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Influence of Factors on Growth of Off-Congruent LiNbO3 SingleCrystal by Li-Rich/Li-Poor Chemical Vapor Transport Equilibration De-Long Zhang,*,†,‡ Bei Chen,† Dao-Yin Yu,† and Edwin Yue-Bun Pun‡ †

Department of Opto-electronics and Information Engineering, School of Precision Instruments and Opto-electronics Engineering, and Key Laboratory of Optoelectronic Information Technology, Ministry of Education, Tianjin University, Tianjin 300072, People’s Republic of China ‡ Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, People’s Republic of China

ABSTRACT: Influence of factors on growth of off-congruent LiNbO3 single-crystal by Li-rich/Li-poor chemical vapor transport equilibration (VTE) was studied. These factors include the molar ratio of raw materials Li2CO3 and Nb2O5 for preparing twophase crucible, the hollow space volume for VTE growth, and the period of use of two-phase crucible. To achieve the goal, a number of off-congruent LiNbO3 plates were grown by Li-rich/Li-poor VTE technique using four two-phase crucibles having different molar ratios of raw materials, hollow space volumes, and periods of use. The VTE-induced crystal composition alteration was characterized by measurement of birefringence. A comparison of results of different crucibles shows that in the Lirich VTE case the first factor has less effect, while the latter two may cause a difference of Li2O content as much as 0.4 mol %. Larger hollow space volume and older crucible tend to degrade the VTE growth efficiency. Instead, the growth by Li-poor VTE is less affected by the three factors because of much lower Li2O gas pressure and hence slow growth rate in comparison with that by Li-rich VTE.

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(only ∼1.4 mol % at the Curie point7,8) limits demonstration of efficient devices. The solubility of rare-earth ions depends on not only the diffusion temperature but also the Li2O content in crystal. Under the same temperature, the lower the Li2O content in the crystal, the more the intrinsic defects are present in the crystal, the more the rare-earth ions can be accepted, and hence, the higher the solubility would be. To increase the rareearth ion solubility and diffusivity, an off-congruent, Li-deficient LN is desired. Although the LN phase exists over a wide solid solution range, roughly from 44 to 50 mol % Li2O content, almost all commercially produced LN crystals have a congruent composition (Li2O content = 48.4−48.6 mol %). Chemical vapor transport equilibration (VTE) is an effective and practical method used to adjust Li2O content inside a pure or doped LN.

INTRODUCTION LiNbO3 (LN) crystal is extensively studied because of its wide transparency range, high electro-optic and nonlinear optic coefficients, very high electro-mechanical coupling coefficients, and chemical and mechanical stability. It may find wide uses in fields of electro-optics, acousto-optics, nonlinear optics, and guided-wave optics. A near-stoichiometric (NS) LN displays a number of attractive advantages over the congruent material, such as stronger electro-optic1 and nonlinear optical effects,2 and largely lowered coercive electric field needed for ferroelectric domain reversal.3,4 In addition, an NS LN only needs a little amount of MgO (>0.78 mol %) to prevent photorefractive effect.5,6 An NS LN crystal doped with moderate MgO concentration is a more promising material for nonlinear (integrated) optics. On the other hand, an off-congruent, Lideficient LN crystal may find its use in the field of active waveguide devices. Since the optical gain of a laser or an amplifier depends on the concentration of active ions, the relatively low solubility of rare-earth ions in a congruent LN © 2013 American Chemical Society

Received: February 1, 2013 Revised: February 16, 2013 Published: February 18, 2013 1793

dx.doi.org/10.1021/cg4002016 | Cryst. Growth Des. 2013, 13, 1793−1798

Crystal Growth & Design

Article

As an alternative, an off-congruent NS/Li deficient pure or doped LN plate for various application purposes can be grown starting from a commercial congruent LN plate by Li-rich/Lipoor VTE method. Over the past years, the Li-rich or Li-poor VTE technique has been extensively applied to grow offcongruent pure or doped LN crystal plates for various application purposes such as property characterization, substrate material preparation, and passive and active waveguide fabrication.9−32 To fully understand the VTE growth and grow an offcongruent, NS/Li-deficient LN crystal plate with a desired composition profile, it is interesting and crucial to know various possible factors that affect the VTE growth efficiency, defined as the VTE-induced Li2O content alteration after a certain growth time for a given growth temperature. The Li-rich VTE growth involves the lithium diffusion into the bulk, and the Lipoor VTE growth concerns the lithium diffusion out of crystal (i.e., Li2O vaporization via the crystal surface). Both the Li-rich and Li-poor VTE are dynamic processes. The Li composition on crystal surface and in bulk varies with both the VTE growth temperature and time. Thus, the VTE growth temperature and time are the two major factors affecting the Li composition in the crystal. In the earlier papers, studies on Li-rich/Li-poor VTE growth temperature/time dependences of crystal composition have been reported.18,19 The results showed that the VTE-induced alteration of Li2O content obeys a profile of an integral of error function complement in the depth direction. At crystal surface, it has an Arrhenius relationship to the VTE growth temperature and an approximate square root dependence on the VTE growth duration for both cases of Li-rich and Li-poor VTE. In addition to the VTE growth temperature and time, it is unclear if the Li-rich/Li-poor VTE growth efficiency is influenced also by other factors such as the molar ratio of raw materials Li2CO3 and Nb2O5 for preparation of two-phase crucible, the volume of hollow space sealed by two-phase powder and used for VTE growth, as well as the period of use of two-phase crucible. Present study focuses on the effect of these three factors on the Li-rich/Li-poor VTE growth efficiency.

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Figure 1. (a) Experimental setup for Li-rich/Li-poor VTE growth. (b/ c) Schematic principle of Li-rich/Li-poor VTE growth.

For the case of Li-rich VTE, Li-rich two-phase powder releases the Li2O vapor under the elevated temperature according to the solid phase chemical reaction Li3NbO4 = Li2O↑ + LiNbO3. The Li2O gas cannot escape out of the sealed crucible and fills in the hollow space formed by the crucible and the lid as shown in Figure 1a. As a result, a certain pressure of Li2O gas is generated in the hollow space. Because of the Li concentration gradient outside and inside the crystal, the evaporated Li2O gas is taken up by the crystal. At the elevated temperature, the Li ions (from the evaporated Li2O gas) diffuse into the bulk. As a result, the Li2O content in crystal increases gradually as the VTE proceeds. Figure 1b illustrates the schematic of Li-rich VTE growth. For the case of Li-poor VTE, because of the Li concentration gradient inside and outside the crystal, the Li ions in the bulk of crystal out-diffuse to the crystal surface at the elevated temperature and the crystal evaporates the Li2O gas. A low pressure of Li2O gas is generated in the hollow space. To maintain the vapor equilibrium, part Li2O gas sinks in the Nb-rich two-phase powder due to the solid phase chemical reaction Li2O↑ + LiNb3O8 = 3LiNbO3. As the VTE proceeds, the original congruent crystal becomes more and more Li-deficient due to the continuous Li2O loss from the crystal. Figure 1c shows the schematic of Li-poor VTE growth. As the seeding crystals, eight commercial Z-cut congruent LN plates with optical grade surfaces and same thickness of 0.5 mm were used for the present study. These plates are divided into two groups and each group contains four plates. The four plates in group one were annealed in the Li-rich atmosphere using four different Li-rich twophase crucibles including three newly prepared ones and an old one. Each plate was contained in a Li-rich crucible. The three new crucibles were prepared according to three different Li2CO3 and Nb2O5 molar ratios A:B = 68:32 mol %, 60:40 mol %, and 55:45 mol %. The old crucible has been already used for a long time, say 1000 h or longer. It was prepared by calcining a Li2CO3−Nb2O5 mixture with A:B = 68:32 mol %. The four LN plates in their respective crucibles were consecutively subjected to five Li-rich VTE growth cycles. The VTE growth temperature was fixed at 1100 °C. The growth duration varied from 0 to 2, 4, 6, 10, and 12 h. After each VTE growth cycle, the Li2O content on the crystal surface was evaluated from the measured birefringence at the wavelengths of 633, 1311, and 1553 nm. After the 12 h Li-rich VTE growth, the depth profiles of Li2O content in the four samples studied were characterized by alternatively doing at first the grinding and optical polish to one crystal surface and then the measurement of birefringence, from which the Li2O content was evaluated. The thickness of the plate after each polishing procedure was measured by a digital vernier caliper with an accuracy of

EXPERIMENTAL SECTION

Here, the Li-rich VTE experiments employed a Li-rich two-phase (LiNbO3 + Li3NbO4) crucible, while the Li-poor VTE experiments used an Nb-rich two-phase (LiNbO3 + LiNb3O8) crucible. The twophase crucible was prepared by sintering the homogeneous mixture of Li2CO3 (99.99% in purity) and Nb2O5 (99.99% in purity) powder with a desired molar ratio either A:B (for Li-rich) or A′:B′ (for Nb-rich). The mixture was pressurized and molded into a crucible model. A precalcination at 1000 °C for 10 h and an additional calcination at 1100 °C for 1 h produced an Li-rich or an Nb-rich two-phase crucible. The solid phase chemical reaction involved is A mol % Li2CO3 + B mol % Nb2O5 = C mol % CO2↑ + D mol % LiNbO3 + E mol % Li3NbO4 for the Li-rich crucible and A′ mol % Li2CO3 + B′ mol % Nb2O5 = C′ mol % CO2↑ + D′ mol % LiNbO3 + E′ mol % LiNb3O8 for the Nb-rich crucible. Figure 1a shows the schematic of setup for Li-rich or Li-poor VTE growth experiment. First, as the seeding crystal, congruent LN plate was wrapped with a platinum (Pt) wire to avoid contact with the twophase powder and contained in the two-phase crucible. Then, the crucible was covered by a lid, which was made of the same two-phase powder, and tightly sealed by filling some fine two-phase powder into the gap between the crucible and the lid. Subsequently, the crucible was heated up to the target temperature and dwelled for a scheduled duration. 1794



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10 μm. For each sample, the thickness reduction was carried out on the same surface all the time. On the other hand, the four plates in group two were annealed in the Li-poor atmosphere using four different Nb-rich two-phase crucibles including three newly prepared ones and an old one too. Each plate was contained in an Nb-rich crucible. The three new crucibles were prepared according to three different Li2CO3 and Nb2O5 molar ratios A′:B′ = 30:70 mol %, 40:60 mol %, and 45:55 mol %. The old crucible has been already used for 1500 h or longer. It was prepared by calcining a Li2CO3−Nb2O5 mixture with A′:B′ = 40:60 mol %. The four LN plates in their respective crucibles were consecutively subjected to three Li-poor VTE growth cycles. The VTE temperature was also fixed at 1100 °C. The duration varied from 0 to 26, 52, and 120 h. After each VTE cycle, the Li2O content on the crystal surface was evaluated from the measured birefringence, too. After the 120 h Li-poor VTE growth, the depth profiles of Li2O content in the four samples studied were characterized. The characterization method is similar to the Li-rich VTE case. The measurement of birefringence (i.e., the measurement on the ordinary and extraordinary refractive indices) was accomplished by a Metricon 2010 prism coupler (Metricon Corp., Pennington, NJ), which has a working principle of measurement on critical angle of total reflection. Note that the refractive index measured by this method should be the value at the crystal surface because the total reflection phenomenon takes place there. It is convenient to choose a transverse magnetic (TM) or a transverse electric (TE) polarization scheme to measure the ordinary or extraordinary index, depending on the cut of the plate to be measured. All of the measurements were carried out at room temperature (24.5 ± 0.1 °C). In addition, the Li-rich or Nb-rich crucibles have different volumes of hollow space. To examine the hollow space volume effect on the VTE growth efficiency, the hollow space volume in each crucible was roughly evaluated in such a way. At first, the hollow space was filled with fine powdered two-phase mixture. Then, the volume of the twophase powder used for filling the hollow space was simply measured using a volumetric cylinder. The measurement error of volume is estimated to be within ±0.5 mL.

Effect on Li-Rich VTE Growth Efficiency. Figure 2 shows the Li-rich VTE growth duration dependences of VTE-induced

Figure 2. Li-rich VTE growth duration dependence of VTE-induced Li2O content increase on the surface of four 0.5 mm thick, initially congruent Z-cut LN plates, which were VTE-grown at 1100 °C using four different Li-rich two-phase crucibles.

Li2O content increase ΔC(Li2O) (relative to the congruent point, 48.5 mol %) on surfaces of the four 0.5 mm thick, initially congruent Z-cut LN plates, which were VTE-grown at 1100 °C using four different Li-rich two-phase crucibles. Figure 3 illustrates the depth profiles of Li-rich VTE-induced Li2O

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RESULTS AND DISCUSSION As pointed out in the introduction part, the three possible factors that affect the VTE growth efficiency include the molar ratio of raw materials Li2CO3 and Nb2O5, the hollow space volume and the period of use of two-phase crucible. It is evident that the effect on the VTE growth efficiency is reflected by the modification to the VTE-induced Li-composition alteration in crystal. The Li composition in an LN crystal can be determined by many optical and nonoptical methods.14 Here, the optical method of birefringence measurement was used to quantify the Li2O content at the surface and in the bulk of studied samples. Schlarb and Betzler have studied the crystal composition effect on the birefringence of LN crystal and established a Li2O content-dependent Sellmeier equation.33 By utilizing this equation, one can readily evaluate the Li2O content from measured birefringence. Evaluations at the wavelengths 632.8, 1311, and 1553 nm give an averaged Li2O content value. The measurement error of refractive index, ∼10−3, yields a Li2O content uncertainty of ±0.1 mol %. For reference purposes, before the VTE growth, the birefringence measurement was also carried out on the congruent plate, and the Li2O content was also evaluated. The result shows that the as-grown congruent LN plate has a Li2O content of 48.5 ± 0.1 mol %, which is consistent with the nominal value of 48.4−48.6 mol %. The consistency implies that the Li2O content data evaluated from the measured birefringence are sound. Next, we discuss the effect of the three factors on the Li-rich and Li-poor VTE growth efficiency separately.

Figure 3. Depth profile of Li-rich VTE-induced Li2O content increase in four 0.5 mm thick, initially congruent Z-cut LN plates after the VTE growth at 1100 °C for 12 h using four different Li-rich two-phase crucibles.

content increase ΔC(Li2O) in the four plates after the VTE growth at 1100 °C for 12 h. The experimental error is indicated for each data. As shown in Figure 2, the Li2O content increase ΔC(Li2O) on crystal surface shows a similar VTE growth duration dependence for all four cases of different crucibles. In the initial stage, the ΔC(Li2O) increases strongly with the prolonged VTE. The increase then slows down gradually, and a saturation tendency appears as the VTE growth duration is longer than ∼12 h. Note that around 12 h all of the four plots tend to a similar value, 1.45−1.5 mol %, which can be regarded as the ideal stoichiometric composition (1.5 mol %) within the error of ±0.1 mol %. This result is expected and essentially consistent with the previously reported result that under the 1100 °C VTE growth temperature the duration required for surface composition reaching the stoichiometry is around 15.6 h.18 In Figure 3, the ΔC(Li2O) in the bulk of crystal also shows 1795



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a similar depth dependence for all four cases of different crucibles. The ΔC(Li2O) decreases with the increased depth, passes through a minimum at the depth position half of the plate thickness, ∼0.25 nm, and then increases gradually with the further increased depth. This observation is also expected. Next, we turn to discuss the effect of the three factors. One can see from Figure 2 that the four plots show some differences with a largest discrepancy of 0.4 mol % before the saturation phenomenon takes place, indicating that the three factors concerning with the two-phase crucible affect the VTE growth efficiency on the crystal surface before the saturation phenomenon takes place. Similar effect is also observed in the bulk of crystal as shown in Figure 3. It can be seen from Figures 2 and 3 that the newly prepared 60:40 mol % crucible with the smallest volume of hollow space of 12 mL led to the largest VTE growth efficiency (see the full square plot). The old crucible with the higher A:B ratio (= 68:32 mol %) and a volume of hollow space of 19 mL caused the lowest VTE growth efficiency (see the open triangle plot). In contrast, the newly prepared crucible with the same A:B ratio (= 68:32 mol %) and a similar volume of hollow space of 18 mL led to the higher VTE growth efficiency (see the full triangle plot). Among the three new crucibles, the new crucible with the smallest A:B ratio (= 55:45 mol %) and the largest volume of hollow space of 30 mL resulted in the lowest VTE growth efficiency, which is comparable to the growth efficiency of the old crucible (see the full asterisk plot). A comparison of the full square and full triangle plots allows to conclude that the molar ratio A:B is not the major factor affecting the VTE growth efficiency. The reason for this conclusion is that the crucible with the larger (smaller) A:B ratio led to the lower (higher) VTE growth efficiency. Hence, the difference between the full square and full triangle plots must result from the volume of hollow space, which is 12 mL for the crucible with the smaller A:B ratio (= 60:40 mol %) and 18 mL for the crucible with the larger A:B ratio (= 68 mol %: 32 mol %). It appears that the crucible with the smaller volume of hollow space led to the higher VTE growth efficiency. To clearly show the effect of hollow space volume on the VTE growth efficiency, as the representatives, we pick up from Figure 2 the Li2O content increase data after the 2 h (full squares) and 4 h (full circles) Li-rich VTE growth using the three newly prepared two-phase crucibles, which have different hollow space volumes, and draw against the hollow space volume in Figure 4. Because the VTE growth efficiency is less influenced by the molar ratio A:B, one may therefore conclude from Figure 4 that the Li2O content alteration increases with the reduced hollow space volume. This is readily understood. The smaller the volume, the larger the surface area to volume ratio, and hence, the higher the VTE growth efficiency. In this sense, one can also understand why the 55:45 mol % crucible with the largest hollow space volume shows the lowest VTE growth efficiency among the three new crucibles. A further comparison of the open and full triangle plots allows to conclude that the crucible shows slight time-aging effect on the VTE growth efficiency. The largest gap between the two plots is around 0.2 mol % and the old crucible shows a lower VTE growth efficiency than the new one. The long-term use of crucible leads to the reduction of the Li3NbO4 phase content in the two-phase crucible due to the continuous solidphase chemical reaction Li3NbO4 = Li2O↑ + LiNbO3 under the elevated temperature and hence degrades the VTE growth efficiency.

Figure 4. Hollow space volume dependence of VTE-induced surface Li2O content increase resulting from the 2 h (full squares) and 4 h (full circles) VTE growth using the three newly prepared different Lirich two-phase crucibles. All of the data are taken from Figure 2.

Effect on Li-Poor VTE Growth Efficiency. Figure 5 shows the Li-poor VTE growth duration dependences of VTE-

Figure 5. Li-poor VTE growth duration dependence of VTE-induced Li2O content reduction on the surface of four 0.5 mm thick, initially congruent Z-cut LN plates, which were VTE-grown at 1100 °C using four different Nb-rich two-phase crucibles.

induced Li2O content reduction ΔC(Li2O) (relative to the congruent point, 48.5 mol %) on surfaces of the four 0.5 mm thick, initially congruent Z-cut LN plates, which were VTEgrown at 1100 °C using four different Nb-rich two-phase crucibles. Figure 6 illustrates the depth profiles of ΔC(Li2O) in the four plates after the Li-poor VTE growth at 1100 °C for 120 h. The experimental error is indicated for each data. As shown in Figure 5, the Li2O content reduction ΔC(Li2O) on crystal surface shows a similar VTE growth duration dependence for all four cases of different crucibles. In Figure 6, the ΔC(Li2O) in the bulk of crystal also shows a similar depth dependence for all four cases of different crucibles. These growth duration or depth dependences of VTE-induced Li2O content reduction are expected and similar to the case of Li-rich VTE growth. Very different from the Li-rich VTE case, the four plots corresponding to four different Li-poor two-phase crucibles can be considered as the same within the experimental error of ±0.1 mol % as shown in Figures 5 and 6. This is the case whether on the crystal surface or in the bulk of crystal. Note that, like the Li-rich VTE case, the four Nb-rich two-phase crucibles include three newly prepared ones and an old one, 1796



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CONCLUSIONS We have demonstrated the effect of three possible factors on VTE growth efficiency in initially congruent LN crystal. These factors include the molar ratio of raw materials Li2CO3 and Nb2O5 used for two-phase crucible preparation, the volume of hollow space used for VTE growth, and the period of use of two-phase crucible. The results show that for the case of Li-rich VTE the first factor has less effect, while the latter two may cause a difference of Li2O content as much as 0.4 ± 0.1 mol %. It appears that larger hollow space volume and older crucible tend to degrade the VTE growth efficiency. For the Li-poor VTE case, however, all of the three factors have less effect because of much lower Li2O gas pressure and slow growth rate in comparison with the case of Li-rich VTE growth.

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Figure 6. Depth profile of Li-poor VTE-induced Li2O content reduction in four 0.5 mm thick, initially congruent Z-cut LN plates after the VTE growth at 1100 °C for 120 h using four different Nb-rich two-phase crucibles.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 22 18902057533. Fax: +86 22 2740 6726. Notes

The authors declare no competing financial interest.

have three different molar ratios A′:B′ of 30:70 mol %, 40:60 mol %, and 45:55 mol %, and have two different hollow space volumes of 6.5 mL for the two new crucibles with the A′:B′ ratios of 30:70 mol % and 40:60 mol % and of 9.0 mL for the other two crucibles including a new one with the A′:B′ ratio of 45:55 mol % and an old one with the A′:B′ ratio of 40:60 mol %. We may thus conclude that, unlike the Li-rich VTE case, the three factors have less effect on the Li-poor VTE growth efficiency. The Li+ diffusivity decreases with the decreased Licomposition in the crystal.11,21 It can be anticipated that as the Li-poor VTE proceeds, the Li composition in the crystal reduces gradually, and hence, the VTE growth becomes slow more and more. As pointed out by Bordui et al.,12 the Li-poor VTE is far slower than the Li-rich VTE. Actually, this argument is clarified by our experimental data. One can see from Figures 2 and 5 that an Li-rich VTE growth process of only 12 h can lead to a 1.5 mol % surface Li2O content alteration, while a Lipoor VTE growth process as long as 120 h only leads to a 1.0 mol % surface Li2O content alteration. The slow process of Lipoor VTE can be also understood from the viewpoint of Li2O gas pressure generated in the hollow space of two-phase crucible. As described in the experimental section, in the case of Li-rich VTE, the pressure is generated by the Li2O gas from the two-phase powder under the elevated temperature. In the case of Li-poor VTE, however, the pressure is produced by the Li2O gas evaporated from the LN crystal. It is no doubt that the Li2O gas evaporated from the LN crystal in the Li-poor VTE case must be much less than that from the two-phase powder in the Li-rich VTE case. Accordingly, the Li2O gas pressure in the Lipoor VTE case is much lower than that in the Li-rich VTE case. The lower the Li2O gas pressure, the lower the VTE growth efficiency. The Li-poor VTE is therefore much slower than the Li-rich VTE. Because of a rather slow growth rate of Li-poor VTE, the effect of three factors on the Li-poor VTE growth efficiency is therefore not as obvious as on the Li-rich VTE growth efficiency. Here, we exemplify to explain the little timeaging effect of two-phase crucible. Unlike the Li-rich VTE case, the long-term use of Li-poor crucible cannot lead to a considerable reduction of the LiNb3O8 phase content in the two-phase crucible and hence affects the VTE growth efficiency. This is because the LN crystal evaporates much less Li2O gas than does the two-phase crucible in the Li-rich VTE case.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under Project no. 50872089 and 61107056, by the Key Program for Research on Fundamental to Application and Leading Technology, Tianjin Science and Technology Commission of China under Project no. 11JCZDJC15500, and by Specialized Research Fund for the Doctoral Program of Higher Education of China under Project no. 20100032110052.

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