Direct observation of sn adatoms dynamical fluctuations at the Sn/Ge(111) surface

The well-known low-temperature phase transition sqrt[3]xsqrt[3] to 3x3 for the 1/3 monolayer of Sn adatoms on the Ge(111) surface has been studied by scanning tunneling microscopy. The STM tip was used as a probe to record the tunneling current as a function of time on top of the Sn adatoms. The presence of steps on the current-time curves allowed the detection of fluctuating Sn atoms along the direction vertical to the substrate. We discuss the effect of temperature and surface defects on the frequency of the motion, finding consistency with the dynamical fluctuations model.     

Surface diffusion of Cr adatoms on Au(111) by quantum tunneling

The low-temperature surface diffusion of isolated Cr adatoms on Au(111) has been determined using nonperturbing x rays. Changes in the x-ray magnetic circular dichroism spectral line shape together with Monte Carlo calculations demonstrate that adatom nucleation proceeds via quantum tunneling diffusion rather than over-barrier hopping for temperatures <40K. The jump rates are shown to be as much as 35 orders of magnitude higher than that expected for thermal over-barrier hopping at 10 K.     

New surface phases for potassium adatoms on cleaved Si(111)

In a previous study (B. Reihl and K.O. Magnusson, Phys. Rev. B 42 (1990) 11 839) no change was observed in the LEED pattern from the 2 × 1 pattern of the clean Si(111) surface upon potassium adsorption. In contrast to this paper, we observe six surface phases during room temperature dosing of K on the cleaved Si(111) surface. In addition, we observe a 3 × 1 pattern on the cleaved K/Si(111) surface upon annealing. This paper will provide photographs of the new phases observed with LEED. It also will set limits on the K coverage required for these phases using results from photoemission and secondary electron cutoff measurements. Tentative models for these surfaces will be proposed. These new LEED results show a more complex interaction between K and the cleaved Si(111) surface than previously thought and hopefully will encourage further exploration of adsorption on the cleaved Si(111) surface.     

Triangular Mott-Hubbard insulator phases of Sn/Si(111) and Sn/Ge(111) surfaces

The ground state of Sn/Si(111) and Sn/Ge(111) surface alpha phases is reexamined theoretically, based on ab initio calculations where correlations are approximately included through the orbital dependence of the Coulomb interaction (in the local density+Hubbard U approximation). The effect of correlations is to destabilize the vertical buckling in Sn/Ge(111) and to make the surface magnetic, with a metal-insulator transition for both systems. This signals the onset of a stable narrow gap Mott-Hubbard insulating state, in agreement with very recent experiments. Antiferromagnetic exchange is proposed to be responsible for the observed Gamma-point photoemission intensity, as well as for the partial metallization observed above 60 K in Sn/Si(111). Extrinsic metallization of Sn/Si(111) by, e.g., alkali doping, could lead to a novel 2D triangular superconducting state of this and similar surfaces.     

Exchanges between Si and Pb adatoms on Si(111)

Real-time observation by high-temperature scanning tunneling microscopy of exchanges between Si and Pb atoms on a Si(111)-√3 × √3 surface is reported. The exchange rate is obtained as a function of the temperature. The activation energy of the exchange is about 1.2 eV, and the prefactor, shown to depend on the Pb coverage, is from 2 × 10¹⁰ to 8 × 10¹¹ s⁻¹. This prefactor is much larger than that for the exchange between Pb and Ge adatoms on a Ge(111)-c(2 × 8) surface, indicating that the adatom arrangement greatly influences the exchange mechanism. We also report that metastable 9 × 9 reconstruction appears during Pb desorption.     

Metallic nature of the alpha-Sn/Ge(111) surface down to 2.5 K

Low temperature (down to 2.5 K) scanning tunneling microscopy (STM) and spectroscopy (STS) measurements are presented to assess the nature of the alpha-Sn/Ge(111) surface. Bias-dependent STM and STS measurements have been used to demonstrate that such a surface preserves a metallic 3 x 3 reconstruction at very low temperature. A tip-surface interaction mechanism becomes active below about 20 K at the alpha-Sn/Ge(111) surface, resulting in an apparent unbuckled (sqrt[3] x sqrt[3]) reconstruction when filled states STM images are acquired with tunneling currents higher than 0.2 nA.     

Disproportionation phenomena on free and strained Sn/Ge(111) and Sn/Si(111) surfaces

Distortions of the sqrt[3]x sqrt[3] Sn/Ge(111) and Sn/Si(111) surfaces are shown to reflect a disproportionation of an integer pseudocharge, Q, related to the surface band occupancy. A novel understanding of the (3 x 3)-1U ("1 up, 2 down") and 2U ("2 up, 1 down") distortions of Sn/Ge(111) is obtained by a theoretical study of the phase diagram under strain. Positive strain keeps the unstrained value Q=3 but removes distortions. Negative strain attracts pseudocharge from the valence band causing first a (3 x 3)-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a (sqrt[3] x sqrt[3])-3U ("all up") state with Q=6. The possibility of a fluctuating phase in unstrained Sn/Si(111) is discussed.     

Dynamical motion of adatoms and their neighbours at the Ni(111) surface

We have calculated mean square displacements and one atom spectral densities for the adatoms H, O, S, Ni, and their neighbours on a Ni(111) surface. The adatoms are isolated on the substrate. They lie at a site of three-fold symmetry. The upward shift in frequency of the adatom's normal mode of vibration perpendicular to the surface (a shift from the value on an infinitely massive substrate) has been calculated. A simple analytical formula is obtained by means of which the shift can be estimated for any adatom —surface combination.     

Investigation of the (111) surface of P-doped Si by scanning tunneling microscopy

Received: 27 March 1998     

Identification of Ge/Si intermixing processes at the Bi/Ge/Si(111) surface

The chemical contrast between Si and Ge obtained by scanning tunneling microscopy on Bi-covered Si(111) surfaces is used as a tool to identify two vertical Ge/Si intermixing processes. During annealing of an initially pure Ge monolayer on Si, the intermixing is confined to the first two layers approaching a 50% Ge concentration in each layer. During epitaxial growth, a growth front induced intermixing process acting at step edges is observed. Because of the open atomic structure at the step edges, relative to the terraces, a lower activation barrier for intermixing at the step edge, compared to the terrace, is observed.     

Effect of adsorbed Sn on Ge diffusivity on Si(111) surface

The effect of adsorbed Sn as a surfactant on Ge diffusion on a Si(111) surface has been studied by Low Energy Electron Diffraction and Auger Electron Spectroscopy. The experimental dependence of Ge diffusion coefficients on the Si(111) surface versus temperature in the presence of adsorbed Sn atoms has been measured in the range from 300 to 650°C. It has been shown that at a Sn coverage of about 1 monolayer the mobility of Ge atoms increases by several orders of magnitude.      

Sn/Ge(111) surface charge-density-wave phase transition

Angle-resolved photoemission has been utilized to study the surface electronic structure of 1 / 3 monolayer of Sn on Ge(111) in both the room-temperature (sqrt[3]xsqrt[3] )R30 degrees phase and the low-temperature ( 3x3) charge-density-wave phase. The results reveal a gap opening around the ( 3x3) Brillouin zone boundary, suggesting a Peierls-like transition despite the well-documented lack of Fermi nesting. A highly sensitive electronic response to doping by intrinsic surface defects is the cause for this unusual behavior, and a detailed calculation illustrates the origin of the ( 3x3) symmetry.     

Diffusion and aggregation of adatoms on faceted Pd/Mo(111) surface

Diffusion of Pd adatoms on faceted Pd/Mo(111) surface with hill and valley structure is studied by kinetic Monte Carlo method. The surface consists of {211} channeled facets separated by concave and convex edges. It is shown that diffusion on the faceted surface is significantly reduced with comparison to diffusion on (112) surface by the presence of concave edges. A simple formula for the diffusion coefficient is proposed. The growth of linear structures, studied at low coverage θ = 0.1, depends on the cross-channel diffusion, although the cross-channel diffusion coefficient is by 6–8 orders of magnitude smaller than the in-channel diffusion coefficient. We observed that most of atomic structures grow in valleys and the average size of the structure increases with increase of facet size.     

Atomic motion induced by a scanning tunneling microscope tip on the Si(111) surface

Using scanning tunneling microscopy (STM), we have observed the STM-tip-induced atomic motion of the Si adatoms in the Si(111)−(7 × 7) surface structure near out-of-phase boundaries in the structure. In order to excite the motion of Si adatoms, the tip was fixed at a certain height at every pixel in the lateral scan and the sample bias was changed stepwise. Although the motion of Si adatoms occurs in a complicated manner, analysis shows that its elemental process is quite simple. Namely, the Si adatom jumps from an occupied T₄ site to an unoccupied T₄ site.     

Vertical manipulation of native adatoms on the InAs(111)A surface

We achieved the repositioning of native In adatoms on the polar III-V semiconductor surface InAs(111)A-(2?×?2) with atomic precision in a scanning tunnelling microscope (STM) operated at 5?K. The repositioning is performed by vertical manipulation, i.e., a reversible transfer of an individual adatom between the surface and the STM tip. Surface-to-tip transfer is achieved by a stepwise vibrational excitation of the adsorbate-surface bond via inelastic electron tunnelling assisted by the tip-induced electric field. In contrast, tip-to-surface back-transfer occurs upon tip-surface point contact formation governed by short-range adhesive forces between the surface and the In atom located at the tip apex. In addition, we found that carrier transport through the point contact is not of ballistic nature but is due to electron tunnelling. The vertical manipulation scheme used here enables us to assemble nanostructures of diverse sizes and shapes with the In adatoms residing on vacancy sites of the (2?×?2)-reconstructed surface (nearest-neighbour vacancy spacing: 8.57??).     

Drift of adatoms on the (111) silicon surface under electromigration conditions

The existence of the drift of adatoms on the surface that is induced by an electric current heating a sample under the conditions of sublimation, homoepitaxial growth, and quasiequilibrium on the (111) silicon surface at temperatures above 1050°C has been shown by in situ ultrahigh-vacuum reflection electron microscopy and ex situ atomic-force microscopy. It has been found that the direction of the drift of adatoms is independent of the supersaturation value on the surface. Under these conditions, the drift of adatoms occurs towards underlying terraces, i.e., at 1050–1240°C at the resistive heating of the sample by step-up direct electric current and at 1250–1350°C by step-down current. The presence of the drift of adatoms indicates that adatoms have an effective charge whose value at 1280°C is estimated as 0.07 ± 0.01 of the elementary charge.