Twinning structures in near-stoichiometric lithium niobate single crystals

Twinning structures in near-stoichiometric lithium Shuhua Yao,a Xiaobo Hu,b Tao Yan,b Hong Liu,b* Jiyang Wang,b* Xiaoyong Qinc aDepartment of Materials Science and Engineering, Nanjing University, Nanjing, 210093, People’sRepublic of China, bState Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, People’s Republic of China, and cDeqing Huaying Electronics Company, Deqing 313200, People’s Republic of China. Correspondence e-mail: [email protected], [email protected] A near-stoichiometric lithium niobate single crystal has been grown by theCzochralski method in a hanging double crucible with a continuous powdersupply system. Twins were found at one of the three characteristic growth ridgesof the as-grown crystal. The twin structure was observed and analyzed bytransmission synchrotron topography. The image shifts ÁX and ÁY in thetransmission synchrotron topograph were calculated for the 32 12 and 0222reflections based on results from high-resolution X-ray diffractometry. It is # 2010 International Union of Crystallography confirmed that one of the {011 2}m planes is the composition face of the twin and Printed in Singapore – all rights reserved matrix crystals. The formation mechanism of these twins is discussed.
method. From the synchrotron radiation topographs ofLiNbO3, the twin structure has been determined. Finally, the Lithium niobate (LiNbO3, LN) single crystals have been formation mechanism of twins in an NSLN crystal is discussed.
widely used as electro-optic Q-switches and acoustic surfacewave filters because of their good electro-optic, piezoelectric and nonlinear optical properties (Bergman et al., 1968;Carruthers et al., 1971; Tomeno & Matsumura, 1987). As is Lithium niobate crystals have a rhombohedral structure, point well known, congruent LiNbO3 (CLN) crystals are usually group 3m. The cell parameters of NSLN crystals can also be grown from a congruent melt (molar ratio of Li to Nb is described by a hexagonal system with a = b = 5.1485 A 48.6:51.4) by the conventional Czochralski or Bridgman– ˚ (Yao et al., 2007). A crystal boule with a diameter of Stockbarge methods (Carruthers et al., 1971). Thus, there are 50 mm and a length of 50 mm was grown from an Li-rich melt many intrinsic defects in CLN crystals, owing to the lack of Li.
(Li2CO3:Nb2O5 = 58.5:41.5). In order to keep the composition These defects seriously reduce the optical performance of of the melt constant during crystal growth, a hanging double CLN crystals (Ballman, 1965; Byer, 1970).
crucible with a continuous powder supply system was adopted.
Fortunately, it was discovered that near-stoichiometric A molar ratio of Li2CO3:Nb2O5 of 50:50 was used to synthe- lithium niobate (NSLN) single crystals possess a compara- size the polycrystalline NSLN feed material. In the crystal tively perfect lattice in which few Nb anti-site defects and growth procedure, the pulling rate ranged from 0.2 to cation vacancies exist. However, other structural defects, such 0.3 mm hÀ1 and the rotation rate was kept at 5 r minÀ1. After as twins, inclusions and cracks, have often been found in termination of growth, the crystal was cooled to room NSLN crystals. Among these defects, twinning is a particular temperature in air at a rate of 30–40 K hÀ1 (Wu & Wang, problem, and strategies are still proposed on how to reduce or eliminate these defects. To reduce twinning, it is essential to From the morphology of the as-grown NSLN crystal, a understand the structure and formation mechanism of twins.
characteristic boundary was observed in the neighbourhood of Among the structural characterization tools available, white- one of the three growth ridges. In order to understand the beam synchrotron radiation topography is one of the most boundary structure, an (0001) slice (z cut) containing the effective. Compared with conventional X-ray topography, boundary, with a thickness of 250 mm, was cut from the as- synchrotron radiation topography has many advantages, such grown NSLN crystal, and then both sides of the slice were as a continuous spectrum, high intensity and good collimation polished. The rocking curve of the 0006 reflection of the (Huang et al., 1995; Hu et al., 1993). The chemical etching (0001) slice was recorded using a Siemens D5000 high-reso- method is also a useful technique to observe crystal defects. In lution X-ray diffractometer. During the experiment, the X-ray this paper, the twin structure of an NSLN crystal has been beam is incident on the boundary, and the matrix and twin on observed by means of the synchrotron radiation technique, the two sides of the boundary cannot satisfy the reflection high-resolution X-ray diffractometry and the chemical etching condition simultaneously. Because the intersecting line between the twin boundary and the (0001) plane is parallel to reflection. Because of the misorientation of two individuals on the x axis, we guess that the twin and the matrix share the same two sides of the sub-grain boundary, the two peaks in this x axis. In this case, if we take the x axis as the ! axis and rotate figure correspond to the 0006 reflections of the matrix and the sample around this ! axis, the matrix and twin will satisfy twin. Since the two 0006 reflections share the same tilts, the the reflection conditions at different ! positions. Synchrotron two individuals have structurally the same x axis. From Figs. 1 topographic observations were performed using the trans- and 2, we know that the misorientation angle between matrix mission Laue geometry at beamline 4W1A of the Beijing and twin is  = 6.5 (1). A schematic diagram of the orienta- Synchrotron Laboratory (BSRL). Optical microscopic obser- tion relationship between matrix and twin is shown in Fig. 3.
vation was conducted using an Olympus optical microscope.
The sample was etched in an acid solution (HF:HNO3 = 1:2 v/ synchrotron radiation topograph of the (0001) slice containing the twin boundary was recorded. Fig. 4 shows two transmissionsynchrotron topographs of the (0001) slice for differentreflections. From these images, we can see splitting of the reflection spots, which is due to reflections from the matrix and Fig. 1 shows a photograph of an NSLN crystal. It can be seen twin in the NSLN crystal. The magnitude of the image shift that a twin boundary exists at the bottom of the grown crystal varies with the different reflections. An image shift originating and the twin is nucleated at one of the three characteristic from misorientation between matrix and twin can be clearly growth ridges. As described in the previous section, the x axis observed. To characterize the shift value of the topography was selected as the ! axis. Fig. 2 shows the rocking curve of the images quantitatively according to the misorientation of two (0001) slice of the NSLN crystal, corresponding to the 0006 individuals, a reference coordinate system is established inwhich the x and z axes are parallel to the a and c axes,respectively, the y axis is perpendicular to the x and z axessimultaneously, and the angle between y and b is /6. In thissystem, a, b and c can be written as follows: For a twin crystal, a0, b0 and c0 are expressed in the same The morphology of the as-grown NSLN crystal.
We can then obtain two sets of reciprocal space base vectors, a*, b* and c*, and a0*, b0* and c0*: The rocking curve of the (0001) slice of the NSLN crystal, corresponding A schematic representation of the orientation relationship between two Shuhua Yao et al.  Twinning in lithium niobate crystals Similarly, the Bragg angle 0 for an hkl reflection from the twin If the vector of the hkl reflection is further projected on film, the distance between transmission and the hkl reflection can be accurately determined for the matrix and twin, respectively, Here, a and c are the cell parameters of the NSLN crystal and i, j and k are the unit vectors of the reference system along x, yand z, respectively. In our experiment, the sample surface is In order to describe quantitatively the position of a refraction the (h0k0l0) plane and the incident beam is perpendicular to spot on film, a reference system Rx and Ry is defined on the the sample surface. Therefore, the incident vector can be film. R and R0 are projected on the Rx and Ry directions, written as H0 = h0a* + k0b* + l0c* in reciprocal space. For an respectively. The position of an hkl reflection on film can be hkl reflection from the matrix, the Bragg angle  can be determined. In our experiment, the incident beam was perpendicular to the (0001) plane and the distance D betweenthe sample and the film that recorded the images was 55 mm.
Thus, H0 = c*. The image shift is described by the shift valuesÁX and ÁY, which can be calculated by equations (7) and (8).
The shift values of the 32 12 reflection in the film werecalculated as ÁX = 0.299 mm and ÁY = 3.416 mm, and themeasured values are ÁX = 0.185 mm and ÁY = 3.05 mm. Theshift values of the 0222 reflection in the film were calculated asÁX = 0.158 mm and ÁY = 0.183 mm, and the measured valuesare ÁX = 0 mm and ÁY = 0.154 mm. The difference betweenthe calculated values and those measured from the topographsis mainly due to the deviation of the incident beam from thenormal line of the (0001) slice (Hu et al., 2001). We can Figure 4(a) Synchrotron radiation topograph of the (0001) slice of the NSLNcrystal, corresponding to the 32 12 reflection. (b) Synchrotron radiationtopograph of the (0001) slice of the NSLN crystal, corresponding to the The etching patterns of twins on the (0001)m wafer.
Shuhua Yao et al.  Twinning in lithium niobate crystals therefore conclude from the image shifts observed in supercooling. Work to investigate how to control the compo- synchrotron topography that there is a misorientation sition and stabilize the supercooling more precisely is ongoing.
between the matrix and twin crystals.
Fig. 5 shows the development of etching pits in the (0001) wafer. Twin boundaries can be observed using optical micro-scopy, based on the different chemical etching rates between In summary, the twins of an NSLN crystal have been investi- the matrix and twin crystals. The difference in etching patterns gated by white-beam synchrotron radiation topography and on the two sides of the twin boundary is remarkable. The chemical etching methods. From high-resolution X-ray intersecting lines of the twin boundary with the upper and diffraction data, we deduce that the misorientation of the twin lower surfaces can be clearly seen in Fig. 5. The shapes of the and matrix crystals is 6.5 (1) away from the normal to the etching pits are related to the symmetry of the lattice plane.
[0001] direction. The image shift values of the 32 12 and 0222 The [0001] direction is the threefold symmetric operation axis, reflections have been calculated. The etching patterns on both so the triangular etching pits appear in the matrix crystal sides of the twin boundary showed remarkable differences due region, but we could not observe etching pits in the twin to misorientation. The composition plane of the crystal is one of the {011 2}m planes. Twin formation strongly depends on the We measured the thickness of the etched slice as 192 mm stress around the solid–liquid interface.
using a pocket-sized thickness gauge (Shenyang KejingInstrument Company, SCH-1). The twin boundary is 125 mm This research was supported by the National High Tech- from the two sides, as shown in Fig. 5. We can calculate that nology Research and Development Programme of China (863 the twin boundary has an angle of 56.93 with the (0001) plane.
programme, grant No. 2006AA030106) and by the Natural Thus, its composition plane is the (101 2)m plane. We found the Science Foundation of China (grant Nos. 50572052, 50872070 twin nucleated at the three characteristic growth ridges, so the composition plane of the NSLN crystal is {101 2}m in the matrixcoordinate system. Based on the above geometric relationsbetween the matrix and twin crystals, we can also state that thecomposition plane can alternatively be described as {022 3}t in It is generally thought that the formation of mechanical twinning depends strongly on the crystal composition (Park & Kitamura, 1997; Vere, 1968). Park reported that twins formed on the bottom of grown crystals after detaching them from the melt; stress increases when crystal growth has ended and the crystal is detached from the melt. We have found that twins are generated as a result of oscillation of the growth interface during growth. Shape fluctuation is related to rapid super- cooling. Together with these processes, high stress appearing at the diameter on shrinkage makes twins nucleate. We deduce that the twin observed in an NSLN crystal can be considered to be mechanical twinning. Therefore, it is possible that twins can be initiated not only during the tailing process, but also in a high-stress process such as compositional variation and Shuhua Yao et al.  Twinning in lithium niobate crystals

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