On the opposite domain nature of Er-doped lithium niobate crystals

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1 SSC 4519 PERGAMON Solid State Communications 109 (1999) On the opposite domain nature of Er-doped lithium niobate crystals V. Bermúdez, M.D. Serrano, P.S. Dutta, E. Diéguez* Departamento de Física Materiales, Universidad Autónoma de Madrid, Cantoblanco, Madrid, Spain Received 31 August 1998; accepted 25 November 1998 by M. Cardona Abstract It has been shown that Erbium doping affects the domain structure of LiNbO 3 crystals grown from congruent melt. Doping up to 5 mol% produces opposite domains for congruent crystals. Additionally, the monodomain configuration of pure stoichiometric crystal becomes polydomain with the addition of Er in the melt Elsevier Science Ltd. All rights reserved. Keywords: A. Ferroelectrics; B. Crystal growth; C. Point defects 1. Introduction The spatial domain structure of LiNbO 3 (LN) single crystals results in specific applications and was extensively studied in the past [1]. For instance, single domain LN [2,3] is required for optoelectronic devices [4], opposite domain (ODLN) [5] structure is used in acoustic devices [6], and periodic poled LN crystals [7] is an essential requisite for an efficient second harmonic generation via the quasi-phase matched technique [8]. It is also well known that the concentration and sites in the crystal lattice of intentional dopants or impurities alter the domain structure [9]. The domain structure can be tailored during crystal growth and/or by post-growth treatments [2,3]. Temperature oscillations at the growth interface during the growth can induce oscillations in the growth rate and, consequently, a variation in the distribution of the impurities [10]. The variation in dopant concentration can modify the domain structure in a periodic fashion, which is related to the rotational striations [11]. * Corresponding author. Tel.: ; fax: ; ernesto.dieguez@uam.es In general, the domain behavior in LN depends on the position of the Li atoms in the oxygen octahedron [12]. The lattice location of numerous dopants, in particular that of the rare earth (RE) ions, is in the ferroelectric Li position [13]. Amongst the RE impurities, Erbium plays an important role [14,15]. It was established [16] that for a concentration up to 2 mol% the Er ion occupies the ferroelectric Li position displaced from the oxygen octahedron by about 0.2 Å. The occupancy of this position leads to an internal electric field, created by the space charge introduced by the Er 3 in the Li position, which could be responsible for the domain formation. In the present study, we have investigated the effect of Er concentration on the domain formation in congruent and near stoichiometric LiNbO 3 crystals. 2. Experimental Single crystals of Er doped LiNbO 3 were grown by the Czochralski (Cz) technique from congruent melts as described elsewhere [17]. The Er concentration in the melt was in the range mol%, added as Er 2 O 3. The near stoichiometric crystals were grown /99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S (98)

2 606 V. Bermúdez et al. / Solid State Communications 109 (1999) Table 1 Optimum pulling rate for congruent and near stoichiometric Er doped LN crystals and crystal length growth per revolution (mm/rev) Er conc (mol%) wt.% K 2 O wt.% K 2 O v pull (mm/h) mm/rev from a constant 6 wt.% K 2 O flux [18]. In both cases the melt was contained in a Pt cylindrical crucible. Congruent LN oriented seeds along the c direction were used, with dimensions of (mm). The pulling rate was modified depending on the composition of the melt in order to assure the single crystallinity of the ingots, while the rotation rate was kept constant at 30 rpm. Table 1 shows the optimum pulling rate used in this work for each of the crystals. After X-ray Laue orientation [19] of the as-grown Fig. 1. Opposite domain structure of y-cut LiNbO 3 : x Er samples. Er concentration in the melt: (a) 0.5; (b) 1; (c) 2; (d) 3; (e) 5; (f) 8 mol%).

3 V. Bermúdez et al. / Solid State Communications 109 (1999) Fig. 2. Detailed picture of the opposite domain structure for Er doped LiNbO 3 samples :(a) 3 mol%, (b) 5 mol% and (c) 8 mol% concentration. crystals, y-cut samples were obtained from different zones of the crystal, and polished up to optical grade. The domain structures was revealed by etching with an HF : HNO 3 (1 : 2 by vol.) mixture at 110 C for 10 min and observed by optical and scanning electron microscope (SEM) in a Phillips XL-30 equipment. The diffraction patterns were taken using CuK a (8.04 KeV) radiation with a Siemens D-5000 diffractometer. To discriminate the K b (8.91 KeV) radiation, a 240 ev resolution Si (Li) detector supplied by Kevex with an electronically selected window of 380 ev was used. 3. Results and discussion Fig. 1 shows the domain structure for different Erdoped congruent LN crystals observed by SEM after the etching process. For low Er concentration (Figs. 1(a) and (b)) the domain structure exhibits a random behavior, the thickness of the positive domains being much larger than the negative ones. Increasing the Er concentration in the melt, the domain structure becomes more ordered and a clear periodical structure is formed (Figs. 1(c) (e)). However, for an Er concentration of 8 mol% the periodical domain structure disappears (Fig. 1(f)). Detailed pictures for the highest Er concentrations are shown in Fig. 2, where typical etch pattern located at the negative domain of a y-cut face and smooth positive domain can be observed. The best domain structure corresponds to Er 3 mol%. This may be related to the upper limit of 2 3 mol% for the Er incorporation in LN crystals [16], and the formation of clusters for higher concentrations [16]. The present study also shows that the periodical domain structures are not rotational striations (as observed in Cz crystals). They are formed by the presence of internal electric fields created when the Li atoms are in ferroelectric positions [20]. This is evident from the fact that the crystal length grown per revolution (Table 1) and the domain thickness for different Er concentrations, observed in Figs. 1 and 2, are not related. The domain thickness is Fig. 3. Evolution of the positive/negative domain ratio (X) and the c-lattice parameter ( ) with the Er melt concentration.

4 608 V. Bermúdez et al. / Solid State Communications 109 (1999) Fig. 4. Electrical field transient and domain structure on y-cut Er doped LN samples obtained after (a) 15, (b) 40 and (c) 70 min. of poling time. modified with the Er concentration. The ratio of positive to negative period decreases with increasing Er concentration as shown in Fig. 3. The c-lattice parameter of the Er-doped LN [21] crystals follows the same behavior. The shortening of the c-lattice parameter could favor the formation of Er 3 (Li )-Nb v complexes, instead of the one related to the ilmenite structure, as a result of the local attractive electric force created by the electric charges of this complex, i.e., Er( 2) Nb v ( 5) [22], which could be directly related to the decreasing of the negative to positive domain ratio. The domain structure of near stoichiometric Er doped LN crystals shows a poly-domain nature. Taking into account that: 1. Near stoichiometric LN crystals grown from 6 wt.% K 2 O flux are of single domain nature [23]. 2. A near stoichiometric LN crystal has a limited free space to accept any kind of impurities [24], in contrast with the congruent one. 3. The impurities in the LN crystals modify the domain structure [9], one can conclude that the Er incorporation in the near stoichiometric crystals must be of random nature, and that the dopant is not able to create an organized domain structure. Moreover, it is important to note that the LN crystals grown from a 6 wt.% K 2 O with an Er concentration of 1 mol% did not grow as a single crystal even for a low pulling rate of mm/h. This indicates the difficulty of dopant incorporation in near stoichiometric crystals. Moreover, in order to control and modify the thickness of the ODLN configuration, an electric field was applied to the samples. As previously described [25] experimental arrangement was used for the poling process and a 0.5 mol% Er doped LN ingot was heated at 1100 C. At this temperature, 2 V/cm electric field was applied for different periods of time in order to partially repolarize the crystals and modify the domain thickness. Fig. 4 shows the current transient necessary to maintain constant the applied electric field during the poling process [3]. The domain structure of the y-cut samples after the poling process at different periods of time, shows a partial ordering in the domain structure after a short time of poling (Fig. 4(a)), but becomes random with the increase in the poling time (Figs. 4(b) and (c)). Hence, it can be concluded that a real enhancement of the domain thickness could not be reached by a poling process. 4. Conclusions It was observed that Er ions modify the domain structure of congruent and near stoichiometric LN

5 V. Bermúdez et al. / Solid State Communications 109 (1999) crystals. For congruent LN crystals doped with an Er melt concentration in the range of 3 5 mol% the domain structure is periodic. For Er concentrations higher than 5 mol%, the periodic structure is destroyed. For near stoichiometric LN crystals, a multidomain structure is obtained for the Er dopant concentrations studied in this work. The domain period cannot be tailored by a poling process. Acknowledgements This work was partially supported by CAM-06T/ 015/96 and CICYT ( OP and ESP ) projects. The authors acknowledge Prof. L. Viña for critical reading of the manuscript. References [1] A.M. Prokhorov, Y.S. Kuzminov, Physics and Chemistry of Crystalline Lithium Niobate, Adam Hilger, Bristol, [2] V. Bermúdez, P.S. Dutta, M.D. Serrano, E. Diéguez, J. Appl. Phys. 81 (2) (1997) 862. [3] V. Bermúdez, P.S. Dutta, M.D. Serrano, E. Diéguez, J. Cryst. Growth 169 (1996) 409. [4] F. Agulló-López, J.M. Cabrera, F. Agulló-Rueda, Electrooptics: Phenomena Materials and Applications, Academic Press, San Diego, [5] Zhu Yong-yuan, Ming Nai-ben, Jiang Wen-hua, Shui Yongan, App. Phys. Lett. 53 (23) (1988) [6] Zhu Yong-yuan, Cheng Shi-de, Ming Nai-ben, Ferroelectrics 173 (1995) 207. [7] G.A. Magel, M.M. Fejer, R.L. Byer, Appl. Phys. Lett. 56 (2) (1990) 108. [8] M.M. Fejer, G.A. Magel, D.H. Jundt, R.L. Byer, IEEE. J of Quantum Elect. 28 (11) (1992) [9] V. Bermúdez, P.S. Dutta, M.D. Serrano, E. Diéguez, J. Phys.: Condens. Matter 9 (1997) [10] Ming Nai-Beng, Hong Jing-Fen, Feng Duan, J. Mat. Sci. 17 (1982) [11] G. Müller, Crystal Growth from the Melt, Springer Verlag, Berlin, [12] P.W. Haycock, P.D. Townsend, App. Phys. Lett. 48 (1986) 698. [13] J.G. Solé, B. Macalik, L.E. Bausá, F. Cussó, E. Camarillo, A. Lorenzo, L. Nuñez, F. Jaque, A. Monteil, G. Boulon, J.E.M. Santiuste, I. Vergara, J. Electrochem. Soc. 140 (1993) [14] L. Nuñez, B. Herreros, R. Duchowicz, G. Lifante, J.O. Tocho, F. Cussó, J. Of Luminisc (1994) 81. [15] V.T. Gabrielyan, A.A. Kaminskii, L. Li, Phys. Status Solidi A3 K37 (1970). [16] L. Rebouta, M.F. da Silva, J.C. Soares, D. Serrano, E. Diéguez, F. Agulló-López, J. Tornero, App. Phys. Lett. 70 (9) (1997) [17] M.T. Santos, J.C. Rojo, A. Cintas, L. Arizmendi, E. Dieguez, J. Crystal Growth 156 (1995) 413. [18] G. Malovichko, O. Cerclier, J. Estienne, V. Grachev, E. Kokanyan, C. Boulesteix, J. Phys. Chem. Solids 9 (1995) [19] C. Marin, E. Diéguez, Orientation of Single Crystals by Back Reflection Laue Pattern Simulation, Work Scientific, Singapore, in press. [20] L. Huang, N.A.F. Jaeger, Appl. Phys. Lett. 65 (1994) [21] V. Bermúdez, M.D. Serrano, J. Tornero, E. Diéguez, submitted. [22] J. García Solé, Physica Scripta T55 (1994) 30. [23] V. Bermúdez, P.S. Dutta, M.D. Serrano, E. Diéguez, Appl. Phys. Lett. 70 (6) (1997) 729. [24] A. Kling, J.G. Marques, J.G. Correia, M.F. da Silva, E. Diéguez, F. Agulló-López, J. C. Soares, Nucl. Instr. Meth. in Phys. Res. B 113 (1996) 293. [25] K. Nassau, H.J. Levinstein, G.M. Loiacono, J. Phys. Chem. Solids 27 (1966) 989.