2.3.3.3 Epilayer Doping

Doping during CVD epitaxial growth is primarily accomplished by the introduction of nitrogen (from N$_{2}$ or NH$_{3}$), phosphorous (from PH$_{3}$) and arsenide (from AsH$_{3}$) for n-type, and aluminum (from AlCl$_{3}$ or trimethyl- or triethyl aluminum), boron (from B$_{2}$H$_{6}$), and gallium (from trimethyl gallium) for p-type epilayers. Many of these dopants were first investigated in the 1970s. While these dopants could in theory be utilized over a wide doping range, there were difficulties in obtaining lightly doped material at that time. Lightly doped material is necessary to fully take advantage of the superior breakdown field properties of SiC.

While some variation in epilayer doping can be carried out strictly by varying the flow of dopant gasses, the site-competition doping methodology [64] has enabled a much broader range of SiC doping to be accomplished. In addition, site-competition epitaxy has also enabled moderate epilayer dopings more reliably and repeatably. The site-competition dopant-control 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 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. In contrary to nitrogen, aluminum 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$\times$10$^{14}$ to 1$\times$10$^{19}$ cm$^{-3}$ are commercially available, and doping concentrations nearly a factor of 10 larger and smaller than this range for n-type and p-type doping concentrations have been reported [34]. Commercial epilayer thickness and doping tolerances are presently specified at 15% and 50%, respectively for, 76.2 mm diameter substrate [51].

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