Doping-Induced Lattice Mismatch and Misorientation in 4H-SiC Crystals

The c- and a-lattice constants of nitrogen-doped 4H-SiC were measured in the wide temperature range (RT - 1100°C). The samples used in this study were heavily doped substrates and lightly-doped free-standing epilayers. The lattice constants at room temperature are almost identical for all the samples. However, the lattice contraction by heavy nitrogen doping was clearly observed at high temperatures, which indicates that the thermal expansion coefficients are dependent on the nitrogen concentration. The lattice mismatch (Δd/d) between a lightly-doped free-standing epilayer (Nd = 6x1014 cm-3) and a heavily-doped substrate (Nd = 2x1019 cm-3) was calculated as 1.7x10-4 at 1100°C. The authors also investigated lattice constants of high-dose N+, P+, and Al+-implanted 4H-SiC. Reciprocal space mapping (RSM) was utilized to investigate the lattice mismatch and misorientation. The RSM images show the c-lattice expansion and c-axis tilt of the ion-implanted layers, irrespective of ion species. The authors conclude that the lattice expansion is not caused by heavy doping itself, but by secondary defects formed after the ion-implantation and activation-annealing process.