Surface Review and Letters

A quantitative structural determination of the Ga-polar 1×1 (0001) surface of GaN is performed by quantitative low energy electron diffraction (LEED). The global best-fit structure is obtained by a new frozen LEED approach connected to a simulated annealing algorithm. The global minimization frozen (GMF) LEED search finds that the ordered structure consists of 1 ML of Ga adatoms at atop sites above Ga-terminated bilayers. The Ga adatoms are bonded with a Ga–Ga bond length of 2.51 Å. The spacings within surface bilayers show a weak oscillatory trend, with the outmost bilayer thickness expanding to 0.72 Å and the next bilayer thickness contracting to 0.64 Å, compared to the bulk thickness of 0.65 Å. The interlayer spacing between the first and second bilayers is 1.89 Å, while the next interlayer spacing is 1.94 Å, compared to the bulk value of 1.95 Å. These results are compared with data from other theoretical and experimental studies.

With tips of different hardness, we analyzed the effect of the hardness and shape of the actual AFM tip on the measurement of the best GaSb quantum dot (QD) structures induced by low energy Ar⁺ sputtering. The comparison indicated that the complete information on the detailed dot shape and order structure can be determined with the hard and good tip, while with the soft or worn tip some information on the dot height and shape cannot be obtained. Our results suggest that, in order to obtain the complete surface information, the high hardness and good AFM tip should be used for semiconductor QD structures.

The atomic structure of ultrathin lead and antimony layers deposited on the Ni(111) face in ultrahigh vacuum at a substrate temperature ranging from 150 to 900 K was investigated with the use of low-energy electron diffraction (LEED). LEED patterns corresponding to p(1×1), p(3×3), p(4×4), structures and p(1×1), p(2×2), , structures for the adsorption of Pb and Sb, respectively, on the Ni(111) face were observed. Experimental LEED intensity-versus-energy [I(V)] spectra have been collected for the clean Ni(111) and for the , , structures. The I(V) curves obtained for the clean Ni(111) structure are in good agreement with experimental spectra from the literature.

Activation energies and atomic geometries associated with Si adsorption and desorption processes on the Si face of SiC(0001) have been obtained from in situ reflection high-energy electron diffraction measurements and ab initio pseudopotential calculations. During the initial stage of Si deposition, the SiC(0001) surface develops a series of superstructures, depending on temperature and Si coverage. A kinetic diagram of the structural transformations during Si exposure or annealing is outlined and the adsorption energy E_a is determined. The latter is found to be strongly affected by the surface structure and coverage, changing from 4.5 eV to 1.6 eV as the geometry of the Si adlayer changes from to (3×3). These observations are interpreted with the help of ab initio pseudopotential calculations. It is concluded that measured on corresponds to adsorption at adatom vacancy while E_a3=1.6 eV measured on (3×3) corresponds to adsorption on top of a capping adatom. Finally, the development of surface structures from the surface to silicon-rich and carbon-rich extremes is discussed.

The oxidation of the single crystal stepped Pt₃Ti(510) surface at oxygen pressures below 10^-5 mbar and at a temperature of 770 K was studied by means of X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS) and low energy electron diffraction (LEED). Scanning tunneling microscopy (STM) was used to follow the evolution of the surface morphology on the atomic scale. The clean surface studied in ultrahigh vacuum conditions was found by LEIS to be composed of platinum only in the outermost surface plane. LEED and STM indicate that the clean Pt₃Ti(510) surface consists of (100) terraces separated by double atomic steps. The exposure of the clean surface to oxygen at pressures in the range of 10^-7–10^-5 mbar leads to the growth of a titanium oxide layer (with a composition close to TiO) which covers completely the substrate surface. The TiO film has long range order and exhibits complex LEED patterns. The STM measurements indicate that the ordered array of steps is kept in the early stages of the oxide film growth, whereas a change of the step morphology and step orientation is observed during the oxidation process.

The Ziff–Gulari–Barshad model [ZGB; Phys. Rev. Lett.56, 2553 (1986)] is a lattice gas adsorption–reaction model that qualitatively reproduces some relevant features of the oxidation of carbon monoxide on single crystal surfaces. We study the first order irreversible critical behavior of this model by means of two different types of simulation techniques, namely constant coverage simulations and epidemic analysis. Constant coverage simulations allow us to investigate in detail hysteretic and finite size effects within the coexistence region. The position of the coexistence point is accurately determined by introducing a small perturbation into the constant coverage method. Extensive epidemic simulations reveal the absence of power law behavior in the dynamic behavior at coexistence, in contrast to previous claims.

The ion-driven mechanism in hydrogen permeation is substantially modified when iron is coated with palladium. A detailed knowledge of the electronic structure at the metal–metal interfaces is a prerequisite for understanding the process of H permeation. We have selected two low-Miller-index surfaces as a simple model for the interface. The system under consideration has 148 metallic atoms forming an Fe–Pd cluster distributed in six metallic layers.

We have investigated the interaction of atomic hydrogen with the Fe(110)–Pd(100) interface using the semiempirical atom superposition and electron delocalization (ASED-MO) method. The changes in the electronic structures, density of states (DOS) and crystal overlap orbital population (COOP) in two different Fe–Pd interfaces were compared with the bulk ground states of both metals.

The interfacial Fe–Pd distance results in about 1.74 Å, whereas for the Fe–Pd first neighbors distance it is about 1.85 Å. An important conclusion is that the metal–metal bonds at the interface are stronger than those bonds in the pure metal bulk. A favorable metal adhesion is observed, as revealed by the energetic stabilization of the composite metal system.

H is stabilized near the FePd interface and stopped at the first Pd layer. A H–metal bond is developed with both Fe and Pd atoms while Fe–Pd bonding at the interface remains unaltered.

A procedure for the conversion of film thicknesses from monolayers to angstroms is presented. It is applicable to thickness data experimentally obtained from strained epitaxial films and based on the concept of the epitaxial Bain path. In particular, the in-plane lattice spacing of the film, such as that obtained with LEED on wedge-shaped samples, is used to determine the perpendicular lattice spacing, which serves for the conversion into angstroms. Omission of the implications of the Bain path concept would yield erroneous results. This conversion was successfully applied to compare experimental data, (given in monolayers), with data found in the literature (given in angstroms).

The method of unity bond index–quadratic exponential potential (UBI–QEP) is employed to derive the energetic parameters associated with the steps of the pathway which we propose for the catalytic decomposition of azomethane on the Pd(111) surface. According to the energy calculations, azomethane adsorbs molecularly in trans-configuration and then decomposes to CH₃N with no activation energy. The reaction continues by tilting and dehydrogenation to the products (H₂ and HCN). The calculated activation energies at various surface coverages perfectly account for the variation of relative yields of H₂ and HCN with changing of the coverage of azomethane. According to the calculations, desorption of molecular azomethane at high surface coverage occurs, which is in excellent agreement with experimental data.

We have fabricated gold thin films by metal plasma ion deposition on silicon substrates. The roughness of these nanostructured films has been measured by scanning tunneling microscopy (STM) and we have determined the growth dynamics critical exponents. We have also measured the grain sizes as a function of the film thickness.

Several molecular adsorption states are identified following ethylene adsorption on clean and hydrogen-covered Pd(111) using temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). Di-σ-bonded ethylene forms on clean Pd(111) desorbing with an activation energy of 80 kJ/mol at low coverages. The strong intermolecular lateral interactions considerably reduce the desorption temperature at higher coverages. Π-bonded ethylene is formed on hydrogen-covered Pd(111), where the proportion of π-bonded species increases with hydrogen coverage. This species converts to the more stable di-σ-bonded species on heating. Ethane formation is detected in TPD from hydrogen-precovered Pd(111), which is predominantly formed by reaction with π-bonded ethylene.

Photoemission spectroscopy (Ru 3d/C 1s and O 1s regions) has been used to investigate the formation of carbonate species on the Sm- and SmO_x-covered Ru(001) surfaces at small exposures of CO. The results indicate that the reaction of ~ 1 L CO with disordered Sm layers on the Ru surface at 105 K produces surface carbonate and weakly bound CO₂ via the Boudouard reaction (2CO_g→CO_2,ad+C_ad). The latter species desorbs below 250 K, whereas the former is stable up to 500 K. The surface carbonate species is detected by the O 1s features as well as the C 1s feature developed in the Ru 3d region upon CO exposure. The CO reaction with the SmO_x layer also leads to the formation of surface carbonate. The generation of carbonate on Sm and SmO_x films seems to be related to the ionization of Sm atoms and the defects present in the partly oxidized Sm, respectively.

The k derivative spectra (KDS) transform is used for construction of the three-dimensional atomic structure of the C₂H₄/Si(100)-(2×1) system from photoelectron diffraction data. The image function obtained by the KDS transform clearly observes the second-layer Si atoms and the C emitters apart from the first-layer Si atoms. The observations of the second-layer Si atoms and the C emitters make it easy to measure the C–C bond length correctly. Then a conclusive adsorption model — the di-σ model — for the C₂H₄/Si(100)-(2×1) system is established. In comparison with the KDS transform, the normal small-cone transform hardly measures the C–C bond length. The ability to observe more scatterers of a photoelectron emitter by the KDS transform expands the applicability of holographic imaging.

Scanning tunneling spectroscopy at T = 6 K is used to investigate the local density of states (LDOS) of electron systems belonging to the bulk conduction band of InAs. In particular, the three-dimensional electron system (3DES) of the n-doped material, an adsorbate-induced two-dimensional electron system (2DES) and the tip-induced quantum dot (0DES) are investigated at B = 0 T and B = 6 T. It is found that the 3DES at B = 0 T can be described by Bloch states weakly interacting with the potential disorder provided by ionized dopants. The 2DES at B = 0 T exhibits much stronger LDOS corrugations, stressing the tendency for weak localization in the potential disorder. In a magnetic field, 3DES and 2DES show drift states, which are expected in 2D, but are surprising in 3D, where they point to a new electron phase consisting of droplets of quasi-2D systems.

The 0DES at B = 0 T reveals quantized states in accordance with Hartree calculations. At B = 6 T it exhibits Landau states with exchange enhanced spin splitting. These states are used to investigate the influence of potential disorder on the exchange enhancement, which visualizes the nonlocality of the exchange interaction.

For in-situ measurements of the local electrical conductivity of well-defined crystal surfaces in ultrahigh vacuum, we have developed two kinds of microscopic four-point probe methods. One involves a "four-tip STM prober," in which four independently driven tips of a scanning tunneling microscope (STM) are used for measurements of four-point probe conductivity. The probe spacing can be changed from 500 nm to 1 mm. The other method involves monolithic micro-four-point probes, fabricated on silicon chips, whose probe spacing is fixed around several μm. These probes are installed in scanning-electron-microscopy/electron-diffraction chambers, in which the structures of sample surfaces and probe positions are observed in situ. The probes can be positioned precisely on aimed areas on the sample with the aid of piezoactuators. By the use of these machines, the surface sensitivity in conductivity measurements has been greatly enhanced compared with the macroscopic four-point probe method. Then the conduction through the topmost atomic layers (surface-state conductivity) and the influence of atomic steps on conductivity can be directly measured.

We present in this article a comprehensive review of the dynamical fluctuations in the atomic positions that may take place, even at very low temperatures, at the clean silicon or germanium (100) surfaces or at their (111) surfaces decorated with Sn or Ag adatoms. We also elucidate the intriguing, hitherto unexplained differences observed between the two, seemingly similar, Sn/Si(111) and Sn/Ge(111) surfaces. We also describe the surprising behaviors of silver ultrathin films grown on different semiconductor surfaces, displaying in certain cases a bcc phase, a one-dimensional quasicrystalline superstructure and/or well-defined quantum size effects.

Crystal binding energy of a core electron at the 3d_5/2 level of a Pd atom has been estimated to be -4.0±0.02 eV by decoding the X-ray photoelectron spectra obtained from both the Pd surfaces and the Pd nanoparticles. Findings indicate that the increase in the binding energy originates from the effect of coordination number (CN) imperfection of atoms at a flat surface or at the curved surface of a nanosolid. The CN imperfection shortens the remaining bonds of the lower-coordinated atoms spontaneously associated with bond energy increase, which perturbs the Hamiltonian of an extended solid and hence shifts the core level to higher binding energy. Therefore, the significance of atomic CN imperfection cannot be overlooked in dealing with a low-dimensional system, and the recent bond-order–length–strength (BOLS) correlation mechanism is essentially adequate, for the significance of atomic CN imperfection.