Scanning Tunneling Microscopy Topography Research Articles

MEASUREMENTS OF SPECTRAL RESOLVED PHOTON INTENSITY MAP FOR METAL PARTICLES BY STM

We have measured for the first time the spectral resolved photon maps of the light induced by scanning tunneling microscopy (STM) of noble metal particles on the graphite surface. The photon intensity as a whole is linearly correlated with the height of the topography of the samples. The photon map changes depending on the wavelength of the light collected. The position-dependent contrast besides the height-dependent contrast is obtained by normalizing the photon map with the corresponding STM topography. The normalized photon maps show that the spectral modes of the light emitted are varied with the tip position on the particles. The result shows that the contrast of the photon map is dependent on both the sample topography and the tip shape. A measurement of an isochromat photon map is also made for the mixture of gold and silver metals for a chemical analysis.

Elemental contrast of local work function studied by scanning tunneling microscopy

We have obtained images of the local work function on Au Cu(111) and Pd Cu(111) surfaces using scanning tunneling microscopy (STM) and found that they form contrasts depending on the element on the surfaces. A work function image can be obtained simultaneously with a topographic STM image by measuring the response of tunneling current to variation in the tunneling gap distance during scanning. Using this technique, we successfully observed the difference in local work function between the Au Pd overlayers and the Cu substrate. Our results show that the measured value of the work function on the Au overlayer is between those of Au(111) and Cu(111), consistent with the results obtained by another technique. On the other hand, the work function of the first layer Pd is already larger than that of the bulk Pd(111), and it increases further on the second Pd layer. We also observed a dark contrast along the step edge in work function images which is partly due to dipole moment formation by the Smoluchowski effect.

Surface reconstructions of In on Si(111)

The complete phase diagram of In-induced surface reconstructions on Si(111) surfaces has been mapped by scanning tunneling microscopy (STM) and spectroscopy (STS). The spectroscopy results illustrate the transition from low-coverage semiconducting reconstructions via semimetallic phases to the metallic surfaces at monolayer coverages. Electronic effects in the STM imaging process preclude a straightforward interpretation of the STM data in terms of structure models for the surfaces at intermediate coverages, but for the higher-coverage metallic phases a topographic STM analysis is possible and detailed structure models are presented. The In-Si(111) monolayer surfaces are interpreted in terms of regular adlayer structures above the first Si double layer, but discommensurate phases are observed in the presence of an external stress field, introduced by an external perturbation.

Scanning tunneling microscopy characterization of metalorganic chemical vapor deposition grown GaN

GaN films were grown by metalorganic chemical vapor deposition (MOCVD) on (0001) c-sapphire substrates and their scanning tunneling microscopy (STM) characteristics are reported for the first time. X-Ray characterization and Hall measurements revealed a full-width-half-maximum (FWHM) value of 288 arcsec and background doping reduction accompanied with electron mobility enhancement as the growth temperature increased. STM topography revealed island formations on the growth surface the size of which increased again with growth temperature in a manner consistent with the trends of improved electrical characteristics. STM stimulated luminescence showed no significant photon emission in the tunneling mode but moderate value signals in the field-emission mode, where the intensity increased with tip voltage. Finally, scanning tunneling spectroscopy allowed bandgap estimation at 3.5 eV and suggested localized band-bending fluctuations across the surface.

LOCAL WORK FUNCTION MEASURED WITH SCANNING TUNNELING MICROSCOPY

In the present paper we report local work function measurements on the Cu(111)-Au and Pt(111)-Ag surfaces using scanning tunneling microscopy (STM).By measuring the response of tunneling current to the variation of the tunneling gap distance a work function image can be obtained simultaneously with a topographic STM image. By means of this technique,we are able to detect the difference between the local work functions of the overlayer and the substrate in both Cu(111)-Au and Pt(111)-Ag cases.Our results show that the local work function of the Au overlayer is between those of Au(111) and Cu(111) surfaces,supporting the results obtained with other techniques. In the case of the Pt(111)-Ag surface the local work function depends on the local thickness of the Ag overlayer.

Adsorbate-adsorbate interactions from statistical analysis of STM images: N/Ru(0001).

Atomic nitrogen on Ru(0001) was prepared by dissociative chemisorption of ${\mathrm{N}}_{2}$ and studied by scanning tunneling microscopy (STM) at 300 K. Nitrogen occupies the hcp threefold hollow site and is imaged as a depression with a diameter of about 5 \AA{}. Interactions between the adsorbed nitrogen atoms were obtained by statistical analysis of STM images, by extraction of the two-dimensional pair distribution function from the arrangement of the N atoms. Since the nearest-neighbor separations could be identified with atomic precision, the pair distribution function $g$ and hence the potential of mean force ${V}_{\mathrm{eff}}$ were obtained as a function of the discrete neighbor sites $j$ up to the tenth nearest neighbor. A comparison with Monte Carlo calculations for balls with a hard-sphere potential provides information about the pair potential ${V}_{\mathrm{pair}}(j)$: The nearest-neighbor site is strongly repulsive, the second-neighbor site is weakly repulsive, and the third-neighbor site is weakly attractive. These findings rationalize the absence of island formation and of a well-ordered 2\ifmmode\times\else\texttimes\fi{}2 phase for the N/Ru(0001) system: At temperatures \ensuremath{\ge}300 K the attractive interaction on the third-neighbor site is too weak, while at lower temperatures the diffusion barrier of 0.9 eV represents a kinetic obstacle. The fact that the range of the interaction is identical to the diameter of the N-atom features in the STM topographs is taken as evidence that the interaction is caused by substrate-mediated electronic forces.

Isochromat photon map induced by scanning tunneling microscopy from gold particles.

We have measured the maps of isochromat photon intensity induced by scanning tunneling microscopy (STM) from noble metal particles (Au) on a graphite surface. The map is obtained simultaneously with the STM topography image using an optical band-pass filter. The photon map changes depending on the wavelength of the light collected. The isochromat photon maps show that the spectra are varied with the tip position on the particles. This spectra variation is explained by the effect of the change in the tip curvature radius on the spectra of a local plasmon. \textcopyright{} 1996 The American Physical Society.

New model of the initial stages of Si(111)-(7 x 7) oxidation.

By combining scanning tunneling microscopy (STM) and ultraviolet photoelectron spectroscopy, we show that the adsorption of single ${\mathrm{O}}_{2}$ molecules on Si(111)-(7 $\ifmmode\times\else\texttimes\fi{}$ 7) produces essentially molecular sites, seen as bright individual sites in STM topographs, which are stable at room temperature. Atom-resolved STM studies also reveal that these molecular sites react very efficiently with further ${\mathrm{O}}_{2}$ and ${\mathrm{H}}_{2}$O molecules. This is shown to account for the saturation at low exposure of the molecular site density and for the apparent decay lifetime of these molecular sites.

Photoinduced scanning tunneling microscopy of insulating diamond films

Insulating diamond films were imaged by scanning tunneling microscopy (STM) utilizing photoinduced bulk carrier transport to establish tunneling currents. General comparisons of topographic STM images and atomic force microscopy images acquired on the same sample demonstrate that submicrometer structures obtained in the images can be correlated. This observation establishes that the topography of an insulating surface such as diamond can be imaged under illumination by STM. The results suggest that local electronic structure of illuminated insulating surfaces can be probed using scanning tunneling spectroscopy.

LEED structure determination of the c( 8 × 2)Mn phase on Cu(100)

The structure of the Cu(100)c(8 × 2)Mn phase was determined by a quantitative low-energy electron diffraction (LEED) intensity analysis. This structure is formed by deposition of 0.9 ML Mn on Cu(100) at 200 K. Scanning tunneling microscopy topographs and an investigation of the film morphology by high-resolution electron diffraction show that the Mn film wets the substrate and has a low step density. The LEED intensity analysis proves that Mn forms a quasi-hexagonal layer on the Cu(100) substrate. The Mn atoms in this layer have a pronounced buckling of 0.53 Å and an in-plane spacing of around 2.92 Å. This spacing is considerably larger than the nearest-neighbour spacing of 2.7 Å found in γ-Mn.

Experimental and Simulated Scanning Tunneling Microscopy of the Cleaved Rb1/3WO3 (0001) Surface

ABSTRACTAtomic-scale resolution scanning tunneling microscope (STM) images of cleaved (0001) surfaces of the hexagonal tungsten bronze, Rbl/3WO3, show two distinct contrast patterns. We have interpreted these images using simulated constant current STM topographs. These simulations are constructed based on calculations of the tunnel current as a function of the lateral and vertical position above the surface. By calculating simulated images for the limiting cases of different termination layers, different tip sizes, and different electronic structures, it is possible to systematically explore the important parameters and choose a model that most closely matches the experimental observations. In this case, we conclude that two distinct termination layers have been imaged, a W-O terminated surface and a Rb-O terminated surface. Also, we have found that the O atoms on the W-O surface relax to new positions nearer the 6-fold axis of rotational symmetry. Some of the advantages and disadvantages of this model are discussed.

Internal structure of C60 fullerence molecules as revealed by low-temperature STM

We have performed Scanning Tunneling Microscopy (STM) in ultra-high vacuum at low temperatures (5 and 50K) of unordered and ordered C60 layers adsorbed on a Au(110) surface. STM topographs of the frozen C60 molecules reveal four symmetric patterns within single molecules, which may be associated with different orientations of the fullerenes on a highly corrugated gold substrate.

Scanning tunneling microscopy and spectroscopy of iron suicide epitaxially grown on Si(111)

Epitaxial iron suicide films have been grown on Si(111) by solid phase epitaxy (SPE) in UHV. Structural and electronic properties have been investigated with scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). For initial Fe deposition up to 3 Å and annealing at 850 K, metallic γ-FeSi 2 is formed. These films exhibit a perfect (2 × 2) superstructure, which is attributed to γ-FeSi 2(111) with Si termination. SPE at higher initial iron deposition (15 Å) and annealing at 800 K results in ϵ-FeSi showing a (√3 × √3) R30° superstructure. Subsequent annealing above 900 K leads to β-FeSi 2 formation. As by STS, β-FeSi 2 films are semiconducting with E g = 0.85 eV. STM topographs show that SPE produces rough silicide surfaces wit β-FeSi 2(101) [and not β-FeSi 2(110)] epitaxy. The atomic structure on β-FeSi 2 terraces is complex, consisting domain boundaries and defects.

Correlation from randomness: quantitative analysis of ion-etched graphite surfaces using the scanning tunneling microscope

The scanning tunneling microscope (STM) was used to quantitatively examine the surface morphology of highly oriented pyrolytic graphite (HOPG) surfaces bombarded with 5 keV Ar + ions. Constant current topographs clearly showed that the morphology of the resulting nonequilibrium surfaces depended sensitively on the ion flux, the ion fluence and the sample temperature. For low and intermediate ion fluence, an increase in surface roughness with increasing ion flux was observed. For constant ion flux, the surfaces developed structures with heights proportional to the ion fluence and separations characterized by a correlation length that diverged with increasing fluence. Increasing the sample temperature during bombardment produced smoother surfaces as a result of enhanced surface diffusion. The autocovariance function G( L) and the height correlation function 〈 | h( q)| 2 〉 in reciprocal space were calculated directly from the STM topographs. The latter was then compared with a linear response theory for the formation of rough surfaces under nonequilibrium conditions and a scaling analysis. The fluence dependence of the surface roughening of graphite could not be explained by shot noise alone in a linear theory, whereas surface diffusion and redeposition of sputtered material satisfactorily accounted for the correlation observed in the bombardment-induced features.

Modeling Large Surface Reconstructions on the Connection Machine

Using a massively parallel computer, we undertake an ab initio investigation of the Si(111)-(7×7) surface reconstruction. Calculation of the total energy of an ∼700 effective atom supercell at an 8 Ry plane wave cutoff allows us to determine (1) the energy difference between the (7×7) and (2×1) reconstructions, (2) the relaxed atomic geometry, (3) the scanning tunneling microscope topographs as a function of bias voltage and (4) the occupied and unoccupied electronic states.

Lattice mismatch dislocations in a preferentially sputtered alloy studied by scanning tunneling microscopy.

Scanning tunneling microscopy (STM) on a sputtered and annealed ${\mathrm{Pt}}_{25}$${\mathrm{Ni}}_{75}$(111) single crystal reveals a network of subsurface lattice mismatch dislocations caused by platinum enrichment due to preferential sputtering and recoil mixing. Atomically resolved STM topographs are compared with simulations of these dislocations using embedded atom potentials. This allows one to estimate the depth of the dislocations, and thus the thickness of Pt enrichment, which is three monolayers on the 500 eV ${\mathrm{Xe}}^{+}$ sputtered and five monolayers on the ${\mathrm{Ar}}^{+}$ sputtered surface, compatible with the depth of radiation damage.

Interfacial dislocations detected by scanning tunneling microscopy

Thin films of CoSi 2 have been grown on Si(111) and Si(100) by molecular beam epitaxy (MBE). We present the unexpected result that scanning tunneling microscopy (STM) is sensitive to the misfit dislocations present at the CoSi 2/Si interface in the case of overcritical, relaxed layers. Even for layer thickness above 100 Å a surface contrast consisting of protruding lines which is due to the dislocations at the interface is easily visible in STM topographs. Topographic cross-sections perpendicular to these protruding lines can well be fitted to a Lorentzian with a half-width equal to the CoSi 2 layer thickness, i.e., the distance to the dislocation core. This effect is qualitatively explained by simple elasticity theory, but additional mechanisms have to be invoked in order to account for a quantitative explanation of the observed contrast. These measurements establish conventional STM as a viable technique for the imaging of interface dislocations.

Strain-induced (2 × 1) reconstruction on epitaxial CoSi 2/Si(111) observed by scanning tunneling microscopy: structure model and electrical properties

The formation of a (2 × 1) reconstruction on ultrathin ( d<45 A ̊ ), strained layers of CoSi 2 grown epitaxially on Si(111) is reported. Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED) show that the stable low-temperature ( T<450 K) structure of the Si-rich CoSi 2 surface consists of chains perpendicular to the three 〈2 1 1 〉 directions. A structure model for the atomic rearrangement of this surface is presented which agrees with the results of a detailed analysis of high-resolution STM topographs, including point defects, different domain boundaries and I-U tunneling spectroscopy. The results are compared with the π -bonded chain model for the Si(111)2 × 1 reconstruction.

Observation of misfit dislocations in epitaxial CoSi2/Si (111) layers by scanning tunneling microscopy

The surfaces of epitaxial CoSi2 layers grown on Si(111) have been examined by scanning tunneling microscopy (STM) in ultrahigh vacuum. The onset of strain relaxation above the critical thickness of about 40 Å has been monitored by STM for the first time. This relaxation takes place by the formation of a honeycomb network of partial dislocations lying in the interface plane. An associated network of protruding lines has been detected in STM topographs for film thicknesses up to 104 Å. The topographic cross sections perpendicular to the lines are found to have a Lorentzian shape with a height of 0.6 Å and a half-width equal to the layer thickness. Our analysis suggests that similar effects should be observable for a wider class of heteroepitaxial systems.

Submicron-scale surface roughening induced by ion bombardment.

The scanning tunneling microscope (STM) was used to quantitatively investigate nonequilibrium surfaces of graphite roughened by sputter etching with 5-keV Ar ions. The resulting surface morphology depended strongly on the ion flux and fluence, as well as the sample temperature. The height-correlation functions of the roughened surfaces were calculated directly from the STM topographs and compared to linear-response theory and scaling analyses for the propagation of growing fronts. The surfaces developed correlated structures characterized by a length that diverged with increasing ion fluence.