Reorganization of Porous Silicon: Effect on Epitaxial Layer Quality and Detachment

Abstract

Cost reduction is still a main goal in solar cell research and can be achieved by going towards thinner silicon bulk material. One way to avoid kerf loss and to reach thicknesses of less than 50 μm is a lift-off approach using porous silicon and epitaxial thickening for silicon foil fabrication. The porous layer structure requires a reorganization step that was varied in this work to optimize the detachment properties and the crystal quality of the epitaxial Si film. All processes were carried out in a quasi-inline Atmospheric Pressure Chemical Vapour Deposition (APCVD) reactor. Cross–sections were observed to see if the porous layer shows the desired structure. Stacking fault densities in epitaxial layers deposited on porous silicon layers significantly decrease with increasing reorganization time but are at least one order of magnitude higher than in epitaxial layers deposited on polished wafers. Microwave photoconductive decay (MWPCD) measurements and photo luminescence (PL) imaging were carried out to determine the effective carrier lifetimes of the detached foils and to correlate them with stacking faults and cracks. A detached 40 μm thin silicon foil with an averaged effective carrier lifetime of 22 μs is shown which corresponds to a diffusion length of over 200 μm. This investigation shows that silicon foils deposited in a quasi-inline APCVD reactor exhibit good detachment properties and a good crystal quality, which is both needed for high efficiency solar cell processing.