Solar-grade boron emitters by BF3 plasma doping and role of the co-implanted fluorine

Abstract

We investigate the electrical properties and dopant profiles of boron emitters performed by plasma immersion ion implantation from boron trifluoride (BF3) gas precursor, thermally annealed and passivated by silicon oxide/silicon nitride stacks. High thermal budgets are required for doses compatible with screen-printed metal pastes, to reach very good activation rates. However, if good sheet resistances and saturation current densities may be obtained, we met strong limitations of the implied open-circuit voltage of the n-type Czochralski silicon substrates, which is incompatible with high-efficiency solar cells. Such limitations are not encountered with beamline where pure B+ ions are implanted. Efforts on the passivation quality may improve the implied open-circuit voltage but are not sufficient. We provide experimental comparison between beamline and plasma immersion allowing us to discriminate the causes explaining this observation (implantation technique or ion specie used) and to infer our interpretation: The co-implantation of fluorine seems to indirectly impact the lifetime of the core substrate after thermal annealing.