ML Au Research Articles

One-dimensional structures on Si(111) observed by RHEED

A vicinal Si(111) surface covered with a sub-monolayer amount of Au has been studied by reflection high energy electron diffraction (RHEED). At the coverage equal to 0.20 ML of Au, the RHEED pattern showed a reconstruction in the form of regularly distributed monoatomic steps. In order to determine the shape of the surface unit cell, several RHEED patterns obtained within a wide range of the electron incident angle were recorded and analyzed. The surface was composed of flat Si(111) terraces separated by single Si(111) steps. This surface was identified as an ideal Si(755) facet with Au atoms that form quasi-one-dimensional chains running along a Si[11̄0] azimuth. Regularly spaced steps were stable within the investigated range of temperatures from 100 to 470 K.

Gold-induced ordering on vicinal Si(111)

A vicinal Si(111) surface covered with a sub-monolayer amount of Au has been studied by RHEED. For a sample misoriented by 8.46° towards the [1̄1̄2] azimuthal direction at a coverage equal to 0.20 ML of Au, the RHEED pattern showed a regular distribution of monoatomic steps with the staircase width equal to 18.88 ± 0.33 A ̊ . Au-induced order was identified with a (755) facet of Si. Regularly spaced steps were stable within the investigated range of temperatures from 130 to 470 K. Upon further increase of the Au-coverage the periodicity was disturbed and at a coverage of about 0.50 ML of Au onset of (√3 × √3)R37° reconstruction was visible.

Gold-induced one-dimensional ordering on vicinal Si(111)

Behaviour of a sub-monolayer Au-coverage on the vicinal Si(111) has been studied by RHEED. For the sample misoriented by 8.46 ° towards the [1̄1̄2] azimuthal direction at the coverage equal to 0.20 ML of Au, the RHEED pattern showed almost perfect distribution of the monoatomic Si(111) steps with the staircase width equal to 18.52 ± 0.66 A ̊ . Au-induced order was identified with the (755) facet of Si with a one-dimensional chains of Au. Upon further increase of the Au deposition this regular distribution was gradually disturbed and at the coverage of about 0.50 ML of Au onset of an (√3 × √3)R 30 ° reconstruction was visible.

Reactive intermixing at the AuInP(100) interface: new photoemission results

Using photoelectron emission at different photon energies we have studied the interaction of gold (up to ten monolayers nominal coverage) deposited at room temperature and at T = 110 K on InP(100)-(4 × 2) surfaces. From the analysis of core-level and valence-band spectra, we derive information on interfacial reactions and overlayer growth modes. At T = 110 K interface disruption is small and we observed a rather homogeneous gold overlayer with only a minute concentration of In after deposition of 10 ML of Au. At T = 300 K severe disruption occurs, and up to 10 ML of Au an In concentration of at least 10% is clearly identified. Annealing to T = 560 K leads to the formation of In x Au 1− x clusters with x up to 0.3.

Spin-resolved photoemission by circularly polarized light from Au on Ag(111): experiment and theory

The electronic structure of 1 ML and 2 ML Au on Ag(111) has been studied experimentally and theoretically by spin-, angle- and energy-resolved photoemission in the highly symmetrical set-up of normally incident circularly polarized VUV radiation and normal electron emission. The measured spectra agree rather well with their relativistically calculated counterparts and can be consistently interpreted with the aid of spin- and layer-resolved densities of states. The spin polarization of spectral peaks directly reflects the double group symmetry type of the the corresponding initial states. For 1 ML (2 ML) Au on Ag(111) there are two (four) Au-derived states, which are confined to the adlayers (“film states”). For 2 ML Au, the “film-state” photoemission intensity depends strongly on the photon energy, which is understood as an interference effect. Since the agreement with the experimental photoemission spectra is improved by calculations assuming an additional Ag layer on top, and since the Au-derived experimental features are substantially broadened, an intermixing of Au and Ag is very likely.

Ru-promoted alloying of Au and Cu ultrathin films: photoemission studies

For the co-adsorption of Au and Cu on Ru(001), the Ru substrate promotes the alloying of the noble metals by lowering the activation energy required for the process. High-resolution photoemission spectroscopy with synchrotron radiation and X-ray photoelectron spectroscopy have been used to investigate the core-level binding energy shifts and the valence-band behaviour in a series of Au and Cu overlayers on Ru(001): 1 ML Au, 1 ML Cu, 1 ML Au and 1 ML Cu, 3 ML Au and 1 ML Cu, as well as 1 ML Au and 3 ML Cu. A monolayer of Au in contact with Ru(001) has a Au 4 f 7 2 binding energy ∼0.23 eV higher than that of Au surface atoms on pure Au but still ∼0.1 eV smaller than that of the bulk Au. In contrast a monolayer of Cu bonded to Ru(001) exhibits a Cu 2 p 3 2 binding energy ∼0.1 eV lower than that of a Cu surface atom on pure Cu. Most of the co-deposited AuCu systems formed ultrathin alloys on top of the Ru(001) surface. The strain and electronic perturbations induced by the Ru substrate on the Au and Cu overlayers favour the formation of AuCu alloys. Binding energy shifts of the Au 5d band components and the accompanying change to the overall d-band width were found to be very sensitive to the alloying process, as has been observed in bulk AuCu alloys. The order of deposition, annealing, and the temperature of the substrate were all found to have an effect on the formation of AuCu ultrathin alloys. These ultrathin alloys exhibit Au 4 f 7 2 and Cu 2 p 3 2 binding energies that are significantly different from those of bulk AuCu alloys as a consequence of the interactions with the Ru(001) substrate.

Epitaxial growth of silver on an Si( 111) √3 × √3 Au surface at room temperature

Growth of Ag on an Si(111) √3 × √3 Au surface was investigated by reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). At an initial stage of Ag deposition, the √3 × √3 structure fades into the diffuse background. Subsequently, a faint pattern from Ag(111) appears at 0.8 monolayers (ML, 1 ML = 1.39 × 10 15 atoms cm −2). At 7 ML growth the RHEED pattern shows a flat Ag(111) surface, although RHEED oscillations are hardly observed during growth. For Ag deposition onto an amorphous-like surface covered by 0.6 ML Au on the √3 × √3 surface, RHEED intensity oscillations are observed during growth. At an initial stage of Ag growth on the surface, extra streaks are observed in the RHEED pattern, continuing up to about 6 ML deposition. During the appearance of the extra streaks, the Auger spectrum of Si-LVV splits into two peaks at 88 and 92 eV. Small Si-LVV, 92 eV, and Au-NOO peaks remain up to 10 ML and the oscillation amplitude of the RHEED intensity is not damped up to 10 ML coverage. In the case of Ag deposition onto the √3 × √3 Au surface, since no segregation of Si and Au atoms is observed, it is concluded that the silicide and Si segregations induce two-dimensional nucleation on the surface.

Silver growth on [formula omitted] surfaces at low temperature

Silver growth on the Si(111) 3 × 3 - Ag ( 3 - Ag) surface is investigated by RHEED. For Ag deposition on the 3 -Ag surface at 150 K, a 21 × 21 structure appears at a coverage of 0.2 monolayer (ML: 1 ML = 7.8 × 10 14/cm 2). Analyzing the RHEED rocking curves of this structure, we have concluded that Ag atoms are embedded at silicon trimer sites of the 3 -Ag surface. This surface is very similar to the structure of the 21 × 21 surface formed by 0.14 ML Au deposition. In further deposition another 3 structure reappears, and subsequently one-dimensional (1D) islands of Ag are formed along the [11̄0] direction. Besides 1D islands, 3D Ag crystals are also formed on the surface with orientations {100} Ag|{111} Si and {110} Ag|{111} Si. When annealed at room temperature the 1D islands are rearranged into small 3D islands. We discuss the origin of these 1D islands.

Structure and properties of Au overlayers on Ru(100)

The influence of substrate morphology is critical to an understanding of hetero-epitaxial growth in metal-metal systems. We have studied vapour deposition of Au on Ru(100) which differs significantly from the extensively studied Au/Ru(001) system. For example, Au deposition at 300 K results in non-pseudomorphic, simultaneous multilayer (SM) growth in contrast to the layer-by-layer pseudomorphic growth characteristic of Au on Ru(001). SM growth is demonstrated by AES, TDS, XPS, ARUPS and LEED; the latter shows development of a series of (1 × n) patterns (n = 3, 4, 5,...) from 0.5 ML Au upwards, indicating formation of close-packed Au chains along the [010] Ru direction. This is consistent with attractive Au-Au interactions, confirmed by a complete lineshape analysis of the Au thermal desorption spectra which shows an 85 kJ/mol increase in the desorption activation energy in the 0–0.3 ML coverage region. At coverages ≳ 15 ML the Au overlayer adopts an fcc Au(111)-like arrangement oriented with the Au[110] axis parallel to Ru[010]. For coverages < 10 ML, LEED, AES and XPS data obtained after annealing to 1075 K indicate that the observed SM growth is not a result of kinetic limitations and that differences in behaviour between this system and Au/Ru(001) can be attributed to the anisotropic nature of the Ru(100) substrate. Thermal desorption spectra show a multilayer desorption peak only after ∼ 2 ML Au deposition, despite the SM growth mode, indicating quasi-melting and spreading of the overlayer at temperatures close to desorption. Au coverages> 8 ML undergo a complete structural rearrangement involving after annealing to ∼ 1050–1100 K. The rearrangement was indicated by development of a new LEED pattern and changes in the He I valence band spectra

Interfacial strain-enhanced reconstruction of Au multilayer films on Rh(100).

The Au(100) surface reconstructs to a contracted, hexagonal-close-packed (hcp) surface layer at or below 373 K. This study investigates the consequence of film-substrate interactions, in particular lattice mismatch, on Au(100) reconstruction by looking at the Au/Rh(100) interface which has a 7% mismatch strain. Monte Carlo simulations using the Rafii-Tabar-Sutton-Chen parametrization of the Finnis-Sinclair pseudo-many-body potential find that Au films on Rh(100) exhibit a structural transition to a hcp-type adlayer at significantly lower temperatures than pure Au(100). Furthermore, the extent of the hcp regions (observed as striped domains) is temperature and coverage dependent. For instance, for coverages up to 1.0 ML of Au, the structure of the adlayer is pseudomorphic, while multilayer Au films exhibit this reconstruction. These results lead to the conclusion that lattice mismatch provides an additional driving force for Au(100) reconstruction in multilayer Au films.

Photoemission study of Au, Ge, and O2 deposition on NH4F etched Si(111)

We have studied the interaction of a metal Au, a semiconductor Ge, and a nonmetal O2, with the NH4F etched Si(111) surface with photoemission spectroscopy. Two components were present in Si 2p core-level spectra from the H-terminated surface. We observed the flatband condition from the as-etched, n-type, Si(111) surface. We performed stepwise depositions of Au and measured the band bending with photoemission spectroscopy. The Fermi level pinned near midgap as Au was deposited onto the as-etched surface. After the deposition of 1 ML of Au, a Au-silicide layer formed. This interfacial component indicated that the passivating H layer was compromised. As the Au coverage was increased, layers of pure Au formed between the bulk silicon and the Au-silicide layer. The observed behavior was nearly identical to that of Au deposition on the Si(111) 7×7 surface [Phys. Rev. Lett. 67, 2187 (1991)]. Next, we tested the ability of the monohydride layer to sustain surfactant assisted growth of Ge. Ge islanding was observed at 400 °C indicating that good surfactant growth was not obtained. This was consistent with the recent results of Sakai and Tatsumi [Appl. Phys. Lett. 61, 52 (1994)] who reported that the surface roughness was nearly the same for surfaces grown with or without H at this temperature. Although the monohydride layer was not a good surfactant for the Si(111) surface at this temperature, further study at different temperatures is needed to determine the ability of the ideal monohydride layer to act as a surfactant. Finally, we observed no oxidation of the as-etched surface at room temperature upon exposure to molecular oxygen.

Atomic depth distribution analysis of Ag and Au on Si(111) during epitaxial growth by total reflection angle X-ray spectroscopy

When an electron beam is incident upon a surface, the electron trajectories near the surface in the sample change drastically depending upon the change in glancing angle θ g. The X-ray emission from surface atoms changes markedly depending on their atomic depth distribution. Thus, by measuring the θ g dependence of the X-ray emission, the composition in depth direction near the surface can be estimated. This method is very sensitive and a depth resolution better than 1 ML can be attained. By applying this method, it was found that when Ag was deposited on a Si(111)-α-√3 × √3-Au structure at room temperature, Ag grew on Au in the normal fashion; on the other hand, when 1 ML of Au was deposited on a Si(111)-√3 × √3-Ag structure at room temperature, the sites of Ag and Au atoms exchanged with each other and Ag atoms moved to the topmost layer of the surface. At further Au deposition, Au atoms moved continuously below the Ag layer, resulting in a condition as if Ag atoms were floating. These are new epitaxial growth modes, wherein the former should be called the “substitution atom growth mode” and the latter the “floating atom growth mode”. These phenomena were confirmed also by UHV-SEM observation. The results are further explained by Monte Carlo electron trajectory simulation.

Studies on 2D Au-Cu binary adsorbates on Si(111) surfaces by means of combined LEED/AES/RBS techniques

Isothermal annealing at 170°C and subsequent isochronal annealing for 5 min of Au-Cu binary adsorbates on the Si(111) surface have been studied by means of combined LEED/AES/RBS techniques. In cases of deposition of Cu onto the 5 × 1-Au/Si(111) surface as well as of Au onto the quasi-5 × 5-Cu/Si(111) surface it is found that a √3 × √3 structure is formed by the initial isothermal annealing for 3 min at Au coverages between 0.35 and 0.60 ML and that for prolonged isothermal annealing the 3 × 3 structure persists keeping the initial Au coverage, while the Cu content decreases to a certain equilibrium coverage corresponding to a the equilibrium Cu-to-Au ratio of 1 4 − 1 3 . The stabilization of the 3 × 3 structure, which at the Au/Si(111) surface below 0.60 ML of Au, is ascribed to coexistence of Cu. Achievement of the equilibrium Cu coverage is concluded to certify formation of a 2D ordered alloy of Au-Cu on the Si(111) surface. The dynamic behavior of Au and Cu binary adsorbates at the Si(111) surface and the atomic arrangement of Au and Cu at the 3 × 3 surface are also discussed.

Initial stages of the formation of the Au/GaAs(001) interface: A low-energy ion scattering study

Submonolayer coverages of Au on GaAs(001) substrates prepared by various ex situ chemical cleaning procedures using low-energy ion scattering techniques are investigated. This research is compared with previous results for 1 ML of Au deposited on Si(111). A comparison of the energy spectra showed substantial broadening in the Li+ backscattering ion peak from Au deposited on GaAs, as compared to the Au on Si. Li+ impact collision ion scattering spectroscopy (ICISS) results showed that the gold formed an ordered overlayer above the Si surface, which was thermally stable to 500 °C. Even for depositions on room-temperature GaAs(001), shadowing of Au by substrate atoms at low scattering angles showed that Au penetrated below the GaAs surface. The estimate of the average penetration depth into the room-temperature substrate is two to three atomic planes of the GaAs crystal. An ICISS experiment showed that the Au atoms sampled by the ion beam were not ordered on lattice sites, but appeared to occupy multiple and/or random positions within the substrate. Upon annealing to 300 °C, the gold atoms were more ordered, but did not penetrate significantly further into the GaAs. At higher annealing temperatures, the Au either diffused further into the substrate or became incorporated in islands, and occupied favored subsurface sites.

The spin-orbit splitting in the Si bandstructure measured by means of spin-resolved photoemission

Abstract Spin-, angle- and energy-resolved photoemission with circularly polarized synchrotron radiation from BESSY has been applied to deduce information about the electronic structure of silicon in the Λ-direction. We used the highly symmetrical experimental set-up of normal radiation incidence and normal electron emission. This method, which has in the past several times been applied to metals and rare gas crystals with medium and high atomic numbers, has thus been applied now to a semiconductor with a low atomic number ( Z = 14). For photon energies between 5.5 and 10 eV we find a peak V in the photoemission spectra which corresponds to a transition from the valence bands in the bulk band structure of silicon in the Λ-direction. The electrons which contribute to the peak are spin polarized due to a spin-orbit splitting Δ E so smaller than 50 meV. Adsorption of 0.5 ML Ag or Au induces band bendings and work function changes and yields a sharper peak and less secondary electron background than the clean Si(1 1 1)-surface. For both the clean and adsorbate covered surface and k ⊥ away from Γ we determine Δ E so from a spin analysis of peak V and find values between 20 and 35 meV with typical errors of 10 meV. These values in Λ-direction are smaller than the Δ E so -value of 44 meV at the Γ-point [1, 2].

Transmission channeling study of the Au/Si(100) interface

The Au/Si (100) interface has been studied by the transmission channeling technique. The results show that at the very beginning the deposited Au atoms are partially registered with the 〈100〉 axis in the substrate crystal and are partially randomly distributed across the 〈100〉 channel. In addition to the Au–Si atom interaction, the Au–Au interaction already plays an important role in the submonolayer region, which causes the Au atoms to be more and more randomly distributed as monolayer coverage is approached. Cluster formation is a possible mechanism for the interface growth in this region. Through the transmission Si surface peak, evidence has been found that room-temperature intermixing takes place at ∼4 ML Au coverage. The observed initiation threshold appears to favor an electron screening model over a diffusion model to explain room-temperature Au–Si intermixing.