Scanning Tunneling Microscopy Topography Research Articles

Time response in tunneling to a pn junction

We report on the variation in the response time in scanning tunneling microscopy (STM) measurements across a Si pn junction, and its effect on STM images of such devices. The response time of the tip height subsequent to a stepwise change in the voltage at a fixed demanded current varies by almost two orders of magnitude across the junction, with the slowest response of nearly 80 ms corresponding to the tip–sample junction in strong inversion. Measurements at decreasing separation show an increase in response time up to a saturation point, consistent with screening due to the inversion charge. The slow response in regions where the tip–sample junction is in inversion explains observations of anomalously deep features in STM topography images and scan direction-dependent features in STM conductance maps.

Resonant electron injection as an atomic-scale tool for surface studies

Three surfaces---clean C(100)-(2\ifmmode\times\else\texttimes\fi{}1) diamond, hydrogenated C(100)-(2\ifmmode\times\else\texttimes\fi{}1):H diamond and clean Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) silicon---were imaged at the atomic scale using the scanning tunneling microscope (STM) operating in an unconventional resonant electron injection mode. The reflection of electrons, forming a resonance in the tip-simple vacuum gap, would seem to play a crucial role in the STM's ability to visualize surfaces under these conditions. The first barrier resonance was found to be critical for this surface visualization in two respects. First, a necessary condition was that the STM topographies had to be recorded at a bias coinciding with the energy of the first barrier resonance. Second, the corrugation of the STM topographies was found to be directly proportional to the fineness of the first barrier resonance. Barrier resonances were found to be very sensitive to the difference in energy from the bottom of conduction band. The influence of the bulk electronic structure of the particular sample on the resonance fineness is discussed.

Visualization of 0.1-μm-metal-oxide-semiconductor field-effect transistors by cross-sectional scanning tunneling microscopy

The device structure of 0.1-μm-metal-oxide-semiconductor field-effect transistors (MOSFETs) has been examined by cross-sectional scanning tunneling microscopy (STM). Topographic STM images display the source/drain, gate, channel, gate oxide, and spacer of the MOSFETs in terms of height, where these regions appear as though they are a different height from each other. The bias voltage dependence of the STM images shows that the contrast observed by STM reflects the differences in carrier densities between the regions in addition to that in the corrugations. The dimensions of these regions as obtained from the images are consistent with the specifications of devices in feature size that we fabricated.

Diffuse interface electron scattering in epitaxial Co/Cu bilayers

We have examined the origin of diffuse electron scattering at Co/Cu interfaces using in situ resistance measurements and scanning tunneling microscopy (STM) on a model system consisting of epitaxial (111)-oriented Co on Cu and Cu on Co. We grew epitaxial Co/Cu bilayers on 40 Å Cu(111)/Si(111) substrates, and monitored the resistance and surface morphology during the initial stages of growth for both Co on Cu and Cu on Co. For the case of Co on Cu, the resistance initially increases by 10% at submonolayer coverage, and then drops after 1–1.5 ML coverage. In situ STM topographs taken at similar stages of growth reveal that 20–30 Å Co islands initially nucleate above Cu step edges at submonolayer coverage and then grow inward to cover the Cu terraces. These islands introduce new steps at the surface, dramatically reduce the lateral correlation length of the surface profile, and consequently increase its contribution to the surface scattering resistance. We find that the nucleation and percolation of these islands is strongly correlated with the resistance behavior. In contrast, for Cu deposited on Co, we observe no island nucleation, and no corresponding resistance increase.

STM topographic and barrier imaging of self-assembled InAs/GaAs dots

In this paper we present scanning tunneling microscopy (STM) images of self-assembled InAs dots on GaAs(001) surfaces. The STM was operating in air at room temperature using both constant-current as well as gap-modulated modes. The conductivity and surface band bending have been modified by irradiating the surface by an Ar + laser beam during the scan. In this manner, photoconductivity images have been obtained in an attempt to have a contrast between the dots and the InAs wetting layer. Our results reveal a clear difference between STM topographies and ‘barrier’ method. While topographic images show dome-shaped dots, the barrier ones resemble sharp pyramids even if they are acquired on the same place and at the same time. The generation of STM images is considered for both modes of operation and the concepts of topography and barrier height √ ϕ are revised and compared with a numerical simulation of Qdot surfaces.

Electron Tunneling through SiO2/Si Structures in Scanning Tunneling Microscopy

We study electron tunneling through SiO2/Si(001) structures in scanning tunneling microscopy (STM), where the oxide thickness ranges from 0.6 to 2.7 nm. Electron tunneling probabilities through the structures are calculated by employing an exactly solvable one-dimensional model. The calculation reproduces the measured I–V characteristics very well. It is shown by experiment and calculation that a region covered by SiO2 is seen as a higher patch than the Si bare surface in a STM topography (constant current mode measurement). The apparent SiO2 “thickness” in STM topography is comparable to the calculated one for the 0.6 nm SiO2 but appears to be much thinner than the calculated one for the 2.7 nm SiO2. Origin of the discrepancy is discussed in terms of the electron scattering at the interfaces and in the bulk of SiO2 films.

InGaAs/InP quantum well intermixing studied by cross-sectional scanning tunneling microscopy

Cross-sectional scanning tunneling microscopy (STM) is used to study lattice matched InGaAs/InP quantum well (QW) intermixing induced by ion implantation and thermal annealing. Different strain development in QWs (determined by STM topography of elastic relaxation in cross sectionally cleaved samples) is found to be dependent upon the range of the implanted ions relative to the QWs. It is found that the quantum wells remain latticed matched to the barrier layers after intermixing when ions are implanted through the multiple quantum well (MQW) stack. A shallow implantation in which ions are implanted into the cap layer above the MQW stack leads to tensilely strained wells and compressively strained interfaces between wells and barriers. The strain development in the latter case is attributed to different degrees of interdiffusion on the group III and group V sublattices. Finite element elastic computations are used to extract the group V and group III interdiffusion length ratio, and results using different diffusion models are compared. A preferred group V interdiffusion in the case of shallow implantation is explained in terms of faster diffusing P related defects compared to In related defects. Images of as-grown QWs provide useful information about the growth technique related compositional fluctuations at the interfaces.

Interference of Electron Waves on Carbon Nanotubes at Room Temperature

AbstractWe report on the structure and electronic properties of single wall carbon nanotubes (SWNTs) tips with atomically spatial resolution. Scanning tunneling microscopy (STM) show topographic images of closed tips with a variety of geometrical structure; these include round, conical, as well as tips with a messy shape. At low bias voltage, topographic STM images show standing wave pattern of the charge density in the vicinity of nanotube ends. The patterns originate from constructive interference between the electronic states and its reflection on the nanotube tips. The modulations extend along 6-8 nm away from the cap. Atomically resolved images show asymmetry in the charge density that smears out as the bias voltage increases. These distinctive tip states do not exist elsewhere on the tube and are related to the presence of topological defects at tube ends.

Strained InAs nanostructures self-organised on high-index InP(113)B

We investigated self-assembled InAs islands grown on In 0.52Al 0.48As/InP(113)B by molecular beam epitaxy (MBE) using in-situ reflection high-energy electron diffraction (RHEED) and in-situ scanning tunnelling microscopy (STM). For a deposition of 3 monolayers (MLs) of InAs, the STM topography revealed a high density of well-organised hexagonal facetted dots. The dots are truncated (height ∼3 nm) and elongated along the [1 1 0] direction. Their typical base dimensions are approximately 48 and 19 nm. The angles between facet orientation and crystal direction were accurately deduced from RHEED patterns. From these structural characterisations, the dot shape and formation mechanism are discussed using a standard scheme based on surface energy minimisation. A direct correlation is found between the dot shape and the surface reconstruction unit cell.

Growth of silicon nanostructures on graphite

Silicon nanostructures such as small clusters, superclusters, and elongated chains, with an average diameter of a few nanometers, have been synthesized by magnetron sputtering on cleaved highly oriented pyrolytic graphite (HOPG). Scanning tunneling microscopy (STM) reveals that flat, defect-poor areas of the HOPG surface are covered with almost uniformly sized silicon clusters of 0.6±0.2 nm, 5.1±1.2 nm, and 15.4±3 nm diameter. Surface regions with defects such as pits and craters, descending a few layers into the graphite surface, are sparsely covered with silicon. Most of the deposited material, with an average diameter of 2 nm, is found to be attached to the monatomic step edges forming the crater rims. A simulation of the growth process, i.e. deposition of silicon atoms onto a surface with built-in defects, and subsequent surface diffusion and aggregation of the adatoms, convincingly reproduces most of the Si nanostructures observed in the STM topographs.

Statistical analysis of the electronic structure of single-wall carbon nanotubes

The electronic structure of single-wall nanotubes in intact, undissolved ``buckypaper'' has been studied using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) at 23 K. STM topography shows, that single-wall carbon nanotubes (SWNT's) form microbundles that in turn aggregate in ropes. STS allows us to distinguish between room temperature metallic and wide-gap SWNT's. We find a distribution ratio of $(49\ifmmode\pm\else\textpm\fi{}7)%:(51\ifmmode\pm\else\textpm\fi{}7)%,$ respectively. A statistical analysis of the observed band gaps and metallic plateaus within these groups is carried out and used to determine the diameter distribution of the SWNT's in the buckypaper.

Spectra mapping of scanning tunneling microscope-induced light from electrochemically deposited Ag films on Au

We have carried out spectra mapping measurements of scanning tunneling microscope (STM)-induced photon emission from electrochemically deposited Ag films on Au. The luminescence spectra for Ag and Au films are recorded at the same time as the STM topography image in air. The spectrally resolved photon intensity map is obtained from the observed 4096 spectra, which shows a correlation with the topography. The luminescence spectra observed for Ag films exhibit a broader band of spectra with a spectral shift toward a shorter wavelength compared with Au films. The spectral modes are compared with the topography of the films.

Molecular Photography in the Undergraduate Laboratory: Identification of Functional Groups Using Scanning Tunneling Microscopy

A scanning tunneling microscope (STM) investigation of bare graphite and 11-bromoundecanol adsorbed on graphite has been performed in the undergraduate physical chemistry laboratory at Columbia University. In addition to collecting substrate (surface) and adsorbate (molecule) images, students compare their experimental STM topographs with the shapes of the frontier molecular orbitals of a structurally similar brominated alcohol generated using the Spartan computation program. This investigation stresses the use of the STM as a "molecular camera" with which students can visualize large-scale molecular assemblies and identify chemically significant portions of the molecules. Further, it demonstrates an area where theoretical calculations can be coupled with experimental measurements in a unified setting.

Synchrotron-radiation stimulated desorption of SiO2 thin films on Si(111) surfaces observed by scanning tunneling microscopy

Synchrotron radiation (SR) stimulated desorption of silicon dioxide thin films was studied using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy. Reconstructed Si(111)-7×7 patterns were observed by LEED after 2 h SR irradiation at a surface temperature of 700 °C. The STM images show an atomically flat Si(111)-7×7 surface. STM topographs of SR-irradiated surfaces suggest that the oxide desorption mechanism is completely different from that of thermal desorption of SiO2 film. These results indicate that the atomically flat Si surface can be obtained at low temperatures by using this technique.

Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy Studies of Powdery Palladium/Graphite Model Catalysts

Powdery Pd/graphite model catalysts were studied with transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS), with the later two techniques in air. In contrast to TEM images, STM/STS data are representative of the very surface and are therefore more relevant for an interpretation of the catalytic properties of these kind of catalysts. Besides spherical clusters and layered structures, also platelike and diffuse structures could be distinguished in the STM images. An assignment of the different objects to one of the main elementsgraphite and palladiumwas possible with STS. The I(V) curves on graphite are always much narrower on the V axis than those on palladium, and their slope dI/dV is, thus, larger. Utilizing this distinction, “chemical maps” were determined by taking I(V) data over a grid of surface points and assigning each point the value of the integral under its dI/dV curve. The comparison of such a chemical map with the topographic STM picture of the same area shows that only the spherical cluster structures consist of palladium, in accordance with the TEM pictures. The layered and platelike structures are identified as graphite, and the diffuse structures probably belong to impurities. In any case, the latter consist neither of graphite nor of palladium. A statistical analysis of the cluster morphology reveals that at low Pd loads the clusters are of spherical shape whereas the average width-to-height ratio increases at higher concentrations. The clusters at higher Pd concentrations exhibit a more elliptical shape.

Morphology of Quench Condensed Pb Films near the Insulator to Metal Transition

We present in situ scanning tunneling microscopy topographs of Pb films, formed by vapor deposition onto cold $(T<20\mathrm{K})$, inert substrates, near their insulator to metal transition. At the critical mass deposited thickness for conduction, ${d}_{G}\ensuremath{\cong}5.2\mathrm{nm}$, the films consist of approximately two layers of nanoclusters with diameter, $2r\ensuremath{\approx}20\mathrm{nm}$ and height, $3.5\ensuremath{\le}h\ensuremath{\le}5.5\mathrm{nm}$. We discuss how the nanocluster size and formation mechanism dictate the need for two layers to form in order for conduction to commence.

Comparative scanning tunneling microscopic and rocking-curve X-ray characterization of metallic thin films

The surface morphology of thin films has been investigated by means of scanning tunneling microscopy (STM) and X-ray diffraction (XRD). The full width at half maximum of the XRD rocking curve, corrected for substrate and instrumental aberration contributions using a simplified method, is found to correlate well with the fractal dimension which characterizes the surface roughness of the film. In addition, from the rocking curve data we determine the minimum grain sizes parallel to the film and compare these with those obtained from the auto-correlation function analysis of the STM topographies. Results for Au films grown on mica at different temperatures demonstrate the feasibility of quantitative correlation between STM and XRD rocking-curve data in metallic thin films.

Electronic and structural properties of the As vacancy on the (110) surface of GaAs

The isolated As vacancy on the GaAs (110) surface is investigated using ab initio pseudopotentials. The relaxed atomic structure reveals an inward relaxation of the neighboring surface Ga atoms regardless of the vacancy charge state. The stable charge state is determined to be (+1) in p-type material from the formation energies. Three vacancy states, two located within the band gap, are identified. The character of the vacancy wavefunctions is described in detail and discussed in connection with the charge-state dependent relaxation. Despite the inward relaxation of surface Ga neighbors, the theoretical scanning tunneling microscopy topography shows enhanced tunneling into these sites, which is in agreement with existing experiments. The origin of this apparent contradiction is attributed to the nature of the wavefunction for one of the unoccupied vacancy states.

First-principles local-orbital study of the boron-induced reconstruction of Si(001)

A self-consistent multicenter local-orbital formalism is used to investigate the total energy and atomic equilibrium positions of several structural models suggested for the $\mathrm{Si}(001)$ surface covered by half a monolayer of boron. Density-functional calculations based on the Harris functional approximation for the total energy are performed within the local-density approximation and the pseudopotential method. Localized atomiclike orbitals are used for the expansion of the electronic wave functions, together with the slab supercell description for the surface. We focus on the identification of the structural subunits which form the ordered $c(4\ifmmode\times\else\texttimes\fi{}4)$ reconstruction patterns recently observed by scanning-tunneling microscopy (STM). Our calculations favor a structural model which is different than previously proposed geometries. Moreover, the local density of surface states determined for the lowest-energy configuration agrees very well with the corrugation, lateral distribution, and orientation of the protrusions seen in the STM topographs.

Local work function for Cu(111)–Au surface studied by scanning tunneling microscopy

We report results of the measurement of local work function, or apparent barrier height, on the Au/Cu(111) surface using scanning tunneling microscopy (STM). By measuring a response of tunneling current to a change of the tunneling gap distance during scanning, we can obtain a work function image simultaneously with a topographic STM image. In this way, we could successfully observe the difference in local work function due to different elements and atomic structures. Our results show that the mean work function value of the Au overlayer is 7±3% larger than that of the Cu(111) substrate and the value of the work function of the second Au layer is the same as that of the first Au layers within the measurement uncertainty. At the step edges, the work function values are lower than that on the terrace. The width and depth of the low work function trough at Au–Au, and Cu–Au single height steps were also obtained. The measured values are consistent with those measured on vicinal surfaces. This work also suggests that the local work function measurement can be used to identify the element on a surface and that the STM is quite useful to get information on how the work function is related with surface structures and how the work function spatially changes on nanometer scales.