NASA NTRS1998
Presence of different, non-constituent gases may be a critical factor in crystal growth systems. In Physical Vapor Transport processes the cras(es) can be used intentionally (to prevent excessively high, unstable growth conditions), or can evolve unintentionally during the course of the process (which may lead to undesired reduction in the -rowth rate). In melt growth, particularly under low gravity conditions (reduced hydrostatic pressure) the gas present in the system may contribute to formation of voids in the growing crystals and even to a separation of the crystal and the liquid phase [1]. On the other hand, some amount of gas may facilitate 'contactless' crystal growth particularly under reduced gravity conditions [2 - 6]. Different non-constituent gases may be present in growth ampoules, and their amount and composition may change during the crystallization process. Some gases can appear even in empty ampoules sealed originally under high vacuum: they may diffuse in from the outside, and/or desorb from the ampoule walls. Residual gases can also be generated by the source materials: even very high purity commercial elements and compounds may contain trace amounts of impurities, particularly oxides. The oxides may have low volatilities themselves but their reaction with other species, particularly carbon and hydrogen, may produce volatile compounds like water or carbon oxides. The non-constituent gases, either added initially to the system or evolved during the material processing, may diffuse out of the ampoule during the course of the experiment. Gases present outside (e.g. as a protective atmosphere or thermal conductor) may diffuse into the ampoule. In either case the growth conditions and the quality of the crystals may be affected. The problem is of a particular importance in sealed systems where the amount of the gases cannot be directly controlled. Therefore a reasonable knowledge and understanding of the origin, composition, magnitude, and change with time of gases present in sealed ampoules may be important for a meaningful control and interpretation of crystal growth processes. This problem is of a particular importance for processing of electronic materials in space because (i) safety considerations require using sealed systems only, and (ii) high cost of crystal growth experiments in microgravity calls for a throughout, accurate description of the processing conditions necessary for a meaningful, efficient, and conclusive interpretation of the space results. In this paper we present the results of our extensive studies on gases in closed crystal growth systems which include: (a) Degassing properties of fused silica; (b) Generation of inert gases by source materials (CdTe, ZnTe, CdZnTe, ZnSe, PbTe, PbSe, PbSeTe); (c) Diffusive cas losses from silica glass ampoules.