Study of c(2 × 2)-MnAu(0 0 1) layers on Mn(0 0 1) by means of scanning tunneling microscopy/spectroscopy

Intermixing, growth, geometric and electronic structures of gold films grown on antiferromagnetic stacking body-centered-tetragonal manganese (0 0 1) films were studied by means of scanning tunneling microscopy/spectroscopy at room temperature in ultra-high vacuum. We found stable ordered c(2 × 2)-MnAu(0 0 1) alloy layers after depositing Au on pure Mn layers. Since at the fourth layer (5 × 23)-like Au reconstruction appears instead of the c(2 × 2) structure and local density of states peaks obtained on the c(2 × 2)-MnAu surface disappear, pure Au layers likely grow from the fourth layer.     

Electronic properties of Au nano-particles supported on stoichiometric and reduced TiO2(1 1 0) substrates

Growth modes and electronic properties were analyzed for Au nano-particles grown on stoichiometric and reduced TiO₂(1 1 0) substrates by medium energy ion scattering (MEIS) and photoelectron spectroscopy(PES) using synchrotron-radiation-light. Initially, two-dimensional islands (2D) with a height of one and two atomic layers grow and higher coverage increases the islands height to form three-dimensional (3D) islands for the stoichiometric TiO₂(1 1 0) substrate. In contrast, 3D islands start to grow from initial stage with a small Au coverage (?0.1 ML, 1 ML = 1.39 × 10¹⁵  atoms/cm²: Au(1 1 1)) probably due to O-vacancies acting as a nucleation site. Above 0.7 ML, all the islands become 3D ones taking a shape of a partial sphere and the Au clusters change to metal for both substrates. We observed the Au 4f and Ti 3p core level shifts together with the valence band spectra. The Ti 3p peak for the O-deficient surface shifts to higher binding energy by 0.25 ± 0.05 eV compared to that for the stoichiometric surface, indicating downward band bending by an electron charge transfer from an O-vacancy induced surface state band to n-type TiO₂ substrate. Higher binding energy shifts of Au 4f peaks observed for both substrates reveal an electron charge transfer from Au to TiO₂ substrates. The work functions of Au nano-particles supported on the stoichiometric and reduced TiO₂ substrates were also determined as a function of Au coverage and explained clearly by the above surface and interface dipoles.     

A valence band photoemission study of Pb adsorption on Rh(1 0 0) and Rh(1 1 0)

We have studied the adsorption of Pb on the Rh(1 0 0) and (1 1 0) surfaces by photoemission and low energy electron diffraction (LEED), and tested the chemical properties by adsorption of CO. Pb forms two distinct c(2 × 2) phases on Rh(1 0 0), according to the temperature of the substrate. The phase formed below about 570-620 K, denoted α-c(2 × 2), reduces the coverage of adsorbed CO but does not affect the valence band spectrum of the molecule. The phase formed above this temperature, denoted β-c(2 × 2), also reduces the coverage of adsorbed CO but the valence band spectrum of the adsorbed CO is strongly affected. The two phases are also characterised by a slightly different binding energy of the Pb 5d_5/2 level, 17.54 eV for the α phase and 17.70 for the β phase. The Pb/Rh(1 1 0) surface shows two ordered Pb induced phases, c(2 × 2) and p(3 × 1). CO adsorbs on the first with reduced heat of adsorption and with a valence band spectrum that is strongly altered with respect to CO adsorbed on clean Rh(1 1 0), but does not adsorb on the p(3 × 1) structure at 300 K. We compare the present results with previous results from related systems.     

Structure and electronic properties of gold adsorbed on Ti(0 0 0 1)

The Au/Ti(0 0 0 1) adsorption system was studied by low energy electron diffraction (LEED) and photoemission spectroscopy with synchrotron radiation after step-wise Au evaporation onto the Ti(0 0 0 1) surface. For adsorption of Au at 300 K, no additional superstructures were observed and the (1 × 1) pattern of the clean surface simply became diffuse. Annealing of gold layers more than 1 ML thick resulted in the formation of an ordered Au-Ti surface alloy. Depending on the temperature and annealing time, three surface reconstructions were observed by LEED: (√3 × √3) R30°, (2 × 2) and a one-dimensional incommensurate (√3 × √3) rectangular pattern. The Au 4f core level and valence band photoemission spectra provided evidence of a strong chemical interaction between gold and titanium. The data indicated formation of an intermetallic interface and associated valence orbital hybridization, together with diffusion of gold into the bulk. Au core-level shifts were found to be dependent on the surface alloy stoichiometry.     

Formation of oxygen-induced Si(1 1 3)-3 × 2 facets on the Si(5 5 12) surface

We report the formation of Si(1 1 3)-3 × 2 facets upon exposing oxygens on the Si(5 5 12) surface at an elevated temperature. These facets are found to form only for a limited range of oxygen exposure and exhibit a well-defined 3 × 2 LEED pattern. We also find the surface electronic state unique only to the facets in the valence band. The spectral feature of these electronic states and the behavior of a (1/3 1/2) LEED spot upon oxygen contents in the facets indicate that the formation is a heterogeneous mixture of the clean Si(1 1 3) facets free of oxygens and other facets containing oxygen atoms.     

Growth mode and electronic structure of Au nano-clusters on NiO(0 0 1) and TiO2(1 1 0)

The growth mode and electronic structure of Au nano-clusters grown on NiO and TiO₂ were analyzed by reflection high-energy electron diffraction, a field-emission type scanning electron microscope, medium energy ion scattering and photoelectron spectroscopy. Au was deposited on clean NiO(0 0 1)-1 × 1 and TiO₂(1 1 0)-1 × 1 surfaces at room temperature with a Knudsen cell at a rate of 0.25-0.35 ML/min (1 ML = 1.39 × 10¹⁵ atoms/cm²:Au(1 1 1)). Initially two-dimensional (2D) islands with thickness of one Au-atom layer grow epitaxially on NiO(0 0 1) and then neighboring 2D-islands link each other to form three-dimensional (3D)-islands with the c-axis oriented to the [1 1 1] direction. The critical size to form 3D-islands is estimated to be about 5 nm². The shape of the 3D-islands is well approximated by a partial sphere with a diameter d and height h ranging from 2.0 to 11.8 nm and from 0.95 to 4.2 nm, respectively for Au coverage from 0.13 to 4.6 ML. The valence band spectra show that the Au/NiO and Au/TiO₂ surfaces have metallic characters for Au coverage above 0.9 ML. We observed Au 4f spectra and found no binding energy shift for Au/NiO but significant higher binding energy shifts for Au/TiO₂ due to an electron charge transfer from Au to TiO₂. The work function of Au/NiO(0 0 1) gradually increases with increase in Au coverage from 4.4 eV (NiO(0 0 1)) to 5.36 eV (Au(1 1 1)). In contrast, a small Au deposition(0.15 to 1.5 ML) on TiO₂(1 1 0) leads to reduction of the work function, which is correlated with an electron charge transfer from Au to TiO₂ substrate.     

Growth of Au thin film on Cu-modified Si(1 1 1) surface

Miniaturizing of electronic devices requires that conductive elements maintain advanced electrical characteristics upon reducing their geometrical sizes. For gold, which is valued for its high electrical conductivity and stability against ambient conditions, creation of extra-thin films on silicon is hampered by formation of the quite complex Au/Si interface. In the present work, by forming a Si(1 1 1)5.55 × 5.55-Cu surface reconstruction prior to Au deposition we formed Au films with smoother surface morphology and higher surface conductivity compared to Au film grown on a pristine Si(1 1 1)7 × 7 surface. Scanning tunnelling microscopy and four-point probe measurements were used to characterize the growth mode of the Au film on a Si(1 1 1)5.55 × 5.55-Cu reconstruction at room temperature.     

The Na-adsorbed Ge(0 0 1) surface: Structure and STM image simulations

Sodium adsorbed on the Ge(0 0 1) surface causes reconstruction of the surface with the type of reconstruction depending on the amount of the adsorbate. We present theoretical investigations of the structure and electronic properties of Na-adsorbed Ge(0 0 1) for the coverage of 0.5 monolayer using the combination of two methods: a plane-wave basis method and a local-orbital minimal-basis method. Two possible minimum-total-energy atomic configurations have been found, namely, the Na/Ge(0 0 1)-p(2 ×1) and Na/Ge(0 0 1)-p(4 × 1) reconstructions. The surface electronic structure for all calculated configurations occurs to be metallic. Our investigations are completed by a simulation of STM images for the obtained atomic structures.     

Effect of hydrogenation on the electronic structure of the P/Si(0 0 1)-(1 × 2) surface

Ab initio calculations, based on pseudopotentials and density functional theory (DFT), have been performed to investigate the effect of hydrogenation on the electronic properties of P/Si(0 0 1)-(1 × 2) surface. In parallel with this, the electronic band structure of the hydrogenated and non-hydrogenated P/Si(0 0 1)-(1 × 2) surface have been calculated for half- and full-monolayer P. For the mixed Si-P dimer structure, we have identified two occupied and one unoccupied surface state, which correspond to 0.5 ML coverage of P. When this surface is terminated with H, we see that two occupied states completely disappeared and that one unoccupied state is shifted towards the conduction band. A similar calculations for the 1 ML coverage of P have been also carried out. It is seen that the unoccupied state C₁ appeared in the P/Si(0 0 1)-(1 × 2) surface is passivated when this surface is terminated with the H atoms. To explain the nature of the surface states, we have also plotted the total and partial charge densities at the point of the Surface Brillouin Zone (SBZ).     

Surface electronic structure of the (3 × 2) reconstruction induced by Yb on a Si(1 1 1) surface

We have investigated the electronic structure of the Yb/Si(1 1 1)-(3 × 2) surface using angle-resolved photoelectron spectroscopy. Five surface states have been identified in the gap of the bulk band projection. Among these five surface state, the dispersions of three of them agree well with those of the surface states of monovalent atom adsorbed Si(1 1 1)-(3 × 1) surfaces. The dispersions of the two other surface states agree well with those observed on the Ca/Si(1 1 1)-(3 × 2) surface, whose basic structure is the same as that of monovalent atom adsorbed Si(1 1 1)-(3 × 1) surfaces. Taking these results into account, we conclude that the five surface states observed in the band gap originate from the orbitals of Si atoms that form a honeycomb-chain-channel structure.     

Structural stability and electronic structure of iron adsorption on the GaN(0 0 0 1) surface

In this work, we have investigated by means of first-principles spin-polarized calculations, the electronic and magnetic properties of iron (Fe) adsorption and diffusion on the GaN(0 0 0 1) surface using density functional theory (DFT) within a plane-wave pseudopotential scheme. In the surface adsorption study, results show that the most stable positions of a Fe adatom on GaN(0 0 0 1) surface are the H3 sites and T4 sites, for low and high Fe coverage respectively. We found that the Fe-H3 2 × 2 surface reconstruction exhibits a half-metallic behavior with a spin band gap and stable ferromagnetism ordering, which is a desirable property for high-efficiency magnetoelectronic devices. In addition, confirming previous experimental results, we found that the iron monolayers present a ferromagnetic order and a large thermal stability. This is interesting from a theoretical point of view and for its technological applications.     

Electronic properties of clean unreconstructed 6H-SiC(0 0 0 1) surfaces studied by angle resolved photoelectron spectroscopy

The properties of the clean and unreconstructed 6H-SiC(0 0 0 1) and 6H-SiC surfaces were investigated by means of angle-resolved photoelectron spectroscopy (ARPES). These highly metastable surfaces were prepared by exposing hydrogen terminated surfaces to a high flux of synchrotron radiation. On both surfaces we find a band of surface states with 1 × 1 periodicity assigned to unsaturated Si and C dangling bonds located at 0.8 eV and 0.2 eV above the valence band maximum, respectively. Both states are located below the Fermi level. The dispersion of the surface bands amounts to 0.2 eV for the Si derived band and 0.7 eV for C derived band. It is suggested that the electronic properties of these surfaces are governed by strong correlation effects (Mott-Hubbard metal insulator transition). The results for the (0 0 0 1) surface are directly compared to Si-rich (√3 × √3)R30° reconstructed surface. Distinct differences in electronic structure of the (√3 × √3)R30° and 1 × 1 surfaces are observed.     

Atomic and electronic structure of Sr/Si(0 0 1)-(2 × 2)

The adsorption of Sr on the Si(0 0 1) surface with the semiantiphase dimer (2 × 2) reconstruction is studied, based upon the ab initio pseudopotential calculations. It is calculated that the semiantiphase dimer (2 × 2) reconstruction (2 dimers per unit cell) is more favorable than the (2 × 1) phase (1 dimer per unit cell) by an energy of about 0.24 eV/dimer. Considering the energetically more stable reconstruction, we have assumed four possible locations for 1/4 monolayer (ML) Sr adsorption on this surface: (i) bridge, (ii) cave, (iii) pedestal, and (iv) valley-bridge. We find that Sr adsorption on the valley-bridge site is energetically more favorable than all other cases studied here. Interestingly, one of the dimers becomes symmetric, but the other one is still asymmetric with the buckling angle reduced from 18° to 14°, when compared with the clean Si(0 0 1)-(2 × 2) surface. The calculated bond length between Sr and Si in the case of valley-bridge adsorption site is 3.05 Å, and in good agreement with other theoretical calculations. We also present and compare the electronic band structures for the clean and covered surfaces as well as the corresponding charge density plots.     

Surface energy of M2AC(0 0 0 1) determined by density functional theory (M = Ti, V, Cr; A = Al, Ga, Ge)

We have studied the correlation between the valence electron configuration and the electronic structure of M₂AC(0 0 0 1) surfaces (M = Ti, V, Cr; A = Al, Ga, Ge) by density functional theory. The A surface termination is the most stable configuration for all systems studied according to our surface energy data. As the M valence electron population is increased, the surface energy increases by 22% and 12% for A = Al and Ga, respectively, while it decreases by 29% for A = Ge. This can be understood by evaluating the valence electron concentration induced changes in the surface density of states. Antibonding surface Md-Ap states are present as Ti is substituted by Cr in M₂AC(0 0 0 1) for A = Al and Ga, while antibonding surface Md-Ap states are not present as Ti is substituted by Cr in M₂GeC(0 0 0 1).     

Valence band studies of the formation of ultrathin pure silicon nitride films on Si(1 0 0)

The growth of ultrathin films of Si₃N₄ directly on Si surfaces is studied with valence band photoemission. The information from these studies about the growth mechanism and the changes of the electronic structure is enhanced by the use of various photon energies with synchrotron radiation. The silicon nitride films are grown isothermally on the Si(1 0 0) and Si(1 1 1) surfaces by reactions with atomic N. The atomic nitrogen is produced by using a remote, microwave excited nitrogen plasma. The growth under these conditions was earlier shown to be self limiting. The details in the valence band spectra are identified and resolved with numerical methods, and followed systematically during the growth. Thus the identification of Si surface states, Si-nitride interface states and bulk nitride states becomes possible. The previously obtained separation between amorphous and crystalline growth occurring around 500 °C is further supported in the present studies.     

Anisotropy of electron structure at InAs(1 1 1) surfaces by laser pump-and-probe photoemission spectroscopy

The electronic structure and the electron dynamics of the clean InAs(1 1 1)A 2 × 2 and the InAs(1 1 1)B 1 × 1 surfaces have been studied by laser pump-and-probe photoemission spectroscopy. Normally unpopulated electron states above the valence band maximum (VBM) are filled on the InAs(1 1 1)A surface due to the conduction band pinning above the Fermi level (E_F). Accompanied by the downward band banding alignment, a charge accumulation layer is confined to the surface region creating a two dimensional electron gas (2DEG). The decay of the photoexcited carriers above the conduction band minimum (CBM) is originated by bulk states affected by the presence of the surface. No occupied states were found on the InAs(1 1 1)B 1 × 1 surface. This fact is suggested to be due to the surface stabilisation by the charge removal from the surface into the bulk. The weak photoemission intensity above the VBM on the (1 1 1)B surface is attributed to electron states trapped by surface defects. The fast decay of the photoexcited electron states on the (1 1 1)A and the (1 1 1)B surfaces was found to be τ_1 1 1 A ? 5 ps and τ_1 1 1 B ?  4 ps, respectively. We suggest the diffusion of the hot electrons into the bulk is the decay mechanism.     

Ab initio study of atomic geometry and electronic states of GaSb(0 0 1) reconstructions

We have employed the pseudopotential method and the density functional scheme to study the atomic geometry and electronic states of the GaSb(0 0 1) surface such as (1 × 3), c(2 × 6) and (4 × 3) reconstructions. It is found that both of (1 × 3) and c(2 × 6) reconstructions are characterised by metallic band structures, and thus violate the so-called electron counting rule, one of the main building principles of the stability of compound semiconductor surfaces. We establish that the stability of these reconstructions results from significant elastic deformation in the top atomic layers of the surface, a process which overcomes the penality incurred by the violation of the electron counting rule. The atomic geometry and electronic states for the two reconstructions are compared and contrasted with each other. The α and β phases of the (4 × 3) reconstruction also show large atomic relaxations but are semiconducting and obey the electron counting rule.     

Eu- and Yb-induced reconstructions on a vicinal Si(1 0 0) surface

Early stages of rare-earth metal (Yb and Eu) growth on a vicinal, single-domain Si(1 0 0)2 × 1 surface have been studied in the coverage range of 0.1-0.3 monolayer (ML) by low energy electron diffraction, scanning tunneling microscopy, and synchrotron radiation photoemission spectroscopy. We show that Yb induces the 2 × 3 periodicity in the whole range of coverage studied. The 2 × 3 reconstruction coexists with the local 3 × 2/4 × 2 structure at about 0.2 ML of Yb. In contrast, Eu forms the 3 × 2 periodicity at 0.1-0.2 ML, whereas this structure is converted into the 2 × 3 phase at about 0.3 ML. The atomic arrangement and electronic properties of these reconstructions and the adsorbate-mediated modification of surface morphology are investigated.     

Electron surface states in short-period superlattices: (GaAs)2/(AlAs)2(1 0 0)-c(4 × 4)

The electronic structure of (GaAs)₂/(AlAs)₂(1 0 0)-c(4 × 4) superlattice surfaces was studied by means of angular-resolved photoelectron spectroscopy (ARUPS) in the photon energy range 20-38 eV. Four samples with different surface termination layers were grown and As-capped by molecular beam epitaxy (MBE). ARUPS measurements were performed on decapped samples with perfect c(4 × 4) reconstructed surfaces. An intensive surface state was, for the first time, observed below the top of the valence band. This surface state was found to shift with superlattices’ different surface termination in agreement with theoretical predictions.