Vibrational properties of GaN(0001) surfaces

Differently treated hexagonal GaN(0001) epilayers on Al2O3 substrates were investigated by high-resolution electron energy-loss spectroscopy (HREELS) to determine their vibrational properties. X-ray photoemission spectroscopy (XPS), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED) were utilized to monitor the surface composition and structure, respectively. A comparison was made between samples which were cleaned solely by a wet chemical etch prior to insertion into the vacuum system and such which were additionally heated in situ to 800°C first in a Ga-beam and subsequently in UHV. LEED investigations exhibited 1 × 1 patterns for both of the preparation procedures, whereas only for the latter no oxygen contamination was detected by XPS and AES. The AES spectra revealed the presence of C which cannot be traced back to hydrocarbon contamination, since the HREEL spectra showed no such loss features. Electron energy-loss structures at 700 cm−1 (86.8 meV) which are attributed to optical Fuchs-Kliewer phonons were found for both preparation methods. Spectra recorded from heat-treated samples exhibited additional loss features at 565 cm−1 (70 meV). These may be attributed to vibrations between Ga and residual C atoms.