PHONONS IN (110)SURFACES OF III-V COMPOUND SEMICONDUCTORS

High-resolution electron energy-loss spectroscopy was applied to deter mine phonon dispersions of cleaved AlSb(110), GaP(110), GaSb(110), and InAs(110) surfaces. Besides the acoustic surface waves and the optica l Fuchs-Kliewer modes additional flat phonon bands were detected in al l materials. The experimental results agree very well with recent ab i nitio calculations of surface phonon dispersion curves. For GaP(110), several phonon modes are observed within the fundamental gap between a coustic and optical bulk phonon bands. In AlSb(110) a gap mode is dete cted very close to the bulk optical band edge whereas the GaSb gap is free of surface-localized states. The measurements confirm theoretical predictions that significant gap modes exist at these surfaces if the anion mass is smaller than the cation mass. For the acoustic and the Fuchs-Kliewer modes chemical trends are demonstrated. The differences of the excitation energies in the materials considered depend only on the anion and cation masses as well as the nearest-neighbor distances.