High resolution study of thermal interfacial oxides in doped polycrystalline silicon passivating contacts

This study examines changes in the thickness of thermally grown ultrathin interfacial oxide layers in doped poly-Si passivating contact structures for high efficiency solar cells. A comparison of interfacial oxide thickness measured by spectroscopic ellipsometry and Transmission Electron Microscopy (TEM) shows that ellipsometry overestimates the oxide thickness by approximately 0.3 nm when compared to TEM. We also investigated changes in the thickness of interfacial oxide layers after each high-temperature step during the formation of doped poly-Si passivating contacts. The TEM studies demonstrate that the interfacial oxide thickness remains largely unchanged after an intrinsic poly-Si layer deposition on top of the oxide, and also after ex-situ dopant diffusion to form n⁺ and p⁺ poly-Si contacts. However, employing a pre-annealing step of the intrinsic poly-Si films at 1000 °C prior to dopant diffusion, thickens the interfacial oxides by approximately 0.4 nm, and improves the crystallinity of the doped films. Finally, we investigated the impact of oxide thickness and pre-annealing on the surface passivation and contact resistivity, revealing a positive impact of the pre-annealing step in both cases. Increasing the oxide thickness up to 1.5 nm had minimal impact on the surface passivation but led to a significant increase in contact resistivity.