Optimization of the crystallization rate in growing Bi 4 Ge 3 O 12 crystals by the LTG CZ technique
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Optimization of the Crystallization Rate in Growing Bi4Ge3O12 Crystals by the LTG CZ Technique N. V. Ivannikova, V. N. Shlegel, and Ya. V. Vasiliev Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, pr. Akademila Lavrent’eva 3, Novosibirsk, 630090 Russia email: [email protected] Received November 28, 2012
Abstract—The influence of the growth rate on the shaping and quality of Bi4Ge3O12 (BGO) crystals during their growth by the lowthermalgradient Czochralski technique has been studied. Several series of BGO crystals have been grown with the rate varying from 0.3 to 15 mm/h. The limited growing rates at which the crystal quality is not deteriorated are established. The results of the study make it possible to increase the growth rate in commercial systems by a factor of 1.5–2 and thus significantly increase the growth efficiency. DOI: 10.1134/S1063774514040075
INTRODUCTION Bismuth orthogermanate Bi4Ge3O12 (BGO) is a scintillation material widely used in highenergy phys ics, positronemission tomography, and other fields. The optimization of its growth conditions is an urgent problem for expanding the area of its application and reducing the cost of commercially produced crystals. BGO crystals, which are characterized by a com paratively high crystallization heat (44.6 kcal/mol) [1] and relatively low melting temperature (1040°С), are easily faceted during the crystallization from melt. According to Jackson’s criterion, the crystallographi cally strong facets become rough when the melting heat and crystallization temperature obey the relation ΔH/RT < 2–5. For BGO ΔH/RTmelt ~ 16, which indi cates a strong tendency of growing crystal to faceting. BGO crystals are grown mainly by the Czochralski and Bridgman techniques. The crystal growth rates reported in the literature vary from 0.6 to 3 mm/h; however, the most popular rates are in the range from 1.5 to 3 mm/h. In this case, an increase in the growth rate is generally accompanied by enhancement of faceting, the formation of a “growth core” and the deterioration of crystal quality [2]. It was shown in [3] that inclusions and other defects are formed in the regions where different (normal and tangential) growth mechanisms coexist, i.e., at the boundary between the rounded and faceted crystallization fronts. In the traditional Czochralski technique, face ting is suppressed by increasing temperature gradients and/or reducing the crystallization rate; this approach provides a unified growth mechanism for the entire surface. BGO crystals of high quality can be grown alternatively using the lowtemperaturegradients Czochralski (LTG Cz) technique. In this case crystals with the highest structural quality are obtained when
facets occupy the entire area of the crystallization front, and, in contrast to the traditional approach, the problem is to exclude the formation of rounded parts growing according to the normal mechanism. The purpose of this study was to investigate the influence of the growth rate on the quality of
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