BBr 3 diffusion with second deposition for laser-doped selective emitters from borosilicate glass

Abstract We study a boron tribromide (BBr 3 ) diffusion process with second deposition realized by active nitrogen flow through the BBr 3 bubbler at the end of the process with respect to laser doping. Compared to the reference BBr 3 diffusion without second deposition, the new BBr 3 diffusion provides a two times higher boron dose within the borosilicate glass (BSG)/silicon dioxide (SiO 2 ) stack layer grown on the silicon surface. The second deposition leads to both thinning of the intermediate SiO 2 layer and 11 nm-growth of the BSG layer, which results in an 8 nm thicker BSG/SiO 2 stack layer with a total thickness of 42 nm. These altered properties of the BSG/SiO 2 layer facilitate the formation of laser-doped selective boron emitters, while the second deposition hardly affects the as diffused charge carrier concentration profile. The obtained emitter properties, sheet resistance R sh ≈ 100 Ω/sq and emitter dark saturation current density j 0e ≈ 25 fA/cm 2 (textured surface, Al 2 O 3 /SiN X passivation) for the photoactive part of the emitter are not affected by the introduced second deposition step. For the laser-doped part of the emitter, the charge carrier concentration after laser doping is higher for the BBr 3 diffusion with second deposition resulting in stronger local doping with up to 10 Ω/sq lower R sh . The application of an analytical model to calculate specific contact resistances ρ C in dependence of e.g. dopant concentration of the laser-doped profiles and crystallite penetration depth d cryst reveals that ρ C is expected to benefit largely from the altered profile shapes due to laser doping for the BBr 3 diffusion with second deposition. A relative decrease in ρ C of up to 90% and 50% is found for small d cryst (depth of several tens of nm) and large d cryst (depth of several hundreds of nm), respectively. Another potential advantage of the second deposition holds for emitter dark saturation current densities at the metal-silicon interface j 0,met . A 3D simulation model that is based on metal crystallites penetrating into the emitter considering the different doping profiles after laser processing yields a j 0,met ≈ 740 fA/cm 2 for small d cryst that is 14% relatively lower for the BBr 3 diffusion with second deposition.