Microstructural characterization of thin SiOx films obtained by physical vapor deposition

X-ray diffraction and reflectivity, X-ray photoelectron spectroscopy and spectroscopic ellipsometry were applied to study the initial composition, thickness, lattice structure and refractive index of ‘fresh’ and annealed thin SiOx films (∼15 nm) on crystalline silicon substrates, prepared by thermal evaporation of SiO in vacuum. It has been ascertained that the film thickness and composition (x = 1.3) of the ‘fresh’ films are very close to the values set during the deposition. It has been shown that furnace annealing of the films at 1000 °C causes phase separation, film densification and small modification of the Si–SiOx interface. Transmission electron microscopy results have proven that a self-assembling process leads to formation of Si nanocrystals with a diameter of ∼4–5 nm and to epitaxial overgrowth of the Si substrate, increasing the c-Si/SiOx interface transition region to 6–7 monolayers. The nanocrystals are randomly distributed in an amorphous SiO2 matrix being closer to the Si–SiOx interface. Formation of tunnel oxide layer with a thickness of 3–5 nm has been found upon annealing. Clockwise hysteresis has been observed in the capacitance-voltage characteristics measured which has been explained by assuming charging and discharging of the nanocrystals with holes, which tunnel from the Si substrate.