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 AIGaN/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 degrees 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 been evaluated for films deposited after both A and B treatments and better dielectric properties have been observed for film fabricated after B treatment. Moreover, dielectric properties improved after post-deposition annealing at high temperature. (C) 2016 Elsevier B.V. All rights reserved.