Reinterpretation of the MolecularO2Chemisorbate in the Initial Oxidation of theSi(111)7×7Surface

Abstract

The initial oxygen adsorption on the Si(111)7 x 7 surface was investigated by high-resolution x-ray absorption spectroscopy. Below 220 K, a molecular adsorption species is identified by distinctive absorption resonances due to the 1 pi(g) molecular orbitals. The molecular species is metastabilized to have a lifetime of 15-35 min at 135 K only with the presence of atomic adsorbates of more than 0. 1 ML (monolayer). It is thus clearly evidenced that the very initial adsorption is dissociative even at 100 K and the molecular species is not a precursor state. The molecular adsorption structures with the coadsorbed oxygen atoms are suggested.

Adsorption and reaction processes of physisorbed molecular oxygen on Si(111)-(7×7)

“…The Si͑111͒-͑7 ϫ 7͒ sample was then cooled down to 45 K before the oxygen exposure, i.e., 10 K above the desorption temperature of condensed O 2 multilayers. 15,20,21 The quality of the O 2 gas was confirmed by a quadrupole mass spectrometer. Figure 2 shows the O 1s core-level spectrum of a 10 Langmuir ͑L; 1 L=1ϫ 10 −6 Torrϫ s͒ O 2 exposed Si͑111͒-͑7 ϫ 7͒ surface, recorded 45 min after the exposure using h = 665 eV.…”

“…6 In contrast to metal surfaces, chemisorbed oxygen molecules were reported to be unstable on a clean Si͑111͒-͑7 ϫ 7͒ surface, [7][8][9][10][11][12][13][14][15][16][17] and to be stabilized only by the presence of atomic oxygen in the form of ins-paul. [12][13][14][15][16][17] That is, there is no local minimum in the potential energy diagram for the molecular paul and grif configurations ͑E d =0͒, and oxygen molecules approaching the Si͑111͒-͑7 ϫ 7͒ clean surface go directly to the atomic insϫ 2 and/or ins-ad configurations, 7 and as a consequence of the presence of these atomic species E d becomes 0. The notations "paul" and "grif" correspond to oxygen molecules adsorbed on top of adatoms of the dimer-adatom-stacking fault structure 18 of the Si͑111͒-͑7 ϫ 7͒ surface, and "ad" and "ins" correspond to an oxygen atom bonding on top of an adatom and an oxygen atom inserted into a back bond, respectively ͓Fig.…”

Determination of the bonding configuration of the metastable molecular oxygen adsorbed on a Si(111)-(7×7) surface

“…The absence of the molecular precursor was supported by a Cs ion scattering experiment. 17 However, quite recently, one theoretical 18 and three spectroscopic [19][20][21] studies have suggested the adsorption of metastable molecular oxygen on a Si͑111͒ surface. These recent studies 18 -21 reported that the metastable molecular species adsorb on a Si͑111͒ surface, on which oxygen atoms are already adsorbed, and they showed the necessity to consider at least three oxygen atoms with different chemical environments.…”

Observation of two metastable oxygen species adsorbed on aSi(111)−(7×7)surface: Reinterpretation of the initial oxidation process

“…[3][4][5][6][7] These reports indicate that metastable oxygen is formed by a secondary reaction process and that the oxidation path should be different from the paths suggested based on the metastable species as a precursor with either molecular paul [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] or atomic ins-ad and ins ϫ3-ad configurations [23][24][25] ͑Fig. 1͒ in the former studies.…”