Ultrasmall Ge islands with low diameter-to-height aspect ratio on Si(1 0 0)-(2 × 1) surfaces

Scanning tunneling microscopy (STM) and high resolution cross-sectional transmission electron microscopy (XTEM) studies have been used to investigate the formation of Ge nanocrystals grown on Si(1 0 0)-(2 × 1) surfaces by molecular beam epitaxy (MBE). We observe relatively high density of Ge islands where small ‘pyramids’, small ‘domes’ and facetted ‘domes’ of various sizes co-exist in the film. As revealed from XTEM images, a large fraction of islands, especially dome-shaped Ge islands have been found to have an aspect ratio of ∼1 (diameter):1 (height). Observation of truncated-sphere-shaped Ge islands with a narrow neck contact with the wetting layer is reported.     

STM study of acetylene reaction with Si(1 1 1): observation of a carbon-induced Si(1 1 1)√3×√3R30° reconstruction

The first stages of acetylene reaction with the Si(1 1 1)7 × 7 reconstructed surface kept at 600 °C are studied by recording scanning tunneling microscopy (STM) images during substrate exposure at a C₂H₂ pressure of 2 × 10⁻⁴ Pa (2 × 10⁻² mbar). We observed the progressive substitution of the 7 × 7 reconstruction with a carbon induced Si(1 1 1)√3×√3R30° reconstruction characterized by an atomic distance of 0.75 ± 0.02 nm, very close to that of the silicon 7 × 7 adatoms. This means that a carbon enrichment of the silicon outermost layers occurs giving rise to the formation of a Si-C phase different from the √3×√3R30° reconstruction typical of Si terminated hexagonal SiC(0 0 0 1) surface with an atomic distance of 0.53 nm. To explain STM images, we propose a reconstruction model which involves carbon atoms in T₄ and/or S₅ sites, as occurring for B doped Si(1 1 1) surface. Step edges and areas around the silicon surface defects are the first regions involved in the reaction process, which spreads from the upper part of the step edges throughout the terraces. Step edges therefore, progressively flakes and this mechanism leads, for the highest exposures, to the formation of large inlets which makes completely irregular the straight edge typical of the Si(1 1 1)7 × 7 terraces. These observations indicate that there occurs an atomic diffusion like that driving the meandering effect. Finally, the formation of a few crystallites is shown also at the lowest acetylene exposures. This is the first STM experiment showing the possibility to have carbon incorporation in a Si(1 1 1) matrix for higher amounts than expected, at least up to 1/6 of silicon atomic layer.     

Signatures of epitaxial graphene grown on Si-terminated 6H-SiC (0 0 0 1)

Epitaxial graphene layers thermally grown on Si-terminated 6H-SiC (0 0 0 1) have been probed using Auger electron spectroscopy, Raman microspectroscopy, and scanning tunneling microscopy (STM). The average multilayer graphene thickness is determined by attenuation of the Si (L₂₃VV) and C (KVV) Auger electron signals. Systematic changes in the Raman spectra are observed as the film thickness increases from one to three layers. The most striking observation is a large increase in the intensity of the Raman 2D-band (overtone of the D-band and also known as the G′-band) for samples with a mean thickness of more than ∼1.5 graphene layers. Correlating this information with STM images, we show that the first graphene layer imaged by STM produces very little 2D intensity, but the second imaged layer shows a single-Lorentzian 2D peak near 2750 cm⁻¹, similar to spectra acquired from single-layer micromechanically cleaved graphene (CG). The 4-10 cm⁻¹ higher frequency shift of the G peak relative to CG can be associated with charge exchange with the underlying SiC substrate and the formation of finite size domains of graphene. The much greater (41-50 cm⁻¹) blue shift observed for the 2D-band may be correlated with these domains and compressive strain.     

A low temperature surface preparation method for STM nano-lithography on Si(1 0 0)

Registration markers are crucial in connecting scanning tunneling microscope (STM) lithographed nano- and atomic-scale devices to the outside world. In this paper we revisit an ultra high vacuum annealing method with a low thermal budget that is fully compatible with etched registration markers and results in clean 2 × 1 reconstructed Si(1 0 0) surfaces required for STM lithography. Surface contamination is prevented by chemically stripping and reforming a protective silicon oxide layer before transferring the sample to the vacuum system. This allows for annealing temperatures of only 900 °C, where normally carbon contaminants result in the formation of SiC clusters on the surface at annealing temperatures below 950 °C. Reactive ion etched marker structures with an etch depth of 60 nm and a typical lateral dimension of only 150 nm survive a 900 °C flash anneal.     

Nanoscale dislocation patterning in Bi(1 1 1)/Si(0 0 1) heteroepitaxy

Continuous, atomically flat, and epitaxial Bi(1 1 1) films could be grown on Si(0 0 1). The inherent strain of 2.3% between the Bi(1 1 1) and Si(0 0 1) lattices is relieved by the formation of a grating like one-dimensional misfit dislocation array at the heterointerface. The lattice distortions around each dislocation give rise to a pronounced height depression Δh = 0.12 nm of the surface, which results in a spot splitting in low-energy electron diffraction and a height contrast in scanning tunneling microscopy (STM). Using STM surface profiles across these depressions, the Burgers vector of the underlying isolated non-interacting dislocations is estimated to be 0.377 nm. For thicker Bi films the ordering of the dislocation network is increased. This reflects an increase of repulsive interaction between neighboring dislocations.     

Atomic structure of the carbon induced Si(0 0 1)-c(4 × 4) surface

The atomic and electronic structures of the Si(0 0 1)-c(4 × 4) surface have been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). To explain the experimental bias dependent STM observations, a modified mixed ad-dimer reconstruction model is introduced. The model involves three tilted Si dimers and a carbon atom incorporated into the third subsurface layer per c(4 × 4) unit cell. The calculated STM images show a close resemblance to the experimental ones.     

Real time investigation of the growth of silicon carbide nanocrystals on Si(1 0 0) using synchrotron X-ray diffraction

The growth of silicon carbide nanocrystals on Si(1 0 0) is studied by synchrotron surface X-ray diffraction (SXRD) during annealing at high temperature. A chemisorbed methanol monolayer is used as carbon source, allowing to have a fixed amount of carbon atoms to feed the growth. At room temperature, minor changes in the 2 × 1 reconstruction of silicon are observed due to the formation of Si-O-CH₃ and Si-H bonds from methanol molecules. When annealed at 500 °C, carbon incorporation into the silicon leads only to local modifications of the surface structure. Above 600 °C, tri-dimensional silicon carbide nanocrystals growth takes place, together with surface roughening and sharp decrease of domain sizes of the 2 × 1 reconstruction. The different processes taking place at each temperature are clearly distinguished and identified during the real time SXRD measurements.     

Interpretation of initial stage of 3C-SiC growth on Si(1 0 0) using dimethylsilane

The initial stage of cubic silicon carbide (3C-SiC) growth on a Si(0 0 1) surface using dimethylsilane (DMS) as a source gas was observed using scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). It was found that the dimer vacancies initially existing on the Si(0 0 1)-(2 × 1) surface were repaired by the Si atoms in DMS molecules, during the formation of the c(4 × 4) surface. From the STM measurement, nucleation of SiC was found to start when the Si surface was covered with the c(4 × 4) structure but before the appearance of SiC spots in the RHEED pattern. The growth mechanism of SiC islands was also discussed based on the results of RHEED, STM and temperature-programmed desorption (TPD).     

The structures of physisorbed and chemisorbed formic acid on Si(1 1 1)-7 × 7

The reaction of formic acid on Si(1 1 1)-7 × 7 was investigated using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and high-resolution electron energy loss spectroscopy (HREELS). The hydroxyl and carbonyl O 1s core levels of chemisorbed formic acid display chemical shifts of 2.4 and 0.2 eV respectively, compared with those of physisorbed molecules. The HREELS spectra of chemisorbed formic acid show the absence of stretching and bending modes of the O-H bond, the appearance of Si-H (2089 cm⁻¹) and the Si-O (680 cm⁻¹) stretching modes and the retained stretching mode of CO at 1703 cm⁻¹. Our results clearly suggest that formic acid dissociates to form monodentate formate species and H-atom on the adatom-rest atom pair of Si(1 1 1)-7 × 7.     

Effect of different Ge predeposition amounts on SiC grown on Si (1 1 1)

The SiC films were grown by solid source molecular beam epitaxy (SSMBE) on Si (1 1 1) with different amounts of Ge predeposited on Si prior to the epitaxial growth of SiC. The samples were investigated with reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), and X-ray diffraction (XRD). The results indicate that there is an optimized Ge predeposition amount of 0.2 nm. The optimized Ge predeposition suppress the Si outdiffusion and reduce the formation of voids. For the sample without Ge predeposition, the Si outdiffusion can be observed in RHEED and the results of XRD show the worse quality of SiC film. For the sample with excess amount of Ge predeposition, the excess Ge can increase the roughness of the surface which induces the poor quality of the SiC film.     

Buffer layer free large area bi-layer graphene on SiC(0 0 0 1)

The influence of hydrogen exposures on monolayer graphene grown on the silicon terminated SiC(0 0 0 1) surface is investigated using photoelectron spectroscopy (PES), low-energy electron microscopy (LEEM) and micro low-energy electron diffraction (μ-LEED). Exposures to ionized hydrogen are shown to have a pronounced effect on the carbon buffer (interface) layer. Exposures to atomic hydrogen are shown to actually convert/transform the monolayer graphene plus carbon buffer layer to bi-layer graphene, i.e. to produce carbon buffer layer free bi-layer graphene on SiC(0 0 0 1). This process is shown to be reversible, so the initial monolayer graphene plus carbon buffer layer situation is recreated after heating to a temperature of about 950 °C. A tentative model of hydrogen intercalation is suggested to explain this single to bi-layer graphene transformation mechanism. Our findings are of relevance and importance for various potential applications based on graphene-SiC structures and hydrogen storage.     

Selforganized nanopatterning of Si(0 0 1)

The (2 × n) superstructure of Si(0 0 1) consists of elongated (2 × 1) reconstructed stripes separated by a dimer-vacancy line every few nanometers, thus offering a means to obtain a nanopattered Si(0 0 1) surface. Scanning tunneling microscopy (STM) investigations of Si(0 0 1) substrates that were deoxidized at 880-920 °C reveal that the formation of the (2 × n) depends strongly on the Si coverage of the topmost surface layer. It forms only in a narrow coverage window ranging from 0.6 to 0.8 ML. Systematic Monte Carlo simulations by an algorithm that combines the diffusion of monomers and dimers with the simultaneous deposition of Si onto the Si(0 0 1) surface, corroborate the STM results and suggest Si deposition as a viable alternative for introducing dimer vacancies in a well-defined manner.     

Effects of sic buffer layer on the optical properties of ZnO films grown on Si (1 1 1) substrates

ZnO films have been grown by a sol-gel process on Si (1 1 1) substrates with and without SiC buffer layers. The influence of SiC buffer layer on the optical properties of ZnO films grown on Si (1 1 1) substrates was investigated. The intensity of the E₂ (high) phonon peak in the micro-Raman spectrum of ZnO film with the SiC buffer layer is stronger than that of the sample without the SiC buffer layer, and the breadth of E₂ (high) phonon peak of ZnO film with the SiC buffer layer is narrower than that of the sample without the SiC buffer layer. These results indicated that the crystalline quality of the sample with the SiC buffer layer is better than that of the sample without the SiC buffer layer. In photoluminescence spectra, the intensity of free exciton emission from ZnO films with the SiC buffer was much stronger than that from ZnO film without the SiC buffer layer, while the intensity of deep level emission from sample with the SiC buffer layer was about half of that of sample without the SiC buffer layer. The results indicate the SiC buffer layer improves optical qualities of ZnO films on Si (1 1 1) substrates.     

Core level photoemission and STM characterization of Ta/Si(1 1 1)-7 × 7 interfaces

We present the results of scanning tunneling microscopy (STM) and photoemission spectroscopy (PES) of the Ta/Si(1 1 1)-7 × 7 system after deposition of Ta at substrate temperatures from 300 to 1250 K. The coverage of Ta varied from 0.05 up to 2.5 of a monolayer (ML). STM shows that at 300 K and coverage less than 1 ML, a disordered chemisorbed phase is formed. Deposition on a hot surface (above 500 K) produces round 3D clusters randomly distributed on the surface. Cluster height and their diameter are found to change drastically with annealing temperature and the Ta coverage. Analysis of photoemission data of the Si 2p core levels shows that at room temperature and at coverage ?1 ML core level binding energy shifts and intensity variations of Si surface related components are observed, which clearly indicate that the reaction starts already at 300 K. Shifts in the binding energy, changes of the peak shapes and intensity of the Ta 4f doublet at higher temperatures can be explained by the formation of stable silicide on the surface.     

Fabrication and scanning tunneling microscopy studies of the Si(1 1 1)-(√31 × √31)-In surface

We report on the fabrication of single phase of the Si(1 1 1)-(√31 × √31)-In reconstruction surface, observed by scanning tunneling microscopy (STM) at room temperature. By depositing specific amounts of indium atoms while heating the Si(1 1 1)-(7 × 7) substrate at a critical temperature, the single phase of Si(1 1 1)-(√31 × √31)-In surfaces could be routinely obtained over the whole surface with large domains. This procedure is certified by our high-resolution STM images in the range of 5-700 nm. Besides, the high resolution STM images of the Si(1 1 1)-(√31 × √31)-In surface were also presented.     

Engineering of nearly strain-free ZnO films on Si(1 1 1) by tuning AlN buffer thickness

ZnO properties were investigated as a function of AlN buffer layer thickness (0–100 nm) in ZnO/AlN/Si(1 1 1) structures grown by metal organic vapor phase epitaxy. A significant improvement of ZnO film crystallinity by tuning AlN buffer thickness was confirmed by x-ray diffraction, topography and photoluminescence measurements. An optimal AlN buffer layer thickness of 50 nm is defined, which allows for growth of nearly strain-free ZnO films. The presence of free excitons at 10 K suggests high crystal quality for all ZnO samples grown on AlN/Si(1 1 1) templates. The intensities of neutral and ionized donor bound exciton lines are found to correlate with the in-plane and out-of-plane strain in the films, respectively.     

(6 × 2) Reconstruction of the Ag/Si(1 1 1) surface at 77 K

The ground state of the Ag/Si(1 1 1)-(3 × 1) has been investigated by low temperature scanning tunneling microscopy (STM) and density-functional theory. The Fourier transform of the STM image reveals a (6 × 2) reconstruction, which is theoretically found to yield a reconstruction with lower energy than the (3 × 1). The most stable (6 × 2) structural model leads to excellent correspondence between experimental and simulated STM images, and reveals a dimerization of the silver atoms in the channels formed by neighbouring honeycomb Si chains.     

Formation and stabilization of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template

We demonstrate the growth of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template by in situ scanning tunneling microscopy (STM). These clusters form near a dimer row at one side of the half-unit cell (HUC); and with three different equivalent orientations. A cluster model comprising three top layer Si atoms bonded to six Fe atoms at the next layer in the 7 × 7 faulted-half template is proposed. The optimized cluster structure determined by first-principles total-energy calculation shows an inward-shifting of the three center Fe atoms. The clusters and the nearby center-adatoms of the next HUCs appear with a significantly reduced height below bias voltages 0.4 V in high resolution empty-state STM images, suggesting an energy gap opening near the Fermi level at these localized cluster and adatom sites. We explain the stabilization of the clusters on the 7 × 7 template using the gain in electronic energy as the driving force for cluster formation.     

High resolution X-ray diffraction of GaN grown on Si (1 1 1) by MOVPE

High temperature GaN layers have been grown on Si (1 1 1) substrate by metalorganic vapor phase epitaxy (MOVPE). AlN was used as a buffer layer and studied as a function of thickness and growth temperature. The growth was monitored by in situ laser reflectometry. High resolution X-ray diffraction (HRXRD) revealed that optimized monocrystalline GaN was obtained for a 40 nm AlN grown at 1080 °C. This is in good agreement with the results of morphological study by scanning electron microscopy (SEM) and also confirmed by atomic force microscopy (AFM) observations. The best morphology of AlN with columnar structure and lower rms surface roughness is greatly advantageous to the coalescence of the GaN epilayer. Symmetric and asymmetric GaN reflections were combined for twist and stress measurements in monocrystalline GaN. It was found that mosaicity and biaxial tensile stress are still high in 1.7 μm GaN. Curvature radius measurement was also done and correlated to the cracks observations over the GaN surface.