The mechanism and kinetics of the oxidation of Cr(100) single-crystal surfaces studied by reflection high-energy electron diffraction, x-ray emission spectroscopy, and secondary ion mass spectrometry/Auger sputter depth profiling

Abstract

Reflection high-energy electron diffraction, x-ray emission spectroscopy, and ex situ secondary ion mass spectrometry (SIMS)/Auger analysis by sputter depth profiling have been used to study the oxidation of Cr(100) single-crystal surfaces in the oxygen partial pressure range 10−9–10−7 Torr from room temperature to 700 °C. It was found that at all temperatures and for oxygen partial pressures as high as 10−7 Torr, that the initial sticking probability for oxygen was unity. The rate of oxidation was then found to decrease exponentially to a limiting thickness determined by the sample temperature. Various oxide phases have been observed depending on the temperature and the oxide thickness: two face centered cubic phases with a0’s of 4.2 and 11.8 Å, a tetragonal phase, and finally hexagonal α-Cr2O3. Initial oxidation of the Cr(100) surface results in conversion of bcc Cr to fcc oxide. The fcc and tetragonal structures are thought to arise from the formation of different Cr vacancy superlattices due to oxygen incorporation, which leads eventually to the precipitation of α-Cr2O3. Oxygen 18/16 isotope experiments coupled with ex situ SIMS/Auger analysis have clearly demonstrated that oxygen uptake is accompanied by oxygen movement into the fcc Cr–O lattice for temperatures up to about 500 °C. After the appearance of α-Cr2O3, growth is primarily by cation transport.