FLUORINE ADSORPTION ON GAAS(110) SURFACES AND THE ONSET OF ETCHING AFTER XEF2 EXPOSURES

The interaction of XeF2 molecules with GaAs(100) surfaces was investig ated with high-resolution electron energy-loss (HREELS) and Auger elec tron spectroscopy (AES) in the exposure range between 1 and 1000 langm uir (L). The AES results show that up XeF2 doses of approximately 100 L, fluorine atoms are mainly chemisorbed on GaAs(110) surfaces. After XeF2 exposures above 100 L, etching of the GaAs(110) surface occurs by removing arsenic atoms. Adsorbed xenon atoms were never detected. In the chemisorption range an energy-loss structure which is attributed t o As-F valence vibrations is observed in the HREEL spectra. The excita tion energy of the As-F vibration shifts from 670 cm(-1) (84 meV) at v ery low fluorine coverages to 720 cm(-1) (90 meV) at 0.3 monolayers. T his finding may be explained by a mutual electrostatic interaction bet ween As-F surface dipoles and their images. From that and by use of di electric theory an effective dynamic charge of the vibrating As-F surf ace molecules is estimated. In the etching regime this As-F loss struc ture disappears and a broad feature at about 575 cm(-1) (71.3 meV) is observed which may be due to vibrations of GaFx groups.