Growth of 3-inch diameter near-stoichiometric LiTaO3 by conventional Czochralski technique
Introduction
The usual, congruent LiTaO3 (CLT) and LiNbO3 (CLN) have been shown to suffer detrimental photorefractive damage in high-power optical conversion application involving visible wavelengths. It was found that near-stoichiometric LiTaO3 (SLT) crystals grown from lithium-rich solutions appear to be less sensitive to photorefractive damage [1], [2], [3]. In addition, the coercive field required for domain inversion in SLT is 6–20 times lower than in CLT and the domain shape is hexagonal. The combination of these features provides the opportunity of developing high-power quasi-phase-matched (QPM) devices for the visible wavelengths [4], [5].
We report here on the growth of 3-in-diameter SLT crystals using conventional Czochralski method. The growth procedure is described in detail and some characteristics of the grown crystals are presented.
Section snippets
Crystal growth
A common Czochralski design was used as it is shown schematically in Fig. 1. The RF coil was connected to 25 kW, 20 kHz radio-frequency generator. Iridium crucible 150 mm in diameter by 150 mm high was placed in the coil center on ZrO2 bubbles inside a ZrO2 tube. Additional alumina tube was enclosing a ZrO2 tube for better insulation. The furnace was placed in a water-cooled chamber and an oxygen and nitrogen mixture was flowed inside in order to sustain 1% O2 atmosphere during the growth process.
Characterization of the grown crystals
At the conditions previously described, colorless and crack-free crystals, 80 mm in diameter and 25–30 mm long were grown with repeatability. Fig. 3, Fig. 4 show typical examples of as-grown boules pulled out of 58 and 60.5 mol% Li2O starting melt correspondingly. As one can see the first crystal has mostly smooth surface while the second one is totally covered with rough defective layer. The as-grown crystals had a ferroelectric multidomain structure and needed to be poled. Poling was
Thermal gradients in the growth system
Thermal gradients in our furnace appropriate for the successful growth of 80 mm diameter SLT from 58 mol% Li2O starting melt were achieved when the center of the crucible was positioned 50 mm above the center of RF coil. The axial and radial gradients below the liquid surface, and were 38 and 3 °C/cm, respectively. The axial gradient above the liquid surface was 42 °C/cm (Fig. 2). Our gradient in the liquid was close to the reported gradient for the
Conclusions
Near-stoichiometric LiTaO3 single crystals, 80 mm in diameter and 25–30 mm in length, were successfully grown by conventional Czochralski method. The SLT crystals grown from a 58 mol% Li2O starting melt had a coercive field that varies in the range of 3.1–2.5 kV/mm and a composition of 49.75–49.8 mol% Li2O. In addition, the coercive field and the composition of crystals obtained from a 60.5 mol% Li2O starting melt were 950–450 V/mm and 49.93–49.97 mol% Li2O. This work proved that large SLT crystals
Acknowledgements
The authors would like to thank Rich Russell and Steve Rutkowski for preparing the samples and Dr. Alexei Alexandrovski for helpful discussions.
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