III-N surface polarization compensating charge referred here to as 'surface donors' (SD) was analyzed in Al2O3/AlGaN/GaN metal-oxide-semiconductor (MOS) heterojunctions using scaled oxide films grown by metal-organic chemical vapor deposition at 600 degrees C. We systematically investigated impact of HCl pre-treatment prior to oxide deposition and post-deposition annealing (PDA) at 700 degrees C. SD density was reduced down to 1.9 x 10(13) cm(-2) by skipping HCl pre-treatment step as compared to 3.3 x 10(13) cm(-2) for structures with HCl pre-treatment followed by PDA. The nature and origin of SD was then analyzed based on the correlation between electrical, micro-structural, and chemical properties of the Al2O3/GaN interfaces with different SD density (N-SD). From the comparison between distributions of interface traps of MOS heterojunction with different N-SD, it is demonstrated that SD cannot be attributed to interface trapped charge. Instead, variation in the integrity of the GaOx interlayer confirmed by X-ray photoelectron spectroscopy is well correlated with N-SD, indicating SD maybe formed by border traps at the Al2O3/GaOx interface. (C) 2017 Elsevier B.V. All rights reserved.