Lifetime Analysis for Defect Characterization in Kerfless Epitaxial Silicon from the Porous Silicon Process

Abstract

Kerfless epitaxial silicon from the porous silicon (PSI) process is a promising alternative for standard wafers. They allow the reduction of PV costs by combining high material quality at reduced production costs. We evaluate the minority carrier lifetime of p-type and n-type epitaxial silicon layers fabricated with the PSI process by means of photoconductance decay measurements. For p-type layers we observe a strong injection dependence of the lifetime that we attribute to bulk Shockley-Read-Hall (SRH) recombination. We determine two limiting defects K3.6 and K157 that describe the injection dependence of 9 samples grown in one batch. Defect K3.6 has a symmetry factor of k=3.6 and is similarly concentrated in all 9 investigated samples. Its concentration decreases upon high temperature processing with and without phosphorous diffusion. The defect K157 has a symmetry factor of k=157 and a higher concentration in samples with a higher porosity in the starting layer. As a consequence of the k-factors being larger than unity the identified defects are less detrimental in n-type silicon than p-type silicon. Accordingly, we fabricate n-type epitaxial layers for which we measure effective lifetimes up to 1330±130μs at Δp = 1015 cm –3.