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.     

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.     

Reconstructed structures of nanosized co islands on Ag/Ge(111) mean square root of 3 x mean square root of 3 surfaces

Structural evolution of Co/Ag/Ge(111) at high temperatures was studied by using scanning tunneling microscopy and low energy electron diffraction. The mean square root of 3 x mean square root of 3-Ag layer between the substrate Ge( 11) and Co adatoms can avoid the formation of Co-Ge compounds below 800 K. The Co atoms nucleate to form islands where mean square root of 13 x mean square root of 13 or 2 x 2 reconstructions were observed after annealing between 373 K and 737 K. The mean square root of 13 x mean square root of 13 structure with mirror symmetry relative to [-211], [11-2], and [1-21] axes was observed for 1-2 layer Co islands. Co islands with over 2 layers appear 2 x 2 structure. All reconstruction structures of the nano-sized Co islands and substrate Ag/Ge(111) mean square root of 3 x mean square root of 3 surface were analyzed using the atomic hard sphere model. The bright protrusions of these reconstructions all sit in the centers of Ag or Ge trimers, which were predicted to have maximum binding energy.     

Adsorption/desorption kinetics of Na atoms on reconstructed Si (111)-7 × 7 surface

Self-assembled nanostructures on a periodic template are fundamentally and technologically important as they put forward the possibility to fabricate and pattern micro/nano-electronics for sensors, ultra high-density memories and nanocatalysts. Alkali-metal (AM) nanostructure grown on a semiconductor surface has received considerable attention because of their simple hydrogen like electronic structure. However, little efforts have been made to understand the fundamental aspects of the growth mechanism of self-assembled nanostructures of AM on semiconductor surfaces. In this paper, we report organized investigation of kinetically controlled room-temperature (RT) adsorption/desorption of sodium (Na) metal atoms on clean reconstructed Si (111)-7 × 7 surface, by X-ray photoelectron spectroscopy (XPS). The RT uptake curve shows a layer-by-layer growth (Frank-vander Merve growth) mode of Na on Si (111)-7 × 7 surfaces and a shift is observed in the binding energy position of Na (1s) spectra. The thermal stability of the Na/Si (111) system was inspected by annealing the system to higher substrate temperatures. Within a temperature range from RT to 350 °C, the temperature induced mobility to the excess Na atoms sitting on top of the bilayer, allowing to arrange themselves. Na atoms desorbed over a wide temperature range of 370 °C, before depleting the Si (111) surface at temperature 720 °C. The acquired valence-band (VB) spectra during Na growth revealed the development of new electronic-states near the Fermi level and desorption leads the termination of these. For Na adsorption up to 2 monolayers, decrease in work function (−1.35 eV) was observed, whereas work function of the system monotonically increases with Na desorption from the Si surface as observed by other studies also. This kinetic and thermodynamic study of Na adsorbed Si (111)-7 × 7 system can be utilized in fabrication of sensors used in night vision devices.     

The behavior of Co atoms on Si (111)-7 × 7 surfaces at low temperatures

The behavior of Co atoms on Si (111)-7 × 7 surfaces at low temperatures was studied by using a variable-temperature scanning tunneling microscopy (VT-STM). Co atoms deposited on Si (111)-7 × 7 surfaces are randomly adsorbed at 100 K. Co atoms start to react with adatoms of Si (111)-7 × 7 surfaces at temperatures between 126 K and 130 K. The reaction transfers the bright dots of Co atoms to dark dots under the STM observation of negative bias. Analysis of the reaction occurrence sites and comparing with the results of room temperature deposition shows that the Co atoms tend to diffuse and react with the adatoms of Si (111)-7 × 7 surfaces at the center sites of unfaulted half unit cell (UHUC) at higher temperatures.     

Oxygen treatment effects on properties of MgO thin films grown on single-crystalline diamond (1 0 0)

Oxygen post-treatment effects on the electronic structure and electrical properties of MgO films grown on homoepitaxial single-crystalline (1 0 0) diamond have been studied. MgO films examined were deposited at room temperature (RT) using an electron beam evaporator and were subsequently either annealed at 573-773 K for 12 h in oxygen ambient or treated by O₂ plasma for 10-40 min. RT resistivities remarkably increased after the O₂ annealing and plasma treatment, indicating that the post treatments play an essential part on the formation and positioning of bandgap states. Cathodoluminescence (CL) spectra had a broad band feature in a wavelength region from 360 to 530 nm, which were decomposed to several peaks originating mainly from the oxygen-vacancy-related F and F⁺ centers and the interstitial vacancies of MgO film. A prominent rectifying behavior of I-V property was observed for a Au/MgO/p-diamond layered structure. Based on temperature dependences of the electrical properties in a temperature region from RT to 600 K, the electrical conduction mechanism in the MgO films is discussed in relation to polaron-related conduction as well as the ionic conduction.     

Surface phase transitions upon reduction of epitaxial WO3(1 0 0) thin films

The surface structure and morphology of WO₃(1 0 0) thin films were studied using scanning tunneling microscopy (STM) and low-energy electron diffraction. The films experienced a net-reducing environment when annealed in oxygen at 800 K leading to surface phase transitions from p(2×2) to p(4×2), and from p(4×2) to a mix p(4×2) and p(3×2). Increasing the annealing temperature to 830 K in ultra-high-vacuum (UHV) led to a fully p(3×2) reconstructed surface. Continued UHV annealing above 800 K caused (1×1) islands to appear on the p(3×2) surface and the film color to darken. Eventually, prolonged UHV annealing led to a (1×1)-terminated surface with straight steps oriented in [0 0 1] or [0 1 0] directions due to crystallographic shear planes. The randomly spaced steps on the (1×1) surface indicated variations in the local stoichiometry in the film. An added row model proposed for the p(4×2) structure is also shown to be consistent with the p(3×2) structure. The formation of the p(4×2) structure from the p(2×2) structure was attributed to W⁵⁺ migration into the bulk to form the troughs between the added rows. Reduction of the p(4×2) structure caused the troughs to narrow rather than deepen, suggesting that W⁵⁺ or added row surface diffusion competes with migration of reduced W ions into the bulk when the p(3×2) structure forms. The STM images also show evidence that the (1×1) structure forms through coalescence of the added rows.     

Structure of Pt3Co(1 1 0)c(2×4)-O surface studied by scanning tunneling microscopy

A Pt₃Co(1 1 0)c(2×4)-O surface has been investigated by scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. At a very initial oxidation stage exposed at 500°C, creation of missing and/or added row structures running to the [0 0 1] direction on clean Pt₃Co(1 1 0)2×1 surface was imaged from the steps. The surface is fully covered by a well-ordered c(2×4) structure at 2 L oxygen exposure. Atomic-resolution STM image shows the added row-type anti-phase Co-O zigzag chains along the [0 0 1] direction. Based on the images, the structure model for the c(2×4) surface was suggested as a first oxidized layer, which comes from the chemical reaction forming stoichiometric Co monoxide. Further oxygen exposure above 5 L, Co-O clusters imaged to large dots were formed on the surface with the size of about 5-7 Å.     

Ordered Ge-dot arrays in a Si-waveguide for 1.5 µm detectors

In the present work we demonstrate a Si rib waveguide photodiode based on a vertical p-i-n junction with Ge islands operating in the near infrared spectral region around λ = 1.55 µm at room temperature. The device employs Ge islands as infrared absorbing medium, in which the electron-hole recombination is indirect in k- and real space resulting in long radiative life times. The 5 µm × 230 nm waveguide-like design of the photodiode ensures, on the one hand, a sufficient light path in order to get complete incident light absorption and, on the other hand, efficient and fast separation of the electron-hole pairs by using thin p-i-n junction perpendicular to the direction of light propagation. The Ge islands are precisely positioned in a 2-dimensional grid of periodicity at 460 nm matching the maxima of a standing wave, which can form in the waveguide, thus further enhancing the efficiency of the device.     

Crystal orientation changes of Ag thin films on the Si(111) substrate due to tribo-assisted recrystallization

Crystal orientation changes of Ag thin films due to the tribo-assisted recrystallization have been studied using grazing incidence X-ray diffraction with synchrotron radiation. After preparation of an Si(111) √3 × √3-Ag surface, a 5-nm-thick Ag film was deposited on the surface at the substrate temperature of 303 K in an ultra-high vacuum chamber. The friction experiments were carried out using a diamond pin-on-plate type tribometer just after the Ag deposition in the same UHV chamber. We found that the coefficient of friction of the Ag films on the Si(111) √3 × √3-Ag surface decreases from 0.07 to 0.03, with increasing reciprocal sliding cycles. In synchronization with the coefficient change, Ag{100} grains are gradually disappearing. As a result, the Ag{111} grains cover the entire surface after 50 sliding cycles. Moreover, we found that the domain size of the Ag{111} grains increases with increasing reciprocal sliding cycles by measuring the rocking curve width. These results directly show that the Ag(111) plane is the sliding plane of friction and the coefficient of friction of Ag films is determined by the fraction of the Ag(111) grains in the Ag films. Moreover, to clarify the reaction between the Ag film and the Si substrate due to the tribo-assisted recrystallization, the substrate strain has been studied by an extremely asymmetric X-ray diffraction technique using synchrotron radiation.     

Ordered mixed surface structures formed by coadsorption of dissimilar metal atoms on Cu(0 0 1)

We have found that various ordered mixed surface structures are formed by coadsorption of two dissimilar metal atoms on Cu(0 0 1) at room tepmerature, using low-energy electron diffraction (LEED) I-V analysis. As coadsorbates, we employed Mg, Bi, Li and K, and surface structures formed by the coadsorption systems of (Mg, Li), (Mg, K) and (Mg, Bi) are presented. A tensor LEED analysis provided detailed geometries of the coadsorbates and the substrate surface. It was found that the surface structures in the above three coadsorption sytems exhibit the restructuring of the Cu(0 0 1) surface. The phase separation into individual adsorbates does not take place, implying that some additional stabilization arises. We demonstrate two origins for the stabilization of the ordered mixed surface structures on Cu(0 0 1). Structures and features formed by the individual adsorption of Mg, Bi, Li and K atoms on Cu(0 0 1) are described first, then those of (2√2×√2)R45°-Mg,Li, (√5×√5)R26.7°-Mg,K, c(2×2)-Mg,Bi, and c(6×4)-Mg,Bi structures formed by the coadsorption are presented. We consider on the basis of the determined structural parameters the question why ordered mixed surface structures are formed instead of the phase separation.     

Rapid solidification and magnetism characteristics of Co-11.8 at%Gd and Co-7.5 at%Gd dual-magnetic ribbons at different wheel speeds

The rapidly solidified ribbons of Co-7.5 and 11.8 at%Gd alloys were obtained by the melt spinning technique. The magnetic properties were measured and the effects of the wheel speed and the heat treatment on the coercivity were examined. SEM and EDS results show that the microstructures of melt-spun ribbons vary at different wheel speeds and after the heat treatment (1073 K, 30 min), and the soft magnetic properties of annealed ribbons are better than those of quenched ones. For Co-11.8 at%Gd ribbons, the maximum saturation magnetization reaches to 65.11 emu/g at the wheel speed of 20 m/s, while the minimum value of coercivity is 75.32 Oe. For Co-7.5 at%Gd ribbons, the minimum value of coercivity is 43 Oe in the annealed ribbons at 30 m/s and the maximum value of saturation magnetization is 106.93 emu/g from the annealed ribbons at 40 m/s. Theoretical analysis indicates that the exchange coupling length of (Co) and Co₁₇Gd₂ phases is 65.3 nm, and the exchange coupling coefficient of Co-7.5 and 11.8 at%Gd ribbons is in the range of 0.023–0.089. The exchange coupling effect in Co-11.8 at%Gd ribbons is stronger than that in Co-7.5 at%Gd ribbons.     

Microstructural properties of Co thin films grown on p-GaAs (1 0 0) Substrates

Microstructural properties of Co thin films grown on p-GaAs (1 0 0) substrates at room temperature by ion beam-assisted deposition were investigated. An atomic force microscopy image showed that the root mean square of the average surface roughness of the Co film was 32.2 Å, and X-ray diffraction and selected area diffraction pattern measurements showed that Co film layers grown on GaAs (1 0 0) substrates were polycrystalline. A bright-field transmission electron microscopy image showed that the Co/p-GaAs (1 0 0) heterointerface grown at room temperature was sudden. These results provide important information on the microstructural properties for Co thin films grown on p-GaAs (1 0 0) substrates at room temperature.     

Structural properties of the formation of yttrium germanides thin films on the Si(111) surface

We have performed first principles total energy calculations to investigate the deposit of yttrium digermanide on the Si(111) surface. We have used the periodic density functional theory as implemented in the Quantum-ESPRESSO package. For the adsorption of a monolayer of yttrium digermanide on the Si(111)-(1 × 1) surface, we have found that the most stable geometry corresponds to a configuration with Y atoms occupying the T4 site above a second layer Si atom, and with a Ge bilayer on top of the structure. The atomic structure of the Ge bilayer is similar to that of Si (Ge) in the bulk but rotated 180° with respect to the crystal. For the three dimensional growth of a few layers of yttrium digermanide on Si(111) we have considered a hexagonal structure with (√3 × √3) periodicity, similar to the one found in the growth of few layers of YSi₂ on Si(111): graphite-like Ge planes (with vacancies) intercalated with yttrium planes. As in the case of a single layer of YGe₂, there is a formation of a Ge bilayer on top of the structure. In this case, the Ge_down atoms of the bilayer, which are on top the vacancies, move down towards the vacancy, while Ge atoms in the graphitic layer, which are below the Ge_up atoms of the bilayer, are displaced towards the vacancy.     

Surface structure of Au/InSb(0 0 1) system investigated with scanning force microscopy

Dynamic force microscopy and Kelvin probe force microscopy (KPFM) have been used to study 0.2 ML Au deposited on clean c(8×2) InSb(0 0 1) surface. Rectangular islands of typical size of 9 nm across have been observed. Upon annealing at 650 K these islands preserve their initial shape. KPFM has shown that the islands are made of material chemically different from that of the surrounding substrate surface. However, the change of LEED pattern to c(4×4) strongly suggests conversion of substrate surface between the islands from In-rich to Sb-rich probably due to alloying of surface indium with gold in the islands.     

A study on phase relations and texture of Co1 − xPtx (0.09 < x < 0.86) nanowire arrays

Based on Rietveld refinement of X-ray diffraction patterns, the phase structure and microstructural parameters of Co_1 − xPt_x nanowires are determined for a range of Pt content. The phase structure of the as-deposited Co_1 − xPt_x(0.09 < x < 0.86) nanowire arrays changes progressively from hcp ε-Co to a mixture of the hcp ε-Co and fcc α-Co,Pt solid solution and finally to pure fcc Co,Pt solid solution with Pt content increasing . Moreover, the texture parameter P₍₁₁₁₎ has a maximum value with Pt content of 50% confirmed by the (111) pole figure measurement. It is suggested that this contributes to enhance magnetocrystalline anisotropy, resulting in a relatively high squareness and coercivity for the nanowires.     

Theoretical investigation of atomic structure and electronic properties of Ca/Si(110)-(2 × 1) reconstruction

We have presented first-principles total-energy calculations for the adsorption of Ca metals onto a Si(110) surface. The density functional method was employed within its local density approximation to study the atomic and electronic properties of the Ca/Si(110) structure. We considered the (1 × 1) and (2 × 1) structural models for Ca coverages of 0.5 monolayer (ML) and 0.25 ML, respectively. Our total-energy calculations indicate that the (1 × 1) phase is not expected to occur. It was found that Ca adatoms are adsorbed on top of the surface and form a bridge with the uppermost Si atoms. The Ca/Si(110)-(2 × 1) produces a semiconducting surface band structure with a direct band gap that is slightly smaller than that of the clean surface. One filled and two empty surface states were observed in the gap; these empty surface states originate from the uppermost Si dangling bond states and the Ca 4 s states. It is found that the Ca-Si bonds have an ionic nature and complete charge being transferred from Ca to the surface Si atoms. Finally, the key structural parameters of the equilibrium geometry are detailed and compared with the available results for metal-adsorbed Si(110) surface, Ca/Si(001), and Ca/Si(111) structures.     

Microstructure tuning of epitaxial BaTiO3 − x thin films grown using laser molecular-beam epitaxy by varying the oxygen pressure

Microstructural properties are found to be variant in the BaTiO_3 − x films grown on SrTiO₃(001) substrate under various oxygen pressures from 2 × 10^− 2 Pa to 2 × 10^− 5 Pa by laser molecular-beam epitaxy. Transmission electron microscopic studies reveal that the predominant defects in the films change from threading dislocations into (111) planar defects (i.e. stacking faults and nanotwins) by lowering the oxygen pressure. High density of these defects was observed in the BaTiO_3 − x film prepared at the oxygen pressure of 2 × 10^− 5 Pa, which shows metallic behavior. The relationships between oxygen pressure, microstructure, and electrical properties are established on the basis of oxygen deficiency. The formation of nanotwins in highly oxygen-deficient BaTiO_3 − x epitaxial thin films results from accommodating excess oxygen vacancies induced by lowering oxygen pressure.     

Structural study of CH3S self-assembled monolayers on Au(111)

We present preliminary results of an experimental investigation of ordered phases of CH₃S chemisorbed on Au(111). The self-assembled monolayer has been grown by dosing dimethyl disulfide in ultrahigh vacuum at different substrate temperatures between 200 K and 320 K and following different protocols. The monolayers have been characterized by means of low-energy He atom scattering with time-of-flight detection in a temperature range between 150 K and 300 K. The observed diffraction patterns show that the main periodicity is well-described by the (3 × 4) overlayer of the Au(111) lattice, coexisting with the (√3 × √3) periodicity, in agreement with previous results obtained by means of low-energy electron diffraction.     

Removal of stacking faults in Ge grown on Si through nanoscale openings in chemical SiO2

Nucleation and eventual coalescence of Ge islands, grown out of 5 to 7 nm diameter openings in chemical SiO₂ template and epitaxially registered to the underlying Si substrate, have been shown to generate a low density of threading dislocations (?10⁶ cm^− 2). This result compares favorably to a threading dislocation density exceeding 10⁸ cm^− 2 in Ge films grown directly on Si. However, the coalesced Ge film contains a relatively high density of stacking faults (5 × 10⁷ cm^− 2), and subsequent growth of GaAs leads to an adverse root-mean-square roughness of 36 nm and a reduced photoluminescence intensity at 20% compared to GaAs grown on Ge or GaAs substrates. Herein, we find that annealing the Ge islands at 1073 K for 30 min before their coalescence into a contiguous film completely removes the stacking faults. However, the anneal step undesirably desorbs any SiO₂ not covered by existing Ge islands. Further Ge growth results in a threading dislocation density of 5 × 10⁷ cm^− 2, but without any stacking faults. Threading dislocations are believed to result from the later Ge growth on the newly exposed Si where the SiO₂ has desorbed from areas uncovered by Ge islands. The morphology and photoluminescence intensity of GaAs grown on the annealed Ge is comparable to films grown on GaAs or Ge substrates. Despite this improvement, the GaAs films grown on the annealed Ge/Si exhibit a threading dislocation density of 2 × 10⁷ cm^− 2 and a minority carrier lifetime of 67 ps compared to 4 to 5 ns for GaAs on Ge or GaAs substrates. A second oxidation step after the high temperature anneal of the Ge islands is proposed to reconstitute the SiO₂ template and subsequently improve the quality of Ge film.