Depletion of boron-doped surfaces protected with barrier layers during POCL3-diffusion

Different diffusion barrier layers applied on the boron-doped surfaces of silicon wafers with sawdamage etched surfaces are investigated with respect to their applicability in the fabrication process of n-type silicon solar cells using sequential tube furnace diffusions with a first BBr3- and a subsequent POCl3-diffusion. The barrier layers consist of silicon oxide and/or silicon nitride deposited either by plasma-enhanced chemical vapor deposition or sputter technology. The layer itself must act as barrier against phosphorus diffusion into the silicon wafer during POCl3-diffusion. Furthermore, it has to ensure that no substantial depletion of boron occurs at the wafer surface. The boron doping surface concentrations and profile depths measured after POCl3-diffusion depend on the applied diffusion barrier. If solely silicon oxide barrier layers are used, depletion of boron at the wafer surface and deeper profiles are observed, which we attribute to oxygen diffusion through the barrier and growth of a thin thermal oxide film at the silicon. With a thin silicon nitride layer incorporated into the diffusion barrier system, no significant change in the boron doping profile is detected. Numerical simulations of the boron diffusion during the POCl3- process agree well with the measurements and support these findings.