In situ excimer laser irradiation as cleaning tool for solid phase epitaxy of laser crystallized polycrystalline silicon thin films

Abstract

This study demonstrates an innovative approach to clean a polycrystalline seed layer surface for solid phase epitaxy of amorphous silicon (a-Si). The excimer laser cleaning (ELC) makes use of in situ excimer laser irradiation during the first stages of a-Si deposition leading to melting of the as deposited a-Si and parts of the seed layer. The increased diffusion in liquid silicon allows for "smearing out" of surface contamination species. After liquid phase epitaxy is finished, further a-Si is deposited and crystallized by solid phase epitaxy. Thanks to the "smearing out," the interface between the crystalline and amorphous silicon exhibits less contamination locally. SIMS measurements demonstrate a reduced carbon concentration at the seed layer surface after ELC. Numerical simulations, taking into account heat transfer and temperature dependent diffusion, support the reduction of carbon concentration. The simulations agree very well with experimental results. To get optimal solid phase growth conditions, the a-Si deposition temperature has to be below 200 8C. Above 200 8C deposition temperature, defective growth occurs resulting in poor crystallinity or even in nonepitaxial growth.

Multicrystalline silicon thin film solar cells on glass with epitaxially grown emitter prepared by a two-step laser crystallization process

“…of the silicon nitride or silicon carbide layer, by the Piranha cleaning and hydrofluoric acid etching steps. Referring to reference a contaminated interface leads to defective epitaxial growth during excimer laser crystallization with lower fluences.…”

“…The liquid phase epitaxial process was numerically simulated using commercial software (Comsol). The simulation takes account of heat conduction with the absorbed light of the excimer laser as heating source and the Jackson–Chalmers model for the epitaxial crystallization .…”