Evidence for a 2D-metallic state of the clean 7 × 7 Si(111) surface

Backes, U.; Ibach, H.

doi:10.1016/0038-1098(81)90577-9

Cited by 75 publications

(9 citation statements)

References 12 publications

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“…1a). The loss continuum is mainly attributed to the excitations of two-dimensional plasmons associated with the dangling-bond electrons and to low-lying interband and intraband transitions with significant electron-phonon coupling [14][15][16]. Upon exposure of the adsorbates, the loss continuum becomes reduced, indicating the involvement of the danglingbond electrons in the chemisorption process.…”

Section: Resultsmentioning

confidence: 97%

Room-temperature chemisorption and thermal evolution of perchloroethylene and trichloroethylene on Si(111)7×7: Formation of chlorinated vinylene and vinylidene and acetylide adspecies, and thermal etching reactions

Leung

2005

Surface Science

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Section: Resultsmentioning

confidence: 97%

Room-temperature chemisorption and thermal evolution of perchloroethylene and trichloroethylene on Si(111)7×7: Formation of chlorinated vinylene and vinylidene and acetylide adspecies, and thermal etching reactions

Leung

2005

Surface Science

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“…1 " 4 These features, which have been interpreted to be indicative of a metallic band of surface states contributing to the surface loss function, 1,4 have appreciably hindered the observation of intrinsic vibrations of the surface. In this Letter, we demonstrate that it is possible to suppress both the high electronic background and the broadening of the elastic reflected signal, by allowing small amounts of impurities on the surface.…”

Section: W Daum and H Ibachmentioning

confidence: 99%

Adatom vibrations on Si(111) reconstructed surfaces

1987

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The vibrational electron energy-loss spectrum of the Si(l 1 0(7x7) surface exhibits a well-defined loss at 570 cm -1 and a structure at 200-270 cm -1 . A similar spectrum is displayed by a Si surface with a pseudo (1 x l) structure. Ab initio calculations of the dynamical matrix show that these features are due to localized vibrations involving Si adatoms and the surface atoms lying underneath. The latter are fivefold-coordinated Si atoms exhibiting the electronic structure of an acceptor impurity localized at the surface.PACS numbers: 68.35.Ja, 68.35.BsRecent studies by high-resolution electron energy-loss (EEL) spectroscopy of clean Si(ll 0(7x7) surfaces have found a continuum of energy losses with increasing intensities towards smaller loss energies accompanied by a large broadening of the elastic peaks. 1 " 4 These features, which have been interpreted to be indicative of a metallic band of surface states contributing to the surface loss function, 1,4 have appreciably hindered the observation of intrinsic vibrations of the surface. In this Letter, we demonstrate that it is possible to suppress both the high electronic background and the broadening of the elastic reflected signal, by allowing small amounts of impurities on the surface. We have obtained Si(ll 0(7x7) surfaces with some carbon that exhibit a sharp loss at 570 cm -1 and a broad weaker feature near 220 cm -1 . We have also produced SiC111) surfaces with a pseudo (lxl) low-energy electron diffraction (LEED) pattern, stabilized by traces of Ni, that display nearly the same EEL spectra, indicating that the local arrangements of the surface atoms are similar to that of the (7x7) surface. By comparison to ab initio calculations of the vibration spectrum of a Sij5 cluster we show that the observed EEL spectrum is a clear signature of Si adatoms. The 570-cm _1 loss, which has an unusually high frequency for a localized Si vibration, involves the stretching of two Si-Si bonds and is thus inconsistent with tetrahedral coordination. The calculation reveals a new bond connecting the adatom with the surface Si atom lying underneath, which is therefore fivefold coordinated. This Si atom has the electronic structure of an acceptor impurity localized at the surface and we suggest that it may play a role in explaining the metallic character of the clean Si(l 1 0(7x7) surface.The experiments have been performed in ultrahigh vacuum (UHV), in a recipient equipped with an air-lock system which enabled us to change the samples quickly without breaking the vacuum. For a coarse determination of the sample temperature a NiCr-Ni thermocouple was fixed to the sample holder. Figure 1 (a) shows the EEL and Auger spectra of a SiC 11 0(7x7) surface containing small amounts of carbon. The samples, which have been cut from 400-800-n cm /2-type doped Si(l 11) wafers, have been introduced in the recipient through air and flashed to 800 °C in UHV in order to remove the oxide. The Auger spectrum shows that besides some carbon (/c272/^si92 = 0.014) no other contaminants were on the su...

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“…We also find a nearly identical image at the lowest positive bias, +0.15 V. We cannot probe states exactly at £ F , but the similarity between images at the smallest positive and negative bias voltages suggests that they involve tunneling through the same metallic state, in agreement with UPS, 10 -11 IPS, 11 ' 12 and energy-loss studies. 13 This is the only state which at negative sample bias exhibits an asymmetry of the two halves of the unit cell and is thereby responsible for the asymmetry in the STM topographs of Binnig et al 14 and of Tromp, Hamers, and Demuth. 5 A differential current image between -1.0-and -0.6-V bias [ Fig.…”

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confidence: 96%

Surface Electronic Structure of Si (111)-(7×7) Resolved in Real Space

1986

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We have obtained the first energy-resolved real-space images of the filled and empty surface states of the Si(l 1 l)-(7x7) surface, with 3-A lateral resolution. This ability to resolve spatially these surface states with a scanning tunneling microscope depends upon a new method to acquire and separate geometric and electronic information. Our results not only are in good agreement with previous spectroscopic studies but also directly reveal the atomic location and geometric origin of the Si (111) -(7 x 7) surface states.Surface states play an important role in the physical properties of solid surfaces and have been the subject of numerous theoretical and experimental studies. While these states have been extensively studied in reciprocal space, real-space observation and determination of the geometric origins of surface states have hitherto not been possible. We have atomically resolved and directly identified the physical origin and nature of the various surface states of the Si(lll)-(7x7) surface found in previous photoemission and inverse-photoemission studies. These include surface states due to dangling bonds on the twelve adatoms, several states localized on the atoms in the layer beneath the adatoms, a state due to Si-Si backbonds, and a state localized in the deep corner hole of the Si(lll)-(7x7) surface. In effect, we have mapped out the electronic states of the Si(lll)-(7x 7) surface with a lateral resolution of 3 A,The utilization of scanning tunneling microscopy (STM) to probe surface electronic structure in real space has been plagued by a number of experimental problems. 1 In one reported approach, 2 " 4 / vs V or dl/dV vs V curves are measured without scanning. While such curves contain electronic structure information, the lateral position of the tip is uncertain and real-space images of the surface states are not obtained. Alternatively, images of dljdV may be acquired at various bias voltages. 4 However, at each voltage the tip follows a different contour 5 so that the dl/dV images contain a mixture of geometric and electronic structure information, complicating interpretation of the observed features. 6 Ideally, one would like to know the complete I-Vcharacteristics, at constant sample-tip separation, at each point in a topographic image.Here we describe a new method, current-imagingtunneling spectroscopy (CITS), which overcomes the problems inherent in the aforementioned techniques and allows real-space imaging of surface electronic states. In this method the feedback control circuit used for constant-current STM is gated so that it is only active about 30% of the time. When the feedback loop is active, a constant voltage is applied to the sample; when inactive, the position of the tip is held stationary and the tunneling current is measured at various different bias voltages. By the repeating of this sequence at 2.2 kHz a constant sample-tip separation is maintained during all I-V measurements and the I-V characteristics of each point along the raster scan are determined simultaneously with t...

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Evidence for a 2D-metallic state of the clean 7 × 7 Si(111) surface

Cited by 75 publications

References 12 publications

Room-temperature chemisorption and thermal evolution of perchloroethylene and trichloroethylene on Si(111)7×7: Formation of chlorinated vinylene and vinylidene and acetylide adspecies, and thermal etching reactions

Room-temperature chemisorption and thermal evolution of perchloroethylene and trichloroethylene on Si(111)7×7: Formation of chlorinated vinylene and vinylidene and acetylide adspecies, and thermal etching reactions

Adatom vibrations on Si(111) reconstructed surfaces

Surface Electronic Structure of Si (111)-(7×7) Resolved in Real Space

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