Energy-band alignment of atomic layer deposited (HfO₂)_x(Al₂O₃)_1-x gate dielectrics on 4H-SiC

We study a series of (HfO₂)_x(Al₂O₃)_1-x/4H-SiC MOS capacitors. It is shown that the conduction band offset of HfO₂ is 0.5 eV and the conduction band offset of HfAlO is 1.11-1.72 eV. The conduction band offsets of (HfO₂)_x(Al₂O₃)_1-x are increased with the increase of the Al composition, and the (HfO₂)_x(Al₂O₃)_1-x offer acceptable barrier heights (> 1 eV) for both electrons and holes. With a higher conduction band offset, (HfO₂)_x(Al₂O₃)_1-x/4H-SiC MOS capacitors result in a ～ 3 orders of magnitude lower gate leakage current at an effective electric field of 15 MV/cm and roughly the same effective breakdown field of ～ 25 MV/cm compared to HfO₂. Considering the tradeoff among the band gap, the band offset, and the dielectric constant, we conclude that the optimum Al₂O₃ concentration is about 30% for an alternative gate dielectric in 4H-SiC power MOS-based transistors.