Intrinsic and extrinsic effects in surfaces - acoustic-phonon softening of capped SI(111) surfaces

Bond-charge-model calculations of phonon dispersion in the Si(111):As(1×1) are compared with inelastic helium scattering measurements. The agreement is very good with no surface fitting of model parameters. This permits a novel means of assessing the adlayer-induced changes in the surface: We recalculate the phonon spectrum with the adlayer mass reduced from mAs to mSi but with all other parameters unchanged. The resulting dispersion curves agree extremely well with measurements and calculations for a (1×1) hydrogen-terminated Si(111) surface, demonstrating that the surface force constants are not appreciably altered by these chemisorption processes. Hence the acoustic phonon dispersion in the Si(111):As(1×1) surface can be fully understood in terms of (1) the intrinsic perturbation of breaking the (111) covalent bond to create the surface and (2) the ''mass defect'' due to As-Si substitution. © 1992 The American Physical Society.