The structural aspects of non-crystalline SiO2 films on silicon: a review

Thermal oxidation of silicon results in vitreous (v) SiO2 films that are non-crystalline (nc) but have a high degree of short-range-order and bond ordering that is usually not the case with amorphous solids. This combination results from the small variation of the Si–O bond energy as the Si–O–Si bond angles (φ) vary from less than 120° to ∼180°. The flexibility of the nc structure is responsible for the crucially important low density of interface states resulting from the quasi-epitaxial accommodation of the growing oxide to the Si substrate. The grown oxide is in a non-elastically compressed (densified) state which profoundly affects various properties. The bond energy increases rapidly as φ becomes less than ∼120°. These ‘strained’ bonds are responsible for several electrical effects as, for instance, hole trapping. SiO2 films prepared by deposition or O+ implantation techniques are similar to thermally grown oxides, as they are also non-crystalline, but some of their properties are significantly different.