Suitability of POCL3 diffusion processes with in-situ oxidation for forming laser-doped selective emitters with low carrier recombination

Laser-doped selective emitters feature the advantage of more effective shielding of minority charge carriers from the metal contacts while allowing for low emitter saturation current density j0e in the photoactive area. The formation of emitters by diffusion processes using phosphorus oxychloride (POCl3) with incorporated in-situ oxidation gains more and more attention as it allows for low j0e below 90 fA/cm² on textured surface with silicon nitride passivation in industrial cycle times. Hence, the combination of both-POCl3 diffusion with in-situ oxidation and laser doping-is very interesting. We examine four different POCl3 diffusions with in-situ oxidation and one reference POCl3 diffusion without in-situ oxidation in terms of their suitability for selective emitter laser doping. Detailed characterizations of the grown layers on the silicon surface are performed after diffusion with respect to the individual layer thicknesses of the phosphosilicate glass (PSG) and the intermediate silicon dioxide (SiO2) layer as well as the stacks' total phosphorus doses. The as-diffused depth-dependent charge carrier concentration profiles show that a second PSG deposition step attached after drive-in to the diffusion processes hardly impacts their course. We find that POCl3 diffusions with in-situ oxidation-especially those with second deposition step-allow for effective laser doping. Thereby, the intermediate SiO2 layer thickness plays a key role: the thicker the layer is the less phosphorus can be incorporated additionally from the PSG layer into the silicon.