Energy band alignment and interface states in AlGaN/4H–SiC vertical heterojunction diodes

The effects of Al_xGa_1?xN aluminum fraction x and SiC surface pre-treatment on AlGaN/4H–SiC heterojunction interfaces are experimentally investigated. From capacitance vs. voltage measurements, the conduction band offsets are found to be ΔE_C ≈ 0.30 for x ≈ 0.3 and ΔE_C ≈ 0.56 for x ≈ 0.5. Forward bias ideality factors are reasonable at 3.3 for Al_0.3Ga_0.7N diodes, but >9 for Al_0.5Ga_0.5N diodes, suggesting a higher level of interface charge related to the higher aluminum fraction. Reverse bias leakage is acceptably low, with breakdown occurring at V_A > 200 V reverse bias for all tested devices. The effect of 1500 °C hydrogen etching of the SiC substrate prior to Al_xGa_1?xN growth is also investigated, and found to have little effect for x = 0.3 but a beneficial effect for x = 0.5.