Comparison of site-specific valence band densities of states determined from Auger spectra and XPS-determined valence band spectra in GeS (001) and GeSe (001)

Auger lineshapes of the Ge M₁M_4,5V and M₃M_4,5V and Se _M1M_4,5V transitions in GeS (001) and GeSe (001) are measured and compared to XPS valence band spectra. Distortions in both types of spectra due to inelastic scattering, analyzer and source broadening, and core level lifetime broadening are removed by deconvolution techniques. The valence band consists of three main peaks at ?2 eV, ?8 eV, and ?13 eV. There is excellent agreement of peak positions in AES and XPS spectra. The Auger lineshapes can be interpreted in terms of site-specific densities of states. They indicate that the states at ～?8 eV and at ～?13 eV are associated with the cation and anion sites respectively. The bonding p-like states at the top of the valence band have both cation and anion character. The Auger lineshapes indicate that the states closest to the valence band maximum are preferentially associated with Ge.     

Infrared modulated reflectance spectra of n and p type gallium antimonide and n type indium antimonide

We have observed the modulated reflectance spectra of n and p type GaSb at 300, 80, and 5 K from 0.56 to 2 eV. The modulated reflectance of intrinsic n type InSb was measured at 80 K from 0.2 to 2 eV. The “dry sandwich” vapor deposition technique was used to make the electroreflectance (ER) samples. The low-temperature spectrum of the undoped p type GaSb sample shows three peaks at the band edge that could be associated with transitions from the top of the valence band, the light (0.903 eV) and heavy (1.014eV) hole state Fermi levels to the conduction band. The energies of the observed peaks are in agreement with the Fermi level determination from Hall effect and Faraday rotation measurements. This modulation mechanism is based on band population effects. The ER signal of InSb under flatband condition at 80 K has five half oscillations at the direct band gap. The contribution of piezoelectric strain to ER is present since the dc bias required to achieve flatband condition is different at the band gap than at E₁. The ER signal corresponding to the direct gap energy E₀ and to the spin-orbit energy E₀ + Δ₀ was determined in the n and p type samples of GaSb at different temperatures. We have measured the intrinsic energy gap in GaSb at room temperature. E_g = 0.74 eV. The corresponding spin-orbit splitting was found to be Δ₀ = 0.733 ± 0.002 eV.     

Thermoreflectance of IV–VI compounds

We have measured thermoreflectance spectra of epitaxial PbS, PbSe, PbTe, and SnTe films from 0.7 to 5.6 eV, at room temperature and 77 K. Fine structures associated with the usual reflectivity peaks are observed, most of which agree in spectral position with electric field and wavelength modulation results. Δε_r and Δε_i have been obtained through a Kramers-Kronig analysis of our Δ R/R spectra. In the absence of experimental symmetry information, tentative assignments are made based on (1) comparison of temperature coefficients with theoretical deformation potentials, (2) comparison of δε curves with theoretical lineshapes, and (3) matching the structures with existing band structure calculations and related information.     

Spin-resolved photoemission study of in situ grown epitaxial Fe layers on W(110)

Spin- and angle-resolved photoemission spectroscopy of in situ grown epitaxial Fe layers on W(110) shows bulk-like behavior for more than two atomic Fe layers. For about ten and more atomic layers of Fe we find a spin polarization to be about -100% near the Fermi energy and +80 % between 1 eV and 3 eV binding energy. For the bilayer of Fe drastic changes in the spin-resolved spectra and a 20 % enhancement of the spin polarization compared to the bulk value are observed. The monolayer of Fe is ferromagnetically ordered with a spin polarization reduced by about 50%. A switching of the easy magnetization axis from [001] to [11̄0] is observed in the spin polarization with decreasing Fe layer thickness near d = (65±5) Å.     

Influence of the surface on the quantum P.E.M. effect in InP

The photoelectromagnetic effect of InP is studied in quantizing magnetic fields at 4·2 K in an energy range 1·4–1·5 eV for linearly polarized light. Depending on the sample surface condition two types of spectral oscillations may appear, those associated with interband transitions between Landau levels or the LO phonon type usually seen in photoconductivity. An analysis of the spectral oscillations gives: E₀ = 1·423±0·001 eV; Δ₀ = 0·102±0·006 eV; hω_L = 0·036 eV.     

Optical transitions on GaAs [110] surface

Modulation techniques have been employed in optical reflection measurements on GaAs [110] cleaved surface. Spectral bands at 2.62 and 2.83 eV observed for the light polarization $\text{E6[}\text{1}\text{10]}$ have been assigned to the surface state transitions. The E₁ and E₁ + Δ₁ bulk transitions were found to be sensitive to the surface conditions.     

Orientation of adsorbed ethylene oxide on Ni(110) by X-ray photoelectron diffraction

The adsorption of ethylene oxide on Ni(110) was studied at 95 K and monolayer coverage by angle-resolved X-ray photoelectron spectroscopy. A slow radiation-induced decomposition at hv = 1486.7 eV to most likely methoxy was noted. The orientation of the adsorbed ethylene oxide was determined by measuring forward scattering enhancements in the O 1s intensity distribution. Peaks in polar (θ) as well as azimuthal (φ) scans occurred at four angular positions in 2π above the surface: (θ = 54°, φ = 36°, 144°, 216°, 324°). These positions were evaluated to yield the tilt angle of the molecule at 48°_relative to normal, and the COC bond angle of adsorbed C₂H₄O of about 57°. The molecule is tilted towards the [001] and [001̄] directions (two domains), with a mirror plane in the [001] azimuth.     

Polarized optical studies of ordered InGaP2 under pressure

In this paper, we discuss polarized photomodulated reflectivity (PR) and photoluminescence (PL) studies of MOCVD grown InGaP₂ epilayers lattice-matched to a GaAs substrate. These structures were grown on a (001) face with a misorientation of two degrees along <110>. The PR spectra of the ordered sample show a strong polarization dependence. For the electric field E parallel to [1̄10] two features in the PR spectra are seen; for E∥[110], however, additional features are observed. A comparison with the spectra of disordered samples of the same alloy composition has enabled a determination of the band gap reduction due to ordering. The linewidths of the PR peaks are approximately 5-10 meV which has enabled us to study them in detail as a function of hydrostatic pressure at cryogenic temperatures. The pressure dependence is slightly sublinear with the first order term of 8-9 meV kbar^-1 for pressures well below the Γ-X crossover. Also observed is the indirect level crossing which occurs under pressure at about 40-kbar. An analysis of PR lineshapes at 1-bar is also presented at several commonly used experimental temperatures.     

Intermediate phases in [111]- and [001]-oriented PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>–29PbTiO<Subscript>3</Subscript> single crystals

Phase transformations in [111]- and [001]-oriented PbMg_1/3Nb_2/3O₃–29PbTiO₃ single crystals have been studied using dielectric and optical measurements before and after applying an electric field. It is shown that the subsequence of phase transitions rhombohedral (R)—tetragonal (T)—cubic (C) phases is observed in nonpolarized samples of both orientations as temperature increases. In the [111]-oriented crystal, an additional intermediate monoclinic phase (it is possible, M _a ) is induced after preliminary polarization at room temperature and the R–M _a –T–C phase transitions are observed on heating. In the [001]-oriented crystal, after its polarization, the monoclinic phase forms instead of the rhombohedral phase even at room temperature and the M _a –T–C transitions occur on heating. The results are discussed from the point of view of the existence polar nanoregions with different local symmetries in a glasslike matrix.     

Electroreflectance from flatband

One strategy for achieving quantitative electroreflectance, electroreflectance from flatband, is reviewed. In flatband electroreflectance, major emphasis is placed on determining the conditions for zero space charge, flatband. Modulations are then done from this condition to large depletion or accumulation band bending. An electrolyte technique is employed that enables the attainment of relatively uniform field strength across the light penetration depth. Results obtained by this experimental approach applied to Ge and GaAs are discussed. At the direct edge in these materials an interference between light and heavy hole contributions to E₀ is observed. Both one electron and exciton theories are shown to have enough flexibility to fit the direct edge lineshapes; however, neither theory yields completely consistent parameters. The E₁, E₁+ Δ₁ lineshapes are shown to fit a two-dimensional critical point theory. Modulations of highly doped n type Ge and GaAs into accumulation produce lineshapes dissimilar to the usual Franz-Keldysh and can be explained by band-filling effects.     

Angle-resolved photoemission from Fe(110): Determination of E()

Using angle-resolved photoemission spectroscopy utilizing polarized synchrotron radiation with a clean Fe(110) sample, we identified each of the Σ₁-, Σ₃- and Σ₄-band peaks with certainty using symmetry selection rules. We also determined energy band dispersion along the γ-Σ-N direction in the bulk Brillouin zone assuming direct transitions. There are small discrepancies between the observed bands and recent ground-state band calculations possibly due to Coulomb correlation effects. A surface state with odd symmetry about the [001] crystal direction and a binding energy of 0.15 eV at γ in the surface Brillouin zone was observed. Our results show that the ground-state calculations of ferromagnetic iron based on the local exchange correlation potential are basically in agreement with experimental dispersion relationship measurements.     

High-resolution normal emission UV-photoelectron spectra and band structure of Cu

The energy distribution curves of photoelectrons emitted along the normal direction from Cu (111), (001) and (110) surfaces using unpolarized HeI (hv=21.2 eV) and HeII (hv=40.8 eV, 48.4 eV) radiation incident at 45° to the surface were measured. For the interpretation the band structure as calculated by Janak et al. and by the combined interpolation scheme was used. The high resolution spectra along the [111] and [001] directions show dominant features from direct transitions, indicating strong residualk _z conservation, and density of states features, too. The band gap emission along the [110] direction using HeI radiation can be explained with direct transitions into evanescent states with nearly free electron bands. From the width of the peaks electron escape depths were calculated.     

A new spin effect in photoemission with unpolarized light: Experimental evidence of spin polarized electrons in normal emission from Pt(111) and Au(111)

Unpolarized light ejects spin polarized electrons from Pt(111) and Au(111) even if the electron emission occurs normal to the surface. For off normal incidence of 11.8 eV, 16.9 eV, and 21.2 eV radiation, and for the main peaks in the photoemission spectra, a degree of spin polarization of up to 30% or more is found for the spin polarization component P _y perpendicular to the reaction plane. A crystal rotation about its surface normal does not change P _y . P _y is largest for transitions from bands with symmetry ₆ ³ . All these experimental findings agree with a recent theoretical prediction [1] of a new spin effect by Tamura and Feder.     

Leed studies of vicinal surfaces of silicon

Clean silicon surfaces inclined at small angles to (111), (100) and (110) planes were investigated by LEED. Surfaces oriented at low angles to the (111) plane contain steps with edges towards [2&#x0304;11] or [21&#x0304;1&#x0304;]. Steps with edges towards [2&#x0304;11] have a height of two interplanar distances d₁₁₁ at low temperatures. At 800°C the reversible reconstruction of this step array into the steps of monolayer height takes place. Steps with edges towards [21&#x0304;1&#x0304;] can be seen at low temperatures only. They are of monolayer height and disappear at annealing in vacuum. Surfaces oriented at low angles to the (100) plane contain steps with (100) terraces and have a height of about two interplanar distances d₁₀₀. Surfaces at low angles to (110) planes are facetted and contain facets of the (47 35 7) type. The information about surface self-diffusion of silicon may be obtained using the kinetic data of structural reconstructions on surfaces close to (111) at different temperatures.     

Strain-split energy bands and piezooptical properties of Si

The energy bands of a crystal of Si under an axial force are calculated using a pseudopotential method including spin-orbit coupling effects. Good numerical agreement with experimentally determined deformation potentials is obtained. The wavelength modulated reflectivity spectra for several uniaxial forces along the [001] and [111] directions are calculated and compared with available experiments. The structure of the spectra is analyzed in terms of the shifts and splittings of the energy bands. It is shown that the optical structure corresponding to the main peak in reflectivity around 3·4 eV is due to transitions associated with several initial points related to the A line and it is well reproduced by an energy band model where the energy value of the L₃—L₁ transition is slightly larger than that corresponding to the Γ_25′–Γ₁₅ one.     

Study of W bulk bands with normal (001) photoemission using synchrotron radiation

Photoemission normal to the (001) face of W was measured using polarized synchrotron radiation at normal incidence for 10 eV ?hv? 30 eV. All the major features observed agree very well with the predictions of three-dimensional, bulk band calculations for direct transitions along the line Δ, even for final energies well above E_F. No surface resonances were observed with s-polarized radiation.     

Oxygen adsorption on silver (110): Dispersion,bonding and precursor state

Angle-resolved photoemission, both in the laboratory and with synchrotron radiation, has been used to investigate three states of adsorbed oxygen on Ag(110): chemisorbed atomic oxygen, undissociated chemisorbed dioxygen and physisorbed molecular oxygen. For atomic oxygen, dispersion of adsorbate-induced levels was observed indicating strong oxygen-oxygen interaction in the [001] direction. Polarised light combined with selection rules was used to determine the symmetry of the adsorbate-derived bands. The adsorption geometry and symmetry of the oxygen-induced levels for the chemisorbed dioxygen species were also investigated with the selection rules. The molecule appears to lie parallel to the surface with the OO axis oriented in the [11&#x0304;0] direction. The adsorbate-induced feature of lowest ionisation potential at 1.1 eV relative to E_F is 1 π_g-derived. The very low frequency reported for the OO stretch and the analogy with coordination chemistry also suggest that the chemisorbed dioxygen species is lying down on the surface. At sufficiently low temperature, oxygen was found to physisorb on the bare metal also with its OO axis parallel to the surface. We identify physisorbed oxygen as an intrinsic precursor state for chemisorption.     

Excited states of dinaphth[de-1, 2, 3; d′e′-5, 6, 7]azulene: Polarized absorption in stretched polymers and magnetic circular dichroism

UV-visible polarized absorption spectra of dinaphth[de-1,2,3; d′e′-5,6,7]azulene I have been determined from measurements of linear dichroism in stretched polymer sheets. Magnetic circular dichroic (MCD) spectra have been measured in solution and help to distinguish individual electronic transitions. π-Electron calculations of transition energies, oscillator strengths, and polarization directions are in good agreement with the experimental data and permit a straightforward correlation of the low-energy part of the spectrum of I with that of the parent pleiadiene II.     

Magnetostrictive effects at a spin reorientation transition in a cubic crystal

Mössbauer studies of spin reorientation transitions in the high magnetostrictive materials RFe₂(R = rareearth) were performed. The transitions studied were [100] → [110] (Ho_0.9Tb_0.1Fe₂ at 240 K), [100] → [111] (Dy_0.7Tb_0.3Fe₂ at 220 K), [110] → [100] (Ho_0.5Er_0.5Fe₂ at 135 K and Ho_0.3Tm_0.7Fe₂ at 110 K), [111] → [100] (Dy_0.2Er_0.8Fe₂ at 140 K) and [110] → [111] (Ho_0.7Tb_0.3Fe₂ at 120 K). The first four transitions were found to be of second order, continuous reorientation of magnetization. In Dy_0.2Er_0.8Fe₂ and Ho_0.7Tb_0.3Fe₂ a sharp discontinuous first order transition is observed. In all systems the Mössbauer recoil free fraction, its anisotropy, the isomer shift and hyperfine interaction parameters change sharply through the spin reorientation transition. All phenomena observed can be understood in terms of changes in static magnetostrictive distortions combined with critical phonon softening at the spin reorientation phase transitions. The system Ho_1-xTb_xFe₂ is unique in behaviour as Ho_0.9Tb_0.1Fe₂ shows the highest magnetostrictive effects, whereas Ho_0.7 Tb_0.3Fe₂ shows almost none.     

Polarization anisotropy in room temperature contactless electroreflectance spectrum of self-organized lateral compositional superlattices

We report the results of room temperature contactless electroreflectance measurements on (GaP)₂(InP)₂superlattices grown on GaAs (001) substrates which show lateral composition modulation along the [110] direction in the (001) plane. The strong polarization anisotropy of some features in the spectrum has been explained by identifying them as arising from transitions in the lateral superlattice formed due to the composition modulation. The measured optical transition energies are considerably lower than theoretical estimates based on the random alloy model, suggesting the need for a better model to describe the electronic band structure of this system.