Growth of Bi 12 GeO 20 and Bi 12 SiO 20 crystals by the low-thermal gradient Czochralski technique
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Growth of Bi12GeO20 and Bi12SiO20 Crystals by the LowThermal Gradient Czochralski Technique V. N. Shlegel and D. S. Pantsurkin Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia email: [email protected] Received April 13, 2010
Abstract—Bi12SiO20 crystals have been grown for the first time by the lowthermal gradient Czochralski technique in the 〈111〉 and 〈110〉 directions. The conditions for reproducible crystal growth with a highqual ity polyhedral faceted front are found. The systematic features of shaping Bi12SiO20 and Bi12GeO20 crystals, grown by the lowthermal gradient Czochralski technique, are compared. The defect formation in these crys tals is studied and their optical homogeneity is analyzed by interferometry. DOI: 10.1134/S1063774511010226
INTRODUCTION Sillenite crystals have a number of properties important for practical applications, such as piezo electric sensors, filters and delay lines of electromag netic signals, electrooptical and magnetooptical field strength meters, space–time modulators, etc. It is known [1] that these devices require crystals with a low dislocation density and high optical homogeneity. Crystals of congruently melting sillenites (Bi12SiO20 (BSO), Bi12GeO20 (BGO), etc.) are gener ally grown by the conventional Czochralski method [1]. In this case, one of the main problems is their opti cal homogeneity [2–11]. The two most characteristic types of “optical” defects are selected: (i) inclusions of foreign phases and crucible material and (ii) the pres ence of regions with enhanced optical density in the crystal bulk, which can manifest themselves as stria tions and the socalled “growth column” [1]. Accord ing to [7–9, 12], the formation of the growth column and the effect of selective decoration in the form of a threebladed propeller during growth in the 〈111〉 direction are related to the growth rate anisotropy and the difference in the distribution coefficients of “pho tochromic” impurities for polar {110} and nonpolar {100} faces present at the crystallization front. Regions with enhanced optical density may also arise due to the coexistence of two (normal and layerbylayer) growth mechanisms at the crystallization front [13]. Steiner et al. [12] suggested that optical inhomogeneity may arise during layerbylayer growth due to deviations in the crystal composition from stoichiometric within the homogeneity range and variations in the impurity con centration at temperature fluctuations. According to [14], when the abovementioned factors are mini mized, the optical density of sillenite crystals may change because of the implementation of equilibria in these systems, leading to the formation of metastable
phases, which are fairly stable in melts of oxide bis muthcontaining systems. It was shown that single crystals of stable compounds containing inclusions of metastable phases or products of their decomposition can be obtained from superheated melts at crystalliza tion rates somewh
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