Silicon Doping Performed by Different Diffusion Sources Aiming Co-Diffusion

Abstract

In this work several diffusion sources are investigated, aiming at simultaneous phosphorus and boron doping of silicon from different diffusion sources. This challenging process step is called co-diffusion. In the context of this work boronand phosphorus-doped silicate glass (BSG and PSG) as solid diffusion sources and phosphorus oxychloride (POCl3) as gaseous precursor for phosphorus diffusion are investigated. The combination of solid and gaseous diffusion sources leads to a high flexibility in adjusting sheet resistances on the one hand and to a simplification of process flows on the other. In this work the sheet resistance of p-doped areas is varied in the range of Rsh = 50 – 300 Ω/sq by varying composition and thickness of the solid diffusion sources. The deposition of these solid diffusion sources is carried out by plasma-enhanced chemical vapor deposition (PECVD) using two different process equipments. The excellent passivation capability of the highly p-doped silicon surfaces has lead to dark saturation current densities J0 = 29.6 fA/cm. In the case of silicon n-doping with gaseous diffusion sources, the variation of sheet resistance is possible by adjusting the gaseous composition in the ambience during diffusion. The sheet resistance has been varied in the range of Rsh = 50-500 Ω/sq. In the case of n-doping the measured dark saturation current densities have been J0 = 58.6 fA/cm and thus show the high potential of these diffusions for implementation in process sequences of high efficiency silicon solar cells.