Polycrystalline silicon foils by flash lamp annealing of spray-coated silicon nanoparticle dispersions

A novel technique for the preparation of thin silicon foils (d ≈ 10 µm) starting from silicon nanoparticle dispersions is reported. The basic steps are ultrasonic spray coating of mixtures of silicon nanoparticles and organosilicon compounds and subsequent flash lamp annealing (FLA). The organosilicon compounds are used to stabilize the silicon nanoparticles in dispersion. The FLA induces melting of the silicon nanoparticles which transform into polycrystalline silicon thin films. Parameter adjustment (e.g., pressure, flash duration, flash energy, and substrate temperature) allows for the preparation of thin films as well as free-standing and flexible silicon foils. If the sprayed film thickness reaches a critical value (d ≈ 20 µm) and the FLA is performed under vacuum conditions (5 × 10⁻³ mbar), then the film peels off from the molybdenum substrate. In a following step, the foil is flashed by FLA from the backside. This method targets thin silicon foils exhibiting thicknesses between 5 and 10 µm. The lateral size of silicon foils depends on the setup used. In this work, samples with a maximum area of approximately 5 × 5 cm² were produced. The silicon foils were investigated by scanning electron microscopy and spectroscopic ellipsometry to determine thickness, surface structure, and the effective dielectric response. The compositions (Si, C, and O) and the bond characteristics of Si–O and Si–C were analyzed by means of energy dispersive X-ray spectroscopy and X-ray photoemission spectroscopy. Transmission electron microscopy and X-ray diffraction provided the average sizes of the silicon nanoparticles before and after FLA. A rough estimation of the free charge carrier concentration [n _el ≈ (10¹³–10¹⁴) cm⁻³] was possible by electrical four-point probe measurements taking into account information on crystallite sizes.