Structural Features of Boron-Doped Si（113） Surfaces Simulated by ab initio Calculations

Based on ab initio calculations, boron-doped Si（113） surfaces have been simulated and atomic structures of the surfaces have been proposed. It has been determined that surface features of empty and filled states that are separately localized at pentamers and adatoms indicates a low surface density of B atoms, while it is attributed to heavy doping of B atoms at the second layer that pentamers and adatoms are both present in an image of scanning tunnelling microscopy. B doping at the second layer should be balanced by adsorbed B or Si atoms beside the adatoms and inserted B interstitials below the adatoms.     

One-dimensional diffusion of vacancies on an Sr/Si(100)-c(2×4) surface

An Sr/Si(100)-c(2×4) surface is investigated by high-resolution scanning tunneling microscopy(STM) and scanning tunneling spectroscopy(STS).The semiconductor property of this surface is confirmed by STS.The STM images of this surface shows that it is bias-voltage dependent and an atomic resolution image can be obtained at an empty state under a bias voltage of 1.5 V.Furthermore,one-dimensional(1D) diffusion of vacancies can be found in the room-temperature STM images.Sr vacancies diffuse along the valley channels,which are constructed by silicon dimers in the surface.Weak interaction between Sr and silicon dimers,low metal coverage,surface vacancy,and energy of thermal fluctuation at room temperature all contribute to this 1D diffusion.     

Surface diffusion of Si,Ge and C adatoms on Si （001） substrate studied the molecular dynamics simulation

Depositions of Si, Ge and C atoms onto a preliminary Si (001) substrate at different temperatures are investigated by using the molecular dynamics method. The mechanism of atomic self-assembling occurring locally on the flat terraces between steps is suggested. Diffusion and arrangement patterns of adatoms at different temperatures are observed. At 900 K, the deposited atoms are more likely to form dimers in the perpendicular [110] direction due to the more favourable movement along the perpendicular [110] direction. C adatoms are more likely to break or reconstruct the dimers on the substrate surface and have larger diffusion distances than Ge and Si adatoms. Exchange between C adatoms and substrate atoms are obvious and the epitaxial thickness is small. Total potential energies of adatoms and substrate atoms involved in the simulation cell are computed. When a newly arrived adatom reaches the stable position, the potential energy of the system will decrease and the curves turns into a ladder-like shape. It is found that C adatoms can lead to more reduction of the system energy and the potential energy of the system will increase as temperature increases.     

The non-destructive threshold of the graphite surface by STM in the ultra-fast pulse mode

In this paper single ultra-fast voltage pulses are introduced to the Pt/Ir tip of a scanning tunnelling microscope (STM), and the non-destructive threshold of the graphite surface is studied systematically in a wide range of pulse durations (from 10$^{4}$ to 8\,ns). Considering the waveform distortion of the pulses at the tunnelling region, this paper gives the corrected threshold curve of pulse amplitude depending on pulse duration. A new explanation of threshold power has been suggested and fits the experimental results well.     

Formation of graphene on Ru（0001） surface

We report on the formation of a graphene monolayer on a Ru(0001) surface by annealing the Ru(0001) crystal. The samples are characterized by scanning tunnelling microscopy (STM) and Auger electron spectroscopy (AES). STM images show that the Moir\'{e} pattern is caused by the graphene layer mismatched with the underlying Ru(0001) surface and has an $N\times N$ superlattice. It is further found that the graphene monolayer on a Ru(0001) surface is very stable at high temperatures. Our results provide a simple and convenient method to produce a graphene monolayer on the Ru(0001) surface, which is used as a template for fabricating functional nanostructures needed in future nano devices and catalysis.     

Structural transformation of Ge dimers on Ge（001） surfaces induced by bias voltage

Scanning tunnelling microscopy is utilized to investigate the local bias voltage tunnelling dependent transformation between （2×1） and c（4×2） structures on Ge（001） surfaces, which is reversibly observed at room temperature and a critical bias voltage of -0.80 V. Similar transformation is also found on an epitaxial Ce islands but at a slightly different critical bias voltage of -1.00V. It is found that the interaction between the topmost atoms on the STM tip and the atoms of the dimers, and the pinning effect induced by Sb atoms, the nacancies or the epitaxial clusters, can drive the structural transformation at the critical bias voltage.     

Formation mechanism of Ge nanocrystals embedded in SiO2 studied by fluorescence x-ray absorption fine structure

This paper reports that the Ge nanocrystals embedded in SiO2 matrix are grown on Si（100） and quartz-glass substrates, and the formation mechanism is systematically studied by using fluorescence x-ray absorption fine structure （XAFS）. It is found that the formation of Ge nanocrystals strongly depends on the properties of substrate materials. In the as-prepared samples with Ge molar content of 60%, Ge atoms exist in amorphous Ge （about 36%） and GeO2 （about 24%） phases. At the annealing temperature of 1073 K, on the quartz-glass substrate Ge nanocrystals are generated from crystallization of amorphous Ge, rather than from the direct decomposition of GeO2 in the as-deposited sample. However, on the Si（100） substrate, the Ge nanocrystals are generated partly from crystallization of amorphous Ge, and partly from GeO2 phases through the permutation reaction with Si substrate. Quantitative analysis reveals that about 10% of GeO2 in the as-prepared sample are permuted with Si wafer to form Ge nanocrystals.     

IN SITU STM STUDY OF THERMAL ANNEALING OF Au THIN FILMS:AN INVESTIGATION ON DECAY OF NANOMETER Au CLUSTERS AND 2D ISLANDS

An in situ, ultrahigh vacuum scanning tunneling micrmcopy(UHV STM) study of thermal annealing of gold thin films is presented in this paper. The gold thin films was heated and annealed in the UHV chamber in the temperature range From room temperature to maximum of 300℃, while a consecutive STM imaging was performed on the surface of the gold films during the heating and annealing. The STM results showed that the surface corrugation changes became more apparent after the temperature increased above 100℃, whereas much smoothened surface and large Au (111) crystalline terrace(>200nm) formed at temperature of 160℃ of above. Betides the surface morphology change, our images clearly revealed the melting of multilayer gold clusters and shrinking of monolayer gold islands in a nanometer scale. It was shown that the decay of the gold clusters and islands constitute the microscopic processes contributing to the thermal activated surface morphology change. A classical theory of mass flow kinetics was adopted in analyzing the decay processes. The results showed that surface diffusion is the dominate mechanism in the thermal annealing of the gold thin films. This study presents the first microscopic investigation of thermal annealing processes of metal thin films observed by in situ and real-time STM.     

Fabrication and measurement of optical characterization of one dimensional photonic crystal with defect

Numerical calculations based on the transfer matrix method are carried out, and the results of band gap with resonance peaks are obtained. The electron beam lithography technology （EBL） and induction coupling plasma （ICP） etching are used to make the photonic crystal （PC） structures, and from several scanning electron microscope images, the PC structures are observed with features closing to the design. In order to create the tiny PC structures in the right places of the waveguide by the EBL technology at different time, some alignment markers are deposited on the chip, which are made of gold that deposited on titanium for its good adhesion to the underlying Si. An optical testing bed is designed for measurement of the optical characterization of PC structures. Through the analysis of the measured data, a △λ value of 0.8 nm is obtained and for the centre frequency of 1547 nm, a very high quality factor value of 1933 can be obtained. The 3-rim difference represents only a 0.2% error from the theoretical centre.     

Cu原子掺杂调控三聚氰胺在Au(111)表面自组装结构

The doping effect of Cu on the self-assembly film of melamine on an Au(111) surface has been investigated with scanning tunneling microscopy (STM). The evaporated Cu adatoms occupy the positions underneath the amino groups and change the hydrogen bonding pattern between the melamine molecules. Accordingly, the self-assembly structure has changed stepwise from a well-defined honeycomb into a track-like and then a triangular structure depending on the amount of Cu adatoms. The interaction between Cu adatom and melamine is moderate thus the Cu adatoms can be released upon mild heating to around 100℃. These findings are different from previous observations of either the coordination assembly or the physically trapped metal adatoms.     

Ge在Si(111)7×7表面的选择性吸附

利用超高真空扫描隧道显微镜研究了室温条件下Ge在Si(111)7×7表面上初期吸附过程.在Ge所形成团簇中存在一个临界核.这些Ge团簇的吸附中心总是在三个增原子所围成的区域中.它们的电子结构具有类似半导体的性质,即其局域态密度在远离费米面的能级处很大,而在费米面附近的能级处非常小. 关键词： 扫描隧道显微镜 Si(111)7×7表面 Ge团簇     

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.     

Early stages of vanadium deposition on Si(1 1 1)-7 × 7

We investigate the low-coverage regime of vanadium deposition on the Si(1 1 1)-7 × 7 surface using a combination of scanning tunnelling microscopy (STM) and density-functional theory (DFT) adsorption energy calculations. We theoretically identify the most stable structures in this system: (i) substitutional vanadium atoms at silicon adatom positions; (ii) interstitial vanadium atoms between silicon adatoms and rest atoms; and (iii) interstitial vanadium - silicon adatom vacancy complexes. STM images reveal two simple vanadium-related features near the Si adatom positions: bright spots at both polarities (BB) and dark spots for empty and bright spots for filled states (DB). We relate the BB spots to the interstitial structures and the DB spots to substitutional structures.     

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.     

Scanning tunnelling microscopy imaging and modification of hydrogen-passivated Ge(100) surfaces

Scanning tunnelling microscopy (STM) study and modification of hydrogen (H)-passivated Ge(100) surfaces have been investigated. Thermal oxidation procedures were used to minimise surface roughness. Ge samples were passivated in HF solution after thermal oxidation. STM and atomic force microscope (AFM) imaging showed that, using HF etching after thermal oxidation, we can obtain a natural H-passivatedtopographically and chemically flat Ge(100) surface. The root-mean-square (rms) roughness ofa H-passivatedGe(100) surface measured both by STM and AFM is less than 2 ?. Electric properties of H-passivatedGe(100) surfaces were studied by scanning tunnelling spectroscopy (STS) in nitrogen ambient. STS showed that the H-passivated Ge surfaces were not pinned. Modification on H-passivated Ge(100) surfaces was carried out using STM by applying an electric voltage between the sample and tip in air. Modified features were characterised by STM and AFM imaging. On the H-passivated Ge(100) surfaces, stable, low-voltage, nanometer-scale modified features can be produced.     

Electronic stabilization of the Si(111)5 × 2-Au surface: Pb and Si adatoms

Using scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES) and first-principles density functional theory (DFT), we study the structural and the electronic properties of the Si(111)5 × 2-Au surface decorated with Pb adatoms. The STM topography data reveal that Pb adatoms form a similar superstructure to that observed in the case of Si adatoms on a bare Si(111)5 × 2-Au surface. The DFT calculations show that preferential adsorption sites of Pb atoms are located near the double Au chain. Bias dependent STM topography and spectroscopy together with the DFT calculations allow us to distinguish Pb from Si adatoms. Both the Si and Pb adatoms modify the electronic properties in the same way, which confirms the electronic origin of the stabilization of the surface.     

Scanning tunneling microscopy of N2H4 on silicon surfaces

The chemistry of N₂H₄ on Si(100)2 × 1 and Si(111)7 × 7 has been studied using scanning tunneling microscopy. At low coverages on Si(100)2 × 1 at room temperature the adsorption sites are distributed randomly on the surface and are imaged as dark spots in the dimer row by the STM. Upon annealing the substrate at 600 K, both isolated reaction products, as well as clusters of reaction products are formed on the surface. The STM images show that the majority of the isolated reaction products are adsorbed symmetrically across the dimers. Based on previous HREELS data, these are most likely NH_x groups. However, the clusters are not well resolved. Because of this we speculate that they are not simply symmetrically adsorbed NH_x groups, but likely have a more complicated internal structure. At higher coverages, the STM images show that the predominant pathway for adsorption is with the N---N bond parallel to the surface, in agreement with HREELS studies of this system. On Si(111)7 × 7, the molecule behaves in a manner which is similar to NH₃. That is, at low coverages the molecule adsorbs preferentially at center adatoms due to the greater reactivity of these sites, while at higher coverages it also reacts with the corner adatoms.     

Structure determination of the Si3N4/Si(111)- (8 x 8) surface: a combined study of Kikuchi electron holography, scanning tunneling microscopy, and ab initio calculations

A comprehensive atomic model for the reconstructed surface of Si3N4 thin layer grown on Si(111) is presented. Kikuchi electron holography images clearly show the existence of adatoms on the Si3N4(0001)/Si(111)-(8x8) surface. Compared with the ab initio calculations, more than 30 symmetry-inequivalent atomic pairs in the outmost layers are successfully identified. Scanning tunneling microscopy (STM) images show diamond-shaped unit cells and nine adatoms in each cell. High-resolution STM images reveal extra features and are in good agreement with the partial charge density distribution obtained from total-energy calculations.     

Origin of the stability of Ge(105) on si: a new structure model and surface strain relaxation

The structure of Ge(105)-(1 x 2) grown on Si(105) is examined by scanning tunneling microscopy (STM) and first-principles calculations. The morphology evolution with an increasing amount of Ge deposited documents the existence of a tensile surface strain in Si(105) and its relaxation with increasing coverage of Ge. A detailed analysis of high-resolution STM images and first-principles calculations produce a new stable model for the Ge(105)-(1 x 2) structure formed on the Si(105) surface that includes the existence of surface strain. It corrects the model developed from early observations of the facets of "hut" clusters grown on Si(001).