Automated pulling of large alkali halide single crystals

Abstract

A technique is described for pulling single crystals with simultaneous feeding of starting material to the melt; automatic control of the growth process is ensured by regulating the heater temperature in response to signals of a highly sensitive melt-level contact gauge. Growing crystal diameter d is preset by the ratio of the feed rate ṁ to pulling rate v p, axial growth rate being equal to v p. Crystal broadening from the seed up to the preset final diameter is automatically ensured by programming the ratio m ̇ v p . The stability of melt level in the crucible renders it possible to achieve a reproducible control over the shape of temperature field in the melt and in the growing crystal and hence, over the shape of interface. This technique was used for growing KCl single crystals up to 450 mm in diameter and 140 kg, and CsI:Na single crystals up to 300 mm in diameter and 115 kg. In the case of 300 mm dia. crystals a relative deviation of Δd d = ±1.5% was achieved over crystal length of 750 mm.