Electronic structure of Co islands grown on the √3 × √3-Ag/Ge(111) surface

By means of room temperature scanning tunneling spectroscopy (RT STS), we have studied the electronic structure of two different Ag/Ge(111) phases as well as Co islands grown on the √3 × √3-Ag/Ge (111) forming either √13 × √13 or 2 × 2 patterns. The spectrum obtained from 4 × 4-Ag/Ge(111) structure shows the existence of a shoulder at 0.7 V which is also present in the electronic structure of the Ge(111)-c2 × 8 and indicates donation of Ge electrons to electronic states of the Ag-driven phase. However, this fact is not supported by the electronic spectrum taken from the √3 × √3-Ag/Ge (111). The complexity of the Co-√13 × √13 islands bonding with the substrate is mirrored by a large number of peaks in their electronic spectra. The spectra obtained from the Co-2 × 2 islands which had grown on the step differ from those taken from Co-2 × 2 islands located along the edge of the terrace by a number of peaks at negative sample bias. This discrepancy is elucidated in terms of dissimilarities of Co-substrate interaction accompanying Co islands growth on different areas of the stepped surface.     

Germanium adsorption on Ag(111): an AES-LEED and STM study

The adsorption of germanium on Ag(111) has been investigated using Scanning Tunneling Microscopy, Auger Electron Spectroscopy and Low Energy Electron Diffraction. From the shape of the Auger peak-to-peak versus time curves, we deduce that at room temperature the growth mode is nearly layer-by-layer at least for the first two layers. In the sub-monolayer range, the growth starts by the formation of a (mean square root of 3 x mean square root of 3)R30 degrees surface superstructure which is complete at 1/3 monolayer coverage. Beyond this coverage a rectangular c(mean square root of 3 x 7) superstructure is observed. STM images reveal that this last reconstruction is formed by an ordered arrangement of self-assembled Ge tetramers giving rise to a surprising undulation of the surface.     

Immobilization of methylene blue on self-assembled iodine monolayers on gold

The immobilization of methylene blue (MB) on iodine-covered Au(111) is studied by electrochemical techniques, scanning tunneling microscopy (STM), Auger Electron Spectroscopy (AES), and Raman spectroscopy. Results show that MB species are efficiently adsorbed on the square root of 3 x square root of 3 R30 degrees I lattice on Au(111). The electrochemical behavior of the adsorbed MB molecules is reversible, indicating a relatively fast electron transfer from the Au(111) surface to the immobilized MB species through the iodine layer. STM images with molecular resolution are consistent with adsorption of MB dimers on a square root of 3 x square root of 3 R30 degrees I lattice placed atop of the Au(111) substrate. Results are compared to those obtained for MB immobilized on Au(111) covered by S(n) (n = 3-8) surface structures.     

Manifestations of strain-relaxation in the structure of nano-sized Co-2 × 2 islands grown on Ag/Ge(111)-√3 × √3 surface

We have examined strain-relaxation of Co-2 × 2 islands grown on the Ag/Ge(111)-√3 × √3 surface by analyzing scanning tunneling microscopy images. We have found that the Co-2 × 2 islands commonly adopt a more compact arrangement as compared to that of the Ge(111) substrate, however they differ in a degree of an atomic compactness. We have not found a distinct relation between strain-relaxation and the island height. Three groups of islands have been identified upon analyzing a correspondence between strain-relaxation and the island size: (i) small islands (not bigger than 80 nm²) with a high atomic compactness, displaying fixed inter-row distances, (ii) small islands with unfixed distances between atomic rows, and (iii) big islands (bigger than 80 nm²) with fixed inter-row distances, but with a less compact atomic arrangement compared to that of the first two groups. We propose a model to account for the relation between the relaxation and the island size.     

Growth of oriented Ag nanocrystals on air-oxidized Si surfaces: An in-situ reflection high energy electron diffraction study

Growth of Ag nanoislands on air-oxidized Si(001), (111) and (110) surfaces has been investigated by reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and cross-sectional transmission electron microscopy. We have shown that the oriented nanocrystalline Ag, similar to the epitaxial growth of Ag on clean Si surfaces, can be grown on oxide-covered Si surfaces. A thin oxide layer (~ 2-3 nm thick) is formed on ultra-high vacuum (UHV)-cleaned Si surfaces via exposure of the clean reconstructed surface to air. Deposition of Ag was carried out under UHV at different substrate temperatures and monitored by RHEED. RHEED results reveal that Ag deposition at room temperature leads to the growth of randomly oriented Ag islands while, in spite of the presence of the oxide layer between Ag islands and Si, preferred orientations with an epitaxial relationship with the substrate evolve when Ag is deposited at higher substrate temperatures. STM images of the oxidized surfaces, prior to Ag deposition, apparently do not show any order. However, Fourier transforms of STM images show the presence of a short range order on the oxidized surface following the unit cells of the underlying reconstructed Si surface. It is intriguing that Ag nanoislands follow an epitaxial orientational relationship with the substrate in spite of the presence of a 2-3 nm thick oxide layer between Ag and Si. Apparently, the short range order existing on the oxide surface influences the orientation of the Ag nanoislands.     

Size control of Co islands grown on √3 x √3-Ag/Ge(111) surface

The growth of uniform nanostructures requires simple and reproducible ways to control the size. It is found that Co atoms can form two-dimensional structural islands on √3 x √3-Ag/Ge(111) surfaces. Temperature and Co coverage are two factors to modulate the island size. By using scanning tunneling microscopy, the surface structures and morphology for different annealing temperatures and variable Co coverage have been investigated. For 100 degrees C annealing temperature, Co atoms are difficult to condense into structural islands at 0.35 ML whereas several structural Co islands are found at 1.4 ML. This difference is due to the quantity of Co atoms per unit area for forming structural islands. As the temperature increases, Co atoms get more energy to diffuse. Therefore, the average island size increases with rising temperatures until the coverage of 3.5 ML. Yet, the island size stops growing above the coverage of 3.5 ML because of the limitation for the Co covered area. Therefore the Co islands increase their height rather than their size. In addition, the shape of Co islands can also be controlled. It transforms from random shapes to the hexagonal shape with increasing temperature.     

Effect of Si(100)-<Emphasis Type="Italic">c</Emphasis>(4 × 12)-Al and Si(111)-(5.55 × 5.55)-Cu reconstructions on the deposition of cobalt onto silicon surface

The use of surface reconstructions for modifying properties of single crystal silicon substrates with a view to the creation of new nanostructures is a promising direction in the development of nanotechnologies. Systems Si(100)-c(4 × 12)-Al and Si(111)-(5.55 × 5.55)-Cu occupy special positions among stable reconstructions of the silicon surface, which have been recently demonstrated to be promising templates. The adsorption of cobalt on these surfaces at various temperatures has been studied using scanning tunneling microscopy. The room-temperature deposition leads to the formation of a weakly ordered layer of metallic Co with retained initial reconstructions at the Co/Si interface. An increase in the temperature leads to the formation of faceted cobalt silicide islands on both reconstructed surfaces.     

Control of the axial coordination of a surface-confined manganese (III) porphyrin complex

The organization and thermal lability of chloro(5,10,15,20-tetraphenyl porphyrinato)manganese(III) (Cl-MnTPP) molecules on the Ag(111) surface have been investigated under ultra-high vacuum conditions, using scanning tunnelling microscopy, low energy electron diffraction and x-ray photoelectron spectroscopy. The findings reveal the epitaxial nature of the molecule-substrate interface, and moreover, offer a valuable insight into the latent coordination properties of surface-confined metalloporphyrins. The Cl-MnTPP molecules are found to self-assemble on the Ag(111) surface at room temperature, forming an ordered molecular overlayer described by a square unit cell. In accordance with the threefold symmetry of the Ag(111) surface, three rotationally equivalent domains of the molecular overlayer are observed. The primitive lattice vectors of the Cl-MnTPP overlayer show an azimuthal rotation of ±15° relative to those of the Ag(111) surface, while the principal molecular axes of the individual molecules are found to be aligned with the substrate (0(-)11) and ((-)211) crystallographic directions. The axial chloride (Cl) ligand is found to be orientated away from the Ag(111) surface, whereby the average plane of the porphyrin macrocycle lies parallel to that of the substrate. When adsorbed on the Ag(111) surface, the Cl-MnTPP molecules display a latent thermal lability resulting in the dissociation of the axial Cl ligand at ~423 K. The thermally induced dissociation of the Cl ligand leaves the porphyrin complex otherwise intact, giving rise to the coordinatively unsaturated Mn(III) derivative. Consistent with the surface conformation of the Cl-MnTPP precursor, the resulting (5,10,15,20-tetraphenyl porphyrinato)manganese(III) (MnTPP) molecules display the same lattice structure and registry with the Ag(111) surface.     

Crystallographic tilt in GaN-on-Si (111) heterostructures grown by metal–organic chemical vapor deposition

We report on the studies of crystallographic tilt induced by miscut of the Si (111) substrate in GaN-on-Si (111) heterostructures grown by metal–organic chemical vapor deposition. By employing high-resolution X-ray diffraction, we found that the onset of crystallographic tilt occurred at the interface between the AlN nucleation layer and the Si (111) substrate. The orientation of the GaN overlayer always follows that of the AlN nucleation layer irrespective of its quality and miscut of the substrates. The resultant GaN [0002] is tilted toward GaN (11?20) and (10?10) atomic planes for the miscuts of Si (111) toward Si [1?10] and [11?2], respectively. In both cases, the misorientation of GaN (0002), i.e., the tilt of GaN [0002] from the surface normal direction, is in the same direction of the miscut of Si (111). The misorientation angle of the GaN epilayer is generally smaller than the miscut angle of the substrate. However, the crystallographic tilt, i.e., the angle formed between GaN [0002] and Si [111], is always much larger than the Nagai tilt. These observations are attributable to misfit dislocations that are anisotropically generated at the AlN/Si (111) interface. This mechanism is discussed based on recent microscopic observations of in-plane misfit dislocations at the interface near the atomic step edges.     

Zinc porphyrin-driven assembly of gold nanofingers

Nanofingers of gold covered by porphyrins are prepared by a combination of atomic manipulation and surface self-organization. A submonolayer of zinc(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin (ZnTBPP) axially ligated to a self-assembled monolayer of 4-aminothiophenol (4-ATP) on Au(111) is prepared and studied using a combination of ultrahigh vacuum techniques. Under the electric field produced by the STM tip, the relatively weakly bound Au surface atoms along the discommensuration lines become mobile due to the strong bond to 4-ATP, while the tendency of the porphyrins towards self-assembly result in a collective motion of gold clusters. The clusters diffuse onto the surface following well-defined pathways along the [112] direction and then reach the step edges where they assembled, thus forming nanofingers. First-principles density functional theory calculations demonstrate the reduction of the binding energies between the surface gold clusters and the substrate induced by adsorption of thiols. Scanning tunneling microscopy images show assemblies across three adjacent discommensuration lines of the Au(111)-(22 x square root 3) reconstruction, which collectively diffuse along these lines to form islands nucleated at step edges.     

Real-time studies of Ge growth on nanostructured Si substrates

Nanostructured substrates are an interesting path towards the production of quantum dots devoted to microelectronic applications. In this work we report results on the effects of different nanopatterning methods to obtain lateral ordering of Ge islands grown on Si. By using Scanning Tunneling Microscopy we have studied in real-time the wetting layer growth and islands formation on nanopatterned Si substrates at 500 °C. We compare results obtained on Si substrates nanopatterned by using two different techniques: STM lithography and natural patterning induced by surface instabilities such as step bunching. Different issues on both cases have been addressed: substrate preparation, Ge dots placement and growth mode. We have observed that, on Si(001), the Ge islands nucleate near the holes and on Si(111) step bunching can guide the growth of aligned rows of islands.     

Electrical characteristics of Ge-based metal-insulator-semiconductor devices with Ge3N4 dielectrics formed by plasma nitridation

We have fabricated pure germanium nitrides (Ge3N4) using high-density plasma nitridation and investigated electrical properties of Au/Ge3N4/Ge capacitors. We achieved equivalent oxide thickness (EOT) of 1.4 nm, and dielectric constant of Ge3N4 was estimated to be 9.7. The gate leakage current density of 4.3 A/cm2 in the accumulation condition at V(fb)-1 V, where V(fb) is the flatband voltage, was one order of magnitude lower than that of conventional poly-Si/SiO2/Si stacks. The interface state density (D(it)) of Ge3N4/Ge interfaces evaluated by a low-temperature conductance method exhibited a minimum value of 9.4 x 10(11) cm(-2)eV(-1) at E - E(v) = 0.27 eV. Furthermore, the insulating property and interface quality of Ge3N4/Ge system was found to be thermally stable up to 650 degrees C. These results indicate that Ge3N4 is a promising candidate for either a gate insulator or an interfacial layer under high-k dielectrics for Ge-MIS devices.     

Si_3N_4/Si表面Ge生长过程的STM研究

利用STM和LEED分析了Ge在Si3 N4 /Si(111)和Si3 N4 /Si(10 0 )表面生长过程的结构演变。在生长早期 ,Ge在两种衬底表面上都形成高密度的三维纳米团簇 ,这些团簇的大小均在几个纳米范围内 ,并在高温退火时体积增大。当生长继续时 ,Ge的晶体小面开始显现。在晶态的Si3 N4 (0 0 0 1) /Si(111)表面 ,Ge的 (111)晶向的小面生长比其他方向优先。最后在大范围内形成以 (111)方向为主的晶面。相反 ,在非晶的Si3 N4 表面 ,即Si3 N4 /Si(10 0 ) ,Ge晶体的高指数侧面生长较顶面快 ,最终形成金字塔形的岛结构。对这样的表面生长过程进行了探讨并给出了合理的物理解释     

Micromethod for the investigation of the interactions between DNA and redox-active molecules

A novel microscale and surface-based method for the study of the interactions of DNA with other redox-active molecules using DNA-modified electrodes is described. The method is simple, convenient, reliable, reagent-saving, and applicable for DNA studies, especially those involving microsamples. Information such as binding site size (s, in base pairs), binding constant (K), ratio (K0x/KRed) of the binding constants for the oxidized and reduced forms of a bound species, binding free energy (delta Gb), and interaction mode, including changes in the mode of interaction, and "limiting" ratio K0x0/KRed0 at zero ionic strength can be obtained using only 3-15 micrograms of DNA samples. The method was developed using [Co(Phen)3]3+/2+ (Phen = 1,10-phenanthroline)/double-stranded DNA (dsDNA)-modified gold electrodes and [Co(bpy)3]3+/2+ (2,2'-bipyridyl)/dsDNA-modified gold electrodes as model systems. For the [Co(Phen)3]3+/2+/dsDNA-modified gold electrode system, a K2+ of (2.5 +/- 0.3) x 10(5) M-1 and an s of 5 bp were obtained in 5 mM pH 7.1 Tris-HCl buffer solution containing 50 mM NaCl. For [Co(bpy)3]3+/2+/dsDNA-modified gold electrodes, K3+ and s values of (1.3 +/- 0.3) x 10(5) M-1 and 3 bp, respectively, were obtained. While the s values are consistent with those reported in the literature obtained by solution methods, the K values are almost an order of magnitude larger. A transition in the nature of the interaction between dsDNA and [Co(Phen)3]3+/2+, from electrostatic to intercalative with increasing ionic strength, was found in our studies. Negative values of delta E0' for [Co(bpy)3]3+/2+ bound to dsDNA suggest that its interaction with dsDNA is predominantly electrostatic over the ionic strength range of 5-105 mM. The "limiting" ratio K3+0/K2+0 of 22 obtained for [Co(Phen)3]3+/2+ bound to dsDNA at zero ionic strength suggests that electrostatic interactions are predominant over intercalative ones under these limiting conditions. The ratio for [Co(bpy)3]3+/2+ of 16 also indicates that the 3+ form binds to dsDNA more strongly than the 2+ form at zero ionic strength. For [Co(Phen)3]3+/2+/single-stranded DNA (ssDNA)-modified gold electrodes, the nonuniformity of the surface structure of ssDNA-modified gold electrodes greatly complicates the analysis. A system consisting of a dsDNA-modified gold electrode and [Co(tppz)2]3+/2+ (tppz = tetra-2-pyridyl-1,4-pyrazine) was studied by this method, with a K2+ value of (5 +/- 1) x 10(5) M-1 and an 8 value of 7 bp being obtained.     

Thermal evolution of the morphology of Ni/Ag/Si(111)-√3 × √3 surface

The temperature-driven changes in morphology of the interface formed by room temperature (RT) deposition of Ni atoms onto an Ag/Si(111)-√3 × √3 surface were investigated by scanning tunneling microscopy. Roughly 70% of Ni deposition diffused into bulk substrate within the temperature range between RT and 573 K. The images as obtained after annealing up to 670 K correspond to the formation of nano-sized islands of nickel silicides. Two types of islands, large triangular islands typical of the whole range of applied coverage, and smaller islands of different shapes, coexist at Ni coverage higher than 1 monolayer. Annealing above 870 K led to the formation of a 7 × 7 phase in coexistence with small 5 × 5 domains at the expense of a complete disappearance of the √3 × √3 phase. Also, formation of Ni,Si alloy was observed at the temperature, along with segregation of bulk-dissolved Ni species onto the surface.     

Magnetic Ordering and Exchange Interaction of Laves Compounds Sm0.9Pr0.1（Fe1—xCox）2

Structure,magnetic properties and magnetostriction of Sm0.9Pr0.1(Fe1-xCox)2 compounds have been investi-gated by means of X-ray diffraction,a.c. initial susceptibility, extracting sample magnetometer,Mossbauer spec-troscopy and standard strain gauge techniques.The lattice parameter a of the MgCu2-type Laves compounds Sm0.9Pr0.1(Fe1-xCox)2 decreases nonlinearly with increasing Co concentration,deviating from the Vegardˊs law.Curie temperature Tc increases initially from 668 K for x=0 to 694 K for x=0.2 and then decreases to 200 K for x=1.0.The saturation magnetization Ms at temperatures 1.5K, 77K and 300K have the same variation tendency as the composition dependence of Curie temperature,in consistence with rigid-band model.The easy magnetization direction(EMD) od Sm0.9Pr0.1(Fe1-xCox)2 lies along [111] direction in the range x≤0.6,and changes to [110] for x=0.8 ,while Sm0.9Pr0.1(Fe1-xCox)2 stays in the paramagnetic state at room temperature.The composition dependence of the average hyperfine field,Hhf,demonstrates a similar variation tendency as that of the saturation magnetiza-tion Ms and Curie temperature Tc .The spontaneous magnetostricton λ111 increases with increasing Co content.The saturation magnetostriction λs decreases monotonically with increasing x,which is caused by the increase of magnetostriction constant λ100 with opposite sign to that of λ111.A two-sublattice model has been proposed to understand the intermediate region between the [111]and [110] spin configurations ,which can also be used to explain the temperature dependence of magnetization.     

缓冲层Ta对退火Co/Cu/Co薄膜微观结构和界面互扩散的影响

用直流磁控溅射法在Si/SiO₂基底上制备了Co/Cu/Co薄膜和加入缓冲层的Ta/Co/Cu/Co薄膜,用扫描电子显微镜、原子力显微镜、X射线衍射和俄歇电子能谱研究了薄膜的微观结构、表面形貌、织构和界面互扩散现象。结果表明:退火后薄膜中均存在{111}和{002}衍射峰,加入缓冲层Ta后,Co/Cu/Co薄膜的衍射峰强度明显增强,并存在较强的{111}纤维织构,薄膜表面孔洞及粗糙度大幅减小。退火后薄膜界面处产生互扩散现象,层状结构被破坏。缓冲层Ta提高了薄膜与基底材料间的润湿性,可有效缓解界面互扩散现象。     

Auger analysis of thick silver films and of silver layers grown on copper

A sensitive new measure of the line broadening of an Auger doublet, called the R factor, was applied to the growth of (1) thick silver films and (2) thin silver films on a thick copper substrate. It is shown that periodic fluctuations in R with thickness give evidence for monocrystalline layer growth while the absence of such fluctuations is characteristic of polycrystalline growth. By combining measurements of the Auger amplitude and the R factor as functions of the silver film thickness for silver films deposited onto copper, it was shown that a Stranski-Krastanov growth mode is characteristic of the epitaxial growth of Ag(111) on Cu(111) at 210°C. The initial layer in this case is two (111) atom spacings thick on which high flat islands form. Continued growth of these islands is shown to take place by two-atom-thick flat islands.     

The Dislocation Structure of a Single-Crystal γ + γ′ Two-Phase Alloy After Tensile Deformation

The dislocation structures of an industrial single-crystal γ + γ′ two-phase alloy DD3 after tensile deformation from room temperature to 1273K were studied by transmission electron microscopy. The strength of this alloy decreased with an increase in the temperature, and showed a strength peak at 1033K. At room temperature, the dislocations shearing the γ′ particles were found to be 1/3<112> partial dislocations on the dodecahedral slip system <112>{111}. Some dislocation pairs on the cubic <110>{100} system that blocked the glide of dislocations were found at a medium temperature of 873K. As a result, dislocation bands were formed. Shearing of γ′ particles by 1/3<112> partial dislocations on the dodecahedral slip system <112>{111} was also found at this temperature. At the peak temperature of 1033K, because of the strong interaction between dislocations on the {111} and {100} planes, the extent of dislocation bands with high dislocation densities was extensive. The 1/3<112> partial dislocations on the dodecahedral slip system <112>{111} also existed. When the temperature reached the high temperature of 1133K, the range of dislocation bands was limited. The γ′ particles were sheared by <110> dislocation pairs on the octagonal <110>{111} system and the cubic <110>{100} system. At 1273K, the regular hexagonal dislocation networks were formed in the γ matrix and at the γ/γ′ interface. The Burgers vectors of the network were found to be b₁ = 1/2[110], b₂ = 1/2[1–10], b₃ = [100], and the last one was formed by the reaction of b₁ + b₂ → b₃. Dislocations shearing the γ′ particles were found to be <110> dislocation pairs on the octagonal system <110>{111} and cubic slip system <110>{100} at 1273K.