Identification of the Initial-Stage Oxidation Products on Si(111)-(7×7)

Abstract

Density functional theory calculations are used to study the initial-stage adsorption of O 2 molecules on the Si͑111͒-͑7 3 7͒ surface. Contrary to experimental suggestions, we find no evidence of metastable O 2 molecular states on this surface, i.e., O 2 molecules dissociate spontaneously without any barrier. Our electronic and vibrational analysis reveals that the resulting atomic-oxygen products can explain the "molecular" features reported in previous experiments. [ S0031-9007(99) PACS numbers: 68.35.Bs, 68.35.Ja, 73.20.At Chemisorption of oxygen molecules on silicon surfaces has been a subject of both fundamental and technological importance as a model for the initial stage of silicon oxidation. In most cases, however, the adsorption-induced chemical and geometrical complexity has greatly hindered experimental or theoretical identifications of the atomic structure of the reaction products, and thus many structural and spectroscopic issues still remain unresolved.Of particular interest among such examples is the socalled "molecular precursor" model for the initial-stage chemisorption of O 2 molecules on the Si͑111͒-͑7 3 7͒ surface, which was proposed to explain the observed oxygen-induced metastable features: In their vibrational electron energy loss spectroscopy (EELS) studies, Ibach et al. [1] and Edamoto et al.[2] found a loss peak at 149-155 meV that was extinguished by 400 K annealing. Later Höfer et al. [3,4] reported in their ultraviolet photoelectron spectroscopy (UPS) studies two metastable spectra at 2.1 and 3.9 eV which were also quenched by ϳ400 K annealing. These metastable features have been ascribed to chemisorbed O 2 states (i.e., molecular precursors), and the reported lifetime at room temperature varies from ϳ10 min in early studies [3,4] to several hours in recent studies [5][6][7]. The concept of O 2 molecular precursors on Si͑111͒-͑7 3 7͒ has been put forward in a more concrete but controversial way in recent scanning tunneling microscopy (STM) studies: Oxygeninduced bright sites in STM topographs are the most numerous product at low O 2 exposures and are known to be quite stable at room temperature and to survive heating up to 600 K [8][9][10][11][12][13][14]. These bright sites have usually been assigned to atomic-oxygen species [8][9][10][11][12]. Recently, however, Dujardin et al.[13] emphasized their molecular origin by identifying the bright sites with the metastable state that gives a 3.9 eV UPS peak. Furthermore, initiated by the Dujardin et al.'s molecular assignment, Hwang et al. [14] interpreted even the motion of the bright sites at 300 365 ± C as hopping of single O 2 molecules. Despite these experimental proposals, however, the structural identification of the O 2 molecular precursors is yet to be done. Previous theoretical studies [15][16][17][18] were based either on the unreconstructed Si surface structure or on a qualitative theoretical scheme, and so more elaborate theoretical studies are needed.In this Letter, we report an atomistic identification of the initial-sta...

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

“…In studies based on density-functional theory ͑DFT͒ calculations, 1,2 it was stated that the metastable oxygen has an atomic oxygen configuration. These DFT calculations 1,2 reinterpreted the earlier spectroscopic [3][4][5][6][7][8][9][10] and microscopic [11][12][13][14][15][16] results in which the metastable oxygen was reported to be a molecular species, a so-called molecular precursor.…”

“…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. This number is different from that used for the previous models, i.e., only two oxygen atoms with different chemical environments have been considered for the atomic configuration 1,2,17 and the molecular precursor. [3][4][5][6][7][8][9][10][11][12][13][14][15][16] This obvious conflict calls for an urgent and careful reinvestigation of the structural model of the metastable species, which is essential for the understanding of the oxidation process of Si on an atomic level.…”

Ab initiostudy of the oxidation on vicinal Si(001) surfaces: The step-selective oxidation

“…The structural similarity of the S B -O2b structure and the oxidation product of the adatoms on Si͑111͒-7 ϫ 7 looks quite reasonable. 47 Further inspection of the atomic geometry of the clean vicinal Si͑001͒ surface showed that the rebonds at the S B step are significantly elongated compared to the atomic bonds in bulk Si as much as 0.2 Å. As such, the RSAs become more flexible with respect to oxygen-induced strains.…”

Photoemission study of metastable oxygen adsorbed on aSi(111)−(7×7)surface

“…The lifetime obtained in the valence-band studies might therefore be overestimated. In fact, although the 3.8 eV peak observed in the valence-band spectra was assigned to the * orbital of a molecular "paul" species in the early valence-band studies [5][6][7][8]10,11 and theoretical investigations, 3,21 recent valence-band 12 and theoretical 22,23 studies suggest that the origin is the p z orbital of an "ad" oxygen atom. The ad denotes an O atom bonding on top of a Si adatom of the dimer-adatom-stacking fault structure 25 of the Si͑111͒-͑7 ϫ 7͒ surface (Fig.…”

“…This metastable species, which has a finite lifetime and disappears after annealing the sample at 600 K, was observed in various spectroscopic [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and microscopic studies, [17][18][19][20] and it has also been treated theoretically. 3,[21][22][23][24] Despite these extensive research efforts, a complete picture of the processes determining the lifetime has not yet been obtained.…”