Fluorination-induced back-bond weakening and hydrogen passivation on HF-etched Si surfaces

Abstract

The mechanism of hydrogen termination of silicon surface after HF etching has been elucidated using a model of weakened Si--Si back bond which complements the previous model of charge polarization. By calculating the bond energy and Mulliken bonding population of Si--Si bonds on silicon (111) surfaces at the level of density functional theory with substantiation using Hartree--Fock theory, and by analyzing net charges on select atoms, we show that as the surface silicon is bonded with fluorine, the bonds between surface silicon atoms and the silicon atoms underneath are significantly weakened due to the high electronegativity of fluorine and the extraordinary strength of Si--F bonds. The weakened Si--Si back bonds would facilitate the reaction between the HF molecule and the Si surface, peeling off the surface layer, thereby leaving behind a hydrogen-terminated Si surface. The reaction may be initiated by a dipole--dipole interaction on the trifluorine-terminated surface or an insertion-type interaction on the monofluorine-terminated surface due to a fluorination-induced bond polarization. The argument is further supported by studies on reaction pathways using various cluster models of fluorine- and hydrogen-terminated silicon surfaces.