Diode laser array used for decomposition of SiOx into sponge-like Si-SiO2 nanocomposites

Line-shaped light beams from diode laser arrays permit homogeneous and long-term stable processing of surface layers. Here we report on thermally activated decomposition of SiOx using a commercial diode laser of the LIMO GmbH, with cw-operation at a wavelength of 808nm, a maximum power density of 30 kW/cm², a minimum line focus < 100 µm, and a variable scan speed. Dwell times from < 1ms to 100ms can be realized by adjusting the scan speed and focus width, which allows very Rapid Thermal Processing (vRTP) of surfaces layers. SiOx layers of < 1µm thickness have been grown on quartz by sputter deposition. Two different modes of thermal treatment have been used: Furnace annealing at 950°C for 90 minutes and vRTP with 17ms dwell time. Energy-Filtered Transmission Electron Microscopy (EFTEM) reveals that in both cases the homogeneous SiOx has been transformed into a sponge-like Si-SiO2 nanocomposite. Raman spectroscopy shows that the crystallinity of spongy Si is higher for the laser treated sample. Whereas the characteristic structure size of the spongy Si of the furnace annealed sample amounts to a few nm only, it is a few tens of nm for vRTP. It will be shown that in the furnace the phase separation proceeds in the solid state, whereas the more complete phase separation by laser treatment can only be understood by liquid state processes. The laser-synthesized Si nanosponge can be applied as new material in next generation solar cells.