4H to 3C Polytypic Transformation in Al+ Implanted SiC During High Temperature Annealing

Polytypism in SiC has created interest and opportunity for device heterostructures and bandgap engineering in power electronic applications. As each SiC polytype possesses a different bandgap, electron mobility, and degree of anisotropy, unique interfaces can be created without changing its chemical composition. The 4H polytype is commonly used, but the 3C polytype offers high surface electron mobility with isotropic properties as the only cubic polytype. This has driven research on heteroepitaxy with limited success in traditional chemical vapor deposition chambers. Discussion on polytype control and stability has been restricted to bulk and epitaxial crystal growth, despite numerous reports of polytypic transformations occurring during other processing steps. This study revealed the polytypic transformation of 4H-SiC to 3C-SiC after high temperature annealing using high resolution cross-sectional transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Above 1750 degree celsius, the surface significantly roughened under a reduced pressure of Ar, whereas surface planarity was maintained under Ar atmospheric pressure. The formation of 3C-SiC islands occurred adjacent to large surface pits through an epitaxial growth process for the reduced pressure condition only. Loss of SiC stoichiometry at the surface with Si enrichment and availability of on-axis terraces enabled 3C nucleation. 3C-SiC growth was retarded using a protective carbon cap (C-cap) where defect-free single crystal 3C-SiC has a coherent interface with the 4H-SiC substrate underneath. These findings demonstrate that the 3C polytype can be stable at high temperatures, encouraging the need for a better understanding of polytype stability and control. [GRAPHICS] .