Doping-induced strain and relaxation of Al-doped 4H-SiC homoepitaxial layers

Aluminum-doped 4H-SiC epilayers with Al concentrations in the 7.4×1018–3.8×1020cm−3 range were deposited on off-orientation (0001) wafers by chemical vapor deposition method and analyzed using high-resolution x-ray diffraction, transmission electron microscopy, and KOH etching. Reciprocal space maps of (0008) reflection revealed two distinct peaks originating from the substrate and doped epilayer. For Al concentration below 3.3×1020cm−3, 10μm thick layers were fully strained with the a-lattice parameter of the layer matching that of the substrate. The equilibrium c-lattice parameter change versus doping was determined to be 1.3±0.3×10−24cm3. The basal planes of the epilayers were tilted in respect to the substrate in the direction of the offcut with the tilt magnitude proportional to the doping concentration. The 10μm thick layers with Al concentration above 3.3×1020cm−3 underwent partial relaxation. The a-lattice parameter of the epilayer was higher than that of the substrate, the width of ω and 2θ scans of (0008) x-ray peaks broadened by a factor of 2 compared to strained layers, and the threading dislocation density increased by several orders of magnitude. Since no inclusions have been found in the relaxed epilayer, we interpret the above changes as due to strain relaxation by nucleation of dislocations.