In this work, we present a systematic study on the effects of surface nature of two different substrates such as Si(100) and AlGaN/GaN (0001) before atomic layer deposition (ALD) growth of Al2O3 thin films. The Si and AlGaN/GaN surfaces were treated either with: H2O2:H2SO4 (1:3 piranha solution) for 10 minutes and H2O:HF (10:1) for 5 minutes. After surface pretreatment, Al2O3 was immediately deposited at 250 °C by Plasma Enhanced ALD from trimethylaluminum precursor. The film thicknesses were measured to be 28 nm using transmission electron microscopy and different structural evolution has been observed under electron beam analysis, rearraging from amorphous as‐deposited films to epitaxial films depending on the substrate type. Surface morphology obtained by atomic force microscopy in tapping mode shows scattered three‐dimensional nucleation of Al2O3 thin films on Si(001) substrate, while …