Atomic layer deposited (ALD) Al2O3/Ga-polar GaN(0001) metal-oxide-semiconductor (MOS) capacitors have been prepared with surface pretreatments including ex-situ wet sulfide passivation and in-situ cyclic trimethylaluminum (TMA)/hydrogen plasma exposure. Capacitance-voltage characterization showed that the two-step surface preparation led to reductions in the densities of both interface traps and border traps. The influence of the pretreatments on interfacial bonding was investigated by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). DFT calculations correlate the electronic properties with the interfacial bonding from the two-step surface preparation. The results are consistent with the surface preparations influencing the nucleation density of the ALD films and, therefore, the defect density of the interface and neighboring oxide. (C) 2014 Elsevier B.V. All rights reserved.