Compensation of the sputter damage during a-Si deposition for poly-Si/SiO_x passivating contacts by ex-situ p-doping

Abstract

The rf magnetron sputter deposition of a-Si during fabrication of passivating contacts based on poly-Si on top of an interfacial silicon oxide (poly-Si/SiO_x) and a possible sputter damage is investigated, as it is also observed during transparent conductive oxide sputtering for heterojunction solar cells. It is shown that the high temperature anneal for partial crystallization of the sputtered a-Si has a detrimental effect on passivation quality. However, doping during partial crystallization of the a-Si layer by a POCl₃-diffusion compensates this decrease of passivation quality by a field effect passivation. Moreover, a subsequent hydrogenation of the interface leads to implied open circuit voltages of up to 719 mV and saturation current densities of down to 9 fA/cm². Furthermore, depth profiles of the dopants measured by glow discharge optical emission spectroscopy (GD-OES) and electrochemical capacitance-voltage profiling (ECV) reveal a significant difference between total (∼3.5·10²¹ cm⁻³) and electrically active (∼5·10²⁰ cm⁻³) phosphorous atomic density in the poly-Si layer. From these depth profiles also a pile-up of electrically inactive phosphorous close to, but a few nanometers off the interfacial oxide was observed.