Formation and local electronic structure of Ge clusters on Si（111）-7×7 surfaces

We report the formation and local electronic structure of Ge clusters on the Si(111)-7$\times $7 surface studied by using variable temperature scanning tunnelling microscopy (VT-STM) and low-temperature scanning tunnelling spectroscopy (STS). Atom-resolved STM images reveal that the Ge atoms are prone to forming clusters with 1.0~nm in diameter for coverage up to 0.12~ML. Such Ge clusters preferentially nucleate at the centre of the faulted-half unit cells, leading to the `dark sites' of Si centre adatoms from the surrounding three unfaulted-half unit cells in filled-state images. Bias-dependent STM images show the charge transfer from the neighbouring Si adatoms to Ge clusters. Low-temperature STS of the Ge clusters reveals that there is a band gap on the Ge cluster and the large voltage threshold is about 0.9~V.     

The calculation of transition probabilities for atomic oxygen

In this contribution we present scanning tunnelling microscopy (STM) and spectroscopy (STS) investigations on isolated cobalt clusters in contact with Ge(001). Mass-filtered nanoparticles with diameters ranging from 3 to 11 nm are generated using an arc cluster ion source (ACIS) and deposited under soft landing conditions (E_kin/atom < 0.5 eV). Since the tip radius is of the same order as the nanoparticle diameters the recorded STM images are significantly affected by tip folding. By means of the “blind reconstruction method" it is possible to approximate the tip shape. After a respective deconvolution of the image structural features of the particle facets become observable. According to the equilibrium shape of the clusters being a truncated octahedron in the size range under investigation, hexagonal and rectangular features appear in the images. STS is sensitive to occupied and unoccupied states near the Fermi level and reveals the existence of distinct states in the tunnelling conductivity of the substrate as well as on the clusters. The richly structured density of states of the germanium surface serves here as tip condition test. First measurements of the tunnelling conductivity of the Co_N/Ge(001) are presented and discussed.     

The passivation of atomic scale defects present on III–V semiconductor laser facets: an STM/STS investigation

The work presented in this paper is based on the use of scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) to study the passivation of atomic scale defect-induced surface states on cleaved III–V (1 1 0) surfaces. This is based on the use of thin Si layers deposited in situ on to the atomically clean surface. The simultaneous STM and STS measurements allowed direct correlation of the structural and electronic properties at the nanoscopic level. The preferential adsorption of Si clusters onto surface defects was achieved using elevated temperature growth on the GaAs(1 1 0) substrate. The STS results clearly indicated local electronic passivation of both step defects and vacancy clusters when the interface is formed at 280 °C. This observation was also confirmed on a macroscopic level using X-ray photoelectron spectroscopy (XPS) under identical conditions. The results are interpreted in terms of the surface bonding of Si with the defect sites. Furthermore, this STM/STS study has been extended to real laser devices where comparable defect features are observed. The implications of defect passivation in nanotechnology are also discussed.     

On the topographic and optical properties of SiC/SiO2 surfaces

The roughness of the semiconductor surface substantially influences properties of the whole structure, especially when thin films are created. In our work 3C SiC, 4H SiC and Si/a-SiC:H/SiO₂ structures treated by various oxidation a passivation procedures are studied by atomic force microscopy (AFM) and scanning tunnelling microscopy (STM). Surface roughness properties are studied by fractal geometry methods. The complexity of the analysed surface is sensitive to the oxidation and passivation steps and the proposed fractal complexity measure values enable quantification of the fine surface changes. We also determined the optical properties of oxidized and passivated samples by using visual modelling and stochastic optimization.      

Room and high-temperature scanning tunnelling microscopy and spectroscopy (HT-STM/STS) investigations of surface nanomodifications created on the TiO2(1 1 0) surface

High-temperature scanning tunnelling microscopy, scanning tunnelling spectroscopy and current imaging tunnelling spectroscopy (HT-STM/STS/CITS) were used to study the topographic and electronic structures changes due to surface modifications of the TiO₂(1 1 0) surface caused by the STM tip. In situ high-temperature STM results showed that the created modifications were stable even at elevated temperatures. The STS/CITS results showed the presence of energy gap below the Fermi level on the untreated regions. The disappearance of energy gap below the Fermi level on the modifications created by the tip was observed. It is assumed that the presence of the tip can change the chemical stoichiometry of the surface from TiO_2−x towards Ti₂O₃.     

Remote projection and quantum mirages in STS and STM

The phenomena of remote projection and quantum mirages are investigated using standard quantum mechanics. The information inherent in delocalized wave functions in the vicinity of the Fermi level, including contributions from localized states, is available wherever the waves propagate coherently and have a non-vanishing amplitude and therefore can be probed remotely. This can explain the observation of a "quantum mirage" by Manoharan et al.: Nature 403, 512 (2000), i.e., the Kondo antiresonance due to a single adsorbed Co atom on Cu (111) far from the location of the cobalt atom. Similar quantum effects can give rise to "mirage" features in the scanning tunnelling spectrum (STS) both on clean and adsorbate-covered metal surfaces, features which are not resolved in other surface-sensitive spectroscopies (IPES, 2PPE). Within a theory based on a many-particle treatment of the tunnelling phenomena in STS and in scanning tunnelling microscopy (STM), the unexpected features in the scanning tunnelling spectra are associated with the spectral weight of transient ion-resonance states generated in the process of electron injection. They transport in a coherent way the information from the tip towards the sample and vice versa over distances of the order of 10 Å or more, generating spectroscopic structures. These "mirage" states are important for the tunnelling current and the imaging properties.     

Plasma nitridation of Ge(100) surface studied by scanning tunneling microscopy

The geometric and electronic structures of the surface species on Ge(100) after plasma nitridation were investigated in this study. An electron cyclotron resonance (ECR) plasma source was used to directly nitride Ge(100), and scanning tunneling microscopy and spectroscopy (STM/STS) were employed to study the structures of the nitrided surface. Nitridation at room temperature generated a large diversity of adsorbate sites on the surface containing N, O, and displaced Ge atoms, differentiated by annealing between 200 °C and 450 °C. Conversely, nitridation at 500 °C produced Ge–N adsorbate sites which formed ordered and disordered structures on the surface free from oxygen. Density functional theory (DFT) simulations were performed focusing on the ordered nitride structure, and the simulated surface structure showed a good correspondence with the STM data. DFT calculations also found an increase of density of states near the Fermi level on the ordered nitride structure, which is consistent with the Fermi level pinning observed in the STS results. The DFT results predict H-passivation can unpin the Fermi level of the nitrided surface by reducing the dangling bonds and the bond strain, but the residual plasma damage and the low nitridation rate in UHV are challenges to obtain complementary experimental results.     

Ge quantum dots growth on nanopatterned Si(0 0 1) surface: Morphology and stress relaxation study

The self-organized growth of germanium quantum dots on square nanopatterned Si(0 0 1) substrates is investigated by scanning tunnelling microscopy (STM) and grazing incidence X-ray diffraction (GIXRD) techniques. A regular surface patterning in the 10-100 nm period range is obtained by etching an interface dislocation network obtained by the controlled molecular bonding of Si substrates. The depth of the silicon surface profile is increased by a double etching process. Growth experiments are performed by solid source molecular beam epitaxy (MBE), and for deep trenches, germanium growth conditions are optimized to obtain one Ge dot per Si mesa. It is shown that the trench depth and the mesa profile strongly affect the dot size and its coincidence with the initial regular surface network. Anomalous GIXRD measurements are performed to highlight the Ge elastic relaxation and intermixing during heteroepitaxial growth. We report a significant modification in the stress state of Ge dots as a function of thermal annealing after growth.     

Tunnelling spectroscopy on silver islands and large deposited silver clusters on Ge(001)

Morphology and electronic properties of silver islands and deposited silver clusters on Ge(001) have been studied using scanning tunnelling microscopy (STM) and spectroscopy (STS) at low temperatures. Already the clean surface bears an interesting electronic structure, which is obvious from the STS. The tunnelling spectra exhibit strong peaks associated with dangling bond-derived surface states and an antibonding σ-state of the surface dimer. For silver islands of only few monolayers in height, complex spectra are interpreted to be dominated by metal–semiconductor interface states. These islands show energy gaps which are not observed for larger ones beyond 1 nm in height. Spectra of the larger islands contain a series of distinct peaks originating from lateral and three-dimensional electron confinement, respectively. Silver clusters – preformed in the gas phase using a cluster source – have been fabricated, size selected and deposited onto germanium(001). In tunnelling spectra dips at the Fermi level are accompanied by two maxima. These characteristics seem to be almost independent from the cluster size. Additional weak structures are found at higher bias voltages, which are understood in terms of quantized states. PACS 36.40.Cg; 61.46.+w; 73.20.At     

Degenerate electronic structure of reconstructed MnSi(1.7) nanowires on Si(001)

Higher manganese silicide nanowires have been grown on the Si(001)-2 × 1 surface by the pre-growth of Bi nanolines. Scanning tunnelling microscope (STM) observations show that the nanowire has a linear surface reconstruction with a periodicity of 0.56 nm, and we propose a reconstruction on their surface to reduce the density of dangling bonds, which forms linear structures matching the dimensions from STM. Scanning tunnelling spectroscopy (STS) data agree with previous calculation results and reveal that the nanowires are degenerate semiconductors, with potential application for spintronics.     

Role of patterning in islands nucleation on semiconductor surfaces

Quantum dots (QDs) grown on semiconductors surfaces are actually the main researchers' interest for applications in the forthcoming nanotechnology era. New frontiers in nanodevice technology rely on the precise positioning of the nucleation site and on controlling the shape and size of the dots. In this article we will review some recent studies regarding the control of the nucleation process on semiconductor surfaces. A few approaches to form ordered patterns on surfaces are described: natural patterning induced by surface instabilities (as step bunching or step meandering), in situ substrate patterning by Scanning Tunneling Microscopy (STM), high resolution patterning by Focused Ion Beam (FIB). Growth of epitaxial layers of semiconductors (Ge/Si(100) and InAs/GaAs(100)) on patterned surfaces has been studied by STM or Atomic Force Microscopy (AFM) unveiling the way in which the first atoms start to aggregate and identifying their exact nucleation site. Control of the dot size to match the patterning typical wavelength has been achieved by using surfactants on misoriented substrates. STM images acquired in real time allows one to identify the mechanism of Ge cluster formation on patterned Si(100), and to follow the island transition from pre-pyramid to pyramid. Nucleation of ordered Ge dots on SiO₂ substrates has been obtained thanks to FIB tight patterning, achieving island densities of 3.5×10¹⁰/cm². To cite this article: N. Motta et al., C. R. Physique 7 (2006).     

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.     

Scanned-probe topological and spectroscopic study of surface states on clean and Si-deposited GaAs (0 0 1)-c(4×4) surfaces

Microscopic topological and spectroscopic properties of MBE-grown GaAs c(4×4) surfaces without and with monolayer Si deposition were investigated by the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Empty state STM images on as-grown surface showed bright and dark cells, and they exhibited strong correlation with the spatial distribution of normal and anomalous conductance gaps of the STS spectra. Bias dependent STM images indicated presence of pinning areas with continuous space and energy distribution of surface gap states. By deposition of monolayer Si, dark areas reduced a great deal and the rate of finding normal STS spectra increased, indicating large reduction of surface states.     

Measuring the implantation depth of silver clusters in graphite

Scanning tunnelling microscopy (STM) and molecular dynamics (MD) simulations have been used to investigate the implantation of Ag₇ ^- clusters into the graphite surface. An experimental measure of the implantation depth of individual clusters is gained via thermal oxidation of the bombarded graphite surfaces. This process results in etching of the cluster-induced defects to form etch pits which grow laterally whilst retaining the depth of the implanted cluster. STM imaging of the etch pits reveals the distribution of implantation depths for deposition energies of 2 keV and 5 keV. Molecular dynamics simulations for clusters of 5 keV energy show that the implantation depth for Ag₇ ^- is largely independent of the impact site on the graphite surface and the cluster orientation. The implantation depth found by MD lies at the upper edge of the experimental depth distribution. Received 30 November 2000     

Rectification behaviour of molecular layers on Si(111)

Reproducible and strong diode-like behaviour is observed for molecular films of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) on n-type Si(111)- 7×7 surfaces studied by scanning tunnelling microscopy (STM) and spectroscopy (STS) at 77 K. The mechanism behind the rectification is likely to be related to the electron distribution at the molecule-silicon interface. We suggest that the adsorption of the molecular layer profoundly modifies the electronic structure of the Si(111)- 7×7 surface.     

Oxidation kinetics of epitaxial Ge-covered Si(100) surfaces

We compared oxidation kinetics on Ge-covered Si(100) surfaces grown at 350 and 600°C for 0.9 and 2.0 ML Ge overlayer thicknesses. The O_KLL intensities showed clear oxidation enhancement on the surfaces grown at 600°C. The oxygen interaction for the surface covered with 2 ML Ge formed at 350°C was weaker than for the Ge(100) surface, indicating that the compressive strain due to the lattice mismatch may suppress the oxygen interaction with surface Ge dimers.     

On the influence of the surface roughness onto the ultrathin SiO2/Si structure properties

The surface roughness of the semiconductor substrate substantially influences properties of the whole semiconductor/oxide structure. SiO₂/Si structures were prepared by using low temperature nitric acid oxidation of silicon (NAOS) method and then the whole structure was passivated by the cyanidization procedure. The influence of the surface morphology of the silicon substrate onto the electrical properties of ultrathin NAOS SiO₂ layer was investigated. Surface height function properties were studied by the AFM method and electrical properties were studied by the STM method. The complexity of analyzed surface structure was sensitive to the oxidation and passivation steps. For describing changes in the oxide layer structure, several fractal measures in an analysis of the STM images were used. This fractal geometry approach enables quantifying the fine spatial changes in the tunneling current spectra.     

Charge transfer in the atomic structure of Ge (1 0 5)

The atomic structure and charge transfer on the Ge (1 0 5) surface formed on Si substrates are studied using scanning tunneling microscopy and spectroscopy (STM and STS). The bias-dependent STM images of the whole Ge (1 0 5) facets formed on a Ge “hut” structure on Si (0 0 1) are observed, which are well explained by the recently confirmed structure model. The local surface density of states on the Ge (1 0 5) surface is measured by STS. The localization of the electronic states expected from charge transfer mechanism is observed in the dI/dV spectra. The surface band gap is estimated as 0.8-0.9 eV, which is even wider than the bulk bandgap of Ge, indicating the strong charge transfer effect to make the dangling bonds stable. The shape of normalized tunnel conductance agrees with the theoretical band structure published recently by Hashimoto et al.     

Amorphous solid foam structures on germanium by heavy ion irradiation

Ge (100) surfaces were irradiated by heavy Bi⁺ and Bi⁺⁺ ions extracted from a Bi-liquid metal ion source in a mass separated focused ion beam system with energies of 30 and 60 keV, respectively. Networks of different nanoporous (or sponge like) structures were found depending on ion energy, fluence, angle of incidence, and irradiation temperature. The porous and amorphous surface structures are explained in terms of high concentration vacancies close to the surface. The surface modification was investigated using SEM and AFM imaging and FIB for cross section preparation.     

Scanning probe microscopy studies of PbS surfaces oxidized in air and etched in aqueous acid solutions

Natural n-type PbS single crystals have been studied using AFM, STM and STS after long-term oxidation in air at ambient temperatures and extensive etching in aqueous acid solutions, in contrast to previous work devoted to initial corrosion of fresh surfaces. The exposure of PbS to atmosphere at high relative humidity for several days yields widespread loose oxidation products; the process is much slower at low humidity. Surface morphologies diverge after the treatment in 1 M perchloric and hydrochloric acid solutions at room temperature and become widely different at elevated temperatures, displaying commonly etch pits up to several micrometers in size and depth along with rather uniformly distributed 20-100 nm protrusions of PbS phase. The changes both in topography and semiconducting properties of PbS found by tunneling spectroscopy have been explained in terms of the non-uniform distribution of donor- and acceptor-type defects D⁺/D⁻ in the metal depleted surface layer, which are generated by chemical reactions and, in turn, determine the rates of the PbS corrosion. In particular, the D⁻ centers exhibit a self-catalyzing effect on the non-oxidative local dissolution of PbS in HCl media, resulting in the deep etch pits.