Study of Ag/Si(111) submonolayer interface: I. Electronic structure by angle-resolved UPS

Angle-resolved ultraviolet photoelectron spectra have been measured for well defined Ag/Si(111) submonolayer interfaces of (1) $\text{Si(111)(}\text{3}\text{×}\text{3}\text{)R30°-Ag}$, (2) “Si(111)(6 × 1)-Ag”, and (3) Ag/Si(111) as deposited at room temperature. Non-dispersive and very narrow (FWHM ～ 0.4–0.5 eV) Ag 4d derived peaks are found at 5.6 and 6.5 eV below the Fermi level for surface (1) and at 5.3 and 6.0 eV for surface (2). Dispersions of sp “binding” states in the energy range between E_F and Ag 4d states have been precisely determined for surface (1). Electronic structures similar to those of the Ag(111) surface, including the surface state near E_F, have been observed for surface (3).     

Gold at the Si(111)-Pd interface; modification of atomic interdiffusion

We present the results of an angle integrated photoemission (hv = 21.2eV) and Auger investigation of the effect of Au interlayers on the growth of the Si(111)-Pd interface. Gold does not remain totally blocked at the interface with increasing amounts of deposited Pd. As a consequence the same amount of Au (around 6 monolayers) acts as a promotor of Si-Pd mixing at low Pd coverages (below about 8 monolayers) and as an inhibitor at higher Pd coverages. The effect is dependent on the amount of Au so that Au at 10 monolayers is an inhibitor independent of the Pd coverage. The implications on the interface growth mechanism are discussed.     

Electronic structure of Si (111) interface between diamond and hexagonal phases

The electronic structure of the Si (111) interface between the diamond and hexagonal wurtzite structure is studied by first-principles LCAO method. No gap interfacial states or resonances are found; instead, the calculated density of states curve shows an oscillatory redistribution of electron states and a reduction of band gap of about 10%. It is concluded that the electronic structure of this interface can be approximated as the average of that of the bulk diamond and the hexagonal Si.     

Local atomic environment of Si suboxides at the SiO2/Si(111) interface determined by angle-scanned photoelectron diffraction

Local environments of Si suboxides at the interface between a thermally grown SiO2 film and Si(111) were studied by angle-scanned photoelectron diffraction. Si 2p core-level spectra containing chemically shifted components were recorded. The components were deconvoluted by least squares fitting and assigned to different Si oxidation states. The obtained diffraction patterns of the various suboxides exhibit different features. Comparison of these patterns with multiple scattering calculations including a multipole R-factor analysis shows that a simple chemical abrupt interface model describes well the environment of the suboxides and indicates ordered SiO2 close to the interface.     

Growth mode and interface structure of Ag on the HF-treated Si(111):H surface

A growth mode and interface structure analysis has been performed for Ag deposited at a high temperature of 300°C on the HF-treated Si(111):H surface by means of medium-energy ion scattering and elastic recoil detection analysis of hydrogen. The measurements show that Ag grows in the Volmer-Weber mode and that the Ag islands on the surface are epitaxial with respect to the substrate. The preferential azimuthal orientation is A-type only when Ag is deposited slowly. The interface does not reconstruct to the √3 × √3-Ag structure, which is normally observed for Ag deposition above 200°C on the Si(111)7 × 7 surface, but retain bulk-like structure. The presence of hydrogen at the interface is demonstrated after deposition of thick (1100 Å) Ag films. However, the amount of hydrogen at the interface is not a full monolayer. This partial desorption of hydrogen from the interface explains why the Schottky barrier heights of Ag/Si(111):H diodes are close to those of Ag/Si(111)7 × 7 and Ag/Si(111)2 × 1.     

Electronic structure of Ag adsorbed on Si(111); experiment and theory

Experimental results (low energy electron loss spectroscopy) and band structure calculations relating to the early stages of Ag growth on a Si(111) surface are presented. Crystallography and thermal desorption kinetics studies of this interface, previously published, gave rise to the following conclusions. At room temperature and below 200°C, two-dimensional (2D) (111) epitaxial layers develop on top of a first ordered layer (√3 × √3), while at higher temperatures three-dimensional (3D) clusters develop over this first layer. Low energy electron loss experiments were performed at various surface coverages θ. They display different evolutions according to the growth modes. For the 2D epitaxial growth, one observes the disappearance of the peaks characteristic of a Si surface and the onset of Ag induced peaks located at 7.1 and 4.6 eV at completion of the √3 layer. These peaks narrow and shift to the bulk Ag excitation energies at 7.5 and 4 eV when a second Ag layer is deposited. In order to explain these results, we present a theoretical calculation of the electronic density of states of the interface using a tight binding approximation. This calculation accounts for the development of the Ag d band from the √3 coverage range to the (111) epitaxial Ag planes. The evolution of the spectra when θ is increased is discussed in view of these results.     

The interaction of Ag with Si(111)

Deposits of Ag on Si(111), at room temperature, have yielded a linear Auger signal-time characteristic to a gradient break point at (7.6 ± 0.9) × 10¹⁴ atoms ofAg cm^?2, which is very close to the Si surface state density of (8–10) × 10¹⁴ cm^?2, and which supports a Stranski-Krastanov growth mechanism. Analysis of the Auger spectra at the monolayer end point revealed a new peak at 82 ± 1 eV. This peak is believed to arise from an Auger process involving an induced Ag-Si interface state. A model is proposed for this state arising from the chemisorption of Ag on Si.     

Study of Ag/Si(111) submonolayer interface: II. Atomic geometry of Si(111) (√3 × √3 )R30°-Ag surface

Final state diffraction of Ag 3d X-ray photoelectrons from the Si(111) (√3 × √3)R30°-Ag surface has been measured. From a kinematical analysis of the diffraction patterns, it is found that a buried honeycomb framework of Ag atoms is formed on the surface with lateral displacement of the first Si layer.     

Initial growth at the F16CoPc/Ag(111) interface

An extended, combined (STM/STS)–(UPS/XPS) study was carried out towards a comprehensive understanding of the mechanism responsible for the F₁₆CoPc/Ag(111) interface formation. The evolution of the morphology and the electronic properties at the organic/metal interface is investigated for the early-stage growth of the ultrathin molecular film. Template-guided molecular structures are formed via a strong molecule–substrate interaction which leads to the formation of a new adsorption-induced interface state close to the Fermi energy (E_F). With increasing the thickness the molecular coupling to the metal surface states becomes less important while the more dominant molecule–molecule interaction governs the second layer formation. The quenching of the interface state upon increasing the molecular thickness, together with the changes observed in the Co 2p and F 1s core levels, is explained based on a charge transfer at the interface and a corresponding charge redistribution within the molecular ligand. A detailed “picture” of the energy level alignment close to E_F is achieved by correlating the high resolution UPS and highly localized STS data.     

Electronic structure of Si(111) surfaces

We report on new angle-resolved photoemission studies of Si(111) 2 × 1 and 7 × 7 surfaces. The emission from the 2 × 1 surface shows much structure. For normal emission the energy positions are insensitive to the photon energy in the range 19–27 eV. The emission has been interpreted as a probe of the surface density of states, SDOS, including both surface states, resonances and bulk-like states. The SDOS was also calculated as a function of parallel momentum k_∥ for a model of the Si(111) 2 × 1 surface obtained from energy minimization considerations. We identify emission from the dangling bond band, which has a positive dispersion of 0.6 eV, and also emission from surface resonances which have some character of the compressed and stretched back bonds. There are also other predicted surface resonances that correspond to experimental peaks which have not been identified in previous work. Except for the dangling bond band, the surface resonances are limited in k_∥ space, so that it is not possible to follow these resonance bands over all angles. Maximum intensity for the normal emission from the dangling bond is obtained at 23 eV, while the emission from the lowest s-like states monotonically increases towards 30 eV photon energy. When annealing the cleaved 2 × 1 surface to the 7 × 7 reconstructed surface, the spectra broaden significantly. The intensity of the dangling bond decreases and we see a very small metallic edge.     

Electronic properties of cleaved Si(111) upon room-temperature deposition of Au

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and photoemission yield spectroscopy (PYS) measurements are performed on a set of 2 × 1 reconstructed silicon (111) surfaces with different bulk dopings as a function of gold coverage θ, from zero to a few hundred monolayers, obtained by UHV evaporation on a sample kept at room temperature. Our measurements show the formation of an Au-Si alloy with the first two monolayers of gold deposit which induces a decrease of the ionization energy Φ by about 0.15 eV while no variation of the work function is observed. In the effective density of states, the double structure related to the 2 × 1 reconstruction is then replaced by a single peak at ? 0.4 eV below the valence band edge. At larger coverages, the Au-Si alloy remains on top of a gold layer which forms an abrupt interface with the Si substrate.