A DIFFUSIONAL MODEL FOR THE OXIDATION BEHAVIOR OF SI1-XGEX ALLOYS

We have developed a kinetic model to describe the oxidation behavior o f 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 oxi dation experiments on Si1-xGex alloys (x<0.17) using a fluorine-contai ning ambient (O-2 and 200 ppm of NF3). The fluorine is known to modify point defect generation during oxidation of pure Si toward vacancy pr oduction, which is also the case for Ge in Si. We demonstrate that flu orinated oxidation of Si1-xGex enhances the oxidation rate by 25%-40% in the temperature range of 700-800 degrees C. Oxides formed at these temperatures were SiO2, while those formed at 600 degrees 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. Con sideration of these data suggests that other factors in addition to ox idation temperature must be considered in predicting which oxide type will be produced, in contrast to most previous reports. Our model, ind eed, shows that alloy composition, oxide thickness, and oxidant partia l pressure are also important parameters. We believe that the model is very useful in predicting the oxide type that should result from a gi ven set of growth conditions, and in particular, it suggests that a ch angeover from SiO2 to mixed oxide formation is likely at some point du ring the oxidation process, particularly if carried to larger thicknes ses. (C) 1997 American Institute of Physics.