2.3.3.4 Epilayer Crystal Defects

Improvements in epilayer quality are needed as there are presently many observable defects present in state of the art SiC homoepilayers. Non-ideal surface morphological features, such as growth pits, 3C-SiC triangular inclusions (triangle defects) introduced in Subsection 2.3.3.2, are generally more prevalent in 4H-SiC epilayers than 6H-SiC epilayers. Most of these features appear to be manifestations of non-optimal step flow during epilayer growth arising from substrate defects, non-ideal substrate surface finish, contamination, and/or unoptimized epitaxial growth conditions. While by no means trivial, it is anticipated that the SiC epilayer surface morphology will greatly improve as refined substrate preparation and epilayer growth processes are developed. Many impurities and crystallographic defects found in sublimation-grown SiC wafers do not propagate into SiC homoepitaxial layers. For example, basal-plane dislocation loops emanating from micropipes and screw dislocations in sublimation-grown SiC wafers (Subsection 2.3.2.2) are generally not observed in SiC epilayers. Unfortunately, however, screw dislocations (both micropipes and closed-core screw dislocations) present in commercial c-axis wafers do replicate themselves up the crystallographic c-axis into SiC homoepilayers grown on commercial wafers [51]. Therefore, devices fabricated in commercial epilayers are still subject to electrical performance and yield limitations imposed by commercial substrate screw-dislocation defect densities.

T. Ayalew: SiC Semiconductor Devices Technology, Modeling, and Simulation