XPS spectra of all samples. Ga 3d XPS spectra for (a) GaN, (c) GaN/AlN, and (e) AlN/GaN samples and Al 2p XPS spectra for (b) AlN, (d) GaN/AlN, and (f) AlN/GaN. Experimental data points are fitted by Voigt (mixed Lorentzian-Gaussian) line shapes (solid lines) after the application of a Tougaard background. Also shown are VB spectra for (g) GaN and (h) AlN. The peak and VBM positions are summarized in Table 1.