Optical and magneto-optical properties of room-temperature ferromagnetic In(Ga)MnAs layers, deposited by pulse laser ablation

Spectral dependences of refractive and absorption indices n(hν), k(hν) (hν=1.2-4.4 eV) and the transversal Kerr effect δ(hν) (hν=0.5-4.4 eV) in In(Ga)MnAs layers fabricated by laser deposition have been investigated. Spectra of the diagonal and off-diagonal components of the dielectric permittivity tensor of these layers have been calculated. Comparison of the spectral dependences δ(hν), ε′(hν) and ε′₂×(hν)² of the In(Ga)MnAs layers with similar spectra for MnAs has been carried out. Particular features in the spectra of the In(Ga)MnAs layers have been explained by a competition of contributions of the In_1−x(Ga_1−x)Mn_xAs host and MnAs inclusions.     

Optical and magneto-optical spectroscopy of thin ferromagnetic InMnAs layers

Spectral dependences of the refractive n(hν) and absorption k(hν) indices (hν= 1.2–4.4 eV) and the magneto-optical constant δ(hν) (hν = 0.5–4.4 eV) of the transverse Kerr effect of the InMnAs layers produced by laser deposition have been studied. The spectra of the diagonal ?(hν) and off-diagonal ?′hν) components of the permittivity tensor of the layers have been found. A comparison of the spectral dependences δ(hν), ?′(hν) and ?′₂ × (hν)² of the InMnAs and MnAs layers have been performed. Features in the spectra of the InMnAs layers have been attributed to a competition between the contributions of the In_{1 ? x} Mn _x As matrix and MnAs inclusions.     

Optical characteristics of MBE grown GaMnAs embedded with MnAs clusters

Photoluminescence (PL) measurements of the GaMnAs layers embedded with MnAs clusters have been performed. It was shown that the presence of MnAs clusters in the semiconducting matrix leads to appearance in the PL spectra a broad peak with local maximums at 1.36 and 1.33 eV, which are related with the defects generated in the phase separation process. The effect of the MnAs clusters on the temperature dependent band gap of GaMnAs was also observed.     

Effects of alloying on the band structure of gold: Piezoreflectance measurements on some AuCo and AuV alloys

Piezoreflectance measurements have been made on a series of gold cobalt and gold vanadium alloys with impurity concentrations of up to 4% using a strain amplitude of 4 × 10^?4 at a frequency of 68 kHz. The spectra show that the main interband transitions occur at 2.4, 3.5 and 4.5 eV. The deformation potentials with impurity concentration have been found for these transitions. An additional interband transition was found to occur at an energy of 1.8eV and this was enhanced significantly with impurity concentration. This may be due to the impurity causing a broadening of the d bands and hence a smearing of the interband threshold or alternatively it may be caused by a d band to Fermi level transition along the Δ direction close to the X point.     

Temperature dependence of aluminum nitride reflectance spectra in vacuum ultraviolet region

Temperature dependence of optical reflectance spectra in vacuum ultraviolet region for aluminum nitride has been measured on high-quality single crystal with synchrotron radiation. The dominant structure due to the interband transition is observed at photon energy around 7.7 eV. With decreasing temperature, the energy position of the dominant structure in the reflectance spectra shifts towards higher energy. The experimental data has been fitted to the Bose-Einstein expression and the obtained parameter related to the strength of the electron-phonon interactions is much smaller than that for the peak at 6.2 eV, suggesting that the higher-lying interband transition energy decreases more slowly with increasing temperature in aluminum nitride (AlN).     

Visible and near ultraviolet photocurrent generation in carbon nanotubes

Multiwall carbon nanotubes are found to generate photocurrent in the visible and near ultra violet spectral range using a photoelectrochemical technique. Peaks in the photocurrent are observed at excitation energies in the visible region. Their electron energy loss spectra exhibit the π plasmon feature, typical of graphite layers, and a peak at lower energy. Features at energies between 0 and 4 eV have been already observed for single wall carbon nanotubes and ascribed to interband electronic transitions due to the reduced dimensionality of these systems. The present measurements suggest that the usual identification of multiwall carbon nanotubes electronic density of states with that of graphite layers is not sufficient and more theoretical investigations are necessary to shed light on this point.     

Electronic excitations and luminescence of SrMgF4 single crystals

The electronic and crystal structures of SrMgF₄ single crystals grown by the Bridgman method have been investigated. The undoped SrMgF₄ single crystals have been studied using low-temperature (T = 10 K) time-resolved fluorescence optical and vacuum ultraviolet spectroscopy under selective excitation by synchrotron radiation (3.7–36.0 eV). Based on the measured reflectivity spectra and calculated spectra of the optical constants, the following parameters of the electronic structure have been determined for the first time: the minimum energy of interband transitions E _g = 12.55 eV, the position of the first exciton peak E _n = 1 = 11.37 eV, the position of the maximum of the “exciton” luminescence excitation band at 10.7 eV, and the position of the fundamental absorption edge at 10.3 eV. It has been found that photoluminescence excitation occurs predominantly in the region of the low-energy fundamental absorption edge of the crystal and that, at energies above E _g , the energy transfer from the matrix to luminescence centers is inefficient. The exciton migration is the main excitation channel of photoluminescence bands at 2.6–3.3 and 3.3–4.2 eV. The direct photoexcitation is characteristic of photoluminescence from defects at 1.8–2.6 and 4.2–5.5 eV.     

Electron energy loss spectroscopic investigation of palladium metal and palladium(II) oxide

Electron energy loss spectra (ELS) obtained from polycrystalline Pd metal and PdO powder using primary electron energies ranging from 100 to 1150 eV have been obtained and examined in an attempt to gain a better understanding of the origins of the loss features and to assess the utility of ELS in investigations of Pd catalysts. The two sets of ELS spectra differ significantly. The ELS spectra from Pd metal exhibit a predominant peak at 6.5 eV, shown to arise from a surface plasmon excitation, and two broad features at 25.1 and 31.9 eV, which originate from bulk loss processes. The broad features consist of several overlapping losses due mainly to interband transitions from the d-band, though a bulk plasmon excitation is believed to produce a feature near 24 eV. Two distinct peaks are present at 3.7 and 7.6 eV in the ELS spectra obtained from PdO, while a broad region of intensity appears over the range from 20 to 40 eV. The peak at 3.7 eV is attributed to a transition between the top of the valence band and the bottom of the conduction band. The feature at 7.6 eV is broad and arises from several overlapping features that are most likely caused by interband transitions rather than collective excitations. Furthermore, the ELS spectra obtained from PdO and oxidized Pd are also quite different indicating that ELS can provide useful information for determining the bonding states of oxygen on Pd-containing catalysts.     

Electronic structure and simulation of the dielectric function of β-FeSi2 epitaxial films on Si(111)

The optical functions of iron disilicide (β-FeSi₂) thin epitaxial films are calculated from the reflectance spectra in the energy range 0.1–6.2 eV with the use of the Kramers-Kronig (KK) integral relations. A comparison of the results of calculations from the transmittance and reflectance spectra and the data obtained from the reflectance spectra in terms of the Kramers-Kronig relations indicates that the fundamental transition at an energy of 0.87±0.01 eV is a direct transition. An empirical model is proposed for the dielectric function of β-FeSi₂ epitaxial films. Within this model, the specific features in the electronic energy-band structure of the epitaxial films are described in an analytical form. It is shown that the maximum contributions to the dielectric function and the reflectance spectrum in the energy range 0.9–1.2 eV are made by the 2D M ₀-type second harmonic oscillator with an energy of 0.977 eV. This oscillator correlates with the second direct interband transition observed in the energy-band structure of β-FeSi₂.     

Electronic excitations on clean and adsorbate covered Pd(111) by angle resolved electron energy loss spectroscopy

Electronic excitations on clean and adsorbate covered Pd(111) have been investigated by angle resolved electron energy loss spectroscopy. Primary energies in the range of 50–1000 eV were chosen for strong specular reflection to emphasize elastic diffraction-before-loss processes. The clean Pd spectra are compared with optical data, and good correspondence is found for the optical limit (q ? 0). The loss features are interpreted in terms of plasmon resonances and interband transitions within the framework of a recent band structure calculation. Virtually no dispersion is observed for the intrinsic Pd losses, but vertical interband transitions decay fast in the dispersive (q ≠ 0) spectra. Two adsorbate systems are investigated in this study: CO in a disordered adsorbate layer and bromine in a well-ordered $\text{(}\text{3×}\text{3 )R30°}$ structure. Adsorbate derived loss features are generally prominent in the nonspecular (q ≠ 0) spectra. While no dispersion is seen for the intramolecular 13.5 eV excitation of adsorbed CO, dispersion up to 1 eV is found for the Br 4p derived loss feature of the ordered overlayer. This is discussed in terms of a two-dimensional adsorbate band structure of bromine.     

Plasmons in monolayer Na,K and Rb films adsorbed on Ni(100)

Electronic excitations in denser monolayer Na, K and Rb films and Na duolayer films adsorbed on a Ni(100) surface have been investigated using Electron Energy Loss Spectroscopy (EELS). Lateral adatom distributions were monitored by LEED. Angular integrated EEL spectra from the ordered c(2 × 2)Na, coverage θ = 0.5, and the ordered hexagonal structures of K and Rb, θ = 0.29, show prominent losses at 3.1, 1.9 and 1.7 eV, of presumably collective nature. The loss energies shift with coverage as ∝ θ^0.4 and as ∝ θ^0.8 for the Na and K, Rb respectively. Angular resolved EEL spectra indicate an only weak dependence of the loss energies on the momentum transfer, Q. In particular the K and Rb losses seem to pass through shallow energy minima, which is predicted by the “box model”. Low energy losses observed at ?1.3 and ?1.0 eV for the c(2 × 2)Na and the hexagonal K and Rb, respectively, are tentatively identified with interband excitations. The observed interband energies yield, when introduced in the “box model”, 3.1., 2.3 and 2.4 eV for the Na, K and Rb, Q = 0 plasmon energies, which is in fair agreement with the observed plasmon loss energies.     

Exciton photoluminescence, photoconductivity and absorption in GaSe0.9Te0.1 alloy crystals

The photoluminescence, photoconductivity and absorption in GaSe_0.9Te_0.1 alloy crystals have been investigated as a function of temperature and external electric field. It has been observed that the exciton peaks shift to lower energy in GaSe_0.9Te_0.1 alloy crystals compared to GaSe crystals. The long wavelength tails of interband photoluminescence, photoconductivity and absorption spectra are determined by the free exciton states and show an Urbach-Martienssen-type dependence to the photon energy. The maxima of the extrinsic photoluminescence and photoconductivity spectra were found to be determined by the acceptor centers with an energy of E_A=E_V+0.19 eV formed by the polytypism and defects complexes that include Se and Te anions.     

Optical absorption of molybdenum thin films between 0.32 and 5.50 eV: Interpretation in terms of energy band transitions

We report, between 0.32 and 5.50 eV, the optical conductivity of polycrystalline thin films of molybdenum. The films have been deposited in ultra high vacuum and the measurements have been carried out in situ. We observe two maxima at 2.75 and 4.0 eV and two shoulders at 1.85 and 5.1 eV. The structure is understood in terms of interband transitions, the conductivity is compared to a joint density of states histogram deduced from Petroff and Viswanathan energy bands.     

Investigation of the optoelectronic properties of columbite ZnNb2O6 crystal

A high-quality ZnNb₂O₆ single-crystal grown by optical floating zone method has been used as a research prototype to analyze the optoelectronic parameters by measuring the absorption coefficient and transmittance spectra along the b-axis from 200 nm to 1000 nm at room temperature. The optical interband transitions of ZnNb₂O₆ have been determined as a direct transition with a band gap of 3.84 eV. The refractive index, extinction coefficient, and real and imaginary parts of the complex dielectric constants as functions of the wavelength for ZnNb₂O₆ crystal are obtained from the measured absorption coefficients and transmittance spectra. In the Urbach tail of 3.16–3.60 eV, the validity of the Cauchy–Sellmeier equation has also been evaluated. Using the single effective oscillator model, the oscillator energy E_o is found to be 4.77 eV. The dispersion energy E_d is 26.88 eV and ZnNb₂O₆ crystal takes an ionic value.     

Leed,auger, and electron energy loss studies of Ni epitaxially grown on Cu(100)

Nickel was epitaxially deposited onto a clean, flat Cu(100) surface. Low energy electron diffraction I(E) curves were recorded for 0.6, 1.1, and 2.7 monolayer (ML) Ni coverage. Multilayer relaxation was considered in theoretical calculations, which were compared with experiment by means of the R|ΔE| factor. The estimated relaxations of the first and second interlayer spacings are estimated to be − 2% and + 1.5% for clean Cu(100), − 2% and − 1.5% for 1 ML Ni coverage, relative to the bulk Cu interlayer spacing of 1.81 Å, and −1% and 0% for 3 ML Ni coverage, relative to the bulk Ni spacing of 1.76 Å. Decreasing the surface Debye temperature of the Ni layer to 268 K from the bulk value of 440 K improves the agreement between theory and experiment. The optimum inner potential values are − 9 and − 10 eV for clean Cu(100) and Ni on Cu(100), respectively. Auger electron spectroscopy was used to determine the thickness of the Ni films, and LEED indicates layer-by-layer growth until about 4 layers, when the LEED spots begin to spread, indicating island formation. Electron energy loss spectra were obtained with primary electron energies of 150 and 300 eV. The 3p core ionization transition was clearly observed after 0.5 ML Ni coverage. Peaks at 3.8 and 7.5 eV for clean Cu are ascribed to interband transitions, and shift to higher energy with Ni coverage. Peaks at 10 and 16 eV for clean Cu (ascribed to an interband transition and a surface plasmon, respectively) disappear with Ni coverage. Bulk plasmon peaks at 19 and 27 eV remain unshifted with Ni coverage. The effect of 0.9 and 1.3 ML Ni coverage of Cu(100) on the chemisorption of Co and oxygen was also studied. The behavior of the surface towards oxygen chemisorption was similar to that of the pure Ni surface. For a large exposure of oxygen (50 L and more) the EEL and Auger spectra are very similar to those observed for NiO. In the case of CO, for submonolayer Ni coverage, the surface shows a more Cu-like behavior, while for larger Ni coverage (a monolayer and more) there is a great similarity with the behavior of the pure Ni(100) surface.     

Synchrotron investigations of the specific features in the electron energy spectrum of silicon nanostructures

The Si L _{2, 3} x-ray absorption near-edge structure (XANES) spectra of porous silicon nanomaterials and nanostructures with epitaxial silicon layers doped by erbium or containing germanium quantum dots are measured using synchrotron radiation for the first time. A model of photoluminescence in porous silicon is proposed on the basis of the results obtained. According to this model, the photoluminescence is caused by interband transitions between the energy levels of the crystalline phase and oxide phases covering silicon nanocrystals. The stresses generated in surface silicon nanolayers by Ge quantum dots or clusters with incorporated Er atoms are responsible for the fine structure of the spectra in the energy range of the conduction band edge and can stimulate luminescence in these nanostructures.     

Photoelectric properties and electroluminescence of <Emphasis Type="Italic">p-i-n</Emphasis> diodes based on GeSi/Si heterostructures with self-assembled nanoclusters

This paper reports on the results of investigations into the photoelectric properties and electroluminescence of p-i-n diodes based on GeSi/Si heterostructures with GeSi self-assembled nanoclusters embedded in the i region. The p-i-n diodes are grown through sublimation molecular-beam epitaxy using a vapor-phase source of germanium. The photovoltage spectra of the p-i-n diodes measured at a temperature of 300 K exhibit a photosensitivity band attributed to interband optical transitions in the GeSi nanoclusters. A new approach to analyzing the photosensitivity spectra of the heterostructures containing GeSi thin layers is proposed, and the energy at the edge of the photosensitivity bands assigned to these layers is determined. The electroluminescence observed in the p-i-n diodes at 77 K is associated with the radiative interband optical transitions in GeSi nanoclusters.     

Electron energy loss spectra from polycrystalline Cr and Cr2O3 before and after surface reduction by Ar bombardment

Electron energy loss spectra (ELS) have been obtained from polycrystalline Cr and Cr₂O₃ before and after surface reduction by 2 keV Ar⁺ bombardment. The primary electron energy used in the ELS measurements was systematically varied from 100 to 1150 eV in order to distinguish surface versus bulk loss processes. Two predominant loss features in the ELS spectra obtained from Cr metal at 9.0 and 23.0 eV are assigned to the surface and bulk plasmon excitations, respectively, and a number of other features arising from single electron transitions from both the bulk and surface Cr 3d bands to higher-lying states in the conduction band are also present. The ELS spectra obtained from Cr₂O₃ exhibit features that originate from both interband transitions and charge-transfer transitions between the Cr and O ions as well as the bulk plasmon at 24.4 eV. The ELS feature at 4.0 eV arises from a charge-transfer transition between the oxygen and chromium ions in the two surface layers beneath the chemisorbed oxygen layer, and the ELS feature at 9.8 eV arises from a similar transition involving the chemisorbed oxygen atoms. The intensity of the ELS peak at 9.8 eV decreases after Ar⁺ sputtering due to the removal of chemisorbed oxygen atoms. Sputtering also increases the number of Cr²⁺ states on the surface, which in turn increases the intensity of the 4.0 eV feature. Furthermore, the ELS spectra obtained from the sputtered Cr₂O₃ surface exhibit features characteristic of both Cr⁰ and Cr₂O₃, indicating that Ar⁺ sputtering reduces Cr₂O₃. The fact that neither the surface- nor the bulk-plasmon features of Cr⁰ can be observed in the ELS spectra obtained from sputtered Cr₂O₃ while the loss features due to Cr⁰ interband transitions are clearly present indicates that Cr⁰ atoms form small clusters lacking a bulk metallic nature during Ar⁺ bombardment of Cr₂O₃.     

Optical absorption and electronic structure of intermetallic compound RuIn3

The optical properties of intermetallide RuIn₃ are investigated by ellipsometry in the spectral range of 0.22–10 μm. The experimental data point to the existence of an energy gap of about 0.5 eV in the electronic spectrum of the compound. The density of the electron states and interband optical conductivity are calculated in terms of the density functional theory. The experimental and theoretical spectra of the optical conductivity are compared. It is found that the formation of basic absorption bands is caused by interband transitions of electrons of the d-band of Ru and p-band of In.     

Optical reflection spectra of cadmium halides

The reflection spectra have been measured at liquid helium temperature in the region 4–11.5 eV for CdCl₂ and CdBr₂ crystals. Exciton bands observed around the optical gap are associated with the halogen ion excitation under the influence of the strong crystal field. Characteristic sharp lines are also observed in the deep interband energy regions.