- Email: [email protected]
Opposite domain formation in Er-doped LiNbO3 bulk crystals grown by the off-centered Czochralski technique
Journal of Crystal Growth 203 (1999) 179}185
Opposite domain formation in Er-doped LiNbO bulk crystals grown by the o!-centered Czochralski technique V. BermuH dez, M.D. Serrano, P.S. Dutta, E. DieH guez* Departamento de Fn& sica de Materiales, Universidad Auto& noma de Madrid, Cantoblanco, 28049 Madrid, Spain Received 13 January 1999; accepted 2 February 1999 Communicated by R.S. Feigelson
Abstract Opposite domain structures of 0.5 mol% Er-doped LiNbO crystals have been grown by the Czochralski technique in an o!-centered geometry by modulating the growth temperature. The period and homogeneity of the opposite domain structures have been related to the growth conditions. The period of the opposite domain structure does not follow the ratio between the pulling and rotation rate. The Er dopant concentration has been found to be constant along the domain structure. 1999 Elsevier Science B.V. All rights reserved. PACS: 77.84.Dy; 66.30.Jt; 77.80.Dj Keywords: Lithium niobate; Periodically poled; PPLN; Erbium
1. Introduction LiNbO (LN) is a very well known material for its electro-optic, acousto-optic and nonlinear properties [1]. Research on this material has been recently revitalized in two ways: on near stoichiometric material [2], and on its domain structure [3]. The domains in LN have been studied from the time of Nassau et al. (1966) [4], and they are well
* Corresponding author. Tel.: #34-913-97-4977; fax: #34913-97-8579. E-mail address: [email protected] (E. DieH guez)
known to be 1803 with the polarization vector (Ps) directed along the c-direction. The LN domain structures can be roughly divided into three classes: single domain, which plays an essential role in the applications and devices of LN crystals [5]; opposite domain (ODLN), with the Ps vector `tail to taila and `head to heada, for acoustic devices [6]; and periodically poled (PPLN), with the domains antiparallel to each other, for second harmonic generation in the blue}green laser radiation [7}9], optical parametric oscillators [10] and recently in order to inhibit the photorefractive damage [11]. In particular, in ODLN structures the number, thickness and quality of the domains are directly related to the desired properties of the technological
0022-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 0 8 7 - 1
180
V. Bermu& dez et al. / Journal of Crystal Growth 203 (1999) 179 }185
devices. One way to produce these ODLN structures is by the Czochralski (Cz) growth technique using the appropriate growth parameters and with impurities such as yttrium (Y) added to the melt [12]. Nevertheless, there is an unclear answer to understand the role played by this impurity in the ODLN formation. In general, it is assumed that the transition metals substitute for Li atoms in the regular position, whereas the rare earths (RE) elements are located in the Li position shifted by a few tenths of an As towards the nearest oxygen plane [13] depending on the atomic radius of the RE ion [14]. In this way an internal electric "eld appears to compensate the space charge created by the RE ion in the LN matrix, which could be responsible for the domain structure [15]. In particular the Er ion is a RE optical ion in which a large amount of useful optical processes take place [16]. While its most useful optical property allows optical gain and laser oscillation for wavelengths around 1.5 lm (of great interest in guided optical communications), other optical properties have been demonstrated as well [17,18]. At low concentrations, the Er ion unambiguously occupies the ferroelectric Li position displaced by about 0.2 As . At higher concentrations, a random position associated within a nonoctahedral location has been proposed [19], which means a solubility limit of about 3 mol% [20]. The total solubility limit in axial positions has been determined to be below 0.5 mol% [20]. On the other hand, in a recent work related with the opposite domain nature of as-grown Er-doped LN crystals [21], it has been shown that for melt concentrations higher than 3 mol%, the domain structure shows a periodical behavior, while for concentrations below 3 mol%, a disordered domain structure has been observed. In this way it is important to develop ODLN structures with the highest dopant concentration showing a total axial incorporation to avoid the formation of clusters and precipitates. In this work the ODLN formation in bulk Er-doped LN crystals grown by an o!-centered Cz technique has been studied, and the growth parameters have been analyzed in order to improve the quality of the ODLN obtained.
2. Experimental procedure The Er-doped LN single crystals were grown by the o!-centered Cz technique [22] in a Cz-UAM equipment described elsewhere [23]. The starting material was LN congruent powder with 0.5 mol% Er, added to the melt as Er O and contained in a Pt crucible of 5 cm in diameter. The LN seed was oriented along the c-direction, with 4;4;20 mm dimensions and displaced 5 mm from the center of the crucible, which means a nonsymmetric temperature "eld for the growing crystal. The pulling rate (v ) was 2 mm/h and the rotation rate (v ) was varied from 10 to 4 rpm during the growth process, as it will be later discussed. Temperature control at the growing solid}liquid interface allowed modulation of the growth temperature.
Fig. 1. Er-LN1 crystal, with the temperature evolution, the growth derivative and the rotation rate (rpm) during the growth process.
V. Bermu& dez et al. / Journal of Crystal Growth 203 (1999) 179 }185
181
Fig. 2. Domain structure of y-cut samples taken from the A(a) and B(b) zones of the Er-LN1 crystal. A magni"ed region of sample B is shown (c).
Several y-cut samples were obtained from di!erent parts of the bulk crystals, and the domain structure revealed by etching in a hot HF : HNO acid mixture. The domains were observed by means of optical and scanning electron microscopes (SEM). The variation of the dopant concentration along the domain structure has been measured by the wave dispersive X-ray analysis (WDX) technique in a Hewlett Packard JXA-8900M microscope.
3. Results and discussion Fig. 1 presents an Er : LN crystal (Er-LN1) which has been grown with a temperature control better than $0.23C, as it has been measured by the control thermocouple located at the bottom of the crucible. In the same "gure is shown the temperature evolution during the growth process, and #uctuations of the growth derivative (mg/min). The rotation rate has been changed during the growth process from 10 to 4 rpm. The samples to be analyzed have been taken from the A, B and C zones in order to compare the e!ect of the crystal length and the rotation rate on the domains structure. Fig. 2 shows the domain structures of y-cut samples
taken from the A (Fig. 2a) and B (Fig. 2b) zones of the Er-LN1 crystal. The arrow in Fig. 2b corresponds with the enlarged region shown in Fig. 2c. It can be observed that the domain structure is inhomogeneous along the whole crystal length. For sample A (Fig. 2a) the domain structure corresponds with the general domain behavior of a LN congruent crystal grown in a symmetric temperature "eld, i.e., with a nonperiodical polydomain structure. For sample B (Fig. 2b) the domain structure was improved and an ODLN structure clearly appears, although it remains inhomogeneous. The domain thickness was about 8 lm which does not correspond with the v /v ratio calculated from the crystal growth parameters. On the other hand, the domain quality for the samples removed from the C zone was worse than the one removed from sample B, showing an average thickness of 10}11 lm. The domain walls for sample B were very rough as can be seen in Fig. 2c, which are not useful from a technological point of view, due to the irregularities in the domain periodicity. Nevertheless the positive and negative domains were well de"ned and periodic. Thus taking into account that a 0.5 mol% Er-doped LN crystal grown in a symmetric temperature "eld has a nonperiodical ODLN structure [21], it can be pointed out that
182
V. Bermu& dez et al. / Journal of Crystal Growth 203 (1999) 179 }185
the domain behavior observed could be related with the o!-centered geometry of the growth axis. In order to improve the quality of the ODLN structure, a regular temperature variation of about 23C at the solid}liquid interface during the growth process was imposed. Fig. 3 presents the Er : LN crystal (Er-LN2) obtained, with the temperature evolution and the growth derivative #uctuations during the growth process. In this crystal there are three parameters which could a!ect the domain structure, (1) the temperature variation, (2) the o!-centered crystal geometry and (3) the variation of the rotation rate. As can be observed in Fig. 3, the temperature #uctuations create a periodical variation in the derivative and thus a periodical change in the crystal diameter. The Er-LN2 crystal was cut, as in the previous case, into three di!erent zones, A, B and C, corresponding to di!erent values of the temperature #uctuation and the rotation rate. Fig. 4 shows the domain structure of the samples taken from these zones. The domain structure at the beginning of the crystal (zone 1 of sample A in Fig. 4a) resembles the one observed in Fig. 2a, i.e., a polydomain structure. However, when the temperature #uctuations began, as it can be observed in the "nal part of zone A in Fig. 3, the domains become an ODLN structure (zone 2 of sample A). As the crystal length increases an improvement in the domain structure was clearly visible in zone B (Fig. 4b), where a large area with uniform opposite domains was obtained. This area preferentially appears in the increasing diameter zones, as it has been previously published [22]. The thickness of the periodic domains was 8.5 lm, which is not related with the v /v ratio of 3.3 lm/rot. When this ratio was changed to 8.3 lm/rot the domain thickness was 10}11 lm (Fig. 4c). The comparison between domains of both crystals indicates an improvement in the quality of the ODLN structure in the Er-LN2 crystal due to the temperature oscillations during the crystal growth. The domain walls for samples removed from Er-LN2 are smoother than those of Er-LN1 crystal, showing a better quality which indicates that the ODLN : Er is improved when a temperature #uctuation is coupled with an o!-centered growth.
Fig. 3. Er-LN2 crystal, with the temperature evolution, the growth derivative and the rotation rate (rpm) during the growth process.
On the other hand one must take into account that in Er-doped PPLN structures the domain thickness is totally related with the v /v ratio as it has been published [22], and which also happens with other kinds of dopants. Moreover, both PPLN and ODLN structures have been grown from the melt with (1) the same dopant concentration, (2) in a nonsymmetrical Cz geometry, (3) with similar v /v parameters, showing a do main thickness of 3.7 and 8.5 lm in the PPLN and ODLN cases respectively, and (4) with di!erent seed orientations, which means an x- and a coriented seed for the PPLN and the ODLN
V. Bermu& dez et al. / Journal of Crystal Growth 203 (1999) 179 }185
183
Fig. 4. Domain structure of the y-cut samples taken from the A(a), B(b) and C(c) zones of the Er-LN2 crystal. A detailed sketch of the samples marked by the arrow is shown.
structures respectively. This last di!erence could be responsible for the disagreement between the domain thickness in both PPLN and ODLN cases. In this way one must point out that the relative growth rates of the crystallographic faces of LN follow the relation: v 'v (where v and v are the V A V A relative growth rates for x- and c-axis respectively) [5]. Thus the dopant incorporation into the crystal
is favored in the growth processes along the c-axis, and as a consequence, the internal electric "eld able to create such structures is di!erent in both cases. More work is underway in this topic. Compositional analysis by WDX was carried out in order to correlate the Er distribution in the crystal with the formation of domains. Both, the ODLN structure (observed by SEM) and the WDX
184
V. Bermu& dez et al. / Journal of Crystal Growth 203 (1999) 179 }185
c-oriented seed from a nonsymmetrical Cz geometry. It has been shown that the o!-centering of the crystal plays an important role in the ODLN structures formation. A temperature #uctuation is neccessary to initiate growth striations in the crystal which improve the quality of the ODLN structures. It has been shown that the domain thickness does not follow the v /v ratio as in PPLN structures. Moreover, it has been shown that there is no variation in the Er concentration along the domain structure.
Acknowledgements The authors would like to acknowledge the partial support to realize this work by CAM-06T/015/96, PETRI-CICYT-95-0086-OP and CICYT-ESP981340.
References
Fig. 5. Domain structure (a) and WDX linescan (b) taken from the ODLN structures of the Er-LN2 crystal.
linescans are presented in Fig. 5. It can be observed that the Er concentration is constant along the domain structure in the range of the detection limit, which is $0.2%. This behavior disagrees with that reported in literature for the ODLN structures obtained from a melt doped with yttrium [22,24]. This fact could be related to the shift of the Er ion from the Li position, which could create a di!erent internal electric "eld from that created due to the yttrium ion in the regular Li position. This result suggets a di!erent polarization mechanism between Y and Er dopants. More work is underway to clarify these di!erences in the polarization mechanism.
4. Conclusions ODLN structures in 0.5 mol% Er-doped LN bulk crystals have been successfully grown with a
[1] F. AgulloH -LoH pez, J.M. Cabrera, F. AgulloH -Rueda, Electrooptics: Phenomena, Materials and Applications, Academic Press, San Diego, 1994. [2] G.Y. Malovichko, O. Cerclier, O.F. Schirmer, Solid State Commun. 89 (1994) 195. [3] V. BermuH dez, P.S. Dutta, M.D. Serrano, E. DieH guez, J. Crystal Growth 172 (1997) 269. [4] K. Nassau, H.J. Levinstein, G.M. Loiacono, J. Phys. Chem. Solids 27 (1966) 989. [5] A.M. Prokhorov, Yu.S. KuzH minov, Physics and Chemistry of Crystalline Lithium Niobate, Hilger, London, 1990. [6] Yong-yuan Zhu, Nai-ben Ming, Wen-hua Jiang, Yong-an Shui, Appl. Phys. Lett. 53 (1988) 2278. [7] G.D. Miller, R.G. Batchko, W.M. Tulloch, D.R. Weise, M.M. Fejer, R.L. Byer, Opt. Lett. 22 (24) (1997) 1834. [8] V. BermuH dez, J. Capmany, J. GarcmH a SoleH , E. DieH guez, Appl. Phys. Lett. 73 (5) (1998) 593. [9] J. Capmany, V. BermuH dez, E. DieH guez, Appl. Phys. Lett., 74 (11) (1999) 1. [10] L.E. Myers, R.C. Eckardt, M.M. Fejer, R.L. Byer, W.R. Bosenberg, J.W. Pierce, J. Opt. Soc. Am. B 12 (11) (1995) 2102. [11] M. Taya, M.C. Bashaw, M.M. Fejer, Opt. Lett. 21 (12) (1996) 857. [12] Yong-yuan Zhu, Nai-ben Ming, J. Appl. Phys. 72 (3) (1992) 904. [13] L. Rebouta, P.J.M. Smulders, D.O. Boerma, F. AgulloH LoH pez, M.F. da Silva, J.C. Soares, Phys. Rev. B 48 (1993) 3600.
V. Bermu& dez et al. / Journal of Crystal Growth 203 (1999) 179 }185 [14] A. Lorenzo, H. Ja!rezic, B. Roux, G. Boulon, J. GarcmH a SoleH , Appl. Phys. Lett. 67 (25) (1995) 3735. [15] L. Huang, N.A.F. Jaeger, Appl. Phys. Lett. 65 (1994) 1763. [16] D.M. Gill, L. Mc Caughan, J.C. Wright, Phys. Rev. B 53 (1996) 2334. [17] W. Sohler, Erbium-Doped Waveguide Ampli"ers and Lasers in LiNbO , Integrated Photonics Research, OSA Technical Digest Series, Vol. 7, 1995, p. 212. [18] L. Nun ez, G. Lifante, F. CussoH , Appl. Phys. B 62 (1996) 485. [19] L. Rebouta, M.F. da Silva, J.C. Soares, D. Serrano, E. DieH guez, F. AgulloH -LoH pez, J. Tornero, Appl. Phys. Lett. 70 (9) (1997) 1070.
185
[20] V. BermuH dez, M.D. Serrano, J. Tornero, E. DieH guez, J. Mater. Res., submitted for publication. [21] V. BermuH dez, M.D. Serrano, P.S. Dutta, E. DieH guez, Solid State Commun. 109 (9) (1999) 1605. [22] Nai-ben Ming, Jing-feng Hong, Duan Feng, J. Mater. Sci. 17 (1982) 1663. [23] M.T. Santos, J.C. Rojo, A. Cintas, L. Arizmendi, E. DieH guez, J. Crystal Growth 156 (1995) 413. [24] Joeg Koen Choi, Keun Ho Auh, J. Mater. Sci. 31 (1996) 643.
Opposite domain formation in Er-doped LiNbO bulk crystals grown by the o!-centered Czochralski technique V. BermuH dez, M.D. Serrano, P.S. Dutta, E. DieH guez* Departamento de Fn& sica de Materiales, Universidad Auto& noma de Madrid, Cantoblanco, 28049 Madrid, Spain Received 13 January 1999; accepted 2 February 1999 Communicated by R.S. Feigelson
Abstract Opposite domain structures of 0.5 mol% Er-doped LiNbO crystals have been grown by the Czochralski technique in an o!-centered geometry by modulating the growth temperature. The period and homogeneity of the opposite domain structures have been related to the growth conditions. The period of the opposite domain structure does not follow the ratio between the pulling and rotation rate. The Er dopant concentration has been found to be constant along the domain structure. 1999 Elsevier Science B.V. All rights reserved. PACS: 77.84.Dy; 66.30.Jt; 77.80.Dj Keywords: Lithium niobate; Periodically poled; PPLN; Erbium
1. Introduction LiNbO (LN) is a very well known material for its electro-optic, acousto-optic and nonlinear properties [1]. Research on this material has been recently revitalized in two ways: on near stoichiometric material [2], and on its domain structure [3]. The domains in LN have been studied from the time of Nassau et al. (1966) [4], and they are well
* Corresponding author. Tel.: #34-913-97-4977; fax: #34913-97-8579. E-mail address: [email protected] (E. DieH guez)
known to be 1803 with the polarization vector (Ps) directed along the c-direction. The LN domain structures can be roughly divided into three classes: single domain, which plays an essential role in the applications and devices of LN crystals [5]; opposite domain (ODLN), with the Ps vector `tail to taila and `head to heada, for acoustic devices [6]; and periodically poled (PPLN), with the domains antiparallel to each other, for second harmonic generation in the blue}green laser radiation [7}9], optical parametric oscillators [10] and recently in order to inhibit the photorefractive damage [11]. In particular, in ODLN structures the number, thickness and quality of the domains are directly related to the desired properties of the technological
0022-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 0 8 7 - 1
180
V. Bermu& dez et al. / Journal of Crystal Growth 203 (1999) 179 }185
devices. One way to produce these ODLN structures is by the Czochralski (Cz) growth technique using the appropriate growth parameters and with impurities such as yttrium (Y) added to the melt [12]. Nevertheless, there is an unclear answer to understand the role played by this impurity in the ODLN formation. In general, it is assumed that the transition metals substitute for Li atoms in the regular position, whereas the rare earths (RE) elements are located in the Li position shifted by a few tenths of an As towards the nearest oxygen plane [13] depending on the atomic radius of the RE ion [14]. In this way an internal electric "eld appears to compensate the space charge created by the RE ion in the LN matrix, which could be responsible for the domain structure [15]. In particular the Er ion is a RE optical ion in which a large amount of useful optical processes take place [16]. While its most useful optical property allows optical gain and laser oscillation for wavelengths around 1.5 lm (of great interest in guided optical communications), other optical properties have been demonstrated as well [17,18]. At low concentrations, the Er ion unambiguously occupies the ferroelectric Li position displaced by about 0.2 As . At higher concentrations, a random position associated within a nonoctahedral location has been proposed [19], which means a solubility limit of about 3 mol% [20]. The total solubility limit in axial positions has been determined to be below 0.5 mol% [20]. On the other hand, in a recent work related with the opposite domain nature of as-grown Er-doped LN crystals [21], it has been shown that for melt concentrations higher than 3 mol%, the domain structure shows a periodical behavior, while for concentrations below 3 mol%, a disordered domain structure has been observed. In this way it is important to develop ODLN structures with the highest dopant concentration showing a total axial incorporation to avoid the formation of clusters and precipitates. In this work the ODLN formation in bulk Er-doped LN crystals grown by an o!-centered Cz technique has been studied, and the growth parameters have been analyzed in order to improve the quality of the ODLN obtained.
2. Experimental procedure The Er-doped LN single crystals were grown by the o!-centered Cz technique [22] in a Cz-UAM equipment described elsewhere [23]. The starting material was LN congruent powder with 0.5 mol% Er, added to the melt as Er O and contained in a Pt crucible of 5 cm in diameter. The LN seed was oriented along the c-direction, with 4;4;20 mm dimensions and displaced 5 mm from the center of the crucible, which means a nonsymmetric temperature "eld for the growing crystal. The pulling rate (v ) was 2 mm/h and the rotation rate (v ) was varied from 10 to 4 rpm during the growth process, as it will be later discussed. Temperature control at the growing solid}liquid interface allowed modulation of the growth temperature.
Fig. 1. Er-LN1 crystal, with the temperature evolution, the growth derivative and the rotation rate (rpm) during the growth process.
V. Bermu& dez et al. / Journal of Crystal Growth 203 (1999) 179 }185
181
Fig. 2. Domain structure of y-cut samples taken from the A(a) and B(b) zones of the Er-LN1 crystal. A magni"ed region of sample B is shown (c).
Several y-cut samples were obtained from di!erent parts of the bulk crystals, and the domain structure revealed by etching in a hot HF : HNO acid mixture. The domains were observed by means of optical and scanning electron microscopes (SEM). The variation of the dopant concentration along the domain structure has been measured by the wave dispersive X-ray analysis (WDX) technique in a Hewlett Packard JXA-8900M microscope.
3. Results and discussion Fig. 1 presents an Er : LN crystal (Er-LN1) which has been grown with a temperature control better than $0.23C, as it has been measured by the control thermocouple located at the bottom of the crucible. In the same "gure is shown the temperature evolution during the growth process, and #uctuations of the growth derivative (mg/min). The rotation rate has been changed during the growth process from 10 to 4 rpm. The samples to be analyzed have been taken from the A, B and C zones in order to compare the e!ect of the crystal length and the rotation rate on the domains structure. Fig. 2 shows the domain structures of y-cut samples
taken from the A (Fig. 2a) and B (Fig. 2b) zones of the Er-LN1 crystal. The arrow in Fig. 2b corresponds with the enlarged region shown in Fig. 2c. It can be observed that the domain structure is inhomogeneous along the whole crystal length. For sample A (Fig. 2a) the domain structure corresponds with the general domain behavior of a LN congruent crystal grown in a symmetric temperature "eld, i.e., with a nonperiodical polydomain structure. For sample B (Fig. 2b) the domain structure was improved and an ODLN structure clearly appears, although it remains inhomogeneous. The domain thickness was about 8 lm which does not correspond with the v /v ratio calculated from the crystal growth parameters. On the other hand, the domain quality for the samples removed from the C zone was worse than the one removed from sample B, showing an average thickness of 10}11 lm. The domain walls for sample B were very rough as can be seen in Fig. 2c, which are not useful from a technological point of view, due to the irregularities in the domain periodicity. Nevertheless the positive and negative domains were well de"ned and periodic. Thus taking into account that a 0.5 mol% Er-doped LN crystal grown in a symmetric temperature "eld has a nonperiodical ODLN structure [21], it can be pointed out that
182
V. Bermu& dez et al. / Journal of Crystal Growth 203 (1999) 179 }185
the domain behavior observed could be related with the o!-centered geometry of the growth axis. In order to improve the quality of the ODLN structure, a regular temperature variation of about 23C at the solid}liquid interface during the growth process was imposed. Fig. 3 presents the Er : LN crystal (Er-LN2) obtained, with the temperature evolution and the growth derivative #uctuations during the growth process. In this crystal there are three parameters which could a!ect the domain structure, (1) the temperature variation, (2) the o!-centered crystal geometry and (3) the variation of the rotation rate. As can be observed in Fig. 3, the temperature #uctuations create a periodical variation in the derivative and thus a periodical change in the crystal diameter. The Er-LN2 crystal was cut, as in the previous case, into three di!erent zones, A, B and C, corresponding to di!erent values of the temperature #uctuation and the rotation rate. Fig. 4 shows the domain structure of the samples taken from these zones. The domain structure at the beginning of the crystal (zone 1 of sample A in Fig. 4a) resembles the one observed in Fig. 2a, i.e., a polydomain structure. However, when the temperature #uctuations began, as it can be observed in the "nal part of zone A in Fig. 3, the domains become an ODLN structure (zone 2 of sample A). As the crystal length increases an improvement in the domain structure was clearly visible in zone B (Fig. 4b), where a large area with uniform opposite domains was obtained. This area preferentially appears in the increasing diameter zones, as it has been previously published [22]. The thickness of the periodic domains was 8.5 lm, which is not related with the v /v ratio of 3.3 lm/rot. When this ratio was changed to 8.3 lm/rot the domain thickness was 10}11 lm (Fig. 4c). The comparison between domains of both crystals indicates an improvement in the quality of the ODLN structure in the Er-LN2 crystal due to the temperature oscillations during the crystal growth. The domain walls for samples removed from Er-LN2 are smoother than those of Er-LN1 crystal, showing a better quality which indicates that the ODLN : Er is improved when a temperature #uctuation is coupled with an o!-centered growth.
Fig. 3. Er-LN2 crystal, with the temperature evolution, the growth derivative and the rotation rate (rpm) during the growth process.
On the other hand one must take into account that in Er-doped PPLN structures the domain thickness is totally related with the v /v ratio as it has been published [22], and which also happens with other kinds of dopants. Moreover, both PPLN and ODLN structures have been grown from the melt with (1) the same dopant concentration, (2) in a nonsymmetrical Cz geometry, (3) with similar v /v parameters, showing a do main thickness of 3.7 and 8.5 lm in the PPLN and ODLN cases respectively, and (4) with di!erent seed orientations, which means an x- and a coriented seed for the PPLN and the ODLN
V. Bermu& dez et al. / Journal of Crystal Growth 203 (1999) 179 }185
183
Fig. 4. Domain structure of the y-cut samples taken from the A(a), B(b) and C(c) zones of the Er-LN2 crystal. A detailed sketch of the samples marked by the arrow is shown.
structures respectively. This last di!erence could be responsible for the disagreement between the domain thickness in both PPLN and ODLN cases. In this way one must point out that the relative growth rates of the crystallographic faces of LN follow the relation: v 'v (where v and v are the V A V A relative growth rates for x- and c-axis respectively) [5]. Thus the dopant incorporation into the crystal
is favored in the growth processes along the c-axis, and as a consequence, the internal electric "eld able to create such structures is di!erent in both cases. More work is underway in this topic. Compositional analysis by WDX was carried out in order to correlate the Er distribution in the crystal with the formation of domains. Both, the ODLN structure (observed by SEM) and the WDX
184
V. Bermu& dez et al. / Journal of Crystal Growth 203 (1999) 179 }185
c-oriented seed from a nonsymmetrical Cz geometry. It has been shown that the o!-centering of the crystal plays an important role in the ODLN structures formation. A temperature #uctuation is neccessary to initiate growth striations in the crystal which improve the quality of the ODLN structures. It has been shown that the domain thickness does not follow the v /v ratio as in PPLN structures. Moreover, it has been shown that there is no variation in the Er concentration along the domain structure.
Acknowledgements The authors would like to acknowledge the partial support to realize this work by CAM-06T/015/96, PETRI-CICYT-95-0086-OP and CICYT-ESP981340.
References
Fig. 5. Domain structure (a) and WDX linescan (b) taken from the ODLN structures of the Er-LN2 crystal.
linescans are presented in Fig. 5. It can be observed that the Er concentration is constant along the domain structure in the range of the detection limit, which is $0.2%. This behavior disagrees with that reported in literature for the ODLN structures obtained from a melt doped with yttrium [22,24]. This fact could be related to the shift of the Er ion from the Li position, which could create a di!erent internal electric "eld from that created due to the yttrium ion in the regular Li position. This result suggets a di!erent polarization mechanism between Y and Er dopants. More work is underway to clarify these di!erences in the polarization mechanism.
4. Conclusions ODLN structures in 0.5 mol% Er-doped LN bulk crystals have been successfully grown with a
[1] F. AgulloH -LoH pez, J.M. Cabrera, F. AgulloH -Rueda, Electrooptics: Phenomena, Materials and Applications, Academic Press, San Diego, 1994. [2] G.Y. Malovichko, O. Cerclier, O.F. Schirmer, Solid State Commun. 89 (1994) 195. [3] V. BermuH dez, P.S. Dutta, M.D. Serrano, E. DieH guez, J. Crystal Growth 172 (1997) 269. [4] K. Nassau, H.J. Levinstein, G.M. Loiacono, J. Phys. Chem. Solids 27 (1966) 989. [5] A.M. Prokhorov, Yu.S. KuzH minov, Physics and Chemistry of Crystalline Lithium Niobate, Hilger, London, 1990. [6] Yong-yuan Zhu, Nai-ben Ming, Wen-hua Jiang, Yong-an Shui, Appl. Phys. Lett. 53 (1988) 2278. [7] G.D. Miller, R.G. Batchko, W.M. Tulloch, D.R. Weise, M.M. Fejer, R.L. Byer, Opt. Lett. 22 (24) (1997) 1834. [8] V. BermuH dez, J. Capmany, J. GarcmH a SoleH , E. DieH guez, Appl. Phys. Lett. 73 (5) (1998) 593. [9] J. Capmany, V. BermuH dez, E. DieH guez, Appl. Phys. Lett., 74 (11) (1999) 1. [10] L.E. Myers, R.C. Eckardt, M.M. Fejer, R.L. Byer, W.R. Bosenberg, J.W. Pierce, J. Opt. Soc. Am. B 12 (11) (1995) 2102. [11] M. Taya, M.C. Bashaw, M.M. Fejer, Opt. Lett. 21 (12) (1996) 857. [12] Yong-yuan Zhu, Nai-ben Ming, J. Appl. Phys. 72 (3) (1992) 904. [13] L. Rebouta, P.J.M. Smulders, D.O. Boerma, F. AgulloH LoH pez, M.F. da Silva, J.C. Soares, Phys. Rev. B 48 (1993) 3600.
V. Bermu& dez et al. / Journal of Crystal Growth 203 (1999) 179 }185 [14] A. Lorenzo, H. Ja!rezic, B. Roux, G. Boulon, J. GarcmH a SoleH , Appl. Phys. Lett. 67 (25) (1995) 3735. [15] L. Huang, N.A.F. Jaeger, Appl. Phys. Lett. 65 (1994) 1763. [16] D.M. Gill, L. Mc Caughan, J.C. Wright, Phys. Rev. B 53 (1996) 2334. [17] W. Sohler, Erbium-Doped Waveguide Ampli"ers and Lasers in LiNbO , Integrated Photonics Research, OSA Technical Digest Series, Vol. 7, 1995, p. 212. [18] L. Nun ez, G. Lifante, F. CussoH , Appl. Phys. B 62 (1996) 485. [19] L. Rebouta, M.F. da Silva, J.C. Soares, D. Serrano, E. DieH guez, F. AgulloH -LoH pez, J. Tornero, Appl. Phys. Lett. 70 (9) (1997) 1070.
185
[20] V. BermuH dez, M.D. Serrano, J. Tornero, E. DieH guez, J. Mater. Res., submitted for publication. [21] V. BermuH dez, M.D. Serrano, P.S. Dutta, E. DieH guez, Solid State Commun. 109 (9) (1999) 1605. [22] Nai-ben Ming, Jing-feng Hong, Duan Feng, J. Mater. Sci. 17 (1982) 1663. [23] M.T. Santos, J.C. Rojo, A. Cintas, L. Arizmendi, E. DieH guez, J. Crystal Growth 156 (1995) 413. [24] Joeg Koen Choi, Keun Ho Auh, J. Mater. Sci. 31 (1996) 643.