Showing papers on "Surface states published in 1976"
TL;DR: In this paper, it was shown that impurities and defects in semiconductors are associated with energy levels in the forbidden gap, and it is well known that impurity and defect in a semiconductor can be associated with the energy levels of surface states.
Abstract: It is well known that impurities and defects in semiconductors are associated with energy levels in the forbidden gap. Similar states occur at the surface of a semiconductor where the crystal lattice and the symmetry are strongly disturbed. These states are called surface states. Owing to the two-dimensional nature of the surface, their density is measured per unit area, in contrast to bulk states, which are measured per unit volume. A third type of states, similar to surface states, occurs at the interface between two adjacent materials. These states are called interface states. Very often they are also simply called surface states.
164 citations
TL;DR: In this paper, the semi-empirical tight-binding method was used to construct accurate valence bands for bulk Si and Ge using Hamiltonian parameters through second neighbors, and the density of surface bands and resonances associated with atomic layers near the surface was calculated.
Abstract: The semiempirical tight-binding method is used to construct accurate valence bands for bulk Si and Ge using Hamiltonian parameters through second neighbors. One additional parameter is used to describe surface relaxation (back-bond contraction). The density of surface bands and resonances associated with atomic layers near the surface is calculated. Surprisingly, the resonances are found to contribute to the surface density of states almost as effectively as proper surface states. Comparison with ion-neutralization, electron-energy loss, and ultraviolet photoemission data is made.
137 citations
TL;DR: In this paper, photoemission studies of valence bands, core levels, empty surface states, and band bending for cleaved GaAs (110) have been conducted, showing that there are no intrinsic band-gap surface states for GaAs, and that large excitronic binding energies are involved in core-level to empty-surface-state transitions.
Abstract: Photoemission studies of valence bands, core levels, empty surface states, and band bending for cleaved GaAs (110) are described. These studies show that there are no intrinsic band‐gap surface states for GaAs (110), and that large (approx. greater than 0.5 eV) excitronic binding energies are involved in core‐level‐to‐empty‐surface‐state transitions (e.g., using yield spectroscopy or electron energy‐loss spectroscopy). Studies of the effects on chemisorbed oxygen and thin Au overlayers on InAs (110) and GaAs (110) surfaces are also described. Trends and correlations in III–V semiconductor empty surface states are discussed.
130 citations
99 citations
TL;DR: In this article, the formation of Schottky barriers between Si(111) 7 and group-III metals has been studied by ultraviolet photoemission spectroscopy, and a two-step empirical model is proposed for the formation.
Abstract: The formation of Schottky barriers between Si(111) 7 \ifmmode\times\else\texttimes\fi{} 7 and group-III metals has been studied by ultraviolet photoemission spectroscopy Modifications of the band bending and of the work function occur for low values of metal coverage (one to four monolayers) The "intrinsic," clean surface states are simultaneously replaced with "extrinsic" metal-related interface states A two-step empirical model is proposed for the formation of the barrier The first step is saturation of interface bonds, and the second step, critical in determining the junction properties, corresponds to the formation of a thin region with properties intermediate between that of a metal and of a semiconductor Our experiments emphasize the need for a detailed theoretical treatment of the interface chemical bonds and underline the inadequacy of "macroscopic" models for metal-semiconductor junctions
99 citations
TL;DR: The presence of steps affects many surface properties like surface states, work function, adsorption and catalytic activity, therefore it is important to know the existence and properties of steps on all investigated surfaces.
Abstract: Among possible structural surface defects atomic steps constitute so far the only well investigated defect With Low Energy Electron Diffraction (LEED) step density, orientation, terrace width, step arrangements and other structural parameters may be measured Steps are present on nearly all surfaces, special step configurations may be produced by proper treatment The presence of steps affects many surface properties like surface states, work function, adsorption and catalytic activity It is therefore important to know the existence and properties of steps on all investigated surfaces
97 citations
TL;DR: In this paper, an energy-level scheme of surface states is proposed for the various surfaces, and room temperature oxidation saturates the dangling Ga bonds and eliminates the empty, intrinsic surface states for coverages as low as 0.3 of the saturation coverage.
Abstract: Electron‐energy‐loss measurements are reported for the polar {111} and (100) surfaces, as well as for the nonpolar (110) surface. The surface composition of the polar faces was changed by various annealing treatments. Surfaces with unsaturated Ga bonds exhibit empty surface states 0.9±0.2 eV above the bulk valence band edge. Their origin is attributed to the dangling Ga bonds. Dangling As bonds, if present, do not contribute to any surface states in the band‐gap region, but seem to be responsible for filled states 0.5–2 eV below the valence band edge—the value depending on the particular surface. An energy‐level scheme of surface states is proposed for the various surfaces. Room temperature oxidation saturates the dangling Ga bonds and eliminates the empty, intrinsic surface states for coverages as low as 0.3 of the saturation coverage. The oxygen is chemically bonded only to the Ga atoms—the As atoms do not seem to play a direct role in the oxidation process. The nature of the bonding seems to be identical for all surfaces investigated. There are indications that empty, oxide‐related states are located near the bottom of the bulk conduction band and extend into the gap region.
96 citations
TL;DR: In this article, contact potential difference (CPD) measurements for GaAs, GaP, and InAs UHV-cleaved single crystals, and for epitaxially grown layers of (110) orientation were reported.
Abstract: Contact potential difference (CPD) measurements are reported for GaAs, GaP, and InAs UHV‐cleaved single crystals, and for GaAs epitaxially grown layers of (110) orientation. For the GaAs and InAs single crystals the CPD between n‐ and p‐type samples of the same material was practically equal to the band gap. This means that the intergap surface state density must be <1012/cm2. For epitaxially grown GaAs layers the CPD appeared to be independent of the volume dope, i.e., the Fermi level is stabilized due to surface states in the forbidden zone. For GaP single crystals the energy bands appear to be flat up to the surface in p‐type material, but for an n‐type sample a band bending of 0.45 eV occurs.
79 citations
TL;DR: In this paper, the electronic structure of polystyrene was investigated using contact charge exchange experiments between various metals and atactic poly styrene, and it was argued that the carriers are electrons injected from the region of the Fermi level of the metal into the polymer.
Abstract: Contact charge‐exchange experiments between various metals and atactic polystyrene are utilized to elucidate the electronic structure of the polystyrene Bulk states present at approximately 1014/cm3 within a 4‐μ‐thick boundary layer, as well as surface states of approximate density 1010/cm2, govern the charge‐exchange characteristics of polystyrene in the metal contacts It is argued that the carriers are electrons injected from the region of the Fermi level of the metal into the polymer, and that the metal/polymer junction remains indefinitely in a nonequilibrium state A steady‐state injection level is reached because of differences in injection and transport properties of the surface boundary layer and the deeper bulk material A significant effect of the substrate metal on the populations of the electronic states at the free surface of the polystyrene films is identified and shown to be long range (at least 15 μ)
79 citations
TL;DR: In this paper, it was shown that a simple space charge current based on trap states in the middle region gives sufficiently high α-values when the injection problem at the space charge boundary is neglected, and that a correct quantitative description of the varistor behaviour must involve a rather complex junction model which includes additional mechanisms, such as tunneling out of surface states.
Abstract: A new type of voltage dependent resistors (varistors), based on zinc oxide ceramics with several other metal oxides as additives, has recently been investigated. The microstructure of these varistors consists of ZnO grains surrounded by thin insulating metal oxide barriers. We present experimental evidence to show that the highly non-linear current-voltage characteristics, with α-values (α = dlogI/dlogV) in excess of approximately 60 at room-temperature, can be described by the properties of an isolated junction between two ZnO grains. Guided by these and other experimental observations, we discuss several simplified versions of a theoretical model based on the assumption of an nnn or npn junction with its associated depletion regions, surface states, donor and acceptor states. We find that none of these simplified models can lead to α ≳ 25 within a current and voltage range that is consistent with the experimental observations. Only an inclusion of tunneling out of the surface states can in principle lead to larger values of α. On the other hand, a simple space charge current based on trap states in the middle region is shown to give sufficiently high α-values when the injection problem at the space charge boundary is neglected. We conclude that a correct quantitative description of the varistor behaviour must involve a rather complex junction model which includes additional mechanisms, such as e.g. space charge effects or tunneling out of surface states.
67 citations
TL;DR: In this article, the barrier height of a metal-polymer contact has been investigated using photoinjection of electrons from the metal into the PET, which indicates the existence of surface states on PET.
Abstract: The properties of a metal-polymer contact have been investigated using photoinjection of electrons from the metal into the PET. Contact barrier heights were determined from the Fowler's plot of photo-injection currents in PET. The barrier heights for Cu-PET and Al-PET were 2.90 and 2.83 eV respectively; however, the workfunctions of Cu and Al were 4.54 and 3.44 eV respectively. The difference in the barrier height between Cu and Al (0.07 eV) is much smaller than that in the workfunction (1.1 eV). This indicates the existence of surface states on PET. From a simplified contact model with a uniform density of surface states Ns, the value of the Ns was estimated as 1.7*1014 cm-3 eV-1. The barrier height was strongly affected by atmospheric conditions. O2 molecules adsorbed on PET can act as electron traps, i.e. surface states, because of their large electron affinity.
TL;DR: In this article, angle-resolved uv photoemission spectra (UPS) of the (110) surface of GaAs were used to study the dispersion information about occupied states and thus to discriminate between surface and bulk transitions.
Abstract: Synchrotron radiation was used to study the angle‐resolved uv photoemission spectra (UPS) of the (110) surface of GaAs. The wide photon energy continuum (hν<30 eV) coupled with 40 angular resolution permitted the identification of two occupied surface states, centered at ∠−1 and ∠−7 eV measured from the valence band (VB) maximum. These states have not previously been observed in photoemission experiments due to their degeneracy with the bulk VB emission. The relatively new technique of angle‐resolved UPS enables one to obtain dispersion information about occupied states and thus lends itself to discriminating between surface and bulk transitions. Since the parallel component of the electron’s momentum is conserved upon leaving the sample, portions of the occupied surface band structure could be identified and mapped out. These were found to be in qualitative agreement with the dispersions predicted by Joannopoulos and Cohen. Sensitivity of the emission to surface contamination served as a test of their su...
TL;DR: In this article, the deuterium absorption and desorption of H2(D2) on Pd layers are controlled successively by a chemisorption step (second order kinetics) in the early stage of the processes and by a surface migration step in the subsequent stage of processes.
TL;DR: In this article, the electronic structure of vacuum and metal-semiconductor interfaces was studied using a method involving self-consistent pseudopotentials, and the Schottky barrier heights were calculated in good agreement with experimental results.
Abstract: The electronic structure of vacuum–semiconductor and metal–semiconductor interfaces has been studied using a method involving self‐consistent pseudopotentials. Our model for an intimate metal–semiconductor interface consists of jellium in contact with a semiconductor described in the pseudopotential formalism. Local density of states and charge densities are used to analyze the electronic properties of the two types of interfaces. For the vacuum–semiconductor case, Si (111) surfaces and the (110) surfaces of III–V and II–VI zincblende semiconductors are investigated, with GaAs and ZnSe considered as zincblende prototypes. For metal–semiconductor interfaces, jellium of Al density in contact with (111) surfaces of Si and the (110) surfaces of GaAs, ZnSe, and ZnS are investigated. The calculated Schottky barrier heights are in good agreement with experimental results. It is shown that the variations in the experimental barrier height for different metals in contact with various semiconductors can be understo...
TL;DR: In this paper, the local densities of states on (110) and (100) facets of Mo and W are studied in the tight-binding approximation, including the spin-orbit coupling term, and using the moment method and the continued fraction technique.
Abstract: The local densities of states on (110) and (100) facets of Mo and W are studied in the tight-binding approximation, including the spin-orbit coupling term, and using the moment method and the continued fraction technique. Surface states are found on the (100) plane in agreement with experiments. They are shown not to be produced especially by spin-orbit coupling.
TL;DR: The GSCH model for surface states on (110) GaAs proposed by Gregory et al. appears (based on a wide range of results) to apply to all faces of all III-V semiconductors as mentioned in this paper.
Abstract: The GSCH model for surface states on (110) GaAs proposed by Gregory et al. appears (based on a wide range of results) to be well established and to apply to all faces of all III–V semiconductors. In this model, the filled and empty surface states are associated with and localized on the column V and III atoms, respectively, and a band gap (greater than 1 eV) separates the filled and empty surface states. For example, detailed studies of GaAs and InP locate the bottom of the empty states at 0.7 and 0.25 eV, respectively, below the conduction band minimum (CBM), whereas in GaSb it lies above the CBM. The filled states in all three cases are located well below the valence band maximum (VBM). In the context of the GSCH model, the details of the first steps in oxidation and formation of Schottky barrier with Cs have been examined experimentally. It is concluded from studies of Ga and As chemical shifts by Pianetta et al. that, in the first step of oxidation, the oxygen gains electrons from As but not the Ga su...
TL;DR: In this article, a commonly assumed model for the relaxed GaAs (110) surface is considered, which can account for the insensitivity of the empty cation-derived surface states to metal overlayers, and the lack of evidence for occupied anion surface states.
Abstract: A commonly assumed model for the relaxed GaAs (110) surface is considered. This model can account for the insensitivity of the empty cation-derived surface states to metal overlayers, and the lack of evidence for occupied anion surface states in recent photoemission measurements. With relaxation of the surface, we find that the empty cation-derived surface band becomes more dispersive. The center of mass lies above the conduction-band minimum; however, the threshold of the surface band remains within the bulk band gap.
TL;DR: In this paper, the authors considered charge buildup in MOS structures due to hole trapping in the oxide and the creation of sheet charge at the silicon interface and found that the contribution of hole trapping causes the flatband voltage to increase with thickness in a manner in which square and cube dependences are limiting cases.
Abstract: The model considers charge buildup in MOS structures due to hole trapping in the oxide and the creation of sheet charge at the silicon interface. The contribution of hole trapping causes the flatband voltage to increase with thickness in a manner in which square and cube dependences are limiting cases. Experimental measurements on samples covering a 200 - 1000 A range of oxide thickness are consistent with the model, using independently obtained values of hole-trapping parameters. An important finding of our experimental results is that a negative interface charge contribution due to surface states created during irradiation compensates most of the positive charge in the oxide at flatband. The tendency of the surface states to "track" the positive charge buildup in the oxide, for all thicknesses, applies both in creation during irradiation and in annihilation during annealing. An explanation is proposed based on the common defect origin of hole traps and potential surface states.
TL;DR: In this article, the relation between the total energy distribution of electrons field emitted from metals and the surface density of states is analyzed in general terms, and it is shown that the peak in the (100) plane at 0.29 eV below the Fermi level is due to virtual surface states which give rise to enhanced tunneling at that energy.
Abstract: The relation between the total energy distribution of electrons field emitted from metals and the surface density of states is analyzed in general terms. Numerical reslts on the total energy distribution of field-emitted electrons from the (100) and (110) planes of tungsten obtained previously by the authors and extended here to a wider energy region are interpreted in terms of surface density-of-states effects. It is shown in particular that the peak in the total energy distribution from the (100) plane at 0.29 eV below the Fermi level is due to virtual surface states which give rise to enhanced tunneling at that energy. For the (100) plane of tungsten virtual surface states are shown to exist in an energy region extending from 0.20 to 0.95 eV below the Fermi level, but only the states at 0.29 eV below ${E}_{F}$ give rise to peaks in the total energy distribution of the emitted electrons. Another broad peak in the total energy distribution from the (100) plane observed at about 1.3 eV below the Fermi level is also reproduced by our calculation. This peak is the cumulative effect of a number of factors which cannot be disentangled in a simple way, but is not a result of surface states.
TL;DR: In this paper, it was found that O 2 exposures above 10 5 L (1LM = 10 −6 Torr sec) were required to produce changes in the energy distribution curves.
TL;DR: In this paper, a detailed photoemission study of the cleavage face of GaAs was performed and it was shown that there are no empty surface states in the lower half of the energy gap.
Abstract: A detailed photoemission study of the cleavage face of GaAs shows that there are no empty surface states in the lower half of the energy gap, but empty surface states extend down to about the middle of the energy gap. The empty surface states cause Fermi-level pinning and band bending on $n$-type GaAs, but the bands are approximately flat for $p$-type GaAs. No evidence for filled surface states is found below the valence-band maximum, indicating that any filled surface states below the valence-band maximum have no strong structure and may be strongly mixed with bulk valence-band states. Several theoretical calculations of the surface states for GaAs (110) have been reported which find an empty surface state in good agreement with the results reported here, but the calculations also find a high density of surface states located approximately 0.5 eV below the valence-band maximum, in poor agreement with our results.
TL;DR: In this paper, a study of the (100 and (110) molybdenum surfaces by directional photo-emission spectroscopy is presented, and the results are discussed in terms of calculated band structure within the framework of the ${K}_{\ensuremath{\parallel}}$-conservation assumption.
Abstract: A study of the (100) and (110) molybdenum surfaces by directional photoemission spectroscopy is presented. Energy distribution spectra formed by photoelectrons emitted normal to the surfaces have been measured for photon energies between 10.2 and 21.2 eV. The results are discussed in terms of calculated band structure within the framework of the ${K}_{\ensuremath{\parallel}}$-conservation assumption. A good agreement is found between the main features of the experimental spectra and the emission expected from the band structure along the corresponding symmetry line in the Brillouin zone, assuming essentially direct-transition and surface-emission processes. Two extra structures are interpreted in terms of surface states or resonances: on the (100) surface, 0.5 eV below ${E}_{F}$; on the (110) surface, 4.5 eV below ${E}_{F}$ in the $s$-$d$ hybridization gap.
TL;DR: In this article, the electronic structure of the (001) ideal surface of niobium is calculated using a self-consistent pseudopotential method, and the surface states are identified and analyzed throughout the two-dimensional Brillouin zone.
Abstract: The electronic structure of the (001) ideal surface of niobium is calculated using a self-consistent pseudopotential method. Surface states are identified and analyzed throughout the two-dimensional Brillouin zone. Charge densities and the local density of states near the surface are presented and discussed. Our calculations predict strong surface features in the density of states in the range 0--2 eV above the Fermi energy. Occupied surface states are also discussed. (AIP)
TL;DR: In this article, a tight-binding calculation of the energy bands of a 29-layer (110) ferromagnetic iron thin film was performed by fitting a bulk calculation of Tawil and Callaway with the diagonal surface matrix elements shifted by a constant amount to obtain surface charge neutrality.
Abstract: We have performed a tight-binding calculation of the energy bands of a 29-layer (110) ferromagnetic iron thin film. The matrix parameters were obtained by fitting a bulk calculation of Tawil and Callaway with the diagonal surface matrix elements shifted by a constant amount to obtain surface charge neutrality. The energy bands were calculated at 117 points in the irreducible (one fourth) two-dimensional Brillouin zone. The planar and total densities of states are also reported and compared to previous results. A discussion of the surface states and energy bands is given and the structure of the energy bands is correlated to structure seen in the planar density of states and the effects of $s\ensuremath{-}d$ hybridization.
TL;DR: In this paper, the surface states in cleaved surfaces of II-VI compounds with zinc-blende structure were studied and the tight-binding method was used to calculate the electronic structure of a layer of twelve (110) planes.
Abstract: The nature of surface states in cleaved surfaces of II–VI compounds with zincblende structure is studied. The tight‐binding method is used to calculate the electronic structure of a layer of twelve (110) planes. The results show various surface bands both in the gap and in empty lenses of the projected bulk band structure. The nature of these states is discussed and a comparison with the available experiments is presented. The results support the model of ionic surface states previously proposed for these surfaces.
TL;DR: In this article, new surface states related to Al overlayers on the (110) surface of GaAs were found, consistent with the conjectures of Rowe, Christman and Margaritondo.
TL;DR: In this paper, various electron spectroscopy techniques, with electrons excited by either photons or ions, have rapidly developed as probes of the electronic structure of surfaces, surface complexes, and surfaces covered with thin overlayers of metals.
Abstract: Within the past 2–3 years, various electron spectroscopy techniques, with electrons excited by either photons,1–6 electrons,7–9 or ions,10 have rapidly developed as probes of the electronic structure of surfaces, surface complexes, and surfaces covered with thin overlayers of metals. This development, which has as its underlying basis the surface sensitivity of electron emission in the ∠10–103 eV range (i.e., hot electron mean free paths are usually in the 3–30 A range), has accompanied parallel developments in electron energy analyzers and sources (e.g., synchrotron radiation).Alternative techniques for probing electron energy levels at solid surfaces include ultraviolet photoemission spectroscopy (UPS),1–6 x‐ray photoelectron spectroscopy (XPS),11 field emission spectroscopy (FES),12 electron‐energy‐loss spectroscopy (ELS),7,8 ion neutralization spectroscopy (INS),10 appearance potential spectroscopy (APS),9 etc. Each technique has its particular advantages and disadvantages. uv photoelectron and electr...