Dependence of (a) N1s and (b) Ga 3d peak positions for a 20-nm-thickness GaN buffer layer on H2 cleaned (open circles) and nitrided (closed circles) sapphire substrates on the annealing time under the N2 and H2 mixed ambient. The positions were detected by XPS analysis. [after Ref. 117 ] The sample on the nitrided substrate evaporated completely, and AlGaN was formed at the interface between the sample and the non-nitrided substrate.
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