Tuning the Limiting Thickness of a Thin Oxide Layer on Al(111) with Oxygen Gas Pressure

We report an x-ray photoelectron spectroscopy study of the oxidation of Al(111) surfaces at room temperature, which reveals that the limiting thickness of an aluminum oxide film can be tuned by using oxygen pressure. This behavior is attributed to a strong dependence of the kinetic potential on the oxygen gas pressure. The coverage of oxygen anions on the surface of the oxide film depends on the gas pressure leading to a pressure dependence of the kinetic potential. Our results indicate that a significantly large oxygen pressure ($>1\mathrm{Torr}$) is required to develop the saturated surface coverage of oxygen ions, which results in the maximum kinetic potential and therefore the saturated limiting thickness of the oxide film.

Figure 1

Oxide film thickness as a function of oxidation time and oxygen gas pressure. The oxidation starts with a clean Al(111) surface which is oxidized first at $p({\mathrm{O}}_2)=1\times {10}^{-8}\mathrm{Torr}$. A stepwise increase in oxygen pressure is applied after a limiting oxide thickness is reached at each oxygen pressure. The stepwise increase in oxygen pressure results in a corresponding increase of the limiting thickness of the oxide film until an oxygen pressure of $p({\mathrm{O}}_2)=1\mathrm{Torr}$ is reached, beyond which the additional oxygen exposure to the surface does not result in any subsequent oxide growth.

Figure 2

Comparison of the oxidation kinetics of a freshly cleaned Al(111) surface to that of an Al(111) surface oxidized by stepwise increases in oxygen pressure. Both give a similar limiting thickness of the oxide films at the same oxygen gas pressure, irrespective of whether the surface is covered with a preexisting oxide layer formed at a lower oxygen pressure or not.

Figure 3

Equilibrium surface coverage of oxygen ions with respect to the oxygen gas pressure. The solid line corresponds to a theoretical fitting to the Langmuir isotherm for dissociative oxygen adsorption. The dashed line indicates the approximate oxygen pressure beyond which the maximum oxygen surface coverage is reached.