Abstract
Core level (CL) and valence band (VB) spectra of heavily Ga-doped ZnO (GZO) films with carrier concentrations (Ne) ranging from 1.8×1020 to 1.0×1021cm−3 were measured by high-resolution Al Kα (hν=1486.6eV) x-ray photoelectron spectroscopy (XPS) and Cr Kα (hν=5414.7eV) hard x-ray photoelectron spectroscopy (HAXPES). The CL spectra of the GZO films measured by XPS had little dependence on Ne. In contrast, clear differences in asymmetric broadening were observed in the HAXPES spectra owing to the large probing depth. The asymmetry in the Zn 2p3/2 and O 1s HAXPES spectra is mainly attributed to the energy loss of the conduction electron plasmon caused by the high Ne of the GZO films. Similar asymmetry was also observed in the VB spectra of these GZO films. It was found that such asymmetry plays a crucial role in the determination of crystal polarity. With increasing Ne, the intensity of the sub-peak at a binding energy Eb of about 5eV in the VB spectrum decreased and the sub-peak became indistinguishable. We clarified the limitation of the criterion using the sub-peak and proposed an alternative method for polarity determination.
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