Structural and microstructural characterization of nanocrystalline silicon thin films obtained by radio-frequency magnetron sputtering

Textured silicon thin films are deposited by reactive magnetron sputtering in hydrogen-rich plasma on (100)-Si and amorphous SiO₂ substrates. We quantitatively determine, combining x-ray texture analysis, x-ray reflectivity, transmission electron microscopy, atomic force microscopy measurements, and Raman and Fourier transform infrared spectroscopy analyses, the structure (cell parameter and mean electron density) and microstructure (crystalline fraction, preferred orientations, anisotropic crystallite sizes, thicknesses, etc.) of these films. For both kinds of substrates, no perfect <111> orientation is observed whereas a systematic elongation of the anisotropic Si crystallites along one [111] direction is present. A small elongation of the Si cell parameter of the nanocrystals is found without internal stress. With the substrate to target distance, the crystalline fraction and mean electron density show an opposite behavior to that of the film porosity. The former increases and the latter decreases, and are correlated to the texture evolution. Preferred orientations are observed with texture strengths around two to three times a random distribution, with a tendency to achieve lower strengths for films grown on SiO₂ substrates. The texture components are evolving with the substrate to target distance, with <110> and (ℓ larger than 2) orientations favored for smaller and larger distances, respectively. All these microstructural properties are correlated with their optical properties and more particularly to their refractive index and their optical band gap.