A controlled AlGaN surface preparation method avails to improve the performance of GaN-based HEMT devices. A comparative investigation of chemical treatments by (1:10) NH4OH:H2O and (1:10) HCl:H2O solutions for AlGaN surface preparation by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) is reported. The XPS data clearly reveal that the native oxide on AlGaN was composed of Al2O3, Ga2O3 and NO compounds. These compounds were etched off partially or completely by both the chemical treatments, namely NH4OH or HCl solutions, independently. The HCl treatment etches out Al2O3 completely from native oxide unlike NH4OH treatment. The HCl treatment results in larger amount of carbon segregation on AlGaN surfaces, however it removes all oxides' compounds faster than NH4OH treatment. The AFM results reveal the improvement of surface morphology by both the chemical treatments leading to the surface roughness RMS values of 0.24 nm and 0.21 nm for NH4OH and HCl treated AlGaN layers, respectively. (C) 2009 Elsevier B.V. All rights reserved.