Understanding phosphorus emitter diffusion in silicon solar cell processing

High concentration in-diffusion of phosphorus from a spray-on source is a promising method for in-line emitter diffusion in p-type silicon solar cell processing. Increasing the understanding of the physical mechanisms behind phosphorus diffusion in silicon is therefore of great importance in order to facilitate optimization of the diffusion process. In this paper, the main mechanisms responsible for dopant transport during phosphorus emitter diffusion in solar cell processing are identified and described, and an integrated diffusion model is presented for simulation of such emitter profiles. It is found that the occurrence of the kink-and-tail profile shape, characteristic of
high concentration phosphorus diffused emitters, is caused by a changeover from a vacancy-mediated diffusion at high concentrations to an interstitially driven diffusion at lower concentrations. This changeover involves a transition via migration of self-interstitials, limiting diffusion in the so-called kink region to restore local point defect equilibrium. It is further found that the three distinct diffusion regimes all exhibit Arrhenius behaviors, thus a model for emitter diffusion involving only three free parameters, namely the diffusion temperature, the diffusion time, and the phosphorus surface concentration, can be employed to accurately simulate emitter diffusion profiles.