Impurity Gettering in Polycrystalline-Silicon Based Passivating Contacts—The Role of Oxide Stoichiometry and Pinholes

Polycrystalline-silicon/oxide (poly-Si/SiO_x) passivating contacts for high efficiency solar cells exhibit excellent surface passivation, carrier selectivity, and impurity gettering effects. However, the ultrathin SiO_x interlayer can act as a diffusion barrier for metal impurities and this potentially slows down the overall gettering rate of the poly-Si/SiO_x structures. Herein, the factors that determine the blocking effects of the SiO_x interlayers are identified and investigated by examining two general types of the SiO_x interlayers: 1.3 nm ultrathin tunneling SiO_x with negligible pinholes and 2.5 nm SiO_x with thermally created pinholes. Iron is used as tracer impurity in silicon to quantify the gettering rate. By fitting the experimental gettering kinetics by a diffusion-limited segregation gettering model, the blocking effects of the SiO_x interlayers are quantified by a transport parameter. Both the oxide stoichiometry and pinhole density affect the effective transport of iron through SiO_x interlayers. The oxide stoichiometry depends strongly on the oxidation method, while the pinhole density is affected by the activation temperature, doping concentration, doping technique, and possibly the dopant type as well. To enable a fast gettering process during typical high-temperature formation of the poly-Si/SiO_x structures, a SiO_x interlayer that is less stoichiometric or with a higher pinhole density is preferred.