Spectrum Of Surface States Research Articles

Kronig-Penney-type model for semi-infinite superlattices: I. A comment on surface-state energy expression

Energy spectrum of surface states in a semi-infinite superlattice represented by a Kronig-Penney-type model, calculated via the direct matching procedure and transfer matrix approach, is examined critically. A possibility of appearance of infinite barrier localized states, not corresponding to true surface states, is discussed in detail.

In 0.52 Al 0.48 As/In 0.53 Ga 0.47 As lateral resonant tunnelling transistor

The first fabrication and characterisation are reported for a lateral resonant tunnelling transistor formed in the In0.52Al0.48As/In0.53Ga0.47As system lattice-matched to InP. Strong multiple negative differential resistances (NDR) and transconductances are observed indicative of resonant tunnelling across depletion-defined tunnel-barriers in the channel of a two-dimensional electron gas. Calculations of the potential energy surface and quasibound state spectrum for this heterostructure provide qualitative understanding of the device operation and close agreement with the measured multiple-NDR peak spacings.

Optical spectroscopy of semiconductor surfaces

The use of optical spectroscopy for the investigation of semiconductor surfaces is reviewed. Its sensitivity to the spectrum of surface states and/or to surface atomic structure is discussed. A comparison between theory and experiment is carried out for Si(111)2×1, Si(110), GaP(110) and GaAs(110).

First-principles study of the electronic and magnetic structure of c(2 x 2) sulfur chemisorbed above Fe(001).

We have performed the first, first-principles study of the adsorption of sulfur above a magnetic, Fe surface. Our results, derived from the all-electron, film full-potential linearized augmented-plane-wave method applied to a seven-layer Fe film with and without c(2\ifmmode\times\else\texttimes\fi{}2) layers of S positioned next to the two surface Fe [Fe(S)] layers, include determinations of the equilibrium sulfur height (${H}_{\mathrm{eq}}$) and vibrational frequency, as well as the associated electronic and magnetic structures. We find excellent agreement between our calculated value (1.12 A\r{}) of ${H}_{\mathrm{eq}}$ with the earlier result [1.09(5) A\r{}] derived by Legg, Jona, Jepsen, and Marcus from a dynamical low-energy electron diffraction intensity analysis. The adsorption induces antibonding minority surface states immediately above and below ${E}_{F}$ which play an important role both in reducing the magnetic moment of the Fe(S) atom (by \ensuremath{\sim}20%) and in the rather small calculated increase (0.85 eV) in work function. These states should be clearly resolvable in both integrated and angle-resolved, spin-polarized photoemission and inverse-photoemission experiments. We present additional predictions, including the adsorption-induced changes in the hyperfine fields and in the angle-resolved, spin-polarized surface state spectra, and relate our findings to questions associated with the sulfur-induced poisoning of an iron catalyst.

Si(110)+Ni system: Structural, vibrational and electronic properties

Vibrational properties (Debye temperature and thermal expansion coefficients) as well as electronic ones (work function, electron affinity, spectra of surface electron states in the gap) were investigated on the clean “16x2” and Ni-induced (4x5), (5x1) and (2x1) structures of the Si(110) face. Atomic structure was monitored with LEED. Special attention was given to (4x5)Ni, (2x1)Ni and (5x1)Ni phases which showed reversible structural phase transitions. All parameters mentioned depend on the surface atomic structure. The elastic forces increase in strength and become more anharmonic upon the diminution of the elementary cell's size. Harmonic and anharmonic force constants have been obtained. The surface state spectra have a gap between donor- and acceptor-type states which varies from 0.2 to 0.4 eV depending upon structural phase transitions. The band bending of all the surface lattices indicates the presence of a depletion layer. Commonly speaking all the characteristics of long period (4x5)Ni and (5x1)Ni structures have been found to be similar but to differ remarkably from the (2x1)Ni one. Surface dimers are suggested to be the structural elements of all the surface phases. Dimers are formed during mutual saturation of dangling bonds on the Si(110) face.

Fermi-level pinning at the Sb/GaAs(110) surface studied by scanning tunneling spectroscopy.

Antimony overlayers on the GaAs(110) surface have been studied by scanning tunneling microscopy and spectroscopy. On ordered Sb terraces, a characteristic spectrum of surface states is observed. Near the edges of Sb islands, additional states are found within the band-gap region. These band-gap states comprise two separate bands of filled and empty states, with the surface Fermi level pinned between the two bands.

Interface state effects in Pd-gate mos hydrogen sensors

The interface state densities for Pd/SiO 2/n-Si structures have been measured by the conductance technique. Au automated instrumentation system has been developed for this purpose. A distinct change in the surface state spectrum is found for devices with thin (∼10 nm) oxides upon exposure to hydrogen; however, no such effect is seen for thick (∼100 nm) oxide structures. It is suggested that the interaction of the hydrogen with interface traps produced during the oxidation of the silicon is responsible for these observations.

Atomic geometry and surface-state spectrum for Ge(111)-(2×1)

The ..pi..-bonded chain model is studied for Ge(111)-(2 x 1). The minimum-energy chain geometry is found with the use of the Hellmann-Feynman forces. A buckling of the surface atoms in the chain is energetically favorable. This buckling is associated with rehybridization (sp/sup 3/ to sp/sup 2/) effects. Theoretical surface-state dispersions for this geometry are compared with recent angle-resolved photoemission experiments.

Barrier height and surface states at cleaned InSb(110) surfaces

The electronic properties of p-InSb(110) surfaces which were cleaned by the ion bombardment annealing technique are investigated by measurements of the integral and differential field effects. The field-induced changes in surface conductivity imply inversion layers with electron conduction in the space charge layer. The analysis of the experimental results yields the barrier height at the surface and the term spectrum of surface states. The general shape of the surface state density per energy interval shows a disorder-dependent continuous distribution with a minimum about midgap and increasing tails towards the bulk band edges. Near the conduction band minimum a peaking distribution of donor-type surface states is superimposed on the continuous distribution depending on the ion bombardment annealing conditions. The nature of the spectrum of surface states is discussed on the basis of intrinsic and extrinsic states.

Interface properties of metal/oxide/semiconductor structures with ultrathin plasma SiO 2

The conductance and capacitance of tunnel metal/oxide/semiconductor (MOS) structures with plasma SiO 2 (thickness 30–80 Å) as the oxide were measured at various frequencies. The experimental current-voltage curves and breakdown oxide field were also investigated. The surface state spectrum in the silicon gap for samples with different thicknesses and at various oxygen pressures is given. The surface state capture cross section for an n-Si(111) MOS with plasma SiO 2 is about 10 −18 cm 2. For all thicknesses investigated negative charge was observed in the surface states.

Optical absorption spectra of surface or interface states in hydrogenated amorphous silicon

The optical absorption of doped and undoped hydrogenated amorphous silicon (a-Si:H) films ranging from 5 nm to 10 μm was measured using photothermal deflection spectroscopy. The absorption spectra show that there is a high defect layer associated with the surface or interface of the film. From comparison of defect absorption and dangling bond spin densities, it is found that a-Si:H films which have ∼1015 bulk defects/cm3 exhibit surface or interface layers with ∼1012 dangling bonds/cm2.

Surface reconstruction and interface formation in Si and GaAs

We investigated the effects of substrate surface reconstruction upon the interface formation between In and Ge overlayers and Si(111)2 × 1, Si(111)7 × 7, and GaAs(110) substrates. Photoemission spectroscopy with synchrotron radiation was used to monitor the evolution of the interface parameters—Schottky barrier height or band discontinuities—as a function of the overlayer thickness. All interfaces involving Si(111)7 × 7 substrates exhibited a characteristic evolution pattern with saturation at 1.4–1.8 monolayers coverage. The interfaces involving Si(111)2 × 1 substrates instead exhibited faster evolution saturating below a monolayer coverage with marked differences between individual cleaved surfaces. A similarly rapid evolution was observed for the interfaces involving GaAs(110) substrates. These results indicate that the spectrum of intrinsic surface states is different for Si(111)2 × 1 and for Si(111)7 × 7. In particular for Si(111)7 × 7 a large density of localized states exists near the Fermi level.

Energy-dependent electron-energy-loss spectroscopy: Application to the surface and bulk electronic structure of MgO

The various effects that can occur when the primary electron energy, ${E}_{p}$, is varied in a reflection-energy-loss experiment are considered. For ${E}_{p}\ensuremath{\ge}100$ eV, the dominant effect is an increase of the electron mean free path with increasing ${E}_{p}$. By performing energy-loss measurements with $100 \mathrm{eV}\ensuremath{\le}{E}_{p}\ensuremath{\le}2000 \mathrm{eV}$, it is possible to unambiguously separate bulk and surface features in loss spectra. That technique has been applied to the MgO (100) surface, and both Mg intraionic and O-to-Mg interionic transitions have been studied. The former transitions are found to agree well with the excited-state spectra of free ${\mathrm{Mg}}^{2+}$ ions, while the latter agree more closely with itinerant-electron calculations of MgO. Intrinsic surface-state transitions are seen in both ${\mathrm{Mg}}^{2+}$ and O-to-Mg spectra. The Mg core-level surface-state spectra can be explained by Stark splitting of the surface ${\mathrm{Mg}}^{2+}$ levels in the intense electric fields at the crystal surface. The surface-state structure seen in O-to-Mg loss spectra agrees with discrete variational $X\ensuremath{\alpha}$ calculations of the MgO (100) surface and disagrees with linear combination of atomic orbitals calculations of the same surface. A low-energy-loss peak, possibly associated with surface defects, is seen on some surfaces; the exact nature of the surface defects involved is not yet clear.

GAP states at cleaved InSb(110) surfaces

Abstract The electronic properties of cleaved InSb(110) surfaces of p-type bulk material are investigated at low temperatures by measurements of integral and differential field effect. The asymmetry in the voltage dependence of the field induced changes of surface conductivity implies depletion layers in the surface depending on the cleavage state. Annealing to temperatures less than 350°C results in weak accumulation layers independently of the cleavage state. The analysis of the experimental results provides the term spectra of surface states within the energy gap. The general shape of the surface state density per energy interval shows a continuous distribution with a minimum near the middle of the gap and increasing tails towards the band edges. Near the conduction band minimum a peaking structure is superposed to the continuous distribution depending on the cleavage state. The nature of the spectrum of surface states within the gap is discussed on the base of intrinsic and extrinsic states.

Gap states at cleaved InSb(110) surfaces

The electronic properties of cleaved InSb(110) surfaces of p-type bulk material are investigated at low temperatures by measurements of integral and differential field effect. The asymmetry in the voltage dependence of the field induced changes of surface conductivity implies depletion layers in the surface depending on the cleavage state. Annealing to temperatures less than 350°C results in weak accumulation layers independently of the cleavage state. The analysis of the experimental results provides the term spectra of surface states within the energy gap. The general shape of the surface state density per energy interval shows a continuous distribution with a minimum near the middle of the gap and increasing tails towards the band edges. Near the conduction band minimum a peaking structure is superposed to the continuous distribution depending on the cleavage state. The nature of the spectrum of surface states within the gap is discussed on the base of intrinsic and extrinsic states.

Properties of rare earth oxide films

We investigated the properties of rare earth oxide films produced by the vacuum evaporation of rare earth metallic films and their oxidation in an oxygen atmosphere; we also studied the silicon-rare earth oxide interface. The rare earth oxide films were characterized by good insulating properties: an electrical resistivity of more than 10 12 Ω cm, a dielectric permittivity of 7–20, a dielectric dissipation factor of 0.01 or less and a breakdown field strength of (2–6) × 10 6 V cm −1. The behaviour of the effective surface charge density curve at flat-band voltages for scandium, yttrium and lanthanum to lutetium was found to be identical with that of the thermal neutron capture cross section for these elements. The presence of a deep level located some 0.3–0.38 eV below the conduction band edge was observed in fast surface state spectra of Si/Ln 2O 3 structures (Ln  Sc, Y, La, Gd, Er). On heating an Si/Y 2O 3/Al structure at 423 K for 20 min with a gate voltage of +5 V we observed a shift in the capacitance-voltage curve of approximately 2.5 V towards more negative voltages; for the same conditions but with a gate voltage of −5 V the shift was about 1 V in the opposite direction.

Self-consistent electron theory of a metallic surface

This is a review of researches on the calculation of the electronic characteristics of a free or perturbed metal surface; these include the electron density, the potential, the spectrum of surface states, the work function, the surface energy, response of the surface to an electric field, the chemisorption of atoms, and the adhesion of two metals. Preference is given to papers that employ the theory of the ground state of an inhomogeneous electron gas, the density-functional method. The various approximations to the density-functional used in the calculations, and the various models of the lattice (jelly, pseudopotential), are analyzed as to their applicability. Various sorts of experimental data are discussed on the basis of the theoretical results.

Surface structure and energy spectra of localized states

AbstractA simple model for surface reconstruction is developed utilizing the next‐nearest‐neighbor approximation within the framework of molecular‐orbital theory. Exact energy expressions for this model and various special cases are derived, and they are illustrated by numerical results. The influence of a very weak surface deformation on the energy spectrum of surface states is also discussed.

Interface states and carrier mobility in gamma irradiated most structures

Abstract Some results about the surface states and carrier mobility in p- and n-channel MOST structures, subjected to gamma radiation are presented. The experimental data are obtained by means of differential MOS inverter characteristics and Hall effect measurements in the conductive channel under the gate. The interpretation of the results is carried out on the basis of a theoretical analysis of the fast surface state spectrum at semiconductor/insulator interface. The assumptions and limitations of the physical model used are also discussed.

Tamm surface states of films Hg 1−xCd xTe

Using Kane model Tamm surface states spectra for semimetal and semiconductor samples Hg 1−xCd xTe with finite thickness were obtained. In the limit of thin film (qL ⪡ 1, q - quasi-momentum of surface state, L - film thickness) only the spectrum of antisymmetric surface states differs significantly from that of bulk electrons. Ratio of effective mass of these states to that of bulk electrons, in the parabolic limit (ε g → ∞), is four ( m s m e =4) . In the nonparabolic limit (ε g → 0) the ratio of Kane parameter of these states to that of bulk electrons is half ( P s P e = 1 2 ) . In the limit of thick film (L → ∞) these ratios differ from unity for some percents (5 + 6%).