Solid state diffusion of metallic impurities from crucible and coating materials into crystalline silicon ingots for PV application

Abstract

A setup for annealing experiments was developed which allows the detailed investigation of metal impurity solid state diffusion into silicon crystals. In order to simulate the impurity diffusion into multicrystalline or mono-cast silicon ingots during directional solidification processes, small cuboidal Czochralski silicon ingots with extremely low metal contamination, placed on various substrates, were annealed for several hours at temperatures just below the melting point of silicon. As substrates, different crucible materials, partly including SiO2 diffusion barrier layers as well as different Si3N4-coatings with widely varying content of metals (Fe, Cr, Ti, Cu, Co and Ni) were used. After ingot annealing, vertical samples were prepared out of the ingot center and the red zone dimensions were determined by lifetime measurements. Additionally, highly sensitive neutron activation elemental analysis was carried out on the silicon samples. In combination with the concentrations of transition elements of various crucible materials and silicon nitride powders the solid state diffusion process of these impurity species into and through the silicon was investigated. The results indicated, that the red zone area mainly contains the fast and intermediate diffusing elements Fe, Co, Cr and Cu. By variation of certain metal element concentration (Fe, Cr, Co, Ni, Cu and Ti) in the substrates it turned out that Fe and Co have the major impact on red zone formation and charge carrier lifetime reduction. Whereas for increasing Fe concentrations in the substrate material (>1017 at/cm3) no further increase of red zone can be observed due to the formation of precipitates in the lower areas of the red zone, fast diffusing Co atoms are able to reduce also the charge carrier lifetime in the center of the silicon samples.