Adsorption of atomic oxygen (N 2O) on a clean Si(100) surface and its influence on the surface state density; A comparison with O 2

Abstract

This paper describes a study concerning the interaction of molecular oxygen (O 2) and nitrous oxide (N 2O) with the clean Si(100) 2 × 1 surface in ultrahigh vacuum at 300 K. Differential reflectometry (DR) in the photon energy range of 1.5–4.5 eV, Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) have been used to monitor these solid-gas reactions. With this combination of techniques it is possible to make an analysis of the (geometric and electronic) structure and chemical composition of the surface layer. The aim of the present study was to give a description of the geometric nature of the oxygen covered Si(100) surface. For that purpose we have used both molecular (O 2) and atomic oxygen (as released by decomposition of N 2O) to oxidize the clean Si(100)2 × 1 surface. In view of the experimental results we propose a model in which the decomposition of N 2O into a chemisorbed oxygen atom and a desorbing N 2 molecule is described by a coupled reaction mechanism. The initial reaction probability was found to be (2.4 ± 0.2) × 10 −5. The principal result is that the decomposition of N 2O only occurs at those Si atoms which have a dimer and dangling bond, i.e., mainly first layer Si atoms. The concentration of defects in our clean Si(100)2 × 1 surface was estimated to be a few percent. Every stage of the reaction involves the formation of bridging oxygen, saturation occurring at monolayer coverage. Thus the interface between the monatomic oxygen adlayer and the underlying Si lattice is essentially abrupt. The interaction of O 2 with the clean Si(100)2 × 1 surface is found to lead to chemisorption, with the initial sticking probability: s(0) = 0.09 ± 0.01. At coverages below 0.5 monolayer the adsorption of O 2 can be described by a dissociative process on the first layer Si atoms. The empty dangling bond orbital which is related to the lowered Si atom of the asymmetric dimer does not show appreciable reactivity towards the O 2 molecule. In the intermediate coverage range (0.5 – 1 monolayer oxygen) incorporation of oxygen into the subsurface Si lattice and adsorption of a molecular oxygen species occur simultaneously. Due to this species, ∽ 20% of the (dimer and dangling bond) surface states are left unsaturated. Beyond monolayer coverage penetration of oxygen into the Si lattice is the limiting process. Careful measurements of the SiL 23VV Auger spectra have led to more definite conclusions about the binding state of oxygen: the SiO 83 eV Auger peak can be exclusively related to Si-O bond formation on the first layer Si atoms.