Occupied Surface States Research Articles

The surface potential of GaN:Si

The surface potential of GaN:Si, of fundamental interest for knowledge of the electrostatic potential and electric field strength, is determined for Si doping in the device relevant range from 6×1017 cm−3 to 2.3×1019 cm−3, in layers grown by low-pressure metal-organic vapor-phase epitaxy. Comparing the Si doping concentration, measured by secondary ion mass spectrometry, to the free electron concentration, measured by Hall effect, shows that all Si doping atoms are activated. We used the sheet resistance of the samples with different thicknesses measured by eddy current, a nondestructive, contactless method, to determine the depleted region. From the width of the depletion layer, which is dependent on the doping concentration, we obtained the GaN:Si surface potential on the basis of the depletion approximation. The surface potential decreases with increasing carrier concentration from about 1.5 eV to 0.2 eV. A qualitative explanation based on the theoretical description of two surface states, an occupied surface state at 1.7 eV and an empty state 0.6 eV below the conduction band edge, is given.

The effect of As, Co, and Ni impurities on pyrite oxidation kinetics: An electrochemical study of synthetic pyrite

Synthetic pyrite crystals doped with As, Co, or Ni, undoped pyrite, and natural arsenian pyrite from Leadville, Colorado were investigated with electrochemical techniques and solid-state measurements of semiconducting properties to determine the effect of impurity content on pyrite’s oxidation behavior. Potential step experiments, cyclic voltammetry, and AC voltammetry were performed in a standard three-electrode electrochemical cell setup. A pH 1.78 sulfuric acid solution containing 1 mM ferric iron, open to atmospheric oxygen, was chosen to approximate water affected by acid drainage. Van der Pauw/Hall effect measurements determined resistivity, carrier concentration and carrier mobility. The anodic dissolution of pyrite and the reduction of ferric iron half-reactions are taken as proxies for natural pyrite oxidation. Pyrite containing no impurities is least reactive. Pyrite with As is more reactive than pyrite with either Ni or Co despite lower dopant concentration. As, Co, and Ni impurities introduce bulk defect states at different energy levels within the band gap. Higher reactivity of impure pyrite suggests that introduced defect levels lead to higher density of occupied surface states at the solid–solution interface and increased metallic behavior. The current density generated from potential step experiments increased with increasing As concentration. The higher reactivity of As-doped pyrite may be related to p-type conductivity and corrosion by holes. The results of this study suggest that considering the impurity content of pyrite in mining waste may lead to more accurate risk assessment of acid producing potential.

High-resolution photoemission spectroscopy study of the single-domainSi(110)−16×2surface

We have investigated the valence band structure and $\mathrm{Si}\phantom{\rule{0.3em}{0ex}}2p$ photoemission spectra of the single-domain $\mathrm{Si}(110)\text{\ensuremath{-}}16\ifmmode\times\else\texttimes\fi{}2$ surface with higher resolution than previous studies. We found that the highest occupied surface state, reported to be dispersive in the previous studies, is resolved into the two surface states with flat energy dispersions. This reveals that the two surface states are not produced by Si $\ensuremath{\pi}$-bonded chains, as suggested in the previous studies, but originate from the building blocks with spatially localized electronic structures such as a Si tetramer and a Si adatom. $\mathrm{Si}\phantom{\rule{0.3em}{0ex}}2p$ line shapes show directly five surface components without any curve fitting. Various atomic structure models, especially the adatom-tetramer-interstitial model, of the $\mathrm{Si}(110)\text{\ensuremath{-}}16\ifmmode\times\else\texttimes\fi{}2$ surface are considered to figure out the atomistic origins of the surface components and states.

Role of surface states in Auger neutralization ofHe+ions on Ag surfaces

Recent measurements of He{sup +} ion fractions that survive to a whole scatterig event when they impinge on Ag surfaces have shown two different and interesting effects: (1) a notable difference of surviving ion fraction depending on which crystallographic face of the target surface is studied [Yu. Bandurin et al., Phys. Rev. Lett. 92, 017601 (2004)], and (2) an uncommonly high ion fraction in the very-low-energy range (tens of eV) [S. Wethekam et al., Phys. Rev. Lett. 90, 037602 (2003)]. Apart from the geometry, one of the differences between the surfaces of a crystal can be seen in the electronic structure: while the (111) surface has an occupied surface state near the Fermi level at the {gamma} point the (110) and (100) faces have not. Motivated by these facts, in this work we study the role that the occupied surface state plays on the Auger neutralization rate and we present an estimation of the ion fractions that survive for the different Ag faces.

Two dimensional behaviour of friction at a metal surface with a surface state

The contribution of electron–hole pair excitations to the friction coefficient of a charged particle traveling parallel to a metal surface is known to decay for large distances z0 to the surface as z0-4, provided the metal is well described by a semi-infinite jellium. Here we study the friction coefficient of a slow proton moving in front of the Cu(111) surface. The intrinsic characteristics of this surface, the existence of a wide energy gap in the band structure for electron motion along the surface normal and the existence of a partially occupied surface state, requires, in principle, a description more realistic than the jellium model. To do so, we use a one dimensional model potential that reproduces these features. The friction coefficient is calculated in linear response theory. We use the electronic states solution of the surface model potential to obtain the surface response function within the random phase approximation. Our results indicate that for large values of z0 the friction coefficient decays as z0-3. We show that this behaviour is due to the two dimensional character of the surface state.

Electronic surface states of Cu(1 1 0) surface

Surface states are a unique and important class of quantum states that shave an important effect on the electronic properties of Cu(1 1 0) surface. The Cu(1 1 0) surface has been studied using ultraviolet photoemission spectroscopy (PES), inverse photoemission spectroscopy (IPES), and reflection anisotropy spectroscopy (RAS), and shows a resonance in the RAS spectra at 2.1 eV due to a transition between occupied and unoccupied surface states. The unoccupied surface state involved in the RAS transition at an energy of 1.7 eV at the Y ¯ point of the surface Brillouin zone has been investigated using IPES and the occupied surface state is seen in PES spectra at 0.45 eV below the Fermi level. The energy difference of the surface states, 2.15 eV, is a good match to the transition energy found in the RAS experiments.

Electronic structures of the oxygenated diamond (100) surfaces

By means of first principles method on the basis of density functional theory (DFT), the equilibrium geometries and density of states (DOS) of the two oxygenated diamond (100) surfaces, bridging model and on-top model are calculated. The results indicate that there are no surface states located in the band gap of the bridging model of oxygenated diamond (100) surface, and the occupied surface states in the valence band are attributed to the non-bonded O 2p orbital, O 2p and C 2p bonding orbitals, and C 2p and H 1s bonding orbitals. By contrast, for the on-top model of oxygenated diamond (100) surface, the unoccupied surface states exist in the band gap, which originate from non-bonded C 2p and O 2p orbitals. In addition, the occupied surface states in the valence band are induced by non-bonded O 2p orbital and the C=O π bond.

Bandgaps, surface states and the anomalous neutralization of Li+ on (111) surfaces of noble metals

The effect of progressive changes in the electronic structure of (111) surfaces of noble metals on neutralization of Li+ ions is studied in relation to recent study on electron transfer on nanoclusters dealing with electronic effects. Strong differences in electron transfer probabilities are revealed and the trends in these changes are related to the progressive changes of the band structure of Ag, Cu and Au surfaces. Experimental data reveal an anomalously high neutralization of ions on these high workfunction surfaces and an unusual ion energy dependence, suggesting a pronounced non-adiabatic character of the electron transfer process, not accountable in a ‘jellium’ model of resonant electron capture. We suggest a mechanism of capture involving non-resonant charge exchange from occupied surface states as well as a strongly reduced loss due to the presence of the L-bandgap.

Angle resolved photoemission study of image potential states and surface states on Cu(1 1 1)

Angle resolved non-linear photoemission induced by short laser pulses is used to investigate both the occupied surface state and the unoccupied image potential states on Cu(1 1 1). The image potential state effective mass measured at zero parallel momentum with a non-resonant photon energy between the surface state and the image potential state is significantly different from the effective mass measured at resonance in the same conditions. The surface state effective mass shows no variation with photon energy. To explain the modification of the image potential state effective mass in the non-resonant excitation, the influence of the high energy electron population in the bulk bands near the image potential state must be taken into account.

Metallization of the3C−SiC(001)−(3×2)surface induced by hydrogen adsorption: A first-principles investigation

We report ab initio calculations on the atomic and electronic structure, formation energies and surface vibrational modes of a variety of hydrogenated $3C\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}(001)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ surfaces to shed more light on the recently discovered hydrogen-induced metallization of this semiconductor surface. The calculations are carried out employing the local density approximation of density functional theory. The adsorption systems are described using supercells, nonlocal norm-conserving pseudopotentials and Gaussian orbital basis sets. First, we show in agreement with other most recent ab initio calculations that Si dangling bonds at the third layer, originally suggested for explaining the experimentally observed surface metallization, are not stable. Instead, for low H exposure, we find H atoms to form monohydrides at the top layer and to become adsorbed in angular Si-H-Si bonds on the third layer. Due to these bonds the surface becomes metallic. We scrutinize further possibilities of surface metallization investigating numerous conceivable other H configurations on the first three layers of the $\mathrm{Si}\mathrm{C}(001)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ surface. It turns out that H atoms can also occupy bridge positions in angular Si-H-Si bonds on the second layer yielding a locally stable metallic surface. For larger H exposure, we find H atoms to form dihydrides at the top layer and to become adsorbed on the second and/or third layer in angular Si-H-Si bridge bonds. These structures are metallic, as well, and they are energetically even more favorable. In all cases considered, the angular Si-H-Si bonds are the origin of the surface metallization giving rise to partially occupied surface state bands which close the gap and overlap in parts with the lower projected conduction bands. Finally we address surface phonons. Some of the calculated frequencies are in very good accord with experiment. Others, particularly characteristic for the Si-H-Si bridge bonds, have not been resolved in experiment, to date.

Ultra-fast dynamics of coherent lattice and spin excitations at the Gd(0001) surface

The Gd(0001) surface is investigated by pump–probe experiments using femtosecond laser pulses at 740–860 nm wavelength. Employing optical second-harmonic generation, spin and lattice dynamics are separated through the symmetry of optical field contributions that are even and odd with respect to magnetization reversal. A coherent phonon–magnon mode at a frequency of 3 THz that is excited through the exchange-split surface state is observed in the time domain. A magneto-elastic phonon–magnon interaction based on spin–orbit coupling is weak for Gd and a modulation of the exchange interaction mediated by the lattice vibration is proposed as a microscopic interaction mechanism of this coupled mode. In parallel, electron–electron and electron–phonon interactions and their magnetic counterparts lead to incoherent dynamics of the electron, lattice, and spin subsystems. Variation of the optical wavelength shows that for longer wavelengths up to 860 nm the coherent mode dominates, while for shorter ones (≥740 nm) incoherent contributions prevail. This dependence indicates that selective depopulation of the occupied surface state component drives the coherent excitation. However, temperature-dependent studies show that the oscillation amplitude of even and odd contributions scales with the spin polarization of the surface state, suggesting that the spin dependence of the ion potentials contributes as well. Furthermore, the frequency of the coherent mode presents a blue shift with a delay of 0.17 THz/ps that saturates at the static frequency of the respective bulk phonon. This behavior is a consequence of equilibration of the screened ion potential at the surface subsequent to the intense laser excitation.

Initial Stage of Si(001) Surface Oxidation from First-Principles Calculations

A comprehensive density-functional theory (DFT) study of the atomic structure, electronic properties, and optical response of the Si(001) surface at the initial stages of oxidation is presented. The most favored adsorption position of a single O atom on top of the (4 x 2)-reconstructed Si(001) surface is found at the back-bond of the "down" Si dimer atom. There is no energy barrier for oxygen insertion into this bond. The ionization energy of the surface reaches a maximum when the oxidation of the second Si monolayer starts. Oxidation leads to an increase of the energy gap between occupied and empty surface states. The calculated reflectance anisotropy spectroscopy (RAS) data in comparison with experiment suggest a considerable amount of surface disorder already after oxidation of the first monolayer.

A simple chemical view of relaxations at stoichiometric (1 1 0) surfaces of rutile-structure type oxides: A first-principles study of stishovite, SiO 2

First-principles electronic structure calculations have been used to examine the geometric and electronic structure of the bulk and (1 1 0) surface of stishovite, the rutile-structure polymorph of SiO 2. The primary changes in geometric and electronic structure associated with surface relaxation are similar to those predicted for stoichiometric (1 1 0) surfaces of other rutile-structure oxides: TiO 2, SnO 2, RuO 2. Occupied surface states can be attributed primarily to changes in the local coordination environment (hybridization) of surface oxygen anions, and the relaxations that lead to “rumpling” of the stoichiometric (1 1 0) surface can be viewed as a change in hybridization of 3-coordinated in-plane oxygen from a planar (sp 2) bulk local coordination environment to a lower-energy, non-planar, pyramidal (sp 3) surface geometry, following earlier descriptions by Godin and LaFemina for SnO 2(1 1 0). It is demonstrated that these descriptions follow naturally from a visual examination of the 3D valence charge density distributions and the electron localization function (ELF) which provide a view of the electronic structure in terms of electron bond pairs and lone pairs. Consideration of the surface relaxations in terms of molecular analogs suggests that the simple valence shell electron pair repulsion (VSEPR) model provides insight into the chemical driving force for surface relaxation and oxygen rehybridization.

Interaction of CO molecules with surface state electrons on Ag(1 1 1)

Like other close-packed noble metal surfaces, Ag(1 1 1) exhibits an occupied Shockley-type surface state that is believed to influence the adsorption of atoms and molecules. Using low-temperature scanning tunneling microscopy, we have directly probed this interaction by investigating the local CO distribution dependent on the Ag(1 1 1) surface state standing wave pattern forming in the neighborhood of strong scattering centers such as step edges or hexagonal holes. A quantitative analysis of the STM data reveals that the CO molecules are not arbitrarily distributed upon adsorption at 5 K; they adsorb preferentially near the minima of the standing wave pattern.

Relationship between strain and the surface electronic structure of Cu(111) films on Ru(0001): Theory and experiment

The relationship between strain and surface electronic structure of Cu(111) films grown on Ru(0001) is studied by a combination of tunneling spectroscopy and ab initio theoretical calculations. Experimentally, the relaxation of the 5.5% in-plane lattice mismatch between Ru(0001) and Cu(111) changes layer-by-layer the lateral lattice parameter of the Cu film, while the surface state, that is above the Fermi level for the pseudomorphic monolayer of Cu, shifts down in energy with increasing thickness until it becomes the occupied surface state of Cu(111). The effects due to strain in the Cu films are distinguished from those due to the proximity to the Ru substrate by detailed comparisons with theoretically expanded Cu(111) and Cu/Ru(0001) surfaces. Our ab initio calculations indicate that the observed energy shift must be essentially assigned to the decreasing tensile stress of the deposited film.

Atomic and electronic structure of theSi(001)−Rbchemisorption system at 0.5 and 1.0 monolayer coverage

The atomic and electronic structure of the $\mathrm{Si}(001)\text{\ensuremath{-}}\mathrm{Rb}$ chemisorption system at 0.5 and 1.0 monolayer coverage has been investigated using the plane wave, pseudopotential density functional method contained in the VASP code. The atomic and electronic structures of the $\mathrm{Rb}$ chemisorption system at 0.5 monolayer coverage are found to be very similar to the $\mathrm{Li}$ chemisorption system at the same coverage. The minimum energy structure determined for the $\mathrm{Si}(001)\text{\ensuremath{-}}\mathrm{Rb}$ chemisorption system at $1.0\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ coverage has been found to be in good agreement with the x-ray standing-wave (XSW) field data. The predicted occupied and unoccupied electronic surface state bands have also been shown to be in excellent agreement with the ARUPS and IPES results. The calculations suggest that the bonding between the $\mathrm{Rb}$ adatoms and the $\mathrm{Si}$ substrate atoms is neither purely ionic nor covalent. No evidence has been found in support of strong $\mathrm{Rb}\text{\ensuremath{-}}\mathrm{Rb}$ interactions and the formation of a metallic overlayer.

Theoretical study of the electronic structure of the Si 3N 4(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-Si 3N 4 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 Si 3N 4. 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 Si 3N 4 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.

First principle study on β-SiC(001)-(2×1) surface structure

We calculate the atomic and electronic structure of β-SiC(001)-(2×1) using augmented plane wave and local orbital (APW+LO) method. The calculated results show that the atomic structure of β-SiC(001)-(2×1) surface can be described by symmetrical Si dimmer model, which is different from the asymmetrical Si dimmer model of Si(001)-(2×1) surface. The bond length of Si dimmer of β-SiC(001)-(2×1) surface is 0.269 nm, which is larger than that of Si(001)-(2×1) surface. The calculated results of electronic structure show that a prominent density of states exists at the Fermi level, so β-SiC(001)-(2×1) surface has the characteristics of metal. There are four surface state bands in the gap. Among them, two occupied surface state bands have been confirmed by valence band spectra of synchrotron radiation photoelectron spectroscopy.

Stability and electronic structure of Ge(1 0 5)1 × 2: a first-principles theoretical study

We studied stability and electronic structure of the Ge(1 0 5)1 × 2 surface by density functional theory (DFT) calculations. We show that the (0 0 1) surface is more stable than the (1 0 5) surface if the in-plane lattice constant is equal to the equilibrium lattice constant of Ge ( a Ge), while (1 0 5) becomes much more stable than (0 0 1) when the in-plane lattice constant is compressed to a Si. This stability change is in agreement with experimental observations that Ge(1 0 5) is stable on the Si substrate, and the reason can be explained from the differences in the surface bond lengths of the two surfaces. Because the Ge–Ge length is considerably elongated near the Ge(1 0 5) surface, the surface bond lengths approach their equilibrium values by compressing the in-plane lattice constant from a Ge to a Si, and (1 0 5) becomes fairly stable. Band calculations revealed that the gap between occupied and unoccupied surface states are quite large, suggesting the stability of the 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 pp σ* 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 pp σ* antibonding orbital characters, respectively, derived from the Bi dimer atoms.