A diffusional model for the oxidation behavior of Si1−xGex alloys

Abstract

We have developed a kinetic model to describe the oxidation behavior of Si1−xGex alloys during Ge segregation, which compares the Deal–Grove flux of oxidant diffusing through the oxide to the maximum flux of Si diffusing through the Ge-rich layer. This is motivated by thermal oxidation experiments on Si1−xGex alloys (x<0.17) using a fluorine-containing ambient (O2 and 200 ppm of NF3). The fluorine is known to modify point defect generation during oxidation of pure Si toward vacancy production, which is also the case for Ge in Si. We demonstrate that fluorinated oxidation of Si1−xGex enhances the oxidation rate by 25%–40% in the temperature range of 700–800 °C. Oxides formed at these temperatures were SiO2, while those formed at 600 °C exhibited a transition from SiO2 to mixed oxide growth at some point during the very early phase of oxidation, depending on the alloy composition. Consideration of these data suggests that other factors in addition to oxidation temperature must be considered in predicting which oxide type will be produced, in contrast to most previous reports. Our model, indeed, shows that alloy composition, oxide thickness, and oxidant partial pressure are also important parameters. We believe that the model is very useful in predicting the oxide type that should result from a given set of growth conditions, and in particular, it suggests that a changeover from SiO2 to mixed oxide formation is likely at some point during the oxidation process, particularly if carried to larger thicknesses.