Dependence of electronic properties of polysilicon grain size and intragrain defects

TLDR: In this paper, it was shown that the minority carrier mobility depends on the grain size even when this size is larger than 0.1 mm, which explains that generally the electronic properties are degraded at the bottom or at the periphery of the ingots where the dislocation density is high and at the top where the impurities are concentraled by the Bridgman process.

Abstract: Lifetimes (τn), diffusion lengths (Ln) and consequently mobilities (μn) of minority carriers and also mobilities (μp) of majority carriers have been measured on small MESA diodes (1.5 × 1.5 mm) revealed on photocells realized on boron-doped cast polysilicon obtained by a fast variant Bridgmen method. The lifetime τn and the diffusion length Ln decrease drastically when the number of grains per diode NG increases. It has been deduced that the minority carrier mobility μn depends on the grain size even when this size is larger than 0.1 mm. This influence of the grain size is often screened by intragrain defects. This explains that generally the electronic properties are degraded at the bottom or at the periphery of the ingots where the dislocation density is high and at the top where the impurities are concentraled by the Bridgman process.