Thermodynamic analyses of gases formed during the EFG sapphire growth process

Abstract

Shaped sapphire crystals obtained by the edge-defined film fed growth (EFG) method almost always contain a typical defect, the so-called “voids”, which affect the optical quality of the crystals. The origin of the trapped voids is not yet clarified. One of the hypotheses is concerned with gas formation in the crystal growth setup chamber. The goal of this paper is to study the species of the gases resulting from various chemical reactions between the different components of the crystal growth apparatus, such as the charge (Al 2O 3), the crucible and the shaper material (Mo or W) and the screens and other details made from graphite (C). In order to obtain the partial pressure of the gas species in the growth chamber the thermodynamic calculations were performed using the complex equilibrium code—Gemini2—that minimizes the free enthalpy formation. Carbon monoxide (CO) becomes the most important gas species when one of the growth setup components is made from graphite. The various gas species resulting from different reactions are trapped in the crystal during the growth process and affect the crystal quality. In order to eliminate the contamination of the crystals, this type of calculation should be profitable for a better choice of the crucible, shaper and other set-up detail material.