Surface optical phonons and hydrogen chemisorption on polar and nonpolar faces of GaAs, InP, and GaP

Surface optical phonons have been detected on both the polar (100) and nonpolar (110) faces of GaAs, InP, and GaP with the use of high-resolution electron-energy-loss spectroscopy. The observed frequencies (GaAs: 291, InP: 337, GaP: 396 ${\mathrm{cm}}^{-1\mathrm{}}$) on all surfaces are in excellent agreement (within 5 ${\mathrm{cm}}^{-1\mathrm{}}$) with theoretical predictions and were found to be independent of crystal face, bulk doping level, or method of sample preparation (cleavage in UHV versus sputter-annealing). The insensitivity of the surface-optical-phonon frequencies to the details of surface orientation, reconstruction, and near-surface stoichiometry is a consequence of the effective depth of atomic displacements (∼200 Å) which contribute to the time-dependent electrostatic potential outside the crystal. Adsorption of atomic hydrogen results in identifiable modes for the Ga-H, As-H, In-H, and P-H stretching vibrations at approximately 1880, 2110, 1700, and 2350 ${\mathrm{cm}}^{-1\mathrm{}}$, respectively.