Theoretical and experimental investigation of aluminum-boron codoping of silicon

We present a detailed study on aluminum-boron codoping of silicon by alloying from screen-printed aluminum pastes containing boron additives (Al-B pastes). We derive an analytical model for the formation of the Al-B acceptor profiles by quantitatively describing (i) the composition of the Al-B-Si melt and (ii) the incorporation of Al and B acceptor atoms into the recrystallizing Si lattice. We show that measured Al-B dopant profiles can be excellently described by this model, which therefore offers a straightforward method for the comprehensive investigation of alloying from Al-B pastes. The formation of a characteristic kink in the Al-B dopant profile curve can thus be ascribed to the exhaustion of the B additive dissolution during alloying. By intentionally adding elemental B powder to an Al paste, we demonstrate that only a low percentage of the B powder actually dissolves into the melt. We show that this incomplete dissolution of the B additive strongly affects the recombination characteristics of Al-B-p(+) regions and, thus, is an important element of alloying from Al-B pastes. This study therefore provides improved understanding of aluminum-boron codoping of silicon.