Impurity Gettering in Polycrystalline‐Silicon Based Passivating Contacts—The Role of Oxide Stoichiometry and Pinholes
Zhongshu Yang, Jan Krügener, Frank Feldmann, Jana‐Isabelle Polzin, Bernd Steinhauser, Tien T. Le, Daniel Macdonald, AnYao Liu
Abstract
Abstract 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.