A systematic study was performed on the effects of surface treatments before Atomic Layer Deposition (ALD) growth of Al2O3 thin films on (0001)AlGaN/GaN substrate. The AlGaN/GaN surface was treated either with: H2O2:H2SO4 (A treatment) and H2O2:H2SO4 + H2O:HF (B treatment). After surface wet-treatments, Al2O3 was immediately deposited at 250 °C by Plasma Enhanced ALD from trimethylaluminum precursor. The film thickness was measured to be about 27 nm using transmission electron microscopy and different structural evolution was observed under electron beam analysis, re-arranging from amorphous as-deposited films to polycrystalline or epitaxial films depending on the pre-deposition treatment. Surface morphology obtained by atomic force microscopy in tapping mode showed smooth Al2O3 layers with lower roughness in the case of films deposited after B treatment. Dielectric properties have …