Theory of Formation of Very Thin Oxide Films on Metals

Abstract

The growth of oxide on initially oxide-free evaporated aluminum films at room temperature and atmospheric pressure has been studied in parallel experiments, using elliptically polarized light to measure the film thickness and vibrating capacitor measurements to measure changes in the Volta potential. The film thickness reaches 21 Å in 106 sec and varies essentially logarithmically with time in the time range from 102 to 106 sec. The Volta potential is almost constant during the initial stages of growth and then varies approximately logarithmically with time. The observed change of Volta potential, interpreted as resulting from changes in the Galvani potential, is approximately 350 mV in 106 sec of oxidation. A logical extension of the Mott theory is presented which includes (1) the effect of electronic space charge resulting from equilibration of electrons between the metal and oxide and (2) the electric field in the oxide, both in its role as a barrier to electron penetration of the oxide and in its control over the migration of interstitial aluminum ions. The extended theory predicts a linear relation between V½X (V is Galvani potential difference across the oxide and X is oxide thickness) and ln (oxidation time) whose slope is 3ℏ/(32me)½, m and e being the mass and charge of the electron and ℏ being Planck's constant divided by 2π. The experimentally determined slope has the value 1.32×10−8 V½·cm as compared to the theoretical slope of 1.46×10−8 V½·cm.