Theoretical study of hydrogen adsorption on the GaN(0 0 0 1) surface

Ab initio density functional theory, using the B3LYP hybrid functional with all-electron basis sets, has been applied to the adsorption of H on the (0 0 0 1) surface of wurtzite GaN. For bulk GaN, good agreement is obtained with photoemission and X-ray emission data for the valence band and for the Ga 3d and N 2s shallow core levels. A band gap of E_g = 4.14 eV is computed vs the experimental value (at 0 K) of 3.50 eV. A simple model, consisting of a (2 × 2) structure with 3/4-monolayer (ML) of adsorbed H, is found to yield a density of states in poor agreement with photoemission data for H adsorbed on surfaces prepared by ion bombardment and annealing. A new model, consisting of co-adsorbed Ga (1/4 ML) and H (1/2 ML), is proposed to account for these data.     

Clean reconstructed InAs(1 1 1) A and B surfaces using chemical treatments and annealing

Well-ordered clean InAs(1 1 1) A and B surfaces have been prepared using HCl-isopropanol solutions and characterized using low-energy electron diffraction and photoemission spectroscopy. The as-treated surfaces are covered by a layer containing arsenic and small amounts of InCl_x. Annealing induces desorption of the overlayer and reveals (2 × 2) and (1 × 1) structures on the A and B surfaces, respectively. For both surfaces, the surface components of the In 4d and As 3d reveal a charge transfer from the electropositive surface indium to the electronegative surface arsenic. The major advantage of this preparation method over conventional thermal cleaning is a significant reduction in the annealing temperature (≈250 °C) thereby avoiding anion evaporation.     

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.     

Theoretical study of the electronic structure of the Si3N4(0 0 0 1) surface

Density functional theory has been applied to a study of the electronic structure of the ideally-terminated, relaxed and H-saturated (0 0 0 1) surfaces of β-Si₃N₄ and to that of the bulk material. For the bulk, the lattice constants and atom positions and the valence band density of states are all in good agreement with experimental results. A band gap of 6.7 eV is found which is in fair accord with the experimental value of 5.1-5.3 eV for H-free Si₃N₄. Using a two-dimensionally-periodic slab model, a π-bonding interaction is found between threefold-coordinated Si and twofold-coordinated N atoms in the surface plane leading to π and π* surface-state bands in the gap. A surface-state band derived from s-orbitals is also found in the gap between the upper and lower parts of the valence band. Relaxation results in displacements of surface and first-underlayer atoms and to a stronger π-bonding interaction which increases the π-π* gap. The relaxed surface shows no occupied surface states above the valence band maximum, in agreement with recent photoemission data for a thin Si₃N₄ film. The π* band, however, remains well below the conduction band minimum (but well above the Fermi level). Adsorbing H at all dangling-bond sites on the ideally-terminated surface and then relaxing the surface and first underlayer leads to smaller, but still finite, displacements in comparison to the clean relaxed surface. This surface is more stable, by about 3.67 eV per H, than the clean relaxed surface.     

Growth mode and novel structure of ultra-thin KCl layers on the Si(1 0 0)-2 × 1 surface

This study investigates ultra-thin potassium chloride (KCl) films on the Si(1 0 0)-2 × 1 surfaces at near room temperature. The atomic structure and growth mode of this ionic solid film on the covalent bonded semiconductor surface is examined by synchrotron radiation core level photoemission, scanning tunneling microscopy and ab initio calculations. The Si 2p, K 3p and Cl 2p core level spectra together indicate that adsorbed KCl molecules at submonolayer coverage partially dissociate and that KCl overlayers above one monolayer (ML) have similar features in the valance band density of states as those of the bulk KCl crystal. STM results reveal a novel c(4 × 4) structure at 1 ML coverage. Ab initio calculations show that a model that comprises a periodic pyramidal geometry is consistent with experimental results.     

Surface electronic structure of α-Mo2C(0 0 0 1)

The electronic structure of α-Mo₂C(0 0 0 1) has been investigated by angle-resolved photoemission spectroscopy utilizing synchrotron radiation. A sharp peak is observed at 3.3 eV in normal-emission spectra. Since the peak shows no dispersion as a function of photon energy and is sensitively attenuated by oxygen adsorption, the initial state of the peak is attributed to a surface state. Resonant photoemission study shows that the state includes substantial contribution of 4d orbitals of the Mo atoms in the second layer. The emissions with constant kinetic energies of 22 and 31 eV above the Fermi level (E_F) are found in normal-emission spectra, and these emissions are interpreted as originating from the Mo N₁N₂₃V and N₂₃VV Auger transitions, respectively.     

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.     

Growth and electronic properties of ultra-thin Ag films on Ni(1 1 1)

We studied the growth mode and electronic properties of ultra-thin silver films deposited on Ni(1 1 1) surface by means of scanning tunnelling microscopy (STM) and angle resolved photoemission spectroscopy (ARPES). The formation of the 4d-quantum well states (QWS) was analysed within the phase accumulation model (PAM). The electronic structure of the 1 ML film is consistent with the silver layer which very weakly interacts with the supporting surface. The line-shape analysis of Ag-4d_xz,yz QWS spectrum support the notion of strong localization of these states within the silver layer. The asymmetry of the photoemission peaks implies that the decay of the photo-hole appears to be influenced by the dynamics of the electrons in the supporting surface.     

Progressive changes in atomic structure and gap states on Si(0 0 1) by Bi adsorption

From ab initio studies employing the pseudopotential method and the density functional scheme, we report on progressive changes in geometry, electronic states, and atomic orbitals on Si(0 0 1) by adsorption of different amounts of Bi coverage. For the 1/4 ML coverage, uncovered Si dimers retain the characteristic asymmetric (tilted) geometry of the clean Si(0 0 1) surface and the Si dimers underneath the Bi dimer have become symmetric (untilted) and elongated. For this geometry, occupied as well as unoccupied surface states are found to lie in the silicon band gap, both sets originating mainly from the uncovered and tilted silicon dimers. For the 1/2 ML coverage, there are still both occupied and unoccupied surface states in the band gap. The highest occupied state originates from an elaborate mixture of the p_z orbital at the Si and Bi dimer atoms, and the lowest unoccupied state has a ppσ* antibonding character derived from the Bi dimer atoms. For 1 ML coverage, there are no surface states in the fundamental bulk band gap. The highest occupied and the lowest unoccupied states, lying close to band edges, show a linear combination of the p_z orbitals and ppσ* antibonding orbital characters, respectively, derived from the Bi dimer atoms.     

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.     

Hopping motion of chlorine atoms on Si(1 0 0)-(2 × 1) surfaces induced by carrier injection from scanning tunneling microscope tips

Injection of tunneling electrons and holes from the probe tips of a scanning tunneling microscope was found to enhance the hopping motion of Cl atoms between neighboring dangling-bond sites of Si dimers on Si(1 0 0)-(2 × 1) surfaces, featured by the rate of hopping linearly dependent on the injection current. The hopping rate formed peaks at sample biases of V_S∼+1.25 and −0.85 V, which agree with the peaks in the local density of states spectrum measured by scanning tunneling spectroscopy. The Cl hopping was enhanced at Cl-adsorbed sites even remote from the injection point. The Cl hopping by hole injection was more efficiently enhanced by sweeping the tip along the Si dimer row than by tip-sweeping along the perpendicular direction. Such anisotropy, on the other hand, was insignificant in the electron injection case. All of these findings can be interpreted by the model that the holes injected primarily into a surface band originated from the dangling bonds of Si dimers propagate quite anisotropically along the surface, and become localized at Cl sites somehow to destabilize the Si-Cl bonds causing hopping of the Cl atoms. The electrons injected into a bulk band propagate in an isotropic manner and then get resonantly trapped at Si-Cl antibonding orbitals, resulting in bond destabilization and hopping of the Cl atoms.     

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.     

First-principles study on the half-metallicity of zinc-blende CrP(1 1 0) surface

We have investigated the half-metallicity and magnetism at the (1 1 0) surface of CrP by using the all-electron full-potential linearized augmented plane wave (FLAPW) method within the generalized gradient approximation (GGA). From the calculated local density of states (LDOS), we found that the (1 1 0) surface of CrP preserves the half-metallicity, but the band gaps (∼1.1 eV) of the minority states for the surface Cr and P atoms are much reduced from the bulk value (∼1.9 eV). The magnetic moment of the P is coupled antiferromagnetically to that of the Cr. The magnetic moment of surface Cr atom is calculated to be 3.31μ_B which is increased by 10% compared to the bulk value, 3.00μ_B.     

Comparative study of the GaAs(1 0 0) surface cleaned by atomic hydrogen

In attempt to correlate electronic properties and chemical composition of atomic hydrogen cleaned GaAs(1 0 0) surface, high-resolution photoemission yield spectroscopy (PYS) combined with Auger electron spectroscopy (AES) and mass spectrometry has been used. Our room temperature investigation clearly shows that the variations of surface composition and the electronic properties of a space charge layer as a function of atomic hydrogen dose display three successive interaction stages. There exists a contamination etching stage which is observed up to around 250 L of atomic hydrogen dose followed by a transition stage and a degradation stage which is observed beyond 700 L of exposure. In the first stage, a linear shift in the surface Fermi level is observed towards the conduction band by 0.14 eV, in agreement to the observed restoration of the surface stoichiometry and contamination removal. The next stage is characterized by a drop in ionization energy and work function, which quantitatively agrees with the observed Ga-enrichment as well as the tail of the electronic states attributed to the breaking As-dimers. As a result of the strong hydrogenation, the interface Fermi level E_F − E_v has been pinned at the value of 0.75 eV what corresponds to the degradation stage of the GaAs(1 0 0) surface that exhibits metallic density of states associated with Ga_As antisites defects. The results are discussed quantitatively in terms of the surface molecule approach and compared to those obtained by other groups.     

Valence band and band gap photoemission study of (1 1 1) In2O3 epitaxial films under interactions with oxygen, water and carbon monoxide

Synchrotron radiation ultraviolet photoemission experiments at photon energies of 150 and 49 eV were performed on an epitaxial layer of (1 1 1) In₂O₃ with good crystallinity as established by a standard scanning probe and diffraction methods. Valence band (VB) and band gap photoemission spectra were monitored under separate oxygen, water and carbon monoxide exposures (100 L) at different activation temperatures within the range utilized for chemiresistive gas sensors (160-450 °C). Large changes in photoemission response within the whole VB were observed for all gases. Regular shifts of the valence band edge relative to the Fermi energy were found under gas exposures on two kinds of surface (partially reduced or partially oxidized), and are interpreted as changes of surface potential. Treatments in oxygen resulted in upward band bending (∼0.5 eV at T = 320 °C). Regardless of activation temperature, treatments in water resulted in downward band bending, but with small changes (<0.1 eV). Reduction properties of carbon monoxide were observed only at high temperatures of T ? 370 °C. At temperatures of 160 and 250 °C unusual “oxidizing” behavior of CO was observed with upward band bending of ∼0.7 eV (160 °C). Oxidizing and reducing effects of the gas interactions with the (1 1 1) In₂O₃ surface in all cases were accompanied by a corresponding behavior, i.e., a decrease or increase in photoemission response from so-called defect states in the band gap near the top of the valence band. The increases of photoemission within a band gap with maxima at binding energies (BE) of 0.4 (O₂-induced peak) and 1.0 eV (CO-induced peak) were, respectively, found for interactions with O₂ and CO for low temperatures (T = 160 and 250 °C). These responses were ascribed to acceptor-like electronic levels of O₂ and CO chemisorption states, respectively. A definite split of the top VB peak (BE ∼ 4.0 eV) was found under CO dosing at 160 °C. Established knowledge of the CO interaction with the (1 1 1) In₂O₃ surface explains earlier revealed acceptor-like behavior of In₂O₃ film conductivity during CO detection at operational temperatures lower than 250 °C through the formation of acceptor-like electronic levels of adsorbed CO molecules.     

Adsorption of Cs on InAs(1 1 1) surfaces

Caesiated InAs(1 1 1)B (1 × 1) and InAs(1 1 1)A (2 × 2) surfaces have been studied by photoelectron spectroscopy. On the InAs(1 1 1)B a new (√3 × √3)R30° reconstruction was observed. During Cs evaporation remarkably small changes are observed in the lone pair states, and no sign of an accumulation layer at the surface can be observed. Instead, the additional charge provided by Cs is rapidly transported towards the bulk. On the InAs(1 1 1)A cesium behaves as a typical electropositive alkali metal donator that enhances the already existing accumulation layer.     

Chemically prepared well-ordered InP(0 0 1) surfaces

In the present work HCl-isopropanol treated and vacuum annealed InP(0 0 1) surfaces were studied by means of low-energy electron diffraction (LEED), soft X-ray photoemission (SXPS), and reflectance anisotropy (RAS) spectroscopies. The treatment removes the natural oxide and leaves on the surface a physisorbed overlayer containing InCl_x and phosphorus. Annealing at 230 °C induces desorption of InCl_x overlayer and reveals a P-rich (2 × 1) surface. Subsequent annealing at higher temperature induces In-rich (2 × 4) surface. The structural properties of chemically prepared InP(0 0 1) surfaces were found to be similar to those obtained by decapping of As/P-capped epitaxial layers.     

Carbon monoxide adsorption on the metastable fcc-Co/Cu(1 0 0) surface

The adsorption properties of CO on the epitaxial five-monolayer Co/Cu(1 0 0) system, where the Co overlayer has stabilized in the metastable fcc-phase, are reported. This system is known to exhibit metallic quantum well (MQW) states at energies 1 eV or greater above the Fermi level, which may influence CO adsorption. The CO/fcc-Co/Cu(1 0 0) system was explored with low energy electron diffraction (LEED), inverse photoemission (IPE), reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Upon CO adsorption, a new feature is observed in IPE at 4.4 eV above E_F and is interpreted as the CO 2π^∗ level. When adsorbed at room temperature, TPD exhibits a CO desorption peak at ∼355 K, while low temperature adsorption reveals additional binding configurations with TPD features at ∼220 K and ∼265 K. These TPD peak temperatures are correlated with different C-O stretch vibrational frequencies observed in the IR spectra. The adsorption properties of this surface are compared to those of the surfaces of single crystal hcp-Co, as well as other metastable thin film systems.     

A DFT periodic study of the vanadyl pyrophosphate (1 0 0) surface

The unique ability of the vanadyl pyrophosphate (1 0 0) surface to activate n-butane and then selectively oxidise the hydrocarbon to maleic anhydride was investigated using modern quantum chemical methods. Bulk (VO)₂P₂O₇, together with stoichiometric and phosphorus-enriched (1 0 0) surfaces, were analysed using periodic density functional theory calculations. Also simulated was surface ionic relaxation from bulk geometry, and surface hydration. Density of states (DOS) plots show that, whether stoichiometric or phosphorus-enriched, bulk terminated or relaxed, bare or hydrated, local covalent reactivity at the (1 0 0) surface is controlled by vanadium species. Terminal P-O oxygen species are the most nucleophilic surface oxygens, as indicated by their predominance of sub-vanadium high-lying valence band levels. A periodic treatment of (VO)₂P₂O₇(1 0 0) hence gives results qualitatively identical to those obtained from earlier cluster calculations. Simulation of surface ionic relaxation shows that in-plane P-O-V oxygens may also be involved in rupture of substrate C-H bonds for mild oxidation, while surface hydration calculations indicate that dissociative chemisorption of water may play a key role in perpetuation of the selective oxidation cycle.     

Electronic structure and electron dynamics at the GaSb(0 0 1) surface studied by femtosecond pump-and-probe pulsed laser photoemission spectroscopy

Transiently excited electron states at the GaSb(0 0 1) surface have been studied by means of time- and angle-resolved photoemission spectroscopy based on a femtosecond laser system. A normally unpopulated surface electron state has been found at ∼250 meV above the valence band maximum with a strong confinement at the center of the surface Brillouin zone. The lifetime of transiently excited carriers at the intergap surface states has been found to be ∼11 ps, associated with rapid carrier diffusion.