Analysis of diffuse reflection and absorption spectra of ZnO in the near-IR region

It is established that the absorption of ZnO in the near-IR region of the spectrum is determined by free electrons and chemisorbed gases. The free-electron absorption spectrum is approximated by a power function with an exponent which varies within the range 2–3, depending on the defectiveness of the specimen. Chemisorbed gases yield absorption bands at 1.37; 1.18; 1.08–1.03; 0.95–0.93; 0.85; 0.75–0.73; and 0.65–0.63 eV. The bands at 1.18 and 0.85 eV are due to chemisorbed oxygen in the states O^– and O₂ ^–, while the other bands can be attributed mainly to atomic hydrogen and OH-groups.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 86–90, July, 1988.We thank E. V. Komarov for his help in conducting the experiments.     

Absorption Spectra of Single Crystals of EuGa2S4 and EuGa2S4:Co

The optical properties of EuGa₂S₄ and EuGa₂S₄:Co single crystals in a range of temperatures from 77 to 300 K are investigated. The single crystals are obtained by the Bridgman method and are characterized by tetragonal syngony. The behavior of the optical transitions in the photon energy range 1.70–2.45 eV and the temperature range 77–300 K is determined. It is established that in the energy range 1.77–1.90 eV absorption is associated with transitions of the Co²⁺ ion, while in the range 2.20–2.40 eV, with indirect allowed optical transitions.     

Dispersive optical constants of a-Se100−xSbx films

Thin films of amorphous Se_100−xSb_x (x=5,10 and 20 at%) system are deposited on a silicon substrate at room temperature (300 K) by thermal evaporation technique. The optical constant such as refractive index (n) has been determined by a method based on the envelope curves of the optical transmission spectrum at normal incidence by a Swanpoel method. The oscillator energy (E_o), dispersion energy (E_d) and other parameters have been determined by the Wemple–DiDomenico method. The absorption coefficient (α) has been determined from the reflectivity and transmitivity spectrum in the range 300–2500 nm. The optical-absorption data indicate that the absorption mechanism is a non-direct transition. We found that the optical band gap, E_g^opt, decreases from 1.66±0.01 to 1.35±0.01 eV with increase Sb content.     

Structural and optical studies of thermally evaporated CoPc thin films

The structure of the thermally evaporated cobalt phthalocyanine (CoPc) thin film in the β-form is investigated, and shows a single strong peak indicating preferential orientation in the (1 0 0) direction. Some structural parameters such as crystallite grain size, dislocation density and the number of crystallites per unit surface area are determined.The spectral parameters are determined by applying the electronic orbital transitions.But the optical parameters are deduced using band-model consideration for thin films of Pc.The spectral and optical parameters have also been investigated by using the spectrophotometric measurements of transmittance and reflectance in the wavelength range 200–2500 nm.The absorption spectra recorded in the UV–VIS region show two absorption bands of phthalocyanine (Pc) molecule, namely the Soret band (B) and the Q-band. The Q-band shows its characteristic splitting (Davydov splitting) with ΔQ=0.23 eV.Some of the important spectral parameters, namely optical absorption coefficient (α), molar extinction coefficient (_molar), oscillator strength (f), electric dipole strength (q²) and absorption half bandwidth (Δλ) of the principle optical transitions have been evaluated.The fundamental and the onset indirect energy gaps could be estimated as 2.90 + or − 0.05 and 1.51 eV, respectively.The refractive index showed an anomalous dispersion in the absorption region as well as normal dispersion in the transparent region. From analysis of dispersion curves, the dielectric constants, the dispersion parameters and the molar polarizability were obtained.All the above parameters were obtained for films as deposited and as annealed. No remarkable annealing effect on many parameters was observed.     

Dispersion studies and electronic transitions in nickel phthalocyanine thin films

Thin films of the organic semiconductor nickel phthalocyanine (NiPc) are structurally investigated using X-ray diffraction and infrared light absorption. The optical absorption and dispersion studies of nickel phthalocyanine were investigated using spectrophotometric measurements of transmittance and reflectance at normal incidence in the wavelength range 190–2100 nm. The absorption spectra recorded in the UV-VIS region show two well-defined absorption bands of the phthacyanine molecules, namely, the Soret and the Q-band. The Davydove splitting of the main absorption peak in the metal phthalocyanines correlates with the relative tendencies of the metal to out-of-plane bonding. The refractive index n as well as the absorption index k were calculated and showed an independent of the film thickness in the film thicknesses range 400–770 nm. The refractive index n showed an anomalous dispersion in the absorption region as well as normal dispersion in the transparent region. Some of the important optical absorption such as the molar extinction coefficient, the oscillator strength, the electric dipole strength have been evaluated. The analysis of the spectral behavior of the absorption coefficient α in the absorption region revealed two indirect allowed transitions with corresponding energies 2.77±0.03 and 1.66±0.02 eV. An energy band diagram has been proposed to account for the optical transitions of NiPc thin film. All previous parameters were as well obtained for films annealed at 523 K for 2 h. Discussion of the obtained results and their comparison with the previously published data are also given.     

Peculiarities of infrared absorption in Ti-based alloys

Complex optical conductivity of Ti-based alloys with V and Nb from the region of β-ω structural instability within the energy range of 0.07 ? ?ω ? 4.92 eV has been measured. It is shown that, in alloys having critical concentration, up to 0.07 eV interband absorption occurs and there are no indications of Drude-Zener type absorption. The possibility of a pseudo-gap appearing in the electronic spectrum is discussed.     

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.     

Photo- and thermostimulated processes in α-Al2O3

We reported on the recombination processes determined by the release of electrons from defects connected with the dosimetric 430 K thermostimulated luminescence (TSL) peak as well as with the 260 K TSL peak. These TSL peaks appear in thermochemically reduced α-Al₂O₃ crystals containing hydrogen and emission of these TSL peaks corresponds to luminescence of the F-center. The X-ray exposure or UV excitation in the absorption band of F-centers at 6.0 eV of reduced α-Al₂O₃ crystals doped with acceptor impurities results in the appearance of a broad anisotropic complex absorption band in the spectral region 2.5–3.5 eV and in the appearance of a predominant TSL peak at 430 K. Above 430 K the above-mentioned broad absorption band disappears. Optical bleaching of the 2.5–3.5 eV band is accompanied by the disappearance of the 430 K TSL peak and results in F-center emission. The X-ray or UV excitation of reduced α-Al₂O₃ crystals with donor-type impurities results in the appearance of an anisotropic absorption band at 4.2 eV and the appearance of a dominant TSL peak at 260 K. Above 260 K the 4.2 eV absorption disappears and photostimulated luminescence (PSL) of the F-center recombination luminescence in the 4.2 eV region is no longer observed. Optical bleaching of the 4.2 eV absorption band is accompanied by the disappearance of the 260 K TSL peak. The successful use of reduced α-Al₂O₃ in dosimetry needs the optimization of the concentration of all components (acceptors, hydrogen, intrinsic defects) involved in the thermo- and photostimulated processes.     

Potential of Si1−xGex alloys for Auger generation in highly efficient solar cells

Recent investigation on Si solar cells demonstrated the utility of Auger generation for the creation of more than merely one electron/hole pair per absorbed photon. The semiconductor Si requires a minimum photon energy of about 3.4 eV for this internal carrier multiplication. The current of a Si cell is therefore not significantly increased by Auger generation when the cell is illuminated by an air mass 1.5 spectrum, which contains only few photons with energies above 3.4 eV. Use of Si_1–x Ge _x alloys promises a lower onset energy. Unfortunately, incomplete data on band structures ofrandom Si_1–x Ge _x alloys preclude a detailed quantitative discussion of the full potential for these materials. Nevertheless, (i) analogies to our own quantum efficiency data from pure Si, (ii) the calculated band structure of the hypothetical,ordered zincblende type Si_0.5Ge_0.5 crystal, and (iii) optical data for Si_1–x Ge _x alloys indicate an optimum Ge content ofx=0.6 tox=0.7.     

Optical absorption studies of copper phthalocyanine thin films

The optical absorption of thermally evaporated copper phthalocyanine (CuPc) was studied in the UV-VIS-NIR region. The absorption spectra recorded in the UV-VIS region show two well-defined absorption bands of the phthalocyanine molecules, namely, the Soret (B) and the Q-band . The Q-band shows its characteristic splitting (Davydov splitting) of the main absorption peak in the metal phthalocyanine correlates with the relative tendencies of the metal to out-of-plane bonding. Some of the important spectral characteristics such as the molar extinction coefficient (ε_molar), the oscillator strength (f), the electric dipole strength (q²) and absorption half-bandwidth (Δλ) of the principle optical transitions were evaluated. The analysis of the spectral behavior of the absorption coefficient α in the absorption region revealed two indirect allowed transitions with corresponding energies 2.95±0.03 and 1.55±0.02 eV.The spectra of the infrared absorption allow characterization of vibration modes for the powder, as-deposited material and thin films of CuPc annealed at 423 K for two hours. Discussion of the obtained results and their comparison with the previous published data are also given.     

Compositional modulation of CuZn,CuAl and CuNi alloys

A differential reflectometer that is capable of measuring small differences in optical reflectivity or transmissivity of two specimens and therefore enhances the structure in the spectral reflectivity of materials was used to study various α-Cu-Zn, α-Cu-Al, and Cu-Ni alloys with the aim of investigating the changes in band structure as a function of composition. In Cu-Zn and Cu-Al alloys three main absorption peaks were identified that were associated with the Δ₅ → Δ₁, X₅ → X_4' and L_2' → L₁sitions. It was found that the Δ₅→ Δ₁transition increases slightly in energy with increasing Zn or Al concentration. These transitional energies are identical for both alloys in the entire α-phase region. The X₅ → X_4' and L_2' → L₁ transitions decrease in energy with increasing solute concentration. The shift in energy is significantly larger for the Cu-Al system. In Cu-Ni alloys no shift of the absorption edge around 2.2 eV was found, which is consistent with the predictions of the virtual-bound-state model.     

The changes in the electronic structure of B2 FeAl alloy with a Fe antisite and absorbed hydrogen

The electronic structure and bonding in a B2 FeAl alloy with and without hydrogen interaction with a Fe antisite were computed using a density functional theoretical method. The hydrogen absorption turns out to be a favorable process. The hydrogen was found close to an octahedral site where one of its Al capped is replaced by a Fe antisite. The Fe–H distance is of 1.45 Å same as the Al–H distance.The density of states (DOS) curves show several peaks below the d metal band which is made up mostly of hydrogen based states (>50% H_1s) while the metal contribution in this region includes mainly s and p orbitals.An electron transfer of nearby 0.21e⁻ comes from the metal to the H. The overlap population values reveal metal–metal bond breaking, the intermetallic bond being the most affected. The H bond mainly with the Al atom and the reported Fe–H overlap population is much lower than that corresponding to FePd alloys and BCC Fe. The changes in the overlap population show the Fe–Al bond is weakened nearly 41.5% after H absorption, while the Fe–Fe bond is only weakened 34.5%. H also develops a stronger bond with the Al atoms. The main bond is developed with Al being twice stronger than Fe–H.     

Low-energy peculiarities of the optical properties of inhomogeneous alloys

The optical properties of PdMn_0.7Fe_0.3, Pd₂AuFe and GdCu alloys of different homogeneity have been investigated. Possible reasons for the peculiarities typical of all alloys studied (deviation from the Drude behavior and low-energy absorption of light at ?ω < 0.5 eV) are discussed. Occurrence of these anomalies is related to general features of studied objects microstructure (phase inhomogeneity of medium in the form of presence in high-resistivity matrix of alloys of small inclusions of metal type.     

Electrical and optical properties of electron-irradiated ZnGeP2

Conclusions and summary The following conclusions are drawn from the reported study:The electrophysical properties of ZnGeP₂ crystals and their optical transparency in the range hG are attributable to the presence of a density-dominant (10¹⁷–10¹⁹ cm^–3) deep [E_v+(0.5–0.6) eV] growth defect associated predominantly with Zn vacancy clusters.Irradiation by high-energy electrons induces a shift of the Fermi level in the direction of E_G/2 and increases the resistivity of ZnGeP₂ to values of approximately 10¹² ·cm at 300 K. Irradiation with high-energy electrons is an effective technique for the optical bleaching of p-ZnGeP₂. The reversible modification of the optical absorption spectra of p-ZnGeP₂ in connection with irradiation and subsequent annealing indicates that the absorption step in the vicinity of h 0.6 eV is not attributable to light absorption by germanium inclusions, but to optical transition from the valence band to the growth-defect level E_v+(0.5–0.6) eV.Enhancement of the optical transmissivity of p-ZnGeP₂ in the range hG can be achieved in two wayss 1) as the result of a decrease in the density of centers with the level E_v+0.6 eV by variation of the growth conditions or subsequent annealing; 2) by shifting the Fermi level above the energy position E_v+0.6 eV through the irradiation-induced injection of compensating donor centers.The injection of radiation defects is an effective technique for controlling the electrical and optical parameters of the compound ZnGeP₂.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 122–130, August, 1986.     

Influence of Sr doping on the free carrier absorption in high-T c superconductors La2−x SrxCuO4

The reflectance spectra of polycrystalline La_2–x Sr _x CuO₄ samples were investigated in the energy range between 50 meV and 4 eV in dependence of the Sr content. The spectra are attributed to free carrier absorption of the Drude type, superimposed by optical phonon excitations below 0.1 eV and intrinsic absorption above 1 eV. From the influence of Sr doping onto the plasma energy it is deduced that La_2–x Sr _x CuO₄ is ap-type conductor with a maximum carrier concentration of 2.0×10²¹ cm^–3 forx=0.15. The results are interpreted in terms of a Hubbard model with an empty upper and ap-doped lower Hubbard band with a width of 1.9 eV.     

TM mode optical characteristics of five-layer MOS optical waveguides

The field distribution and complex eigenvalue equation of the TM mode are solved from the wave equation for a five-layer optical waveguide with finite metal cladding and a dielectric buffer layer. For air–Au–SiO₂–GaAs–AlGaAs MOS waveguides, numerical results for the propagation constant and absorption loss of the TM mode are computed in the complex plane from the eigenvalue equation. The effects of some guided structural parameters on the mode propagation and absorption loss are analysed and discussed.     

Optical spectroscopy and electronic structure of the Er5Ge3 compound

The results of the study of the optical properties and electronic structure of the Er₅Ge₃ compound have been presented. In the wavelength range of 0.22–15 μm (0.083–5.64 eV), the optical constants have been measured, and the spectral and electronic characteristics have been determined. The spin-polarization calculations of the band spectrum have been performed in the local electron spin density approximation (LSDA) with a correction for strong correlations in the 4f shell of the rare-earth atom (LSDA + U method). The main features of the experimental dispersion dependence of the optical conductivity in the region of quantum light absorption have been interpreted based on the results of calculations of the electron density of states.     

Nonlinear optical spectroscopy of electronic transitions in hexagonal manganites

The hexagonal rare-earth manganites RMnO₃ (R = Sc, Y, Ho, Er, Tm, Yb, Lu) are a group of materials with an unusual combination of magnetic, electric and optical properties. The electronic structure of these materials was studied by second harmonic (SH) spectroscopy in the range from 1.2 to 3.0 eV. Faraday rotation and absorption spectra were measured in the range from 1.0 to 1.6 eV. Broad bands at ∼1.7 eV and ∼2.7 eV are assigned to electronic transitions between Mn³⁺(3d⁴) levels. The SH spectra are discussed on the basis of a recently developed microscopic theory. Received: 26 April 2001 / Published online: 18 July 2001     

The absorption of pulsed CO2-laser radiation by tetrafluorosilane at various wavenumbers,fluences, pulse durations,temperatures, optical path lengths and pressures of absorbing and non-absorbing gases

The absorption of three linesP(30) (1037.4 cm^–1),P(36) (1031.5 cm^–1) andP(40) (1027.4cm^–1) of the pulsed CO₂ laser (00⁰1–02⁰0 transition) by SiF₄ was examined at various pulse energies, pulse durations, temperatures, optical path lengths and pressures of this compound and several non-absorbing gases. In addition, the low-intensity infrared absorption spectrum of tetrafluorosilane was compared with high-intensity absorption data for all lines of the laser. The experimental dependences demonstrate nonlinear features of the absorption phenomena originating from the high power of the incident radiation and collisions of absorbing molecules with surroundings. These effects are included in the analytical formula, being an extended form of the Lambert-Beer law. which reasonably approximates all experimental data. The importance of the results obtained for understanding general features of multiphoton absorption and for revealing potential applications of SiF₄ as a sensitizer for the infrared region is presented in brief.     

Electronic structure and optical properties of LaNiO3: First-principles calculations

Using a first-principles method, we investigate the electronic structure and optical properties of rhombohedral LaNiO₃. The total density of states shows that there is no band gap and bulk LaNiO₃ is metallic. There is a strong hybridization between Ni and O orbits near the Fermi level, suggesting that the metallic nature of LaNiO₃ mainly originates from Ni 3d states and La atoms have no noticeable contribution to this. The absorption coefficient of LaNiO₃ is one order of magnitude less than that of nickel in the lower energy region (0–5 eV), and the interband optical transitions are mainly derived from O 2p and Ni 3d states. In reflectivity spectrum of LaNiO₃, there are three main reflectance peaks located at 0 eV, 15.6 eV and 22.9 eV, respectively. In the visible–ultraviolet energy range, the reflectivity of LaNiO₃ remarkably decreases with the increasing photon energy and the value is always smaller than that of nickel in the region.