Electron diffraction methods for the analysis of silicon carbide surfaces and the controlled growth of polytype heterostructures

The growth of different silicon carbide (SiC) polytypes on each other is possible by control of the surface structure and the appropriate thermodynamic parameters. Special ultrahigh vacuum conditions, like those used in solid source molecular beam epitaxy, allow the determination of the species on the surface and also the in situ characterization of the growing polytype by electron diffraction methods. The surface reconstruction which favours the growth of a certain polytype can be controlled by reflection high energy electron diffraction. For a non-destructive determination of the polytype of a grown thin SiC film, methods like x-ray photoelectron diffraction (XPD) and electron channelling can be used. The interaction length of electrons near 1 keV kinetic energy is in the range of 1 nm and therefore sensitive to the stacking sequence of the most common SiC polytypes 3C, 4H, 6H with c-axis dimensions between 0.75 and 1.5 nm. To prepare polytype heterostructures like 4H/3C/4H or 6H/3C/6H, untwinned 3C SiC films without double-positioning boundaries have to be grown. On-axis alpha-SiC substrates with uniform surface stacking termination are a prerequisite for this. Such surfaces can be prepared using high temperature hydrogen etching, sublimation etching or step-flow growth. These equally terminated crystals with threefold surface symmetry are particularly suitable for detailed studies of the atomic-geometric structure and their changes during growth or after certain treatments. Results of surface-sensitive characterization methods like scanning tunnelling microscopy, XPD and low energy electron diffraction are presented.