Influence of nitrogen doping concentration on mechanical properties of 4H-SiC: A comparative study of C and Si faces

This study investigates the impact of varying nitrogen doping concentrations on the mechanical properties and anisotropic behavior of 4H-SiC substrates, with particular emphasis on the differences between the C and Si faces using nanoindentation techniques. The results reveal that the C face exhibits higher hardness and elastic modulus than the Si face at equivalent doping concentrations. Furthermore, an increase in nitrogen doping concentration results in a decrease in hardness and elastic modulus of both the C and Si faces, while significantly reducing the anisotropy between these two surfaces. This underscores the substantial influence of nitrogen doping on the mechanical properties of 4H-SiC, as it transforms Si-C bonds into Si-N bonds, reducing bond strength, which in turn exacerbates lattice distortion and decreases fracture toughness. First-principles calculations based on density functional theory (DFT) indicate that the breaking of Si-N bonds and forming of Si-O, Si-H, or Si-OH bonds on the C face lowers surface energy, which contributes to the observed reduction in anisotropy. The XPS results confirm the formation of a thicker oxide layer on the C face of the heavily doped samples, while the chemical mechanical polishing (CMP) difference between the two surfaces decreases with increasing doping concentration due to reduced mechanical anisotropy. These findings provide valuable insights for enhancing double-sided CMP efficiency and optimizing the processing technology in highly nitrogen-doped 4H-SiC wafers.