Absorption optique du niobium en couches minces entre 0,32 ET 5,5 eV. interpretation par la theorie des bandes d'energie

Niobium optical conductivity has been determined from reflexion and transmission data measured, at room temperature, on polycrystalline thin films in the 0.32–5.50 eV spectral range. The films have been deposited in ultra high vacuum and the measurements carried out in situ. Structures are observed at -.55, 2.3, 3.0 and 4.4 eV and tentatively identified with interband transitions in fairly good agreement with the band structure calculated by Mattheiss. In addition, we compare our results with recent experiments on niobium.     

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.     

Mechanism for the optical absorption of cadmium sulfide

The optical transmission and reflection spectra of polycrystalline cadmium sulfide films heated in sulfur vapor have been measured at 77 and 300 °K. The absorption coefficient of the film is calculated for the range 2.25–3.5 eV. Analysis of the spectral dependence of the absorption coefficient shows indirect optical transitions of an electron from the upper valence band to the conduction band in the range 2.31–2.54 eV and direct substitutional transitions in the range 2.58–3.45 eV. Superimposed on this intrinsic absorption is selective absorption due to superstoichiometric atoms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, Vol. 12, No. 4, pp. 97–103, April, 1969.     

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.     

Intraband and interband optical transitions in Zn3AS2

Absorption measurements were made on single crystals and thin films of Zn₃As₂; within the photon energy range of 0.12–1.16 eV at temperatures of 300, 80 and 5 K and reflectivity was measured in the range of 1.0–5.5 eV at 300 K. Absorption below the fundamental edge has been interpreted as a process involving three mechanisms: (i) free-carrier absorption, (ii) intraband transitions between levels in the valence band, and (iii) direct transitions from valence levels to the acceptor level/band. The fundamental absorption edge has been ascribed to direct interband transitions from three valence levels to one conduction level. An isotropic three-level Kane band model has been used to interpret the experimental data, modified by introducing the light-hole level split from the heavy-hole level due to the tetragonal crystal field. A reasonable fit of the model to the experimental results has been obtained in the region of both intraband and interband absorption for the following set of parameters: E_g = 0.985 eV, Δ_SO = 0.30 eV, Δ_CF = 0.05 eV, m^*_hh = 0.36 m₀ and P = 4 × 10^?10eVm (at 300 K). A proposed Zn₃As₂ energy-band model near the Γ point is described to interpret the observed absorption.     

Optical properties of dangling-bond states at cleaved silicon surfaces

The spectral dependence of surface photovoltage and surface photoconductance both under continuous illumination as well as LEED I/V spectra were studied with cleaved Si(111)-2 × 1 surfaces at 130 K. Between 0.23 and 0.5 eV a doubly peaked absorption band was found with opposite sign compared to the SPV and SPC signals at higher photon energies. This band is due to electronic transitions from occupied to empty dangling-bond states located at the raised and the lowered rows of atoms in the 2 × 1 reconstruction, respectively. This absorption shows a pronounced dependence on the polarization of the incident light which correlates with the spatial symmetry of the dangling-bond states. Anneals at up to 500 K remove the low-energy absorption peak and equalize the 2 × 1 reconstruction: The homogeneous Si(111)-2 × 1 structure exhibits a buckling of 0.3 Å and a dangling-bond absorption with a threshold at 0.42 eV and a maximum at 0.47 eV. An anneal at 750 K, forming the 7 × 7 structure, destroys the peak of opposite sign in SPV and SPC and only leaves a broad tail with a threshold of 0.32 eV.     

Optical properties of the ternary compound Tl Sb S2

We present an investigation of the near band-gap optical properties of TlSbS₂ between 2 and 300 K. We use both transmission and reflectivity measurements. The resolution of the first exciton line permits to obtain an accurate determination of the temperature coefficients of both the direct band-gap E₀ and the second threshold E₁. The absorption curves have been fitted according to the Toyozawa's model. We find a strong interaction with a phonon mode of energy 22 meV for both the E₀ and E₁ thresholds. The low temperature reflectivity spectra reveal clearly several direct transitions in the range 1.5–5.5 eV. All these structures have been identified as transitions between the highest valence band and the lowest conduction band.     

Niveaux d'énergie et spectres de rayons X du rutile

The electronic levels of the complex TiO^?8₆ in the D_2h symmetry are determined according to an extended L. C. A. O. method. The results can explain the X-ray spectra of TiO₂. The absoption L_III and K rays are related to transitions from the 2p_3/2 and 1 s levels to the conduction band levels since the emission L_III and K components are explained by the transitions from the valence band levels to the 2p_3/2 and 1 s states. Interband transitions are related to the components of the optical reflexion spectrum of TiO₂ for the energies 0–20 eV. A comparaison is made with the electronic band structures of SnO₂, TiO₂ and BaTiO₃. At the center of the Brillouin zone, we obtain a forbidden gap of 3,01 eV, the corresponding widths of the valence and conduction band are 4,8 and 2,9 eV.     

Amorphous films of CuS

Thin films of CuS have been prepared by reactive evaporation of copper in a sulphur atmosphere. It is found that films deposited on to substrates kept below 315 K are amorphous in nature. The amorphous films have a resistivity of ～ 10⁵ ohm cm and are n-type. The films are golden yellow in colour and are fairly transparent before the onset of band to band transitions. Optical studies give a band gap of 1.60 eV at room temperature (295 K).     

Calorimetric, optical and catalytic activity studies of europium chloride-polyvinyl alcohol composite system

Polyvinyl alcohol (PVA) films doped with europium chloride (EuCl₃) have been prepared by casting from their aqueous solutions. The phase transitions and thermal decomposition behavior of the prepared samples were investigated by thermal analysis and the interactions between the host PVA and Eu³⁺ were examined by FTIR spectroscopy. The optical absorption was recorded at room temperature in the range of 190-1000 nm. From the absorption edge studies, the values of the Urbach energy (E_e) were found to be 0.56 eV in case of the pure polymer; however, its value increased to be in the range of 1.21-1.75 eV. These energy values indicate that the model based on electronic transitions between localized states is not preferable and transitions are made between band tails. Optical parameters such as refractive index and complex dielectric constant have been determined. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple-DiDomenico model. Color properties of the prepared samples are discussed in the framework of CIE L?u*v* color space. The prepared samples have been used as catalysts in the photocatalytic degradation of p-nitrophenol (PNP) in aqueous solution under UV light irradiation using H₂O₂ as oxidizing agent. The catalytic activity of the Eu-polymer towards the photodegradation of PNP greatly increased after doping with Eu³⁺ ions. The highest catalytic activity was noticed at the optimum pH value of 5.5.     

Recombination centers in phosphorous doped hydrogenated amorphous silicon

Optical absorption below the mobility gap of a-SiH_x:P films is derived from photoconductivity measurements and interpreted in terms of optical transitions from occupied localized states in the exponential valence band tail and dangling bond states 0.8 eV above the valence band edge to unoccupied free electron conduction band states. Collection efficiency measurements of Schottky barrier structures indicate that P doping introduces centers with large capture cross-section for holes.     

F-Zentren und Kolloide in der amorphen,hexagonalen und kubischen Phase von LiJ

By simultaneous evaporation of LiI and Li onto a cooled substrate F centers can be produced in the hexagonal (78 K<T _K <200 K) and amorphous (T _K <78 K) phase of one and the same salt. In both modifications there exist two types of centers F and F^*. The F^* center differs from the cubic F center (T _d -symmetry) by a nearby Frenkel defect. In hexagonal films the normal F band peaks at 2.58 eV, whereas the transitions of the F^* center appear at 2.92 and 2.58 eV too. Polarized irradiation at 20 K causes a dichroic behaviour of the F^* centers. Both types of centers can be transformed into one another photochemically. In the amorphous phase all transitions are shifted to lower energies by about 0.1 eV. After the phase change amorphous→hexagonal the absorption bands shift back by the same amount of energy. AboveT _K =230 K the excess metal forms colloids. The absorption bands are due to colloidal centers embedded in the crystalline material (2.25 eV) and films adsorbed to the crystallites (3.1 eV), respectively. By annealing a particle growth can be observed. After electrolytic colouration cubic single crystals of LiI exhibit an absorption band peaking at 2.36 eV. However, it is not yet sure, if this band is allowed to be ascribed to F centers.     

The extinction coefficient of benzene vapor in the region 4.6 to 36 eV

Extinction coefficient spectra and oscillator strength values for the benzene transitions in the 4.6 to 9.2 eV region have been measured taking into account the effect of the instrumental bandwidth. By normalizing Koch and Otto's (1) uncalibrated absorption spectrum at 6 to 36 eV with our spectrum recorded with the same bandpass we provide values of ? in the region 4.6 to 36 eV. The dependence of ?_max on the bandpass is presented and a comparison is made between theoretical band intensities in a progression and band intensities measured experimentally. A total integrated intensity of 0.075 is found for the Rydberg transitions below the first I.P. and a total value of 17.5 is found for the whole region 4.6 to 36 eV.     

Electronic absorption spectrum of Cs<Subscript>2</Subscript>ZnI<Subscript>4</Subscript> thin films

The absorption spectrum of Cs₂ZnI₄ thin films in the energy range 3–6 eV at temperatures from 90 to 340 K has been investigated. It is established that this compound belongs to direct-gap insulators. Low-frequency exciton excitations are localized in ZnI₄ structural elements of the lattice. Phase transitions at 280 K (paraelectric phase ? incommensurate phase), 135 K (incommensurate phase ? monoclinic ferroelastic phase), and 96 K (monoclinic phase ? triclinic ferroelastic phase) have been found from the temperature dependences of the spectral position and halfwidth of the low-frequency exciton band. Additional broadening of the exciton band is observed for ferroelastic phases; it is likely to be due to exciton scattering from strain fluctuations near domain walls.     

Optical absorption due to transitions between bands and local levels in CdCr2Se4

The theory of optical absorption due to transitions between a valence band and a hydrogen-like local level associated with a conduction band is modified to permit an arbitrary power-law dependence of energy on the magnitude of the wave-vector of carriers in the valence band. The observed absorption for photon energies below 1.6 eV in the ferromagnetic semiconductor CdCr₂Se₄ is discussed in terms of a combination of two types of terms. The first type of absorption is due to transitions to a local level from a band with two branches, in each of which there is an energy region with a width of 0.28 eV or more beginning 0.10–0.16 eV from the band edge, in which the energy measured from some origin near but not necessarily equal to the band-edge is approximately proportional to (wave-vector)^{($\text{1}\text{3}$)}. The second type of absorption has a dependence on photon energy ?ω of the form (?ω ? E₃)², where E₃ is a threshold energy probably connected with indirect transitions between bands as suggested by Sakai, Sugano and Okabe. After constraints on parameters appearing in the theory are imposed by use of results of these authors and of Shepherd, it is found that curves of Harbeke and Lehmann on optical absorption in CdCr₂Se₄ at 4.2, 78, 130 and 298 K in the photon-energy range 1.14–1.42 eV can be fitted to a mean accuracy of 3%, using an average of 3.75 adjustable parameters for each curve. The strength of the indirect band-to-band absorption does not have the temperature dependence expected for phonon-assisted indirect band-to-band transitions, but can be described by a term independent of temperature plus another term proportional to the square of the deviation of the magnetization from saturation. The fitting of the absorption curves requires that the ratio of the widths of the two branches of the bands varies from about 1.6 at low temperatures to 1.35 at 298 K and that the total width of the bands involved is less than 1 eV.     

The band structure and electronic properties of sulphur nitride polymer

A semiempirical calculation of the energy band structure of (SN)_x has been made on a tight-binding model with three p orbitals per atom. An important feature is that the Fermi level crosses two overlapping conduction bands. Measurements are reported of the optical transmission spectrum between 0.2 and 4.0 eV in thin films, the free carrier reflectivity in thick films, and the hydrostatic pressure dependence of the conductivity to 15 kbar. The calculated band structure accounts for experimental results connected with interband transitions (optical absorption) and intraband effects (metallic conductivity, reflectivity, specific heat).     

Optical properties of Cr/Fe(t)/MgO (0 0 1) thin films

In this work we report on the optical properties of single-crystalline iron thin films. For this, Cr-capped Fe films with thickness, t, in the range 30–300 Å were prepared on MgO (0 0 1) by DC magnetron sputtering, and then studied by optical absorption technique within the range from 1.0 to 3.6 eV. All measurements were carried out at room temperature using a fiber optics spectrophotometer. The intensity of the transmitted light decreases with increasing film thickness. The optical constants of the films are deduced from a model that considers the transmission of light by two absorbing films on an absorbing substrate. The absorption coefficient of the Fe films is also calculated from the transmission data. The absorption spectra show the following characteristics: (i) two large absorption peaks centered at about 1.20 and 2.65 eV; and (ii) a sharp step near 1.40 eV. These structures are associated with conventional interband transitions of the iron film.     

Effect of thermal annealing on r.f. sputtering-deposited nanocrystalline GaN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> thin films

GaNAs thin films were deposited on Corning glass substrates by radio frequency (r.f.) sputtering in molecular nitrogen ambient. The stoichiometry in the GaNAs alloy was controlled by changing the nitrogen incorporation in the film during the growth process, through the variation of the r.f. power in the range 30–80 watts which produced films with N concentrations in the range: x = 0.85–0.90. The structural and optical properties of the GaNAs thin films were studied by X-ray diffraction (XRD), photoacoustic (PA) and photoluminescence (PL) spectroscopies. XRD measurements show a broad diffraction band with a peak close to the (002) diffraction line of the GaN hexagonal phase, and a slight shoulder at the position corresponding to the (111) GaAs cubic phase. The PA absorption spectra showed a remarkable shift to higher energies of the absorption edge as the r.f. power decreases corresponding to the films with higher N concentrations. Thermal annealing of the GaNAs films at temperatures of 450 °C produced a GaAs nanocrystalline phase with grain sizes in the range 10–13 nm, as confirmed by the XRD measurements that showed a well-defined peak in the (111) GaAs direction, and also by the PA spectra which showed an absorption band at energies around 1.45 eV due to the quantum confinement effects. PL spectra of thermal-annealed GaNAs films showed a very intense emission at 1.5 eV which we have associated to transitions between the first electron excited level and acceptor states in the GaAs nanocrystallites.     

Two-photon spectroscopy of Hg2Cl2 crystals

Two-photon absorption of Hg₂Cl₂ crystals was investigated using a pulsed dye laser at T ω 8.5 K. Both the indirect method of monitoring the luminescence produced by two-photon absorption and the two-photon attenuation measurements of the laser beam were used. The excitation spectrum of the ∼ 3.13 eV (396 nm) luminescence was measured in the energy range 4.0 ⩽ 2hv ⩽ 5.5 eV. A strong maximum at 2hv ω 5.13 eV dominates the spectrum. Comparison with previous one-photon absorption data allows us to assign the parity of electronic states involved in the transitions. The band gap of Hg₂Cl₂ is found to be direct allowed.     

Annealing effect on the structural and optical properties of a Cd1−xZnxS thin film for photovoltaic applications

Herein is a report of a study on a Cd_1−xZn_xS thin film grown on an ITO substrate using a chemical bath deposition technique. The as-deposited films were annealed in air at 400 °C for 30 min. The composition, surface morphology and structural properties of the as-deposited and annealed Cd_1−xZn_xS thin films were studied using EDX, SEM and X-ray diffraction techniques. The annealed films have been observed to possess a crystalline nature with a hexagonal structure. The optical absorption spectra were recorded within the range of 350-800 nm. The band gap of the as-deposited thin films varied from 2.46 to 2.62 eV, whereas in the annealed film these varied from 2.42 to 2.59 eV. The decreased band gap of the films after annealing was due to the improved crystalline nature of the material.