Bismuth Orthogermanate (Bi₄Ge₃O₁₂) is widely used as a scintillation material. Although this crystal is grown for a long time, unsolved questions about the mechanisms of crystallization remain, in particularly, the influence of the growth rate on the quality of the crystals.
Commonly it is considered that the lower the crystallization rate, the higher crystal perfection. [1].
However, during growing the BGO crystals by the Low Thermal Gradient Czochralski Technique (LTG Cz) [2], just opposite behavior is observed. For the growth rate less than 1.5 mm/h, the amount of defects and impurities is higher than for the case of the growth rate 3.5 - 6 mm/h.
Small crystals (diameter 50 mm, length up to 170 mm) maintained steady growth to 10-11 mm/h. The upper limit of the growth rate of high quality crystals depends on the level of impurities in the charge and the homogeneity of the melt.
For estimation of supercooling the specified growth rate changed stepwise during the growth run while the crystal diameter maintained constant by the automatic weighing control system. Experiments were performed in the range from 0.3 to 15 mm/h for two polar growth directions <111>. Typical dependence of temperature drop at the middle section of three zone heated from the magnitude of step of the growth rated is shown in the Fig.

Change of temperature at the heater versus BGO crystal growth rate

The value of the temperature drop ΔT on the heater with increasing the crystallization rate V_cryst. is determined by two main factors: the increase in supercooling and increase heat flow to remove the heat of crystallization. The contribution of the second factor is a linear function of growth rate and could be estimated from series experiments. The results are compared with data published in [3-5] where the values of supercooling were found by direct measurements of temperature on the crystal’s face.

[1] H.J. Scheel // J.Cryst. Growth, 2000, v.211, p.1.
[2] Yu.A. Borovlev, N.V. Ivannikova, V.N. Shlegel et al. // J. Cryst. Growth. 2001, v. 229, p.305
[3] V.D. Golyshev, M.A. Gonik, V.B. Tsvetovsky // J. Cryst. Growth. 2002, v. 237, p.735
[4] S.V. Bykova, V.D. Golyshev, M.A. Gonik et al. // J. Cryst Growth. 2004, v. 266, p.246
[5] S.V. Bykova, V.D. Golyshev, M.A. Gonik et al. // Heat Transfer Engineering, 2006 v. 27, №2, p.43

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1. Growth of giant CdWO₄ crystals and properties of (010) cleaved surface
2. The low thermal gradient Cz technique as a way of growing of dislocation-free germanium crystals
3. The new approach to the weighing control of the CZ crystal growth