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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

INRIM continuous expansion system as high vacuum primary standard for gas pressure measurements below 9 × 10 Pa

The continuous expansion system can be considered the state of the art for pressure measurement in the ultra high vacuum range. In the last years, at INRIM, a new continuous expansion system was designed, assembled and characterized. The system is the high vacuum primary standard in the pressure range from 1 × 10⁻⁶ Pa to 9 × 10⁻² Pa with relative standard uncertainty ranging from 2.1% at 1 × 10⁻⁶ Pa down to 0.4% at 9 × 10⁻² Pa. The system is based on the passing of a measured gas flow through a fixed and known conductance. The gas flow is generated and measured by a primary gas flowmeter based on the constant-pressure-variable-volume method.In the first part of the paper both a correction for the effect of transitional flow through the orifice and a new analytical evaluation of orifice conductance are presented. In the second part the accuracy of the system and the pressure uncertainty evaluation are described.     

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.     

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.     

Behavior of N atoms after thermal nitridation of Si1 − xGex surface

Behavior of N atoms after thermal nitridation of Si_1 − xGe_x (100) surface in NH₃ atmosphere at 400 °C was investigated. X-ray photoelectron spectroscopy (XPS) results show that N atomic amount after nitridation tends to increase with increasing Ge fraction, and amount of N atoms bonded with Ge atoms decreases by heat treatment in H₂ at 400 °C. For nitrided Si_0.3Ge_0.7(100), the bonding between N and Si atoms forms Si₃N₄ structure whose amount is larger than that for nitrided Si(100). Angle-resolved XPS measurements show that there are N atoms not only at the outermost surface but also beneath surface especially in a deeper region around a few atomic layers for the nitrided Si(100), Si_0.3Ge_0.7(100) and Ge(100). From these results, it is suggested that penetration of N atoms through around a few atomic layers for Si, Si_0.3Ge_0.7 and Ge occurs during nitridation, and the N atoms for the nitrided Si_0.3Ge_0.7(100) dominantly form a Si₃N₄ structure which stably remains even during heat treatment in H₂ at 400 °C.     

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.     

Growth process of CuO(1 1 1) and Cu2O(0 0 1) thin films on MgO(0 0 1) substrate under metal-mode condition by reactive dc-magnetron sputtering

The growth process of CuO and Cu₂O thin films on MgO(0 0 1) substrates by reactive dc-magnetron sputtering was studied by reflection high-energy electron diffraction (RHEED) and atomic-force microscopy (AFM). The RHEED pattern and AFM image showed that (1) three-dimensional Cu(0 0 1) islands grew on MgO under the nonreactive sputtering condition, (2) CuO(1 1 1) was deposited layer by layer on MgO at 400 °C under the reactive sputtering condition, and (3) the film deposited at 600 °C in the initial growth stage was composed of three-dimensional Cu islands because O₂ gas could not be incorporated into them due to the low sticking coefficient of O₂ on MgO under the reactive sputtering condition. The layer-by-layer CuO(1 1 1) thin-film growth process is discussed from the viewpoint that Cu and oxygen species are supplied in stoichiometry onto the MgO substrate to form CuO thin-film crystals while maintaining minimum interfacial energy between CuO and MgO.     

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.     

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.     

Study of GeYSi1 − Y:H films deposited by low frequency plasma

In this work, we report a study of the optical properties measured through spectral transmission and spectroscopic ellipsometry in Ge:H and Ge_YSi_1 − Y:H (Y ≈ 0.97) films deposited by low frequency (LF) PE CVD with hydrogen (H) dilution. The dilution was varied in the range of R = 20 to 80. It was observed that H-dilution influences in a different way on the interface and bulk optical properties, which also depend on incorporation of silicon. The films with low band tail characterized by its Urbach energy, E_U, and defect absorption, α_D, have been obtained in Ge:H films for R = 50 with E_U = 0.040 eV and α_D = 2 × 10³ cm^− 1 (hν ≈ 1.04 eV), and in Ge_YSi_1 − Y:H films for R=75 with E_U = 0.030 eV and α_D = 5 × 10² cm^− 1 (hν ≈ 1.04 eV).     

Study of epitaxial lateral overgrowth of semipolar (1 1 − 2 2) GaN by using different SiO2 pattern sizes

We investigated the growth mode and the crystal properties of lateral epitaxial overgrowth (LEO) semipolar (1 1 − 2 2) GaN by using the various SiO₂ pattern sizes of 6, 8, 10 and 12 μm with the window width of 4.0 μm. By using three-step growth technique, we successfully obtained the fully-coalescenced semipolar (1 1 − 2 2) LEO-GaN films regardless of the SiO₂ pattern sizes. However, the coalescence thickness of LEO-GaN film was decreased with decreasing SiO₂ pattern size, indicating that the coalescence of semipolar (1 1 − 2 2) GaN was easily formed by decreasing the pattern size of SiO₂ mask. The full width at half maximums (FWHMs) of X-ray rocking curves (XRCs) of LEO-GaN films decreased with increasing SiO₂ pattern size. In the pattern size of 4 × 10 μm, we achieved the minimum XRCs FWHM of 537 and 368 arc s with two different X-ray incident beam directions of [1 1 − 2 − 3] and [1 − 1 0 0], respectively. Moreover, the photoluminescence bandedge emission of semipolar (1 1 − 2 2) GaN was 45 times increased by LEO process. Based on these results, we concluded that the LEO pattern size of 4 × 10 μm would effectively decrease crystal defects of semipolar (1 1 − 2 2) GaN epilayer, resulting in an improvement of the optical properties.     

Band structure symmetry effects in Gd multilayers grown on W(1 1 2)

We present evidence of coverage-dependent band structure symmetry effects in scanning tunneling microscopy. We find that the symmetry of the Gd induced band located at about 2 eV below the Fermi energy, at the surface Brillouin zone center, appears to be strongly dependent on the atomic structure in the bilayer regime for Gd on W(1 1 2). Light polarization dependent photoemission from this band indicates more d_z² symmetry character for rectangular Gd structures and rather more d_xzd_yz character for the quasi hexagonal bilayer structure. Band symmetry changes accompanying the structural phase transition from rectangular to quasi-hexagonal Gd bilayer structures are consistent with arguments used to explain the apparent STM corrugation observed for Gd on W(1 1 2) (Surf Sci 520(2002)43).