Lithium Precipitation in Silicon

Abstract

Direct observation of precipitates has been previously reported in silicon diffused with phosphorus, arsenic, copper, aluminum, and lithium. The precipitates observed in these studies varied in shape from square, rod-like, hexagonal or irregular platelets, often exhibiting no preferred alignment, to nearly spherical regions of varying concentration and localization, depending upon the initial defect density and crystalline perfection. In the present study it was desired to examine the precipitation process in lithium diffused silicon in terms of the induced defect morphology and diffusion time. To accomplish this end, n-type polished silicon wafers of (111) orientation were coated with mineral oil containing a suspension of lithium. The samples were subsequently annealed for variable times at 450°(C) in a helium atmosphere. The faces were then cleaned ultrasonically in a solution of methyl alcohol and distilled water. After appropriate chemical-mechanical polishing, the samples were subjected to a 2-hour anneal at 600°(C) to insure homogenization.