Effects of the carrier concentration on polarity determination in Ga-doped ZnO films by hard x-ray photoelectron spectroscopy

Core level (CL) and valence band (VB) spectra of heavily Ga-doped ZnO (GZO) films with carrier concentrations (N-e) ranging from 1.8 x 10(20) to 1.0 x 10(21) cm(-3) were measured by high-resolution Al K-alpha (hv = 1486.6 eV) x-ray photoelectron spectroscopy (XPS) and Cr K-alpha (hv = 5414.7 eV) hard x-ray photoelectron spectroscopy (HAXPES). The CL spectra of the GZO films measured by XPS had little dependence on N-e. In contrast, clear differences in asymmetric broadening were observed in the HAXPES spectra owing to the large probing depth. The asymmetry in the Zn 2p(3/2) and O 1s HAXPES spectra is mainly attributed to the energy loss of the conduction electron plasmon caused by the high N-e 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 N-e, the intensity of the sub-peak at a binding energy E-b of about 5 eV 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. (C) 2017 Elsevier B.V. All rights reserved.