5-4-4-3 SiC Epilayer Doping
In-situ doping during CVD epitaxial growth is primarily accomplished through the introduction of nitrogen (usually) for n-type and aluminum (usually trimethyl- or triethylaluminum) for p-type epilayers . Some alternative dopants such as phosphorus and boron have also been investigated for the n-and p-type epilayers, respectively . While some variation in epilayer doping can be carried out strictly by varying the flow of dopant gases, the site-competition doping methodology has enabled a much broader range of SiC doping to be accomplished . In addition, site competition has also made moderate epilayer dopings more reliable and repeatable. The site-competition dopantcontrol technique is based on the fact that many dopants of SiC preferentially incorporate into either Si lattice sites or C lattice sites. As an example, nitrogen preferentially incorporates into lattice sites normally occupied by carbon atoms. By epitaxially growing SiC under carbon-rich conditions, most of the nitrogen present in the CVD system (whether it is a residual contaminant or intentionally introduced) can be excluded from incorporating into the growing SiC crystal. Conversely, by growing in a carbon-deficient environment, the incorporation of nitrogen can be enhanced to form very heavily doped epilayers for ohmic contacts. Aluminum, which is opposite to nitrogen, prefers the Si site of SiC, and other dopants have also been controlled through site competition by properly varying the Si/C ratio during crystal growth. SiC epilayer dopings ranging from 9 × to 1 × are commercially available, and researchers have reported obtaining dopings over a factor of 10 larger and smaller than this range for the n- and p-type dopings . The surface orientation of the wafer also affects the efficiency of doping incorporation during epilayer growth . As of this writing, epilayers available for consumers to specify and purchase to meet their own device application needs have thickness and doping tolerances of ±25% and ±50%, respectively . However, some SiC epilayers used for high-volume device production are far more optimized, exhibiting <5% variation in doping and thickness .