Showing papers on "Surface states published in 1979"

Electronic surface resonances of crystals

Abstract: Electrons incident on a crystal surface can be temporarily trapped in surface states at energies above the vacuum level. These temporary or nonstationary surface states are observed as narrow fluctuations of elastic scattering intensity with respect to variation of electron energy and incidence direction. The scattering process is called electronic surface resonance scattering. The temporary surface states that are intermediate states in resonance scattering are called electronic surface resonances. The article surveys both experimental and theoretical research on electronic surface resonances as observed by scattering of low-energy (<1 keV) electrons. A critical account of experiments on Al(001), W(001), Ni(001), and oxygenated Ni(001) surfaces is offered together with theoretical commentary. Plots of the electronic surface resonance band structure E (k/sub parallel/) (E=resonance energy, k/sub parallel/ =reduced surface-parallel momentum) are compiled and the signficance of E (k/sub parallel/) plots for surface characterization is indicated.

Photoemission studies of intrinsic surface states on Si(100)

Abstract: We have determined the energy vs. k∥ dispersion of intrinsic surface states on Si(100) (2×1) by means of angle‐resolved photoemission with synchrotron radiation. For k∥=0, there is a very pronounced surface state 0.7 eV below EF(i.e.,∠0.4 eV below the top of the valence band) which disperses downwards with increasing k∥. At J′ on the boundary of the surface Brillouin zone, we find two states with binding energies of 0.7 and 1.2 eV with respect to EF. A number of geometrical models and their corresponding theoretical surface state bands have recently been published for Si(100) (2×1). Our results disagree qualitatively with all surface state dispersions calculated up to date, i.e., the ideal surface, the vacancy model, the pairing model, and the zig‐zag‐chain model. Namely, we observed semiconducting surface state bands with a low state density at EF while all models calculated to date yield metallic surface states with a large state density at EF.

Photoemission studies of the silicon-gold interface

Abstract: The early stages of the formation of the Au-Si interface have been studied with photoelectron spectroscopy of the valence-band and core levels. In this study, the Si sample was prepared by cleavage in ultrahigh vacuum and Au was deposited in a controlled manner at room temperature. By slowly increasing the Au coverage on the surface, the silicon surface states were depleted rapidly (by a factor of 2 at an Au coverage of 0.2 monolayers) without any observable change (g0.1 eV) in the Fermi-level pinning position. Furthermore, at low coverages, the binding energies of the Au core levels and the $5d$ peaks in the valence band were similar to those of atomic Au; however, the width of the structure indicated that the Au is strongly interacting with Si. Measurement of the strength of the Au core levels gives evidence for penetration of a fraction of Au into Si. Thus, at low coverages, the Au is probably dispersed in and on the Si, causing the removal of the surface states and production of new states in the band gap. As the Au coverage is increased, the Au $4f$ and Si $2p$ core levels shift in such a way as to suggest the formation of an alloy with variable composition at the Au-Si interface. At the highest Au coverages (above 15 monolayers), a small amount (less than 1 monolayer) of Si was observed on the surface of the deposited Au overlayer. Thermal annealing of a thin Au (50 monolayers of Au) on Si at 500\ifmmode^\circ\else\textdegree\fi{}C resulted in an increased Si concentration at the surface with the photoelectron spectra resembling those at low coverages (about 1.5 monolayers), thus indicating a high diffusion coefficient of Si through the Au layer. When less than 1 monolayer of ${\mathrm{O}}_{2}$ was adsorbed onto the cleaved Si surface prior to the deposition of Au, the intermixing of Au and Si was significantly reduced.

Electrons in disordered metals and at metallic surfaces

Abstract: The Density Functional Theory of Metallic Surfaces.- Density Functional Calculations for Atomic Clusters.- Impurity Knight Shift and Electric Field Gradients at Al Nuclei in Dilute Substitutional Al-Li Alloys.- First Principles Band Theory for Random Metallic Alloys.- Aspects of the Numerical Solutions of the KKR-CPA Equations.- Bulk Electronic Structure of Disordered Transition and Noble Metal Alloys.- Fermi Surface Studies in Disordered Alloys: Positron Annihilation Experiments.- Optical Absorption and Photoemission from Random Alloys.- Polarized Electrons from Metallic Systems.- Transition from the Ordered Solid to the Disordered Liquid State Viewed by Photoemission.- The Magnetic Properties of Alloys.- The Electronic and Cohesive Properties of Disordered Simple Metals.- A Pseudopotential Approach to Some Problems in Binary Alloys of Simple Metals.- Determination of the Distortion Field in Binary Alloys.- Metallic Glasses: Bulk and Surface Properties.- On the Theory of Disordered Systems: CPA Calculation of (SN)x with Hydrogen Impurities and Hartree-Fock Theory of Surface States of Three-Dimensional Crystals.- On the Lectures Whose Full Text Does Not Appear in This Book.- List of Lecturers.- List of Participants.

Self-consistent pseudopotential calculation for the relaxed (110) surface of GaAs

Abstract: The electronic structure of the (110) surface of GaAs is analyzed using a surface-relaxation model as determined by recent low-energy-electron diffraction studies. A self-consistent pseudopotential calculation based on this model yields no intrinsic surface states within the fundamental band gap: a result not achieved in previous pseudopotential calculations. The calculations appear to be in good accord with recent photoemission measurements and we present an analysis of the surface-state energy spectra. In addition, our studies suggest that pseudopotential calculations coupled with angle-resolved photoemission measurements can be a sensitive probe of surface structural properties.

Surface structure and surface states in magnesium oxide powders

Abstract: The photoluminescent and reflectance spectra of MgO smoke have been compared with the spectra of MgO ex-hydroxide and ex-carbonate. Parallel studies by electron microscopy have shown that a strong correlation exists between the optical spectra and the different morphology of the particles obtained by the different routes. The spectra observed are associated with the surface of the oxide particles and the excitation and absorption bands are associated with oxygen ions in 3- and 4-fold coordination. The same kinds of sites are present on all samples but the ions of lowest coordination are much less abundant on the nearly perfect cubes of MgO smoke. The number of 3-fold coordinated sites on the smoke particles is considerably increased after exposure to water vapour for several h. This picture is consistent with the electron micrographs which show erosion of the edges and corners of the MgO smoke after treatment with water vapour. A model involving the removal of ion pairs from the corners and edges of the cube by water is shown to give the observed increase in the sites of lowest coordination.

Electronic structure of a Ti(0001) film

Abstract: A self-consistent calculation of the electronic structure of a thin Ti (0001) film leads to the prediction of a band of surface states coincident with the Fermi level and extending a few tenths of a eV on either side of it. The calculation is performed in a linear combination of Gaussian orbitals basis. The procedure by which the evaluation of the Hamiltonian matrix is reduced to summing over three-center integrals as well as the method by which Gaussian bases are selected are thoroughly described. Observation of the predicted surface band by angle-resolved photoemission is discussed, as well as the question of how general a surface band near the Fermi energy should be among the hcp d-band metals. A shift of surface atom core levels to 0.25-eV greater binding energy is predicted, and the origin of this shift is discussed.

d -like surface-state bands on Cu(100) and Cu(111) observed in angle-resolved photoemission spectroscopy

Abstract: Surface states at the top of the bulk $d$ bands of Cu(100) and Cu(111) single crystals have been observed using angle-resolved photoemission spectroscopy. The measured dispersion of the surface-state bands falls in absolute energy gaps of the projected bulk band structure near the symmetry point $\overline{M}$ of the two-dimensional Brillouin zone, for both faces. Since the parent energy gaps are not caused by the hybridization of crossed energy bands, the observed states are of the Tamm rather than the Shockley type; surface states of the latter type have been observed before in $\mathrm{sp}$ gaps of several noble metals. Tamm states require a sufficiently strong surface "perturbation." Only the self-consistent calculations of Gay, Smith, and Arlinghaus recently reported for Cu(100) contain surface states in the energy-wave-vector region studied here. The observed peaks are very narrow, due to their energy location above the bulk $d$-band continuum as well as the good angle and energy resolution of our spectrometer.

Theoretical electronic properties of Ti O 2 (rutile) (001) and (110) surfaces

Abstract: We have calculated the one-electron density of states of thick slabs of Ti${\mathrm{O}}_{2}$ (rutile) with (001) and (110) symmetry using the linear combination of muffin-tin orbitals (LCMTO) energy-band method. We have also calculated the density of states of bulk Ti${\mathrm{O}}_{2}$ and of bulk ${\mathrm{Ti}}_{2}$${\mathrm{O}}_{3}$. We found that the local titanium-oxygen coordination is the major factor determining the band structures of these surface and bulk systems. In particular, for the unreconstructed (001) face of Ti${\mathrm{O}}_{2}$, we found that there is a large band of occupied surface states in the bulk band gap while there are no occupied states for the unreconstructed (but slightly relaxed) (110) surface. Our results suggest that the (001) face reconstructs to increase the local coordination of titanium in order to eliminate these band-gap surface states.

The interface between Hg1−xCdxTe and its native oxide

Abstract: The results of a study of the electrical properties of the interface between Hg 1− x Cd x Te with x = 0.21 and its native oxide at 77°K are presented. The native oxide is formed by anodic oxidation and results in an interface with reproducible properties. The surface charge, the surface mobility and the effective lifetime are obtained from galvanomagnetic measurements and are related to the semiconductor bulk parameters, the oxide thickness and the annealing conditions. The surface state charge and the metal-semiconductors work function difference are obtained from the shift of the flat band voltage of metal-oxide-semiconductor (MOS) capacitor characteristics. The interface between Hg 1− x Cd x Te and its native anodic oxide is characterized by a density of fast surface states of the order of 5 × 10 11 cm −2 (eV −1 ) near the middle of the bandgap. The density of states increases towards the band edges to the order of 10 13 cm −2 (eV −1 ). The measured flat band voltage is approximately −0.5 V for an oxide thickness of 500 A and for an n -type semiconductor with an electron carrier concentration in the range 1–3 × 10 15 cm −3 at 77°K. The fixed oxide surface state charge is positive for both p -type and n -type semiconductors and is of the order of 6 × 10 11 charges per cm −2 . The surface properties, the significance and the reproducibility of the results are evaluated.

Initial steps of interface formation: Surface states and thermodynamics

Abstract: A general framework about the influence of reactive particles on surface electronic structure and barrier formation at low coverages (ϑ?10−2) is deduced from experimental results obtained under thermodynamic equilibrium conditions on a suitable compound semiconductor ’’model surface.’’ Adsorption isotherms enable a rough classification of weak and strong electronic interactions into physisorption, chemisorption, and surface reaction steps, which depend on temperatures and partial pressures. (1) Covalent bonding of particles on ionic compound semiconductor surfaces, characterized by small partial charges formally attributed to the adsorption complex, may lead to extremely large dipole effects even at very low coverages [e.g.,dipole moments ≳ 15 Debye may formally be attributed to the adsorption complex ZnO (1010)/CO2,chem at ϑ<10−2]. Correspondingly, drastic changes are found in surface atom relaxation. Sticking coefficients are close to unity. (2) Ionic bonding, characterized by pronounced band bending, ...

Magnetic surface states on Ni(100)

Abstract: Angle-resolved photoelectron spectra, utilizing polarized synchrotron radiation, from Ni(100) reveal a narrow surface-sensitive peak at the Fermi energy. This structure exists near the surface-Brillouin-zone edges in both the [10] and [11] directions. The symmetry of this state is odd (even) with respect to the (100) [(110)] mirror plane. Projections of the bulk bands onto the surface Brillouin zone show that these surface-sensitive energy levels exist in regions of the surface Brillouin zone where there are gaps in only one spin band. The very narrow energy width of these states indicates that they are surface states and consequently magnetic.

Investigation of energetic surface state distributions at real surfaces of silicon after treatment with HF and H2O using large-signal photovoltage pulses

Abstract: It is shown that the field-modulated large-signal photovoltage offers definite methodological advantages in comparison to other methods more frequently used for the determination of surface state distribution. Apparatus and sample preparations are described. The distribution of fast states in the forbidden gap at real silicon surfaces was studied after treatment in HF and rinsing in H2O up to times in excess of 100 h. Whilst for short rinsing times minimum concentrations of 3*1012 states cm-2 eV-1 were found, prolonged rinsing led to an 'intrinsic' distribution where the concentration in the minimum was lower by a factor of ten. The results are compared with those recently obtained at thermally oxidised silicon. A meaningful separation of surface states into three groups of different physical origin is possible both for real and for oxidised silicon surfaces. Fluoridation as a probable cause of extrinsic surface states, model descriptions of intrinsic states and, finally, certain aspects concerning semiconductor device technology are discussed.

Surface states at clean, cleaved GaAs(110) surfaces

Abstract: The work function of cleaved GaAs(110) surfaces was measured in dependence of temperature between room temperature and 400 °C. Surfaces cleaved from p‐type and tellurium‐doped n‐type crystals exhibit a reversible behavior, while silicon‐doped n‐type samples showed an irreversible decrease of the work function after the first heating cycles. In the presence of cleavage steps the work function and its temperature dependence are determined by step‐induced surface states via band bending and by step‐induced changes of the surface dipole.

Interface characteristics of Inp MOS capacitors

Abstract: A new electrolyte based on a solution of 10 g of ammonium pentoborate in 100 ml of ethylene glycol buffered to a ph of 6.0 has been used to produce a dielectric layer on the surface of InP. The relative permittivity of the anodic oxide is 7.9 and its dielectric breakdown strength is 3×106v/cm. The electrical characteristics of the surface states at the anodic oxide–‐type InP interface have been investigated using MOS capacitors. Capacitance–voltage data at 200 kHz indicate that the InP surface is already depleted at zero bias with a flat band voltage of 0.85 V. An inversion‐type low frequency (1 Hz) response has been observed for the first time on an InP MOS structure using an anodic oxide. Our analysis of the admittance data yields a U‐shaped surface state distribution having a minimum density of 4×1011 cm−2 eV−1 at mid‐band gap. Many other fea ures of our results also resemble those previously obtained with InP/organic insulator structures.

The electronic susceptibility and the Mo (001) surface phase transition

Abstract: The electronic susceptibility of the Mo (001) surface is calculated, in order to study the phase transition in which the displacement of the surface atoms couples surface states at the Fermi energy. It is found that there is only a small Kohn anomaly in the response to atomic displacements with the wavevector coupling the surface states-so the phase transition is not due to the temperature dependence of the Fermi distribution as in charge-density-wave systems. Instead it is suggested that the Mo (001) surface is unstable for arbitrary atomic displacements, with the surface states marginally favouring the displacement which couples them together. The surface atoms then move in an anharmonic potential and the phonon frequencies are temperature dependent, the mode with the coupling wavevector going soft first and giving the observed phase transition. The possibility of surface magnetism is also considered.

Chemisorption site geometry and interface electronic structure of Ga and Al on GaAs(110)

Abstract: Valence‐band photoemission spectra taken as a function of overlayer coverage during the formation of Ga–GaAs(110) and Al–GaAs(110) interfaces are analyzed. Possible chemisorption site geometries of Ga on GaAs(110) are studied through total energy calculations and the surface electronic structure for Ga (or Al) in an optimal twofold configuration as well as for the ideal onefold coordinated position is obtained. The calculations show that the interface formation results in a change in the relaxation of the GaAs surface from its vacuum interface configuration. This interfacial relaxation at the initial stage of metal–semiconductor interface formation pulls unoccupied surface states into the gap and pins the Fermi level.

Photoeffects in Fe2O3 sintered semiconductors

Abstract: Polycrystalline sintered pellets of α‐Fe2O3 have been studied by capacitance and photoelectrochemical techniques The pellets exhibit (i) photocurrent saturation at a given wavelength with increasing applied voltage, (ii) large dark currents at low applied voltage (076 μA cm−2 at +02 V SCE), (iii) cathodic and anodic photocurrent transients at low levels of monochromatic irradiation, (iv) photocurrent onset at photon energies greater than 21 eV, and (v) flat‐band potentials of −067 V SCE and pH=4 Capacitance measurements followed a Mott‐Schottky relationship indicating a predominance of the semiconductor space‐charge layer over surface states and electrolyte effects No degradation of the semiconductor properties could be observed at pH=4

Surface State Formation during Long-Term Bias-Temperature Stress Aging of Thin SiO2–Si Interfaces

Abstract: Surface state formation during bias-temperature (BT) stress aging is investigated in polycrystalline silicon (phosphorous doped)–thin SiO2–silicon capacitor fabricated by the standard silicon gate process. The applied stress field was 2.3–4.5 MV/cm. The stress temperature was 150–325°C, and the aging time was 2000 hours. Surface state density distributions were calculated using the quasi-static C-V technique. A slow increase in the sharp surface state density peak at 0.7 eV above the valence band edge without measurable flat-band voltage shift is observed after positive BT stress aging, while a rapid increase in the broad peak at 0.4 eV is observed after negative BT stress aging. The observed relation between the rate of change of surface state density and the stress field suggests that surface states may be induced by hot electrons injected into the oxide from P-type silicon by the Schottky process.