the O/Al ratio in stoichiometric alumina film should be 1.5 but in most cases off stoichiometry is obtained. What then happens to the excess O in the film in cases of O/Al greater than 1.5. What then is the implication for the film and possible explanation of the excess oxygen in case of RF sputtering of alumina film?
In reactive sputtered Al2O3, O/Al ratio is usually more than 1.5. O/Al ratio in this process depends on the arrival rate of Oxygen and Aluminum to the substrate and strongly depends on target-substrate position [Please follow Olsson et.al.]. The extra oxygen might form oxygen interstitials which is responsible for negative oxide charge in Al2O3. However, with annealing O/Al ratio comes down and will become almost stoichiometric.
http://www.sciencedirect.com/science/article/pii/S0257897299003783
http://www.sciencedirect.com/science/article/pii/S1369800112003022
I know that you can get over-stoichiometry in the film. A small excess is possible. Indeed the extra oxygen is positioned interstitially in the Al2O3 lattice. Amounts leading to a ratio of 2.5 I don't think that is possible. Due to the interstitials you will also get a lot of compressive stress in the film.
In amorphous films the excess of oxygen is not interstitial and maybe higher amounts of oxygen are possible. I only studied over-stoichiometry at WO3 films. At these amourphous films, I couldn't measure higher ratio's than 3.3 at increasing oxygen pressure during sputtering.
WO3 is an electrochromic material in which you can switch Li-ions in and out. The amount of trapped iones corrolated with the excess of oxygen in the WO3 layer. That's why I studied the over-stoichiometry.
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