FREQUENCY-SHIFT OF THE SI-H VIBRATIONAL-MODES ON ERBIUM SILICIDE MEASURED BY HREELS

Hydrogen adsorption on two-dimensional (2D) ErSi2/Si(111) as well as o n a 300 Angstrom thick ErSi1.7(0001) film on Si(lll) has been studied by HREELS. Both surfaces exhibit the characteristic monohydride Si-H b ending and stretching modes confirming the structural model of a silic ide terminated by a buckled silicon plane similar to the ideal Si(111) termination. Nevertheless when compared to the Si(111)-delta(7x7)H or Si(001)-(2x1)H surfaces, these modes are found to be shifted towards lower energies: approximately 25 cm(-1) for the 2D silicide and 65 cm( -1) for the thick silicide. This provides an example of a marked chang e in the Si-H surface vibration frequencies induced by a modification in the third atomic layer below the surface i.e. in the third coordina tion shell of the adsorbate. The shift is interpreted as a weaker Si-H force constant on the silicide apparently related to a small reductio n in bond order because of electron donation from the Er layer underne ath.