Figure S1 ALE parameters, Figure S2 AFM characterizations of ALE, Figure S3 C-V measurement, Figure S4 Benchmark of SBD, EDX profile

Figure S1. Schematic illustration of the ALE process on the n-AlGaN sample, highlighting the removal of the defective surface layer through chlorination and subsequent Ar plasma etching. In our recipe, the surface is first chlorinated on the etched surface through the chemisorption of the precursor Cl2. The Cl2 flow rate was set to 7.0 sccm during a 0.05-second dosing period. After a 10-second purging step, the chlorinated layer was sputtered away using Ar plasma for 5.00 seconds. The table RF power was maintained at 10 W during the etching process. Following a 2-second spacer pumping time to clean the chamber, a new cycle was repeated. Throughout the entire ALE process, the chamber pressure was kept at 10 mTorr, the ICP RF power at 500 W, the Ar flow rate at 100 sccm, and the process temperature at 20{degree sign}C. Figure S2. Height information depicted by AFM with etched area and hard mask covered area. The pictures inside the figure show the 3D view of AFM results. Figure S3. 1/C2-V characteristics of reference and ALE treated samples. Open circles represent experimental data, while dashed lines indicate linear fitting. The discrepancy between the C-V SBH and I-V SBH probably arises from the high density of interface states in the surface oxide layer of Ref sample and plasma-chemical process induced interface states at the surface of ALE treated sample, respectively. Figure S4. Benchmark of ideality factor and Schottky barrier height with respect to Al content, including low Al fraction and high Al fraction SBDs. Figure S5. N, Al, and O atomic ratio measured via EDX vs. depth profiles of (a) Ref sample and (b) ALE treated sample. Ni/Au Schottky metal was shown on the left of the graphs and AlGaN was shown on the right. Green dash lines indicated the interface region.