[reference: "Structural Similarities and Dissimilarities between Si O, ed. I'm not sure why, unless the joke about looking under the lamp post (the light is better) applies: n doesn't vary much and can vary for many reasons.
Dielectric constant at DC-to-microwave frequencies, and infrared absorption are much better ways to characterize Si O.
("Devitrification" -- that is, crystallization -- of quartz furnace tubes used for high-temperature oxidation is sometimes observed after thousands of hours of use at temperatures exceeding 1200 °C.) The amorphous structure is tends to be very "open": even in thermally-grown oxides, channels exist through which small positive ions such as Na can readily migrate.
Deposited silicon dioxide, almost always by CVD approaches, is almost as old as thermal growth on the substrate, and has been employed in various ways in IC fabrication due to its familiarity, versatility, and reliability. Si O is formed by strong, directional covalent bonds, and has a well-defined local structure: four oxygen atoms are arrayed at the corners of a tetrahedron around a central silicon atom: (click link for an animated version)Si O2The oxygen atoms are electronegative, and some of the silicon valence electron density is transferred to the oxygen neighbors, but it is incorrect to regard the material as a salt of a Si[ 4] ion with O[-2] ions, as is sometimes seen in the literature: the directionality of the bonds is essential to the observed structures.
The bond angles around O-Si-O are essentially the tetrahedral angle, 109 degrees; the Si-O distance is 1.61 Å (0.16 nm) with very little variation.
The water can migrate through the deposited materials to the gate oxide, there causing drifts in performance of transistors under bias, impairing hot electron reliability, also known as gate oxide integrity or GOI.
The water molecules can, however, be consumed by the reactions with Si-H groups: this is the basis of the use of silicon-rich oxides as water getters or barriers.
Chemically pure silica has been prepared in at least 35 crystalline forms with density varying by more than a factor of 2 (17 to 43 Si O units per 1000 cubic Angstroms).
Chemical properties such as hygroscopicity (tendency to react with ambient water) vary tremendously depending on the structure. Devine, Plenum (NY) 1992]I've seen a lot of work using the refractive index at optical frequencies to characterize silica.
As the concentration of H at the surface is reduced this becomes increasingly improbable. a hydrogen plasma helps desorb the remaining isolated Si OH. 4639 (1995)]Intermetal dielectric: In IC processing it is often important that each deposited layer end up with a flat surface upon which to build and pattern subsequent layers -- planarization.
[reference: "Thermal Stability of Hydroxyl Groups on a Well-Defined Silica Surface", O. One means of planarizing a layer is to melt it and let it flow as a liquid, minimizing surface tension and thus curvature.
Deposited oxides often have strained bonds and reduced density.