In situ micro Raman investigation of the laser crystallization in Si thin films plasma enhanced chemical vapor deposition-grown from He-diluted SiH4

A cw Ar+ laser crystallization has been performed locally (on a 2 μm sized spot), using a Raman microscope, on plasma enhanced chemical vapor deposition-grown Si films. The deposition has been carried out from differently He-diluted SiH4 so that no high temperature dehydrogenation has been required before the laser treatment. X-ray diffraction patterns and Raman spectra of the deposited films reveal their amorphous nature whereas infrared spectra would indicate a larger degree of local order in the high dilution (HD) material (SiH4/He=0.02) if compared to the low dilution (LD) one (SiH4/He≅3). Atomic force microscopy and scanning electron microscopy images show large, well defined outgrowths, few hundreds of nanometers-sized, on the surface of the HD film whereas these are few tens of nanometers-sized in the case of the LD film. The threshold laser power densities (LPDs) required to attain the crystallization of the HD and the LD materials (in the range of times of irradiation investigated) are 1.2x105 and 2.0x105 W cm-2, respectively. The relative ease to crystallize the HD material possibly originates from the fact that the heavy dilution of the reacting gas implies a lower rate of growth and so a larger degree of order. Large crystalline fractions (∼0.8) have been observed for the laser-treated HD material. Using a phenomenological model, the diameters of the nanocrystallites from the Raman shift of the crystalline peaks have been estimated. The size of the small crystals increases with the time of irradiation (up to a certain time). The smallest nanocrystals would have been fabricated irradiating the LD material at the threshold LPD for the shortest time of irradiation considered in this work. This low temperature process is of great technological interest (e.g., optoelectronics, microelectronics) because it allows the patterning down to a micrometric scale of (amorphous) a-Si:H films deposited onto glass and/or plastic substrates