Bias-dependent scanning tunneling microscopy study of the oxygen-adsorbed Si(111)-(7×7) surface: Observation of metastable molecular oxygen

Abstract

, Biasdependent scanning tunneling microscopy study of the oxygen-adsorbed Si (111) We have observed the initial stage of oxygen adsorption on a Si(111)-(7ϫ7) surface using scanning tunneling microscopy. Among the bright sites observed after exposing the surface to oxygen in occupied state images, there are differences in the bias dependence of the brightness. Taking into account the local density of states of the oxygen-adsorbed Si(111)-(7ϫ7) surface, we conclude that the sites appearing brightly only with a tip bias of уϩ2.1 V are the molecular oxygen. The preferred adsorption site of this molecular species is a corner adatom, which has an oxygen atom adsorbed into its backbond, of the faulted half of the (7ϫ7) unit cell.

Electronic Control of Single-Molecule Dynamics

“…However, the interpretation of the nature of the adsorbed oxygen species is not clear. There have been a number of experimental studies using the STM, − synchrotron radiation ,, and Cs + ion scattering, as well as theoretical studies, , which illustrates that it is a challenge to distinguish between molecular adsorption and dissociation even with a multitude of different surface-sensitive techniques, especially at low coverages. The most recent experiment and calculations suggest that there is no molecular species at room temperature at very low coverage. , At low temperatures a molecular species may exist with a short lifetime …”

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

“…The binding energy of the dangling bond state of an adatom with one or more atomic oxygen adsorbed into its back bond-i.e., the dangling bonds of the insϫn configuration-was reported to be between 0.5 and 0.8 eV. 4,7,21,22,30 Thus, the change observed after the 600 K annealing indicates that the origin of the 0.8-eV peak is the dangling bonds of the insϫn configurations and the 1.5 eV component observed in the core-level spectra before annealing originates from oxygen species adsorbed on top of adatoms. Taking into account that no molecular species exists in the spectrum recorded 2 h after exposure and that no oxygen atom can adsorb on the top site of an adatom if there is no oxygen atom in a back bond, 21,30 we conclude that the bonding configuration of the second metastable species is ''insϫn-ad.''…”

“…3. Since the adatom back bonds are the preferable adsorption sites at the initial stage of oxidation, 21,30 the steep increase at small dosages indicates that the origin of the 0.0 eV component is the ''ins'' oxygen. Moreover, taking into account that the spectrum of the annealed surface is well reproduced by using the two O 1s components at 0.0 and 0.6 eV and that the ins and tri oxygen species were reported to be the most thermally stable species, 23,24 we attribute the origin of the 0.6 eV component to the ''tri'' oxygen.…”

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

“…Further, a peak, which is not recognized in spectrum (a), is observed at a binding energy of 0.8 eV in spectrum (b). Since the binding energy of the dangling bond state of an insϫ n ͑n =1, 2, and 3͒ species was reported to be between 0.5 and 0.8 eV, 3,10,20,21 the appearance of the 0.8 eV peak and the fact that the 3.8 eV intensity is the same in the two spectra suggest that oxygen species adsorbed at the on top site of an adatom hardly contribute to the stable 3.8 eV peak. Therefore, we conclude that the stable 3.8 eV peak observed in Fig.…”

“…From a scientific point of view, the presence of the metastable oxygen has particularly attracted a deep interest in the initial oxidation stage of the Si͑111͒-͑7 ϫ 7͒ surface. 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.…”