Properties of As-grown, chemically treated and thermally oxidized surfaces of AlGaN/GaN heterostructure

Surface properties of as-grown, chemical treated and thermally oxidized Al0.25Ga0.75N/GaN heterostructures were investigated by x-ray photoemission spectroscopy (XPS), Auger electron spectroscopy (AES), surface Raman spectroscopy, atomic force microscopy (AFM), Hall-effect and magneto-transport measurements. As-grown AlGaN surface was covered with a natural oxide layer consisting of the Al2O3 and Ga2O3 components. The Al2O3 component was dominant over the Ga2O3 component in spite of the fact that the Al content of the AlGaN layer was only 25%. The HF, KOH and NH,OH treatments reduced natural oxides on the AlGaN surface. However, the HF and KOH treatments left the F- and K-related impurities on the surface, and disturbed the surface stoichiometry. On the other hand, the NH4OH treatment was very effective in realizing nearly oxide-free and well-ordered AlGaN surface. Thermal oxidation process at 900 degreesC in dry O-2 flow formed an oxide layer with spatially uniform distribution of Al2O3 and Ga2O3 components on the AlGaN surface. The composition of Al2O3 in the oxide layer was estimated to be 22%. This value was close to the Al content of the AlGaN bulk.