Elsevier

Journal of Crystal Growth

Volume 218, Issues 2–4, 15 September 2000, Pages 327-333
Journal of Crystal Growth

Structural defects in flux-grown stoichiometric LiNbO3 single crystals

https://doi.org/10.1016/S0022-0248(00)00600-XGet rights and content

Abstract

The effect of growth conditions on the crystal perfection of stoichiometric lithium niobate LiNbO3 prepared by the high-temperature top-seeded solution growth technique (HTTSSG) from potassium containing flux was investigated. Flux compositions between 0.09 and 0.21 K2O/LiNbO3 ratio were tested, and it was found that LiNbO3 forms an eutectic system with the K2O, which is considered as the solvent in the system. The exact composition of the eutectic is close to the 1 : 5 molar ratio of K2O and LiNbO3. Stoichiometric crystals with homogeneous composition were grown from fluxes in the range of 0.16–0.195 K2O/LiNbO3 ratio. The formation of the structural defects (faceting and ferroelectric domain reversal) have been examined for growth along the Z=0001, Y=011̄0, X=21̄1̄0, 011̄1 and 02̄21 directions. The development of facets occurred primarily along the {0 1 .2} pyramidal planes. Close connection was found between faceted growth and the appearance of domain reversal.

Introduction

Lithium niobate, LiNbO3 (LN) is an attractive ferroelectric material and has found application in a wide range of optoelectronic devices. LN is a typical non-stoichiometric material, melting congruently at about 48.60 mol% of Li2O, and usually grown by the Czochralski method. Compositional variations of the Li2O content in the crystals lead to local birefringence inhomogeneities degrading the crystal quality. Beside the extraordinary refractive indices, many other physical properties have turned out to be dependent upon the crystal composition and the corresponding defect structure [1], [2], [3], [4], [5], [6], [7]. Therefore, stoichiometric or quasi-stoichometric crystals, having a perfect crystal lattice, are expected to show improved performance for a number of cases.

In stoichiometric crystals, the amount of Nb5+ in Li positions (anti-site Nb5+) is about 0.01%, which is less than in congruent crystals by at least two orders of magnitude. This difference in the crystal defects results in the blue shift of the absorption edge [2] and the narrowing of the spectral lines in EPR and NMR [3], Raman [4], OH vibration [1], and luminescence of Cr-doped samples [5]. The perfect structure allows a significant reduction of the electric field required for domain reversal [6], advancing the preparation of thick, periodically poled sample [7].

There are different methods for the preparation of stoichiometric LiNbO3 crystals [1], [8], [9]. Since the conversion of congruent single crystals by the vapor transport equilibration (VTE) method is effective only for thin samples (wafers or fibers), the use of the direct growth methods is more beneficial and enables the applications of bulk stoichiometric samples. For the growth of stoichiometric crystals, Kitamura et al. developed a continuous filling double-crucible Czochralski method, and used off-congruent melts (at about 58 Li2O mol% concentration) [9]. In a previous work, we reported the successful high-temperature top-seeded solution growth (HTTSSG) from potassium containing flux [1]. For the growth of stoichiometric LN crystals the latter method seems to be one of the best choice: the HTTSSG method is more convenient, and since the K+ ions practically do not enter the lattice, crystals grown from such flux will be of high purity and suitable for various optical applications. Furthermore, comparing the composition of the stoichiometric crystal samples prepared by the different methods, using the blue shift of the absorption edge data for characterization [2], [9], [10], the composition of the HTTSSG-grown crystals seems to be the closest to the stoichiometric Li/Nb=1 ratio.

Potential applications of the stoichiometric LN crystal require high optical quality and the control of the composition of the crystal. The main issue therefore is the growth of optically homogeneous, single-domain crystals with Li/Nb ratio close to unity. In this work, we report the impact of the growth parameters on the chemical and structural homogeneity of the crystals, including the characterization of the crystals grown from various solution compositions and along different growth directions.

Section snippets

Experimental procedure

A series of LiNbO3 single crystals were grown by the HTTSSG method in a diameter-controlled growth apparatus from K2O–Li2O–Nb2O5 fluxes of different compositions. The technical details of the material synthesis and crystal growth are given in Ref. [1]. The starting materials used were Jonhson–Matthey-grade A1 Nb2O5 and K2CO3, and Merck suprapure Li2CO3, always pre-reacted in solid phase. The crystals were pulled along the Z=0001, Y=011̄0, X=21̄1̄0, 011̄1 and 02̄21 directions with a rate of 0.2 

Stoichiometry

The success of the flux growth depends to a large extent on the choice of the appropriate flux composition and growth conditions. Since there are no available thermochemical data on the system K2O–Nb2O5–Li2O, the optimization of the growth process must be done step by step. The outlined phase relations are deduced indirectly from crystal growth experiments and the characterization of the crystals.

In order to study the effect of the K+ content of the flux (liquidus) on the variation of the Li/Nb

Conclusions

Lithium niobate LiNbO3 single crystals were grown by the high-temperature top-seeded solution growth technique from a potassium containing flux, at flux compositions between 0.09 and 0.195 K2O/LiNbO3 ratio. It has been found that homogeneous crystals, with stoichiometric composition can be prepared from fluxes in the range of 0.16–0.195 K2O/LiNbO3 ratio. In this system K2O·5LiNbO3 can be considered like the solvent with eutectic freezing at about 1050°C. The solute is the extra amount of LiNbO3

Acknowledgements

The research on LiNbO3 was supported by the Hungarian Scientific Research Fund (OTKA) Nos. T024092 and T026647.

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