Microstructural evolution and properties of He-charged a-Si coatings prepared by magnetron sputtering

The introduction of porosity in nanomaterials via magnetron sputtering in helium atmospheres is an interesting strategy for the design of functional materials. Amorphous silicon coatings, with high concentration of He incorporated in the form of overpressurized nano-bubbles, present modified optical properties in comparison with dense coatings of similar composition.In this work, annealing process in He atmosphere from 300 C-degrees up to 1200 C-degrees is performed to evaluate stability after the release of He from the a-Si:He coatings (thickness similar to 1700 nm) and to favor matrix crystallization and defect evolution. The work is focused on these effects on the silicon film and its properties. The composition and microstructural evolution of the coatings annealed to temperatures as high as 1200(degrees) were investigated by electron microscopy and X-Ray diffraction. Annealing effects on the optical properties and bandgap of the silicon coatings were evaluated by ellipsometry and UV-Vis spectrometry. Helium release from the coating, densification, defects evolution and ordering due to crystallization have a small effect on the refractive index, but cause a significant change in the material band gap from 1.37 eV to 1.80 eV.