SURFACE PHONONS IN INP(110)

The surface phonon dispersion of cleaved InP(110) surfaces was measure d by use of high-resolution electron energy-loss spectroscopy (HREELS) along the symmetry directions <(Gamma X)over bar> and <(Gamma X')over bar>, i.e., parallel and perpendicular to the surface In-P chains. Be sides an acoustic surface phonon branch with energy lower than 9 meV, four optical phonon bands with little dispersion were recorded at 9.5, 19.5, 30.5 to 33.5, and 42.3 meV. The band with excitation energies a round 32 meV represents a true surface mode as it is located in the en ergy gap between the acoustic and optical bulk phonon bands. The resul ts are compared with available calculations of InP(110) surface phonon s.