ARE POLYTYPE TRANSITIONS POSSIBLE DURING BORON-DIFFUSION

The influence of boron diffusion on the polytype structure of 3C, 6H and 15R SiC polytypes was studied in the temperature range 1600-1900 degrees C. In the surface region partial polytype transitions 15R-->3C and 6H-->3C were detected by reflection high energy electron diffraction. In the bulk region such diffraction spots could not be detected. A maximum of the diffraction intensities of 3C in alpha-SiC was observed at 1800 degrees C, whereas in the temperature range 1600-1800 degrees C no phase transitions in the 3C SiC crystal were detected. At 1800 degrees C and for long diffusion times a phase transition of the type 3C-->15R occur. This reverse phase transition may be the cause of the decrease in diffraction spot intensities of 3C SiC in 15R and 6H crystals in comparison with crystals treated by boron diffusion at lower temperatures. Reasons for the observed polytype transitions are the local generation of stacking faults which lead to elimination of the hexagonal sites in the stacking order of the alpha-SiC polytypes and their stabilization by the diffusion distribution induced stress field.