Our experience shows it is slightly easier way to oxidize Aluminum in air (natural oxidation) at room temperature and atmospheric pressure. Nevertheless a number of authors use just pure oxygen in a vacuum chamber under pressure. Is there is any experimental circumstances when one method of oxidation is preferred from the other, or experimental evidence that suggest that one of this methods is more reliable from the other to produce good quality Al2O3.
Of course, oxidation process employing pure oxigen in isolated room will produce a better product in quality comapared to open oxidation in air. It is because so many various contaminants present in our air now which make the prouduct can be contaminated.
Remember that air is a mixture of gases, particles, water and so on. Oxygen accounts for only 21% of the mixture and in air there are reducing gases such as N2, Ar and CO. Thus if you want to study the fundamental properties of oxides it is better to isolate the effect of oxygen alone. However, many industrial processes that require oxidation of metals (for example formation of TiO2) use air directly or as much, filtered. Also, many other processes use dry air to avoid for example, pitting corrosion induced by water. On the other side, Al2O3 forms spontaneously onto Al . Good quality Al2O3 means what? crystalline, uniform? For decorative aluminum it is often formed by electrochemical anodization...
Are you trying to oxidise bulk aluminium sheet/foil, or a thin aluminum film on a substrate
It all depends on the thickness of the oxide layer you are plannning to achieve. For example in the case of Silicon, SiO2 growth on silicon wafers which is a standard process in semiconductor industry,
wet oxidation and dry oxidation provide a lot of differences. You can some good idea by consulting standard semiconductor books on IC processing, and learn about the advantages and disadvantages with dry and wet oxidation. Maybe they will apply to your case of aluminum as well.
Thanks all for your replies, the reason I ask if because we have made air oxidiced junctions (for more that a 3 days oxidation) in thin Al films where the junction resistance (resistance times area) much higher than more advanced methods. This was a surprice, any thoughts on this?
I have been using plasma oxidation for the fabrication of tunnel junctions (both magnetic and superconducting). In my case there are two drawbacks for using the air oxidation:
1. interface contamination, mainly due to water but also other adsorbed species, so that's why vacuum is needed.
2. the natural oxidation (either in controlled atmosphere or air) will allow the oxidation of only a fixed thickness of Al, corresponding to the passivation thickness, so if either thicker or thinner layers are needed, then one has to use a plasma.
Your junction resistance will certainly be affected by many parameters, but my guess is that it is due to the interface residues that will act as a thicker effective barrier. The other possibilities would be that the passivation thickness was slightly different or that you got pinholes in the Alox that somehow got 'filled up' in the air process.
For decorative aluminum it is often formed by electrochemical anodisation.....
Actually there is one more issue related to the kinetics of the reaction. I believe the rate of oxidation is a function of partial pressure of oxygen (means that higher pressure of oxygen will give higher reaction rate). Since in partial pressure of O2 in air is much lower than it is in pure oxygen, I personally think that conduct oxidation in pure Oxygen is better, but there is also an economical trade-off (air is much cheaper). And if your material acts as a catalyst to activate Nitrogen (dissociate N2) then there will be many more problems.
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