The relaxation behaviour of supersaturated iron in single-crystal silicon at 500 to 750 °C

Abstract

Iron-related defects cause major problems in silicon for both microelectronic devices and photovoltaics. Iron contamination can occur during high temperature processing or, particularly in the case of low-cost photovoltaics, from the feedstock. In many situations, silicon is cooled too rapidly for the establishment of equilibrium, and so the bulk iron concentration exceeds the solubility value. We have investigated the relaxation of supersaturated bulk iron to the equilibrium solubility in single-crystal silicon. Bulk iron concentrations are measured by analysing the change in minority carrier lifetime that occurs when iron-boron pairs are dissociated. High-purity silicon is rubbed with iron and annealed at 750 °C for 24 h. This process creates an iron silicide phase on the rubbed surface and allows the equilibrium solubility of ∼2 × 1012 cm−3 to be established. Samples are then annealed at lower temperatures (500 to 700 °C) for a range of times. The rate of decay in iron concentration depends upon whether a silicide was formed on one side or two sides, with the kinetics in excellent agreement with iron diffusion to one or both surfaces, respectively. Even for the highest supersaturation (∼2000 times the solubility), the pre-existence of a silicide on one surface means there is insufficient driving force for nucleation of a silicide on the other surface. Relaxation experiments were also performed on contaminated samples for which the iron silicide source at the surface was removed after contamination. The iron concentration decays substantially more slowly in these specimens. The kinetics can be explained by relaxation to bulk voids.