Dielectric response functions of the (0001¯), (101¯3) GaN single crystalline and disordered surfaces studied by reflection electron energy loss spectroscopy

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Polar GaN(0001¯) (1×1), semipolar GaN(101¯3) surfaces prepared in NH3 vapor, and their disordered counterparts are investigated by reflection electron energy loss spectroscopy (REELS) and low-energy electron diffraction. The REELS spectra are measured in a range of polar angles at electron kinetic energies of 200 and 1000 eV. The electron energy loss function is determined from the REELS within the framework of the semiclassical approach. Good agreement between experimental and theoretical functions is achieved at all angles for the disordered GaN surfaces and for the ordered surfaces measured at a kinetic energy of 1000 eV. The agreement is worse for the crystals measured at 200 eV, which is explained by the coherent scattering contributions at low energies. The optical constants of the GaN surfaces are derived from the computed dielectric functions: the optical properties of the (0001¯) and (101¯3) surfaces are similar, except for differences in bandgap values, which may be due to observed steps on the (101¯3) surface. The surface optical properties of a disordered GaN surface are found to be different from the GaN crystals. There are pronounced changes in the electronic band structure for disordered GaN due to the preferential sputtering of nitrogen.