Behavior of Ultrathin Zirconium Films upon Exposure to Oxygen

Abstract

Discontinuous films of zirconium, having surface resistivities between 104 and 106 Ω sq−1, were deposited on glass substrates in ultrahigh vacuum and then exposed to oxygen pressures in the range of 5×10−7 to 4×10−8 Torr at temperatures of 25° and −196°C. The change in film resistance with time was monitored and a plot of logR vs logt was constructed. This graph was initially concave upwards, but became linear after a time which was characteristic of monolayer formation. An interpretation of the film behavior was based on the transport of electrons by tunneling through gaps separating the metal islands. The initial change in film resistance was caused by an increase in the zirconium work function. This increase was due to oxygen monolayer formation. The subsequent linear log-log behavior was caused by an increase in the tunneling distance which resulted from oxidation of the metal islands. Values for both the interisland spacing and the room-temperature oxidation rate constant have been calculated from the experimental data. Both impurities and the heat of adsorption interfered with determining a value for the interisland spacing, but when these interferences were eliminated, a distance of 6.9 Å was found for a 104 Ω sq−1 film. The oxidation rate constant for a comparable film at 25°C was 4×10−9 cm with an activation energy of 0.3 kcal mole−1. Pumping speeds also were determined. Zirconium initially pumped oxygen at ≈1500 ml·sec−1·cm−2. The sticking coefficient for oxygen on a partially oxygen covered zirconium surface was approximately 0.05.