evidence for the presence of dislocations in these samples. However, dislocations were not found in any of the TEM samples. TEM studies of samples prepared from boules containing 1.02% Nd that had fractured during growth revealed the presence of micron-sized particles within the core. From analysis of moire! fringe patterns observed in high-resolution images, it was found that the particles were regions of small angular misorientation and higher Nd concentrations of 2.77%. A simplified model of the stress distribution around a 1 mm particle was developed and the hydrostatic stress was determined to be 79.8 MPa. The actual stress state in the crystal is probably quite complex as separation between particles is often smaller than the individual stress distribution and there are also particles of different sizes. The presence of these enriched regions of Nd, and the associated stress development surrounding these regions, is a likely cause for the difficulty in growing single crystal YAG doped with higher Nd concentrations.
TEM samples made from a boule containing an unusually high Nd concentration of 1.395% showed micron-sized rod-shaped particles throughout. These features have a small rotational misorientation, o1 , from the matrix material. The presence of these features would most likely be deleterious to the laser properties of the crystal and the production yield but their role on the mechanical properties of the boule was not determined.
Acknowledgements This work was performed under the auspices of the Department of Defense, Joint Electromagnetics Technology Program Office, under the Assistant Secretary of Defense for Command, Control, Communications and Intelligence (ASD/ C3I), under Contract No. N66001-00-C-6008, through a subcontract from VLOC Incorporated, a subsidiary of II–VI Incorporated. The highresolution TEM work was performed at the EMSL, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, operated for DOE by Battelle. A portion of this research was supported by the National Science Foundation through the Research Experience for Undergraduates site program (#013912). The authors would like to thank Professor K.G. Lynn and the Center for Materials Research.
References [1] X. Jun, Z. Yongzong, L. Hongjun, D. Peizhen, Proc. SPIE 3889 (2000) 420. [2] Q. Xiao, J. Derby, J. Crystal Growth 139 (1994) 147. [3] Z. Galazka, H. Wilke, J. Crystal Growth 216 (2000) 389. [4] B. Cockayne, B. Lent, J.S. Abell, R. Harris, J. Mater. Sci. 8 (1973) 871. [5] A. Lupei, C. Stoicescu, V. Lupei, J. Crystal Growth 177 (1997) 207. [6] Y. Peizhi, D. Peizhen, Y. Zhiwen, T. Yulian, J. Crystal Growth 218 (2000) 87. [7] K. Mazur, W. Wierzchowski, J. Crystal Growth 148 (1995) 345.
ARTICLE IN PRESS D.E. Eakins et al. / Journal of Crystal Growth 267 (2004) 502–509 [8] M. Katsurayama, Y. Anzai, A. Sugiyama, M. Koike, Y. Kato, J. Crystal Growth 229 (2001) 193. [9] V. Iov, J. Crystal Growth 167 (1996) 180. [10] C. Suryanarayana, M.G. Norton, X-ray Diffraction: A Practical Approach, Plenum Press, New York, 1998, pp. 153–166. [11] M. Gerber, Th. Graf, Optics Laser Technol. 33 (2001) 449. [12] J.J. Gilman, Trans. Met. Soc. AIME 212 (1958) 310. [13] D.B. Williams, C.B. Carter, Transmission Electron Microscopy: Imaging, Plenum Press, New York, 1996, p. 445.
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[14] D.B. Williams, C.B. Carter, Transmission Electron Microscopy: Diffraction, Plenum Press, New York, 1996, pp. 321–323. [15] T. Mura, Micromechanics of Defects in Solids, Martinus Nijhoff Publishers, Dordrecht, 1987, p. 79. [16] S. Timoshenko, J.N. Goodier, Theory of Elasticity, McGraw-Hill Book Company, New York, 1951, p. 358. [17] K.W. Andrews, D.J. Dyson, S.R. Keown, Interpretation of Electron Diffraction Patterns, Plenum Press, New York, 1967 (plate 3).
Journal of Crystal Growth 267 (2004) 502–509
A study of fracture and defects in single crystal YAG D.E. Eakins*, M. Held, M.G. Norton*, D.F. Bahr Center for Materials Research, School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920, USA Received 26 March 2003; accepted 2 April 2004
Communicated by L.F. Schneemeyer
Abstract Single crystals of Czochralski grown Nd:YAG have been observed to crack more frequently during growth as the bulk substitutional Nd concentration is increased. Examination of the fracture surfaces indicated that in some cases the cause of failure was microscopic voids acting as stress concentrators. In other cases, the origin of fracture could not be ascertained directly from observation of the fracture surfaces. High-resolution transmission electron microscope studies of samples taken from the core of boules containing 1.02% Nd suggested that spherical particles rich in Nd may act as sources of stress in these high-Nd-doped YAG crystals. The hydrostatic stress at the particle/matrix boundary has been determined from elasticity calculations to be 80 MPa. In boules containing 1.395% Nd, rod-shaped particles were observed throughout the crystal. From electron diffraction analysis these particles appeared to be regions of small misorientation, o1 . r 2004 Elsevier B.V. All rights reserved. PACS: 61.72.y; 81.10.Fq; 81.05.Je Keywords: A1. Characterization; A1. Defects; A1. Electron microscopy; A2. Czochralski method; B1. Oxides; B2. Laser materials
1. Introduction Neodymium-doped Y3Al5O12 (Nd:YAG) crystals are widely used as the pumping medium in solid-state lasers. The goal for commercial suppliers of Nd:YAG has been to grow highly doped, large-diameter crystals. Increasing the dopant *Corresponding author. Tel.: +1-509-335-4207; fax: +1509-335-8654. E-mail addresses: [email protected] (D.E. Eakins), [email protected] (M.G. Norton).
concentration results in a higher absorption coefficient, lower fluorescence lifetime, and greater overall laser efficiency [1]. However, raising the Nd concentration also increases the frequency of cracking during growth. Failure in this manner is unlike that which may occur when crystals are removed from the furnace. Commercial Nd:YAG is regularly provided with Nd concentrations ranging between 0 and 1.5 substitutional percent of yttrium sites; where from the molecular formula Y3x(Ndx)Al5O12, the substitutional percent Nd is given by x=3: For instance, 1.02 sub%
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.04.011
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Nd=0.153 at% Nd. Few crystals beyond 1.5 sub% Nd are available commercially. Nd:YAG may be grown by the Czochralski (Cz) process where crystal growth is obtained by pulling from a melt at high temperatures. There can be large temperature gradients throughout the crystal and melt resulting from crystal rotation, buoyancy, melt convection and low thermal conductivity [2,3]. Fracture of the boule may occur during cooling and is often assumed to be due to thermal expansion differences between the core and surface of the boule, and thermal shock on extraction from the furnace. Fracture during growth can occur as well, and may be detected by a sudden change in the growth rate. It may also be observed directly through a viewing port. If fracture occurs during growth then the process is halted, and the ruined boules used for defect characterization. There has been very little work reported in the literature concerning the cracking of Nd:YAG single crystals. The causes of cracking during and after production in similarly grown Nd-doped yttrium orthoaluminate (Nd:YAlO3) crystals have been investigated [4]. The factors contributing to cracking were identified as hoop stress, which develops due to the difference in cooling rates between the boule surface and core, surface defects such as iridium particles and decompositional grooving, and thermal shock during seeding. Cracking was minimized in these crystals by maintaining pull rates less than 3 mm/h. An appreciable amount of work has been done on growth defects in YAG [5–9]. Findings include the presence of solute trails resulting from unstable growth conditions, dislocation structures, Nd segregation near voids, and Ir inclusions resulting from incorporation of crucible material. Voids and Ir inclusions act as scatter centers in the crystal. Lowering the pull rate and increasing the speed of crystal rotation may remove these defects. The current study attempts to identify growth defects in Nd:YAG and determine which may be the cause of fracture during growth. Bulk samples were first examined with optical microscopy. The fracture surfaces were characterized using a scanning electron microscope (SEM) in an attempt to locate the origin of fracture. A
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transmission electron microscope (TEM) was used to determine the presence of defects and second phases. In order to determine which features may lead to fracture YAG samples with a range of Nd concentrations were examined.
2. Experimental procedure Single crystal boules of Nd:YAG grown by the Cz process were supplied by a commercial vendor. Typical growth conditions are pull rates of 0.4 mm/h, rotational rates of 20 rpm, and temperatures nearing 2100 K [2]. Unless specified otherwise, Nd concentrations are reported as substitutional percent of yttrium sites. The Nd concentrations in the unbroken boules were 0%, 1.02%, 1.395%, and 1.45% Nd. Since the dopant concentration varies along the boule length, concentration is determined by fitting to a dopant profile, and reported as an average. Additionally, several boules of 1.02% Nd that had fractured during growth were also provided. All samples supplied were produced under similar growth conditions. The fractured boules contained internal fracture surfaces that could be seen with the unaided eye. To expose the fracture surface, the boules were immersed in liquid nitrogen for approximately 60 s then placed in a warm water bath at approximately 80 C. The rapid change in temperature caused the fracture surfaces to extend and reach the free surface. The fractured pieces were collected, sputter coated with approximately 5 nm of gold, and examined using a JEOL 6400 SEM operated at 20 kV. The gold coating was necessary to reduce charging of the sample during electron irradiation. X-ray diffraction (XRD) was used to determine the effect of Nd concentration on the lattice parameter of YAG using the precise lattice parameter determination method [10]. Rods 4 mm in diameter by 20 mm in length of varying Nd concentration (0.0–1.45% Nd) were ground using an alumina mortar and pestle until they had a particle size less than 1 mm. XRD data were collected using a Siemens D-500 diffractometer in
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the 2y range 100–128 , with a dwell time of 2 s for each step size of 0.020 . Samples for TEM analysis were prepared from both unbroken and fractured 1.02% Nd crystals and from unbroken 1.395% Nd crystals. Samples were taken from both the core and the bulk regions of the boules. Discs 3 mm in diameter were cored from the desired areas using a diamondcoated coring bit. The specimens were then thinned by conventional bulk preparation methods. A chemical etch consisting of 1:1 H2SO4:H3PO4 at 180 C was used until perforation of the discs. The samples were allowed to soak in the acid mixture while the bath was heated to the desired temperature, similar to the procedure suggested by Gerber and Graf [11] using H3PO4. The samples were subsequently cleaned in an ion mill, and examined using a Philips CM200 and a JEOL JEM 2100 both operated at 200 kV. Because of fluorescence effects that occurred during electron irradiation, it was not possible to use energy dispersive spectroscopy to obtain quantitative chemical information from the samples. The influence of the emitted light results in degradation of the spectrum at low energies. Consequently, compositional information was obtained using wavelength dispersive spectroscopy (WDS) performed with a Cameca Camebax electron microprobe, which is relatively free of such spectral artifacts. The electron microprobe data were quantified using an undoped YAG crystal and pure Nd2O3 for standardization. Tests were conducted at an operating voltage of 15 kV.
breakdown samples revealed the presence of voids preceding the cracks near the core. Three types of voids were observed; spherical, comet shaped, and faceted. In contrast to voids found in YAG by Katsurayama et al. [8], WDS performed near the edge of faceted voids did not reveal any changes in composition. Results indicated a consistent Nd concentration of 0.975%. Several voids were also noticed in YAG crystals doped with 1.02% Nd that had not fractured during growth. This suggests that the presence of voids near the core does not necessarily result in fracture. Fig. 2 is a composite image of an entire fracture surface of YAG recorded by SEM. Closer
Fig. 1. Nd:YAG boule showing core breakdown.
3. Results and discussion Macroscopic examination of the Nd:YAG boules was initially done to document the appearance of crack planes, and to determine if macroscopic defects such as voids or inclusions were present. In several of the boules, a feature known as ‘‘core breakdown’’ could be observed. In such boules, fracture appears to begin near the core, extending into the bulk towards the melt (Fig. 1). The remaining boules that were examined contained fracture planes originating and ending at the crystal walls. Optical microscopy of the core
Fig. 2. Composite image of an entire fracture surface of Nd:YAG.
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examination of regions of the fracture surface showed the presence of river marks (Fig. 3). River marks on a fracture surface are the result of screw dislocations threading perpendicular to the plane of fracture [12]. The river marks were used to trace the crack path, ultimately determining the origin of failure. In some cases, voids were found at the crack origin as shown in Fig. 3. These voids can act as stress concentrators. It was also noticed that in several places, the crack path did not intersect the voids, but traveled past them. This observation suggests that fracture may also occur due to inhomogeneities in the crystal that are undetectable by SEM. TEM samples obtained from the bulk and core of 1.02% Nd boules revealed the presence of bend contours. Bend contours occur when the sample is bent and the diffracting planes are not parallel and do not all satisfy the Bragg condition. Bend contours were also visible in undoped YAG samples, suggesting that they are due to residual stresses introduced during crystal growth arising from the large temperature gradients and inhomogeneous cooling, rather than due to the addition of Nd. Although indirect evidence in the form of river marks in the fracture surface suggests the presence of dislocations, in all of the samples examined by TEM in this study no dislocations were found.
Fig. 3. SEM image of YAG fracture surface showing river marks indicated by arrows. Voids were also found on this fracture surface.
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Because of the apparent low dislocation density they are unlikely to contribute significantly to the overall stress state in the samples. In a study of Yb-doped YAG dislocation densities, measured by etch pits, were reported to decrease to nearly zero as samples were taken progressively further away from the seed [6]. Samples taken from the core region of the 1.02% Nd crystal contained a number of particles ranging from 0.5 to 4 mm in diameter (Fig. 4). In contrast, TEM samples from the bulk of these crystals appeared to be defect free. The darker contrast of the particles in Fig. 4 compared to the surrounding matrix indicates that they are either thicker or have a higher mass. Convergent beam electron diffraction (CBED) was used to approximate the thickness of the particle region compared to the matrix. The sample was tilted until a 2-beam condition was satisfied, and CBED patterns were captured from the matrix and particle. Only the largest particles, 4 mm in diameter, were examined. The method given by Williams and Carter [13] was used to determine an extinction distance for YAG of B150 nm. Hence the particles are nearly twice as thick as the surrounding matrix. The difference in thickness between particle and matrix is one
Fig. 4. Bright-field image showing particles in the core region.
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possible contrast mechanism between the two areas, though an explanation as to why these regions have different thickness (i.e., ion mill at different rates) cannot be obtained from the CBED patterns. High-resolution TEM was used to determine differences between the particles and the surrounding matrix. Good correlation was observed between the experimentally recorded images and the theoretical unit cell of the same scale and orientation. Moire! fringes spaced 2.44 nm apart were observed in the image of some of the particles as shown in Fig. 5. Moire! fringes are imaging artifacts caused in a TEM image by interference of the electron beam by layered lattices. They may arise because of differences in lattice spacing, misorientation, or a combination of both. Close examination of the moire! fringes in Fig. 5 shows that they result from the third case, i.e., there is both a difference in lattice spacing and orientation between the particle and matrix. The relationship between the fringe spacing, D; the interplanar spacings, d1 and d2 ; of the two lattices and the misorientation angle, b; are given by Eq. (1) [14]: d1 d2 D ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi: ð1Þ ðd1 d2 Þ2 þ d1 d2 b2
Fig. 5. Moir!e fringes observed in a high-resolution TEM image of a particle.
Along with satisfying Eq. (1), the interfering planes must also generate fringes that are aligned in the observed direction. Of all the low-index planes considered, only the (2 1 1) planes satisfy both these criteria. The interplanar spacings of particle ðd1 Þ and matrix ðd2 Þ were determined to be 0.49049 and 0.49037 nm, respectively, and the misorientation angle between the planes is 0.201 . At such a small angle of misorientation, observable splitting of the diffraction reflections due to rotation would not occur. The presence of moire! fringes was not observed in all of the images that were obtained for the many different particles examined in this study. The observation of moire! fringes implies that in the particle shown in Fig. 5 the confining stress of the matrix had been removed, possibly during sample preparation, thus allowing the particle to relax. In images of particles where moire! fringes were not observed, the particles remained under a stress imposed by the matrix. This stress results in equivalent lattice spacings for the particle and matrix and the appearance of bend contours associated with local strains and bending of the foil. The strain between the particle and matrix determined by analysis of the moire! fringe patterns can be used to determine indirectly the Nd concentration of the particle. This determination requires knowledge of the relationship between lattice strain and Nd concentration, which was established using the XRD precise lattice parameter method. Using the XRD data, a lattice parameter of 1.20115 nm was assigned to the matrix for a boule with the nominal bulk Nd concentration of 1.02% Nd. The amount of strain between the particle and matrix obtained by the moire! fringe analysis can then be used to calculate the lattice parameter of the particle, which was determined to be 1.20147 nm. This strain corresponds to a particle concentration of 2.77% Nd. Thus, rather than the particles being second phase precipitates they are actually localized regions of elevated Nd concentration. An analysis of the stress caused by such a region of strain in the crystal was performed. The following assumptions were made to simplify calculations: the particle is spherical, isotropic, and stress free. Again, because of the appearance
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of moire! fringes and the lack of bend contours at the particle boundary, it is assumed that the observed particle has been relieved of most of the stress due to removal of the confining matrix during sample preparation. Therefore, the stress needed to compress a spherical particle of observed diameter until the lattice parameter of the matrix is reached can be calculated. Using the strain determined from the moire! fringe analysis and the observed particle radius of 0.5 mm, the radius of the compressed particle was established. The initial and final particle radii were then used to determine the eigenstrains, e where e11 ¼ e22 ¼ e33 : From the equation for hydrostatic stress on a spherical-shaped inhomogeneity [15], a stress of 79.8 MPa at the particle/matrix boundary was calculated using a value of 115 GPa for the elastic constant of YAG and a value of 0.28 for the Poisson ratio. Since the particle is assumed to be symmetrical and isotropic, s11 ¼ s22 ¼ s33 : An additional calculation for the stress as a function of distance away from the particle was performed following the calculation for stress in a spherical pressure vessel of infinite thickness [16]. A plot of the radial stress as a function of distance from a particle of radius 0.49986 mm with an associated hydrostatic stress of 79.8 MPa is shown in Fig. 6. The stress caused by a particle 1 mm in diameter affects a large volume of the sample with respect to its radius. As shown earlier in Fig. 4, the individual particles can be quite close with respect to their stress distribution and it is expected that this may influence, and complicate, the true stress state of the boule. The actual stress state will be further complicated by the presence of differently sized particles. Particles as large as 4 mm in diameter have been observed, and will affect a greater volume of material than the particle considered. Thus, the presence of an irregular and complex state of stress due to randomly distributed, differently sized sources, in conjunction with hoop stresses arising from large thermal gradients, may be a cause of fracture. Fig. 7 is an image of particles observed in TEM samples prepared from boules containing 1.395% Nd. Such high dopant concentrations are extremely rare, and large boules cannot be formed. The rod-shaped particles, 25 mm in length and 5 mm in
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Fig. 6. Plot of stress as a function of distance from a particle 1 mm in diameter.
Fig. 7. Optical and TEM images of a rod-shaped particle (indicated by the arrow) in a 1.395% Nd:YAG specimen.
width, were observed throughout the bulk of the crystal as well as in the core. Unlike the spherical particles observed only in the core of 1.02% Nddoped boules, defects present in the bulk would reduce product yield and result in decreased laser
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efficiency if included in the laser rods. Electron diffraction patterns obtained from the rods and matrix were similar indicating that they are the same structure and phase. Comparing a diffraction pattern obtained from the matrix at the [1 1 1] zone axis, to another recorded from a rod at the same sample orientation, a shift in the intensity distribution of the reflections was observed. This implies a small angular misorientation between the particle and bulk material of less than 1 [17]. At present, the effect of the rods on the stress state of the bulk crystals and their role in fracture has not been determined.
4. Conclusion Several different types of defects have been found in Cz grown Nd-doped YAG single crystals. Voids of various shapes could easily be identified using light microscopy and they were also present on fracture surfaces examined using SEM. In some cases voids could be identified as a cause of fracture of the boule, while in others it was clear that fracture was initiated by other defects. The appearance of river marks on the fracture surface provided indirect evidence for the presence of dislocations in these samples. However, dislocations were not found in any of the TEM samples. TEM studies of samples prepared from boules containing 1.02% Nd that had fractured during growth revealed the presence of micron-sized particles within the core. From analysis of moire! fringe patterns observed in high-resolution images, it was found that the particles were regions of small angular misorientation and higher Nd concentrations of 2.77%. A simplified model of the stress distribution around a 1 mm particle was developed and the hydrostatic stress was determined to be 79.8 MPa. The actual stress state in the crystal is probably quite complex as separation between particles is often smaller than the individual stress distribution and there are also particles of different sizes. The presence of these enriched regions of Nd, and the associated stress development surrounding these regions, is a likely cause for the difficulty in growing single crystal YAG doped with higher Nd concentrations.
TEM samples made from a boule containing an unusually high Nd concentration of 1.395% showed micron-sized rod-shaped particles throughout. These features have a small rotational misorientation, o1 , from the matrix material. The presence of these features would most likely be deleterious to the laser properties of the crystal and the production yield but their role on the mechanical properties of the boule was not determined.
Acknowledgements This work was performed under the auspices of the Department of Defense, Joint Electromagnetics Technology Program Office, under the Assistant Secretary of Defense for Command, Control, Communications and Intelligence (ASD/ C3I), under Contract No. N66001-00-C-6008, through a subcontract from VLOC Incorporated, a subsidiary of II–VI Incorporated. The highresolution TEM work was performed at the EMSL, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, operated for DOE by Battelle. A portion of this research was supported by the National Science Foundation through the Research Experience for Undergraduates site program (#013912). The authors would like to thank Professor K.G. Lynn and the Center for Materials Research.
References [1] X. Jun, Z. Yongzong, L. Hongjun, D. Peizhen, Proc. SPIE 3889 (2000) 420. [2] Q. Xiao, J. Derby, J. Crystal Growth 139 (1994) 147. [3] Z. Galazka, H. Wilke, J. Crystal Growth 216 (2000) 389. [4] B. Cockayne, B. Lent, J.S. Abell, R. Harris, J. Mater. Sci. 8 (1973) 871. [5] A. Lupei, C. Stoicescu, V. Lupei, J. Crystal Growth 177 (1997) 207. [6] Y. Peizhi, D. Peizhen, Y. Zhiwen, T. Yulian, J. Crystal Growth 218 (2000) 87. [7] K. Mazur, W. Wierzchowski, J. Crystal Growth 148 (1995) 345.
ARTICLE IN PRESS D.E. Eakins et al. / Journal of Crystal Growth 267 (2004) 502–509 [8] M. Katsurayama, Y. Anzai, A. Sugiyama, M. Koike, Y. Kato, J. Crystal Growth 229 (2001) 193. [9] V. Iov, J. Crystal Growth 167 (1996) 180. [10] C. Suryanarayana, M.G. Norton, X-ray Diffraction: A Practical Approach, Plenum Press, New York, 1998, pp. 153–166. [11] M. Gerber, Th. Graf, Optics Laser Technol. 33 (2001) 449. [12] J.J. Gilman, Trans. Met. Soc. AIME 212 (1958) 310. [13] D.B. Williams, C.B. Carter, Transmission Electron Microscopy: Imaging, Plenum Press, New York, 1996, p. 445.
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[14] D.B. Williams, C.B. Carter, Transmission Electron Microscopy: Diffraction, Plenum Press, New York, 1996, pp. 321–323. [15] T. Mura, Micromechanics of Defects in Solids, Martinus Nijhoff Publishers, Dordrecht, 1987, p. 79. [16] S. Timoshenko, J.N. Goodier, Theory of Elasticity, McGraw-Hill Book Company, New York, 1951, p. 358. [17] K.W. Andrews, D.J. Dyson, S.R. Keown, Interpretation of Electron Diffraction Patterns, Plenum Press, New York, 1967 (plate 3).