Theoretical studies of electronic states and phonon modes on the TiC(0 0 1)(1 × 1) surface

We present a comprehensive picture of structural and electronic properties of the TiC(0 0 1)(1 × 1) surface. Our investigations are based on first-principles calculations within the local-density approximation of the density-functional theory. Good agreement has been observed between our calculation and experimental data for the atomic geometry of the surface. In particular, the calculated bond lengths between the first-layer C and the second-layer Ti (d_1C-2Ti = 2.188 Å) and between the first-layer Ti and the second-layer C (d_1Ti-2C = 2.031 Å) are in good agreement with the corresponding experimental values of 2.25 Å and 2.14 Å, respectively. We have also identified surface electronic states and provided clear support for previously available photoemission measurements. We have further calculated surface phonon modes at the zone centre and at the zone-edge point X using a linear response scheme based on the ab initio pseudopotential method. Our calculated surface phonon results are in excellent agreement with electron energy loss spectroscopy results.     

Study of vacancy on diamond (1 0 0) (2 × 1) surface from first-principles

Structure and energy related properties of neutral and charged vacancies on relaxed diamond (1 0 0) (2 × 1) surface were investigated by means of density functional theory. Calculations indicate that the diffusion of a single vacancy from the top surface layer to the second layer is not energetically favored. Analysis of energies in charged system shows that neutral state is most stable on diamond (1 0 0) (2 × 1) surface. The multiplicity of possible states can exist on diamond (1 0 0) surface in dependence on the surface Fermi level, which supports that surface diffusion of a vacancy is mediated by the change of vacancy charge states. Analysis of density of states shows surface vacancy can be effectively measured by photoelectricity technology.     

Mechanisms of preferential adsorption on the Si(1 1 1)7 × 7 surface

The surface relaxation mechanisms governing the preferential adsorption of metal atoms onto the faulted half-cells of a 7 × 7 reconstructed Si(1 1 1) surface are studied by rate equations and kinetic Monte Carlo simulations. The versatility of these mechanisms to control the formation of quasi-perfect 2D arrays of metal clusters is revealed via the optimization of the deposition/annealing conditions as a function of operating mechanisms, the Si(1 1 1)7 × 7 energy landscape, and the thermal stability of the created clusters. The influence on the formation process of such nanoarrays of the balance between kinetic limitations, which are especially relevant on Si(1 1 1)7 × 7, and thermodynamic tendencies is discussed.     

Optical anisotropy of the In/Si(1 1 1)(4 × 1)/(8 × 2) nanowire array

Ab initio calculations of the reflectance anisotropy of Si(1 1 1)-In surfaces are presented. A very pronounced optical anisotropy around 2 eV is found that is related to In-chain states. The distortion of the indium chains characteristic for the (4 × 1) → (8 × 2) phase transition results in a splitting of the 2 eV peak, as observed experimentally. The splitting occurs irrespective wether the phase transition occurs according to the trimer or hexamer model.     

Characteristic energy band values and electron attenuation length of a chemical-vapor-deposition diamond (0 0 1)2 × 1 surface

The characteristic energy band values such as the Fermi-level position with respect to valence band top for a boron-doped p-type hydrogen-terminated chemical-vapor-deposition (CVD) diamond (0 0 1)2 × 1 surface and for a clean CVD diamond (0 0 1)2 × 1 surface have been determined by a new method with an accuracy of ±0.02 eV. The electron attenuation length for the clean diamond (0 0 1)2 × 1 surface for the electron kinetic energy of C 1s X-ray photoemission peak by Mg Kα excitation is experimentally determined to be 2.1-2.2 nm. These values are compared and discussed with the previously reported experimental and simulation values.     

A structural parallel between Ge- and Si-induced 4 × 4 and 3 × 3 reconstructions on SiC(0 0 0 1) drawn from comparative RHEED oscillations

The initial Ge growth stages on a (√3 × √3)R30°-reconstructed SiC(0 0 0 1) surface (√3) have been studied using a complete set of surface techniques such as reflection high energy electron diffraction (RHEED), low energy electron diffraction (LEED), atomic force microscopy (AFM) and photoemission and compared with similar Si surface enrichments in place of Ge. The investigations essentially focus on the wetting growth-regimes that are favoured by the use of the √3 surface as a starting substrate, this surface being the closest to a smooth and ideally truncated Si-terminated face of hexagonal SiC(0 0 0 1). Depending on temperature and Ge or Si coverages, varying surface organizations are obtained. They range from unorganized layer by layer growths to relaxed Ge(1 1 1) or Si(1 1 1) island growths, through intermediate attempts of coherent and strained Ge or Si surface layers, characterized by 4 × 4 and 3 × 3 surface reconstructions, respectively. RHEED intensity oscillation recordings, as a function of Ge or Si deposited amounts, have been particularly helpful to pinpoint the limited (by the high lattice mismatch) existence domains of these interesting coherent phases, either in terms of formation temperature or surface coverages. Prominently comparable data for these two Ge- and Si-related reconstructions allow us to propose an atomic model for the still unexplained Ge-4 × 4 one. It is based on a same local organization in trimer and ad-atom units for the Ge excess as admitted for the Si-excess of the 3 × 3 surface, the higher strain nevertheless favouring arrangements, for the Ge-units, in 4 × 4 arrays instead of 3 × 3 Si ones. Admitting such models, 1.25 and 1.44 monolayers of Ge and Si, should, respectively, be able to lie coherently on SiC, with respective lattice mismatches near 30% and 25%. The experimental RHEED-oscillations values are compatible with such theoretical ones. Moreover, these RHEED coverage determinations (for layer completion, for instance) inform us in turn about the initial Si richness of the starting √3 reconstruction and help us to discriminate between earlier contradictory atomic models proposed in the literature.     

Electron-stimulated desorption from an unexpected source: Internal hot electrons for Br-Si(1 0 0)-(2 × 1)

The desorption of Br adatoms from Br-saturated Si(1 0 0)-(2 × 1) was studied with scanning tunneling microscopy as a function of dopant type, dopant concentration, and temperature for 620-775 K. Analysis yields the activation energies and prefactors for desorption, and the former correspond to the energy separation between the Fermi level and Si-Br antibonding states. Thus, electron capture in long-lived states results in Br expulsion via a Franck-Condon transition. Analysis of the prefactors reveals that optical phonons provide the energy needed for the electronic excitation. These results show that desorption induced by an electronic transition can occur in closed system without external stimulus, and they indicate that thermally-excited charge carriers may play a general role in surface reactions.     

Early stages of vanadium deposition on Si(1 1 1)-7 × 7

We investigate the low-coverage regime of vanadium deposition on the Si(1 1 1)-7 × 7 surface using a combination of scanning tunnelling microscopy (STM) and density-functional theory (DFT) adsorption energy calculations. We theoretically identify the most stable structures in this system: (i) substitutional vanadium atoms at silicon adatom positions; (ii) interstitial vanadium atoms between silicon adatoms and rest atoms; and (iii) interstitial vanadium - silicon adatom vacancy complexes. STM images reveal two simple vanadium-related features near the Si adatom positions: bright spots at both polarities (BB) and dark spots for empty and bright spots for filled states (DB). We relate the BB spots to the interstitial structures and the DB spots to substitutional structures.     

STM image visualization of Si(1 1 1) 7 × 7 surface (graphic simulation and implementation)

The possibilities of graphic STM image simulation of a clean Si(1 1 1) 7 × 7 surface at atomic level are indicated. The presented procedure takes into account various types of deformation on the surface near the Fermi level in order to classify them and explain their origin. It also gives a clear hint to insert relevant physical phenomena in a suggested analysis. This goal is achieved exploiting the results of DAS (dimmer adatom stacing fault) model by means of standard mathematical programmes. A clean Si(1 1 1) 7 × 7 surface is considered as the representative example, but similar evaluation is possible for another non-metal and metal surfaces.     

Olefin bonding mechanism and growth to (1 0 0)(2 × 1):H diamond

The bonding and growth mechanism of photochemically attached olefin molecules to (1 0 0)(2 × 1):H diamond is characterized using atomic force (AFM) and scanning tunneling microscopy (STM) experiments in combination with molecular orbital calculations. To identify growth schemas, diamond surfaces after 10, 40 and 90 min of photo-chemically stimulated growth have been characterized. These data show clearly island formation which is discussed taking into account a growth model from silicon. The island growth shows no directional properties which are attributed to arrangement and geometrical properties of hydrogen terminated carbon bonds at the surface of (1 0 0) oriented (2 × 1) reconstructed diamond.     

Structural study of Si(1 1 1)-6 × 1-Ag surface using surface X-ray diffraction

The surface structure of Si(1 1 1)-6 × 1-Ag was investigated using surface X-ray diffraction techniques. By analyzing the CTR scattering intensities along 00 rod, the positions of the Ag and reconstructed Si atoms perpendicular to the surface were determined. The results agreed well with the HCC model proposed for a 3 × 1 structure induced by alkali-metals on a Si(1 1 1) substrate. The heights of the surface Ag and Si atoms did not move when the surface structure changed from Si(1 1 1)-√3 × √3-Ag to Si(1 1 1)-6 × 1-Ag by the desorption of the Ag atoms. From the GIXD measurement, the in-plane arrangement of the surface Ag atoms was determined. The results indicate that the Ag atoms move large distances at the phase transition between the 6 × 1 and 3 × 1 structures.     

First principles calculations of the adsorption and diffusion of Y on the Si(0 0 1)-c(4 × 2) surface

We have studied the adsorption and diffusion of yttrium on the Si(0 0 1)-c(4 × 2) surface in the early stages of growth. Our first principles total energy calculations are based on the density functional theory as implemented in the SIESTA code. The exchange and correlation energies are treated within the generalized gradient approximation according to the Perdew, Burke and Ernzerhof parametrization. Our results demonstrate that the most favorable adsorption site is in the trench between two silicon dimer rows, identified as valley-bridge (V). Our studies show that the diffusion of an Y adatom on Si(0 0 1)-c(4 × 2) surface presents an anisotropic behavior. We found two values for the barriers along the valley (0.54 and 1.07 eV) and one of 1.24 eV in the perpendicular direction, showing that diffusion along the valley is more probable. The analysis of the Mulliken overlap populations shows that the bonding between an Y adatom and the surface is partially covalent. Two Y atoms on the surface do not form dimers instead they are adsorbed as adatoms.     

Band structure of surface states and resonances for clean Cu(1 1 0) surface

We have used the layer KKR method to calculate the Shockley and Rydberg surface states and resonances for Cu(1 1 0) for a given model of the surface potentials. This method has not been used before to predict all of the surface band structure for the energy range from the bottom of the conduction band to ∼7 eV above the vacuum level. The previous methods that used only local electron interactions in ab initio calculations could not produce the Rydberg surface barrier bands while those relying on nearly-free-electron parameterisation of bands could not deal with d-bands.     

The Si(1 1 1)-(7 × 7) reconstruction: A surface close to a Mott-Hubbard metal-insulator transition?

Li adsorption at extremely low coverages on the “metallic” Si(1 1 1)-(7 × 7) surface has been experimentally studied recently by β-NMR experiments. Instead of increasing linearly with the sample temperature, as expected for a metallic system, the relaxation rate α = 1/T₁ is almost constant in between 50 K and 300 K sample temperature and rises Arrhenius like above. In order to understand this behaviour in a transparent way a closed form analysis is presented using rectangular density of states distributions. The almost temperature independent relaxation rate below 300 K points to an extremely localized and thus narrow band (width about 10 meV) which pins the Fermi energy. Because of the steeply rising relaxation rate beyond 300 K it is located energetically within a gap (about 380 meV wide) in between a lower filled and an upper empty (Hubbard) band. In dynamical mean field theories based on Hubbard Hamiltonians this kind of density of states is typical for correlated electron systems close to a Mott-Hubbard metal-insulator transition.     

Chemisorption of 1,1-dichloroethene on the Si(1 1 1)-7 × 7 surface

Chemisorption of 1,1-dichloroethene (Cl₂CCH₂) to a Si(1 1 1)-7 × 7 surface was studied by means of X-ray photoelectron spectroscopy using synchrotron radiation, recording chlorine 2p and carbon 1s spectra. For carbon 1s, spectral assignment of the chemisorbed species is based on quantum chemical calculations of chemical shifts in model compounds.The results confirm the identity of covalently bonded 1-chlorovinyl (-CClCH₂) and vinylidene (CCH₂) adspecies. Upon chemisorption at room temperature it was found that about one-third of the molecules break one C-Cl bond while about two-thirds of the adsorbates break two C-Cl bonds. We do not, however, find evidence for isomerization of CCH₂ to di-bonded vinylene (-CHCH-).     

Surface quantum well state at the striped Cu(1 1 0)(2 × 1)O surface studied by angle resolved photoemission spectroscopy

We have performed an angle resolved photoemission spectroscopy with high energy and high momentum resolutions and have observed the k dependent energy dispersion curves of the striped Cu(1 1 0)(2 × 1)O surface. It is found that the Shockley surface state electron is confined in the clean surface along the perpendicular direction to the stripes and forms a quantum well state (QWS). It has also been clearly observed that an electron of Cu-O antibonding state is confined within the oxygen covered surface.     

The dissociative adsorption of borane on the Ge(1 0 0)-2 × 1 surface: A density functional theory study

By means of cluster models coupled with density functional theory, we have studied the hydroboration of the Ge(1 0 0)-2 × 1 surface with BH₃. It was found that the Ge(1 0 0) surface exhibits rather different surface reactivity toward the dissociative adsorption of BH₃ compared to the C(1 0 0) and Si(1 0 0) surfaces. The strong interaction still exists between the as-formed BH₂ and H adspeices although the dissociative adsorption of BH₃ on the Ge(1 0 0) surface occurs readily, which is in distinct contrast to that on the C(1 0 0) and Si(1 0 0) surfaces. This can be understood by the electrophilic nature of the down Ge atom, which makes it unfavourable to form a GeH bond with the dissociating proton-like hydrogen. Alternatively, it can be attributed to the weak proton affinity of the Ge(1 0 0) surface. Nevertheless, the overall dissociative adsorption of BH₃ on group IV semiconductor surfaces is favourable both thermodynamically and kinetically, suggesting the interesting analogy and similar diversity chemistry of solid surface in the same group.     

Temperature dependent quantum well state on Cu(1 1 0)(2 × 1) striped surface studied by angle resolved photoelectron spectroscopy

We have investigated the temperature dependence of angle resolved photoelectron spectroscopy for the lateral quantum well states (QWS) on the striped Cu(1 1 0)(2 × 1)O surface. For the striped surface with oxygen coverage of 0.25 ML, we have successfully observed two discrete levels along a perpendicular direction to the stripes in the surface Brillouin zone, which is generated by quantization of the Shockley surface state. We have found that the relative photoelectron intensity of the two discrete levels depends on the temperature. The photoelectron intensity tends to concentrate on the ground level of the QWS with decreasing temperature. Our investigation indicates that the electron population of each quantum well level depends on the temperature.     

Growth of (√3 × √3)-Ag and (1 1 1) oriented Ag islands on Ge/Si(1 1 1) surfaces

We have studied the growth of Ag on Ge/Si(1 1 1) substrates. The Ge/Si(1 1 1) substrates were prepared by depositing one monolayer (ML) of Ge on Si(1 1 1)-(7 × 7) surfaces. Following Ge deposition the reflection high energy electron diffraction (RHEED) pattern changed to a (1 × 1) pattern. Ge as well as Ag deposition was carried out at 550 °C. Ag deposition on Ge/Si(1 1 1) substrates up to 10 ML has shown a prominent (√3 × √3)-R30° RHEED pattern along with a streak structure from Ag(1 1 1) surface. Scanning electron microscopy (SEM) shows the formation of Ag islands along with a large fraction of open area, which presumably has the Ag-induced (√3 × √3)-R30° structure on the Ge/Si(1 1 1) surface. X-ray diffraction (XRD) experiments show the presence of only (1 1 1) peak of Ag indicating epitaxial growth of Ag on Ge/Si(1 1 1) surfaces. The possibility of growing a strain-tuned (tensile to compressive) Ag(1 1 1) layer on Ge/Si(1 1 1) substrates is discussed.     

Theoretical STM images of alkaline-earth metal adsorbed Si(1 1 1)3 × 2 surfaces

We have performed total-energy calculations to study theoretical scanning tunneling microscopy (STM) images of the Si(1 1 1)3 × 2 surfaces induced by the adsorption of alkaline-earth metals (AEMs). Previously, in a series of works on Ba/Si(1 1 1) system, we have found that the observed Si(1 1 1)3 × 1-Ba LEED phase indeed has a 3 × 2 periodicity with a Ba coverage of 1/6 ML and the HCC substrate structure. Based on results of the Ba case, we proposed that the HCC structure is also adopted for other AEM atoms, which was confirmed by our recent work. In this paper, we mainly report the STM simulations for different AEM systems to compare with existing experimental data. We discuss the difference in the detailed STM images for different AEM adsorbates. Especially, the difference in filled-state images between Mg and other AEM atoms is attributed to the strong Mg-Si interaction.