“Liquinert” Process for High-Quality Bulk Single Crystal Growth

Abstract

Bulk crystal growth technologies originate from the Czochralski (CZ) and Vertical Bridgman (VB) methods developed almost one century ago. Both methods were applied to prepare single crystals of many kind of inorganic materials, for example, semiconductors, halides and many oxides. In the VB process, molten raw materials are wetting the crucible wall easily. This phenomenon causes the sticking of grown crystals with crucibles and often leads to the cracking of the crystal and crucible. These issues prohibit us from obtaining high quality single crystals. Therefore, practical application of VB method is limited only on several materials such as CaF₂ and GaAs single crystals. The issue of crucible’s wetting is present in the CZ method as well. For example, the purity of silicon single crystals is degraded from 11N raw material to 5~6N level by the oxygen and carbon contamination caused by the wetting between silicon melt and quartz crucible. These issues are yet to be solved in VB and CZ methods. Many molten materials reach the spherical shape driven by the surface tension when a residual moisture (H₂O) is completely removed from the raw material, the crucible, and atmosphere. We denote this condition as the “Liquinert” state meaning “liquid being in an inert state”, non-wetting and non-reactive with the crucible at high temperature. The author has prepared many high-quality bulk crystals of mainly metallic halides and semiconductors, except oxides, by VB method when applying the “Liquinert” process. This technology is applicable to high quality bulk crystal growth of silicon as well as other inorganic materials of huge industrial interest. In this review, the “Liquinert” process, its background, methodology, examples of applications in fundamental research, and practical development are exposed. In addition, we also discuss the future of this industrial process on bulk silicon crystals for semiconductors and solar cells.