Distribution of fast and slow states by photoelectric measurements on real InSb and Si surfaces

At T = 79 K, X rays change the surface properties of n and p type InSb single cryslals as well as the properties of Si-MOS structures. In order to study the influence of surface parameters the distributions of fast and slow surface states were measured at various stages of irradiation. The distribution of fast states on real InSb surfaces has been investigated by techniques similiar to those employed in measurements of current voltage characteristics. By using a combination of integral ac field effect with differential ac field effect the continuous spectrum of fast states was studied not only within the depletion region but also under strong accumulation and inversion. Analysis of photodischarging spectroscopy gives information on the energetic location of slow surface states on real Si surfaces.     

Surface state spectroscopy on zinc oxide crystals by means of modulated photoconductivity measurements

The spectral distribution of photoconductivity for photon energies between 0.2 and 2.5 eV was measured on ZnO crystals at low temperatures in ultrahigh vacuum (UHV). The prism surfaces (∥c axis) were cleaned by heating whereas the polar surfaces (⊥c axis) were prepared by cleavage in UHV. The spectra obtained on the three types of ZnO surfaces are different. Exposure to oxygen and to atomic hydrogen influences the absolute amount of photoconductivity. Therefore, an excitation from surface states is concluded and transition energies are derived.     

Electronic properties of TiC(100) and polar TiC(111) surfaces

The energy bands of films of TiC have been calculated using the linear-combination-of-atomic-orbitals method with parameters obtained by a fit to the bulk band structure. The Madelung potentials and charge redistribution have been determined self-consistently. For the neutral TiC(100) surface, the density of states (DOS) is similar to that of the bulk. For the non-neutral Ti-covered TiC(111) surface, Ti 3d-derived surface states appear around the Fermi energy E_F. The long-range electric field produced by the polar surfaces is screened by the charge redistribution, and the polar surfaces are stabilized. Characteristic features of TiC(111) compared to other surfaces of TiC are attributed to the high surface DOS at E_F.     

Electrochemical modulation spectroscopy

Applications of modulation spectroscopy techniques in electrochemical kinetic studies are reviewed. These methods permit direct in situ detection and identification of reaction intermediates and the measurement of kinetic rate constants approaching the diffusional limit. The results of spectral studies of adsorbed monolayers on metal surfaces are discussed with particular reference to surface roughness effects and photon-assisted charge-transfer transitions between the metal-adatom complex (considered to be a virtual bound state) and the conduction band of the substrate. Recent developments in the theory of the electroreflectance (ER) of metals are presented. The primary features are shown to arise from free-electron effects. At high positive biases, field-modulated interband transitions are enhanced owing to the reduced shielding of surface d orbitals. Surface plasmon generation in metals is also readily detected by ER measurements because of the ease with which interfacial optical parameters can be varied.     

Electro-optical effects at the discrete and continuum exciton states in GaAs

We report electroreflectance spectra from a GaAs-Au Schottky barrier interface, in the region of the fundamental energy gap, at a temperature of ～ 1.8 K. Both the ground exciton level and the continuum of states are investigated separately, by operating in the small modulation and in the on-off field limit, respectively. The field profile is monitored by I–V, C-V, and photovoltage measurements. It is found that, even at ～ 1.8K, a high surface field is generally present. This moves the actual reflecting boundary of the exciton-polariton away from the surface and affects the ER lineshape through field-induced inhomogeneity effects and interference across the high field layer below the surface. The spectra corresponding to the states of the continuum are discussed in terms of the recent calculations of photon-assisted tunneling with coulomb interaction.     

Thermally stimulated electromagnetic-field spectra in planar structures of complex composition: I. Nonradiative fields

It is demonstrated that a wealth of unambiguous information on the details of the atomic-lattice dynamics of solid-solution films can be acquired from a spectral study of the resonance features of thermal fields in immediate proximity to the planar-structure surface. The calculated spectral densities of the p- and s-polarized states of the nonradiative component of thermal fields in a plane-layered (or Cd _x Zn_{1 ? x} Te solid solution film-on-metal substrate) system are compared using the refraction additivity principle. The spectral densities are calculated for different impurity concentrations and thicknesses of the film and various distances from the structure surface.     

First surface electronic structure of a wurtzite-type semiconductor the polar and nonpolar surfaces of ZnO

We report the first calculations of the surface electronic structure for a Wurtzite-type semiconductor, namely ZnO. The bulk electronic properties of the material are described by a realistic empirical tight binding Hamiltonian which we obtained by fitting a selfconsistent pseudopotential bulk band structure and experimental bulk XPS and UPS data. Using the scattering theoretical method, we have calculated the surface band structures and wavevector-integrated as well as wavevector-resolved layer densities of states for polar and nonpolar ZnO surfaces. In agreement with experiment, we find no surface states in the gap on clean ZnO surfaces. Within the projection of the bulk valence and conduction bands, however, distinct surface features occur. First, there are ionic O-2p and Zn-4s surface states which have predominantly resonance character. In addition, more covalent back bond and anti back bond surface states are found which are occupied and empty, respectively. We find very good agreement with recent EELS and UPS surface data.     

On the preparation and electronic properties of clean W(1 1 0) surfaces

We have studied the influence of oxygen pressure during the cyclic annealing used for the cleaning of W(1 1 0) surfaces. For this purpose the surface morphology and electronic properties are measured by means of scanning tunneling microscopy (STM) and spectroscopy (STS), respectively. It is found that the surfaces with impurity atom densities as low as 2 × 10⁻³ can be obtained by gradually reducing the oxygen pressure between subsequent annealing cycles down to about 2 × 10⁻⁸ mbar in the final cycle. Only on the clean surface a bias-dependent spatial modulation of the local density of states (LDOS) is observed at step edges and around impurity sites by STS. In addition, we find a pronounced peak in the occupied states. In combination with density functional theory calculations these features can be traced back to a dispersive p_z-d_xz-type surface resonance band and the lower band edge of a surface state, respectively.     

Theory of electron states at random alloy surfaces - surface states on Cu-Ni (111)

We present first principles calculations (based on the KKRCPA) of the angle-resolved photocurrent emitted from the (111) surfaces of single crystals of $\text{Cu}$-Ni random alloys, and compare the results with new experimental data. Surface states close to the Fermi level are observed, even for concentrated alloys, and their behaviour as a function of composition and $\text{k}$_∥ is correlated with features in the bulk spectral density. Calculations for alloys with a non-uniform concentration profile at the surface (surface segregation) are described, and the effect on the surface states is discussed.     

Surface photovoltage spectroscopy and surface piezoelectric effect in GaAs

“Real” (111) surfaces of n-type GaAs were investigated employing surface photovoltage spectroscopy and the surface piezoelectric effect. Surface states at the energy position E_c ? E_t ? 0.72 eV were found on both the Ga and the As surfaces. Both types of surfaces exhibited a barrier of about 0.55 V. No variations in the surface barrier or the energy position of the surface states were observed in various ambients at atmospheric pressure (dry air, wet air, ammonia and ozone). However, the capture cross-section of the surface states for electrons, as determined from the surface piezoelectric effect transients (of the order of 10^?13 cm²), was found to be sensitive to the ambient. It decreased in wet air and increased in ozone. This effect was more pronounced on the As than on the Ga surfaces. Additional surface states were found to be present in the energy region of 0.9 to 1.0 eV, below the bottom of the conduction band. However, their exact energy positions could not be determined due to interference caused by the carrier trapping of the surface states at E_c ? E_t ? 0.72 eV.     

The adsorption of oxygen and carbon monoxide on cleaved polar and nonpolar ZnO surfaces studied by electron energy loss spectroscopy

Electron energy loss spectroscopy (ELS) with primary energies e₀ ? 80 eV has been performed on ultrahigh vacuum (UHV) cleaved nonpolar (11?00) and polar zinc (0001) and oxygen (0001?) surfaces of ZnO to study the adsorption of oxygen and carbon monoxide. Except for CO on the nonpolar surface where no spectral changes in ELS are observed a surface transition near 11.5 eV is strongly affected at 300 K on all surfaces by CO and O₂. At 300 K clear evidence for new adsorbate characteristic transitions is found for oxygen adsorbed on the Zn polar surface near 7 and 11 eV. At 100 K on all three surfaces both CO and O₂ adsorb in thick layers and produce loss spectra very similar to the gas phase, thus indicating a physisorbed state.     

Charge accumulation in ultrathin Cs / n - GaN and Cs / n - InGaN interfaces

We report on the observation of new phenomena that arise under Cs adsorption on n-GaN(0001) and n-InGaN(0001) surfaces. First, an extremely highly quantum efficient photoemission has been found by excitation with visible light in the transparency region of GaN and InGaN. The photoemission is revealed to appear due to the formation of an electron accumulation layer in the vicinity of the surfaces. Second, a large variety of band bending and potential wells are provided by the Cs coverages. The accumulated charge density at the n-InGaN surface is much stronger than that at the n-GaN surface. Third, a new effect is revealed, namely, the appearance of an oscillation structure in the spectral dependences of the threshold photoemission. A model concept is proposed for photocurrent oscillations that takes into account the formation of an accumulation layer and the multiple-beam interference in parallel-sided GaN or InGaN samples.     

Strongly correlated electron system in the magnetic field

In the range of hole concentrations 0.08<x<0.18$0.08<x<0.18$ the density of states of the two-dimensional t–J model reveals oscillations with changing the magnetic field. Oscillation frequencies correspond to large Fermi surfaces. However, the oscillations are modulated with a frequency which is smaller by an order of magnitude. The modulation is related to van Hove singularities in the Landau subbands, which traverse the Fermi level with changing the field. The singularities are connected with bending the subbands due to strong electron correlations. The frequency of the modulation is of the same order of magnitude as quantum oscillation frequencies in underdoped cuprates.     

Magnetic surface reconstruction in thin film: Possibilities of production and exploitation

Various theoretical techniques are considered for the production of magnetic surface reconstruction (MSR) in ferromagnetic thin films. The problem is discussed within the framework of the surface field model assuming the existence of a unidirectional anisotropy on the surface described by an effective field K_s It is shown that knowledge of the conditions for the occurrence of MSR together with complementary information accessible from critical spin wave resonance permits the complete determination of the properties of the field K_s.     

Analysis of semiconductor surface phonons by Raman spectroscopy

Only recently Raman spectroscopy (RS) has advanced into the study of surface phonons from clean and adsorbate-covered semiconductor surfaces. RS allows the determination of eigenfrequencies as well as symmetry selection rules of surface phonons, by k-conservation limited to the Brillouin zone-center, and offers a significantly higher spectral resolution than standard surface science techniques such as high-resolution electron energy loss spectroscopy. Moreover, surface electronic states become accessible via electron–phonon coupling. In this article the fundamentals of Raman scattering from surface phonons are discussed and its potential illustrated by considering two examples, namely Sb-monolayer-terminated and clean InP(110) surfaces. Both are very well understood with respect to their atomic and electronic structure and thus may be regarded as model systems for heteroterminated and clean semiconductor surfaces. In both cases, localized surface phonons as well as surface resonances are detected by Raman spectroscopy. The experimental results are compared with surface modes predicted by theoretical calculations. On InP(110), due to the high spectral resolution of Raman spectroscopy, several surface modes predicted by theory can be experimentally verified. Surface electronic transitions are detected by changing the energy of the exciting laser light indicating resonances in the RS cross section. Received: 7 April 1999 / Accepted: 25 June 1999 / Published online: 16 September 1999     

Optical-transmission modulation in CdS caused by acoustoelectric domain propagating along c-axis

When an acoustoelectric domain in a piezoelectric semiconductor traverses the monochromatic light near the intrinsic absorption edge, absorbed and transmitted optical modulations are observed. In order to reveal the mechanisms, we apply an electric field parallel to the c-axis of the CdS single crystal. In this case the electric field dependences of the off-axis angle are different for a high field domain and an acoustic domain. Comparing the electric field dependence of the off-axis angle of the optical modulation with that of the high field domain or the acoustic domain, we conclude that the transmitted and absorbed optical modulations are caused by the acoustic domain. Further, in order to establish the nature and mechanism of the optical modulation in case of E||c, we observe the spectral dependences of the absorbed optical modulation and find that there are two kinds of light absorption; one is due to pure shift in the absorption edge and the other is due to broad tail for low energy side. In case of e||c (e: polarization of incident light) both types of modulation exist at the same time and in case of e⊥c only the latter exists.And it is speculated that the transmitted optical modulation is due to the effect of induced birefringence caused by acoustic strain.