Advanced BBr3 diffusion with second deposition step for selective emitter formation by laser doping
The concept of attaching a second deposition step at the end of boron tribromide (BBr3) diffusion is introduced, where second deposition describes an active nitrogen flow through the BBr3 bubbler. This approach provides a higher boron dose in the borosilicate glass (BSG) which facilitates the formation of laser‐doped selective emitters. It is found that the second deposition hardly impacts the as‐diffused charge carrier concentration profile in comparison to BBr3 diffusion without second deposition. The emitter sheet resistance Rsh ≈ 110 O sq−1 and emitter dark saturation current density j0e ≈ 25 fA cm−2 (alkaline textured, Al2O3/SiNX passivation) are similar for both processes. The BBr3 diffusion process forms a BSG/silicon dioxide (SiO2) stack layer on the silicon. The BBr3 diffusion with second deposition step results in an 8 nm thicker BSG/SiO2 stack layer (total thickness: 42 nm) with factor two higher boron dose compared to the BBr3 diffusion without second deposition. After laser doping, the charge carrier concentration is higher for the BBr3 process with second deposition resulting in stronger local doping with about 10 O sq−1 lower Rsh. For laser‐doped and Al2O3/SiNX‐passivated areas, a promising process combination results in j0e = (250 ± 30) fA cm−2 at Rsh = (65 ± 1) O sq−1.