Influence of nitrogen-containing reducing media on the optical and luminescence characteristics of sapphire

The optical and luminescence characteristics of sapphire crystals grown in nitrogen-containing reducing media based on CO have been investigated. It is shown that the nitrogen dissolution in the oxygen sublattice of sapphire under reducing conditions is accompanied by the formation of F ⁺ centers and aggregate F ₂ centers (defects that significantly influence the optical absorption and X-ray-, photo-, and thermoluminescence in sapphire). It is established that the formation of Al₂O₃:N occurs at fairly low nitrogen concentrations in the growth medium (10?C20 vol % when a crystal is grown at P ?? 0.2 Torr and 1 vol % when growth is carried out at P = 800?C950 Torr).     

Spectroscopic ellipsometry studies of reactively sputtered nitrogen-rich GaAsN films

Nitrogen-rich GaAsN thin films have been deposited at 500 °C by reactive rf sputtering of GaAs target in argon-nitrogen atmosphere. The arsenic content of the films was varied by changing the nitrogen percentage in the sputtering atmosphere and the As/Ga ratio in the films was estimated by X-ray fluorescence measurements. Spectroscopic ellipsometry measurements have been carried out on these films and the measured ellispometric spectra were fitted with theoretical spectra generated by using suitable model sample structures. From the best fit parameters of the dispersion model, band-gap values and variation of refractive index and extinction coefficient as a function of wavelength have been obtained for films deposited with different percentages of nitrogen in the sputtering atmosphere. The films deposited with 12% to 100% nitrogen in the sputtering atmosphere, which are of hexagonal GaN, exhibit GaN-like optical properties, though effects due to excess arsenic in amorphous phase are seen in the films deposited with less than 40% nitrogen. The films deposited with 5% to 12% nitrogen in sputtering atmosphere are dominantly polycrystalline GaAs_xN_1−x (x ≈ 0.01 to 0.08) and exhibit variations in optical parameters, which are consistent with their structure and composition. The films deposited with less than 5% nitrogen in sputtering atmosphere are arsenic-rich and amorphous.     

Optical absorption edge in structure-inhomogeneous a-Si:H-based alloys

In this paper we measure microstructure and optical absorption edge of a-Si:H and silicon-rich a-SiN_r:H films prepared at deposition rates ∼0.8 nm/s by radio frequency plasma enhanced chemical vapor deposition method from hydrogen diluted SiH₄ and SiH₄ + NH₃ mixtures, respectively. Microstructure of films was studied by atomic force microscopy and infrared spectroscopy. Both a-Si:H and a-SiN_r:H films are inhomogeneous on a scale of ∼50 nm and contain Si-rich islands with hydrogen (in a-Si:H) or hydrogen and nitrogen (in a-SiN_r:H) collected at their boundaries. It was found that different atomic configurations of N and H determined from IR data should be attributed to such islands and their boundaries. It was established that the optical gap is determined by the concentration of hydrogen (in a-Si:H) or nitrogen (in a-SiN_r:H) in the islands while it is insensitive to variations of content of these alloy atoms at island boundaries. These results are interpreted in terms of a quantum well model modified to take into account structure of alloy atoms.     

Copper nitride (Cu3N) thin films deposited by RF magnetron sputtering

The copper nitride thin films were prepared on glass substrate by RF magnetron sputtering method. At pure nitrogen atmosphere, the nitrogen flow rate affects the copper nitride thin films’ structures. Only a little part of nitrogen atoms insert into the body center of Cu₃N structure and parts of nitrogen atoms insert into Cu₃N crystallites boundary at higher nitrogen flow rate. But the indirect optical energy gap, E_opg, decreases with increasing nitrogen flow rate. The typical value of E_opg is 1.57 eV. In a nitrogen and argon mixture atmosphere, when the nitrogen partial was less than 0.2 Pa at 50 sccm total flow rate, the (1 1 1) peak of copper nitride appears. Thermal decomposition temperature of Cu₃N thin films deposited in pure nitrogen and 30 sccm flow rate was less than 300 °C. The surface morphology was smooth.     

Radiation-Induced Radicals in Anthracene Single Crystals

EPR and optical absorption studies have been made on anthracene single crystals irradiated with electrons at liquid nitrogen temperature. Studies of annealing of the EPR spectra induced by irradiation at liquid nitrogen temperature revealed that they consisted of a broad singlet and two sets angular dependent lines. The angular dependence of the latter was studied and they were ascribed to the 9- and 1-dibenzo-cyclohexadienyl radicals. From comparison of the change in the EPR spectra with the change in the optical absorption spectra at room temperature, optical absorption bands at 535 and 675 nm were ascribed to 9- and 1-dibenzo-cyclohexadienyl radicals. An optical absorption band at 645 nm, which is created by irradiation at liquid nitrogen temperature and decays within two hours at room temperature was suggested to arise from the 2-dibenzo-cyclohexadienyl radical. The broad singlet, which is annealed below room temperature, was ascribed to the 9-anthracyl radical.     

Photoluminescence and optical absorption in glassy AsxSe1−x

The photoluminescence and optical absorption spectra in glassy As_xSe_1−x with 0.08 ? x ? 0.40 are essentially independent of x. The PL spectra peak at approximately half the optical gap and the exponential slopes of the optical absorption edges are all approximately 75 meV. For x > 0.4, the PL peaks shift to higher energies, the widths of the PL spectra increase, and there is a strong component to the optical absorption well below the optical gap. Comparisons with ESR experiments in the As_xS_1−x system suggest the possibility of two PL peaks. The second PL peak and the optical absorption below the optical band gap for x > 0.4 are attributed to the presence of As-As bonds.     

Thermal stability and crystallization behavior of TiO2 doped ZBLAN glasses

ZBLAN glasses with the composition (in mol%) of (100 − x)(53 ZrF₄ + 19 BaF₂ + 5LaF₃ + 3AlF₃ + 20NaF) + xTiO₂ (x = 0, 1.0 and 2.0 mol%) were prepared using a conventional melting technique in dry nitrogen atmosphere. The thermal stability, glass-forming ability, and crystallization kinetics of the ZBLAN system as a function of the TiO₂ concentrations were investigated by Differential Scanning Calorimetry (DSC). Also, the crystalline phases were determined by X-ray Diffraction (XRD). Our study indicates that adding TiO₂ in a fluoride system improves the thermal parameters of the glass, which is interesting for applications as optical fiber.     

Observation of Meyer–Neldel rule in amorphous thin films of Se100−xSbx in presence of light

In the present paper, we report the thermally activated conductivity in thin films of Se_100−xSb_x in presence of light of different intensities. We found that the conductivity obeys the Meyer–Neldel rule as the pre-exponential factor depends on activation energy. We also found a strong correlation between prefactor σ₀₀ and Meyer–Neldel energy. The origin of the correlation and the MN rule can be described by multiple excitations stimulated by optical phonon energy as described by Yelon and Movaghar. The value of optical phonon energy calculated by experimental data is in good agreement with the YM model which suggests that the optical phonons are the source of the excitation energy in such processes.     

Influence of the nitrogen flow rate on the order and structure of PECVD boron nitride thin films

Three sets of boron nitride (BN) thin films are deposited with different N₂/B₂H₆ flow ratios (r = 4, 10 and 25) by plasma enhanced chemical vapor deposition (PECVD). The variations of physical properties in different deposition sets are analyzed by optical (XPS, FTIR, UV–visible spectroscopies), mechanical and electrical measurements. The films are considered to be deposited in a turbostratic phase (t-BN). Evolution of bonding configurations with increasing r is discussed. Relatively higher nitrogen flow rate in the source gas mixture results in lower deposition rates, whereas more ordered films, which tend to reach a unique virtual crystal of band gap 5.93 eV, are formed. Anisotropy in the film structure and film inhomogeneity along the PECVD electrode radial direction are investigated.     

Effect of the hydrogen content on optical band gap in a-Si:H:N

The effect of the hydrogen content on the optical band gap in glow discharge a-Si:H:N produced from SiH₄ and N₂ mixtures is demonstrated. The hydrogen content in the film can be well controlled by making a non-plasma-excited region above the substrate surface during deposition. When the concentration of hydrogen is high, a large amount of nitrogen atoms included in the film does not cause a widening of the optical band gap. The effects of the hydrogen content on the infrared absorption peak wavelength relevant to SiN lattice vibration and the optical band gap are discussed.     

Compositional dependence of optical parameters in Se–Bi–Te–Ag thin films

Compositional dependence of optical parameters in thermally evaporated amorphous Se_80.5Bi_1.5Te_18 ? yAg_y (for y = 0, 1.0, 1.5 and 2.0 at.%) quaternary thin films has been studied using well established Swanepoel method. The optical properties like, refractive index (n), extinction coefficient (k), absorption coefficient (α) and optical band gap (E_g) have been determined from the transmission spectra in the spectral range from 500 to 2500 nm. The optical band gap (E_g) has been estimated by using Tauc's extrapolation method and is found to increase with an increase in the Ag concentration. Present study shows that the refractive index, extinction coefficient and optical band gap increase with the increasing Ag content which is in agreement with the earlier studies. While the increase in the refractive index with Ag content over the entire spectral range can be attributed to the increased polarizability of larger Ag atomic radius (153 pm) compared to the Te atomic radius (135 pm), the increase in the optical band gap with increasing Ag concentration is correlated to an increase in the cohesive energy and decrease in the electronegativity of the films under study. The dielectric constant and optical conductivity (σ) of the thin films under study are also found to increase with the Ag concentration.     

Correlation between nitrogen incorporation and structural modification of hydrogenated carbon nitride films

The growth and microstructure of hydrogenated carbon nitride a-CN_x:H (0 ? x ? 0.10) films deposited by PECVD have been studied. Upon the analysis of FTIR spectra, Raman spectra and XPS, it is concluded that π doping could take place even at a very low percentage of nitrogen, which favors the formation of sp² carbon clusters. The C 1s peak shifts toward higher binding energy while the N 1s peak remains constant as the nitrogen content in the film increases, which can be considered as a result of the chemical shifts on charge transfer due to the strong electronegativity of the N atom. 3D profile measurements show that there were a great number of particles formed when nitrogen is incorporated in to the films and the particles coalescence when the nitrogen content increases due to enhanced surface diffusion. The stress of the films converts from compressive to tensile stress gradually with increased N content. The elimination of grain boundaries and annihilation of excess vacancies, due to columnar structure increasing by diffusion leads to volume shrinkage of the film, thus causing tensile stress. These analyses were fairly consistent to help understand the effects of nitrogen in hydrogenated carbon films.     

Theoretical study on N-succinimidyl oligothiophenes: A novel class of materials for biological applications

We present a comparative theoretical study on a novel class of materials which can be used as biomarkers, namely the oligothiophene-N-succinimidyl esters. We study by ab initio coupled-cluster and first-principles density-functional theory calculations the electronic structure and the optical properties of a model system, namely bithiophene-N-succinimidyl, both isolated and coupled to a –C(O)–NH–(CH₂)₃CH₃ group at one terminal position. The latter is supposed to simulate the molecule after the binding to a biomolecule. We show that, although the electronic and optical properties of the isolated molecule are different from those of bithiophene, the binding to a biomolecule almost restores the well known electronic and optical properties of bithiophene.     

Electrical conductivity and photoreflectance of nanocrystalline copper nitride thin films deposited at low temperature

Nanocrystalline thin films of copper nitride were grown on Si (1 0 0) wafers at a low substrate temperature by reactive magnetron sputtering of Cu target with the mixture of nitrogen and argon. The influence of nitrogen deficiency upon the structural, optical and electrical properties of as-deposited films was investigated. X-ray diffraction confirms the presence of cubic Cu₃N and Cu biphases irrespective of carefully optimized processing parameters. With a Cu content approaching the stoichiometry for Cu₃N, the films assume a smooth morphology with densely-packed nanocrystallites of about 40–60 nm in size. Those deposits containing more than 79% Cu are metallic conductors with excellent electrical conductivity via a percolation mechanism, whereas the slightly substoichiometric Cu₃N films show a typical behavior of deficit semiconductor, with an optical gap of about 1.85 eV as revealed by photoreflectance measurement. All the observations are discussed in terms of nitrogen reemission from the growing film.     

Undoped amorphous SiNx: H alloy semiconductors: Dependence of electronic properties on composition

The electrical and optical properties of a-SiN_x:H thin films prepared by rf growth-discharge in SiH₄N₂H₂ without B or P doping have been measured for 0 < x ? 0.6. It is observed that the activation energy for extended-state electron conduction as well as the optical gap are unaffected by N content for x ? 0.4. The electron mobility in extended states is improved by as much as a factor of 10 by the N incorporation. There is a rapid conversion from the tetrahedral network to a Si₃N₄ network as x increases above ≈ 0.4.     

Modification of amorphous hydrogenated silicon by co-sputtered aluminum

The electronic and optical properties of a-Si_1?xH_x have been modified by the incorporation of aluminum. Samples were prepared by rf sputtering in a hydrogenated atmosphere from a composite silicon-aluminum target. This paper reports on several modified material parameters including the optical band gap, electrical conductivity, and thermal activation energy. Aluminum concentrations up to 10.6% in the target have been investigated. It is observed that the optical band gap remains constant at 1.83 eV for Al concentrations up to 2.7%. For higher concentrations there is a marked decrease in optical gap. The conductivity initially decreases with small Al concentration and the activation energy increases, characteristic of compensation of the inherently n-type material. For higher Al concentrations the conductivity increases by seven orders of magnitude and the activation energy decreases to a minimum of about 0.2 eV. The increase in conductivity can be explained by both the movement of the Fermi level and the shrinking band gap. Microprobe analyses have also been performed to determine the amount of Al actually incorporated into the films. Finally, implications of these results are discussed and compared to previously reported results on gas phase doping and ion implantation.     

Structural dependence of ultrafast third-order optical nonlinearity of Ge–Ga–Ag–S chalcogenide glasses

Ultrafast third-order optical nonlinearity of Ge–Ga–Ag–S chalcogenide glasses at the wavelength of 820 nm has been measured using femtosecond time-resolved optical Kerr (OKE) technique. The results show that Ge–Ga–Ag–S glasses have large third-order optical nonlinear susceptibility, χ⁽³⁾ and the response time is also subpicosecond, which are predominantly due to the ultrafast distortion of electron cloud surrounding the balanced positions of Ge, Ga, Ag and S atoms. What’s more, a strong dependence of χ⁽³⁾ on the composition and microstructure of these glasses was found which shows that [GeS₄] and [GaS₄] tetrahedra play an important role on the third-order optical nonlinearity. These Ge–Ga–Ag–S chalcogenide glasses would be expected as promising materials applied on all-optical switching devices.     

Compositional dependence of the optical properties of amorphous Sb2Se3−xSx thin films

Thin films of Sb₂Se_3−xS_x solid solutions (x = 0, 1, 2, and 3) were deposited by thermal evaporation of presynthesized materials on glass substrates held at room temperature. The films compositions were confirmed by using energy dispersive analysis of X-rays (EDAX). X-ray diffraction studies revealed that all the as-deposited films as well as those annealed at T_a < 423 K have amorphous phase. The optical constants (n, k) and the thickness (t) of the films were determined from optical transmittance data, in the spectral range 500-2500 nm, using the Swanepoel method. The dispersion parameters were determined from the analysis of the refractive index. An analysis of the optical absorption spectra revealed an Urbach’s tail in the low absorption region, while in the high absorption region an indirect band gap characterizes the films with different compositions. It was found that the optical band gap energy increases quadratically as the S content increases.     

A study of nonlinear optical properties in Bi2O3-WO3-TeO2 glasses

A series of bismuth tungsten tellurite glasses were prepared and their densities, linear refractive indices and transmission spectra were measured. The optical bandgaps E_opt and Urbach energies E_e of glasses were obtained from ultraviolet absorption edges. Both the optical gap (E_opt) and the band tail (E_e) are behaving oppositely. As the value of E_opt decreases with increasing WO₃ content, the degree of disorder increases which causes more defects or localized states resulting in deep localized in the bandgap with the tailing increased. Z-scan technique was carried out to investigate the third-order nonlinear optical properties of Bi₂O₃-WO₃-TeO₂ glasses. The third-order optical nonlinearity increases with decreasing the optical bandgap E_opt, since a increase of WO₃ content can provide the non-bridging oxygen ion content.     

Optical and chiro-optical properties of crystals with sillenite structure

The experimental data on the optical and chiro-optical properties of crystals with the sillenite structure Bi₁₂ M _xO_{20 ± δ} (M= elements of II–VIII groups) have been reviewed. The relations between the changes in the chemical composition, the crystal structure of Bi₁₂ M _xO_{20 ± δ} compounds, absorption, circular dichroism, and optical rotation spectra of sillenites have been established. A model taking into account the contribution of the electronic transitions of optically active tetrahedral [MO₄] ^n? complexes to the total optical rotation in sillenites is suggested. The data on the state (oxidation degree) and the recharging processes of 3d-elements in sillenites are systematized. The models of optical centers responsible for the photochromic effect in sillenites are critically analyzed.