Investigation of Contact Resistivities on APCVD (p) Poly-Si for Fired Passivating Contacts

Abstract

We investigate the properties of boron doped polycrystalline Si (poly-Si) deposited by atmospheric pressure chemical vapor deposition (APCVD) applied to fired passivating contacts (FPC), where no high temperature annealing takes place apart from the contact firing step. X-ray diffraction measurements show that the APCVD poly-Si is already partially crystallized directly after deposition and the crystallite size further increases during firing. Without metallization an implied open circuit voltage of up to 719 mV is achieved. Screen-printing with an Ag paste yields minimal contact resistivities of down to 1 mΩcm² at high firing temperatures. Furthermore, thicker poly-Si layers, accomplished by driving the same wafer multiple times through the APCVD system, generally correspond to lower contact resistivities for the FPC. This can partly be explained by an increasing crystallinity and conductivity during deposition due to the higher thermal budget during deposition for thicker layers as well as by a larger contact area for thicker poly-Si layers. Scanning electron microscopy on sample cross-sections show that almost the entire poly-Si layer is covered with Ag crystallites at high firing temperatures. For lower temperatures a lower density of Ag crystallites in the poly-Si is visible. Both findings hold for planar and textured surfaces.