Analysis of polar GaN surfaces with photoelectron and high resolution electron energy loss spectroscopy

Abstract

The properties of Ga-face and N-face GaN surfaces were studied by X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS), atomic force microscopy (AFM), reflection high energy electron diffraction (RHEED) as well as high resolution electron energy loss spectroscopy (HREELS). The Ga-face GaN was grown on 6H-SiC(0001) and the N-face GaN directly on α-Al2O3(0001) by plasma assisted molecular beam epitaxy (PAMBE) and in situ analysed by photoelectron spectroscopy. The Ga-face GaN surfaces show a 2 × 2 reconstruction measured by RHEED. From the analysis of the XPS spectra a valence band maximum located 2.9 eV below the Fermi level results. Furthermore, the UPS spectra exhibit two surface states. The N-face GaN presented a 1 × 1 surface with one surface state and a valence band maximum located 2.4 eV below EF. These values correspond to an upward band bending of about 0.9 eV for the N-face GaN()-1 × 1 and about 0.4 eV for the GaN(0001)-2 × 2 (Ga-face) results. The different surface band bending is further supported by the evaluation of the FWHM of the quasi-elastically reflected electrons in HREELS.