Experimental Determination of Impact Ionization Coefficients Along ⟨1120⟩ in 4H-SiC

The impact ionization coefficients along the ⟨1120⟩ direction in 4H-SiC were extracted by analyzing photocurrent of mesa epitaxial p-n diodes with punchthrough (PT) structures fabricated on 4H-SiC (1120). Compared with the impact ionization coefficients along 4H-SiC łangle0001⟩, the coefficient for electrons is much higher, whereas the coefficient for holes is similar. As a result, the calculated critical electric field strength along 4H-SiC ⟨1120⟩ is approximately 21% lower than that along 4H-SiC łangle0001⟩. This strong anisotropy of the electron impact ionization coefficient is discussed by taking account of the unique conduction band structure. The extracted impact ionization coefficients were validated by comparing the simulated and experimentally measured breakdown voltages of various 4H-SiC (1120) p-n diodes with non-PT structures.