Thickness effects on microstructural evolution of low-pressure-chemical-vapor-deposited amorphous silicon films during excimer-laser-induced crystallization

The effect of film thickness on the microstructural evolution of low pressure chemical vapor deposited amorphous silicon (a-Si) during excimer-laser-induced crystallization is reported. For film thickness less than 50 nm, homogenous nucleation and recalescence are the conditioning factors for the re-solidified phase. For the 30-nm-thick a-Si films, a wide laser energy fluence (> 100 mJ/cm2) is formed which results in constant grain size distributions. We estimate the homogenous nucleation density for a 30 nm a-Si film to be 2.7-4.7 events/cm3 in the molten Si. Transmission electron microscopy is used to investigate the polysilicon grain microstructure of irradiated films. Specially, thicknesses between 24 and 36 nm are found to be the critical thickness range determining if the molten Si becomes amorphous or crystalline. To understand the crystallization mechanisms, heat flow calculations based on the laser-induced melting predictions are proposed.