The study of optical parameters and DC conductivity of Se100−xHgx thin films

Se_100−xHg_x bulk samples have been prepared by conventional melt quenching technique. The thin films of the material have been prepared on glass substrate using the thermal evaporation technique. The transmission spectra has been studied to measure the optical constants like absorption coefficient (α), extinction coefficient (K), optical band gap (E_g), Urbach energy (E_e). The DC conductivity (σ_dc) of Se_100−xHg_x has been also studied to find the activation energy (ΔE_a)$(\mathrm{\Delta }{E}_a)$. The optical band gap increases and Urbach energy first increases then decreases with increase in Hg concentration. DC conductivity and activation energy increases with increase in Hg concentration. These materials are found suitable for the optical disk materials and in optoelectronic devices due to their high absorption coefficient and dependence of reflectance on composition.     

The absorption edges of GeS and Urbach's rule

The low-energy absorption tail of the E∥a exciton in GeS obeys Urbach's rule over at least three orders of magnitude in absorption coefficient for 4.2 ? T ? 240 K. The E∥b polarization (for which no exciton is observed) does not obey Urbach's rule. The E∥a Urbach edge is caused by interactions between the excitons and a 8.7 meV rigid-layer vibrational mode. Our results are consistent with Sumi and Toyazawa's theory of the Urbach edge, and inconsistent with the models of Dow, Redfield and Skettrup. The absorption edge of GeS is confirmed as a direct edge.     

Low energy tail of the exciton luminescence band in 2H-PbI2 and its relation to Urbach rule

We have measured luminescence spectra of 2H-PbI₂ at different temperatures between 77 and 180 K. The lower energy tail of the luminescence band due to free exciton annihilation decreases exponentially with decreasing the photon energy over an order of magnitude, which corresponds to the Urbach tail in the absorption spectrum. This fact suggests that the exciton state corresponding to the exponential tail is in the thermal equilibrium and the steepness coefficient δ₀ = 1.67 in Urbach rule is obtained also from the luminescence spectra.     

Compositional behaviour of Urbach absorption edge and exciton-phonon interaction parameters in Cu6PS5I1−xBrx superionic mixed crystals

Temperature behaviour of optical absorption edge in Cu₆PS₅I_1−xBr_x mixed crystals is studied in the interval 77-325 K. It is shown that the absorption edge has Urbach shape in the 215-325 K temperature interval. The influence of temperature and compositional disorder on the Urbach absorption edge parameters is presented. The mechanism of the Urbach bundle formation and the effect of I→Br anionic substitution on the exciton-phonon interaction parameters is elucidated.     

Optical absorption near the melting point of LiNO3 crystal

Temperature dependence of the absorption spectrum in the ultraviolet region below 4.2 eV is measured in the temperature range from room temperature to just below the melting point T_M of LiNO₃ crystal. Anomalous behaviors in the observed spectra are found on approaching to T_M. The low energy tails of the spectra are analyzed by using Urbach rule. The steepness parameter σ(T), derived from the experimental results, also shows anomalous temperature change near T_M.     

Electrical and optical absorption studies of Cu7GeS5I fast-ion conductor

Electrical conductivity and fundamental absorption spectra of monocrystalline Cu₇GeS₅I were measured in the temperature ranges 95-370 and 77-373 K, respectively. A rather high electrical conductivity (σ_t=6.98×10⁻³Ω⁻¹ cm⁻¹ at 300 K) and low activation energy (ΔE_a=0.183 eV) was found. The influence of different types of disordering on the Urbach absorption edge and electron-phonon interaction parameters were calculated, discussed and compared with the same parameters in Cu₇GeS₅I, Cu₆PX₅I (X=S,Se) and Ag₇GeX₅I (X=S,Se) compounds. We have concluded that the P→Ge and Cu→Ag cation substitution results in an increase of the electrical conductivity and a decrease of the activation energy. Besides, P→Ge substitution, results in complete smearing and disappearance of the exciton absorption bands and in blue shift of the Urbach absorption edge, an increase of the edge energy width and an electron-phonon-interaction enhancement.     

On the physical basis for Urbach's rule

The low-energy tail of the E 6 a exciton in GeS obeys Urbach's rule, with Urbach parameters σ₀ = 1.45 ± 0.05, and $\text{h}\text{?}\text{ω}{}_{\mathrm{p}}\text{= 8.7 ± 0.6 meV}$. The energy ?ω_p corresponds to a previously measured rigid-layer vibrational mode which has no associated electric field. This finding is inconsistent with Dow and Redfield's unified theory of Urbach and exponential absorption edges. Our results are consistent with Sumi and Toyazawa's theory of Urbach edges, and with Fivaz and Mooser's model for electron-phonon interactions in layered compounds.     

Effect of hydrogen dilution on the optical properties of hydrogenated amorphous silicon-nitrogen films prepared by plasma deposition

The effect of the dilution of silane and nitrogen with hydrogen on the optical properties of hydrogenated amorphous silicon-nitrogen films prepared by plasma deposition has been investigated as functions of the gas-volume ratio γ (= ([SiH₄] + [N₂])/([SiH₄] + [N₂] + [H₂]) and the substrate temperature. The prepared films are characterized by the values of the deposition rate, the optical gap, the Urbach energy, the defect density, the integrated infrared absorption intensity and the refractive index, and by correlations between these parameters and the type of hydrogen- and nitrogen-bonding configurations estimated from infrared absorption spectra. The hydrogen dilution effect is discussed in terms of the above and compared with that in hydrogenated amorphous silicon reported in a previous paper by the present authors. It is pointed out that nitrogen atoms incorporated into the silicon network cause more disorder than incorporated hydrogen atoms, from the γ dependence of the Urbach energy and the integrated infrared intensities associated with the hydrogen and nitrogen bondings.     

Spectral behavior study of 3-(4-dimethylamino-phenyl)-1-{6-[3-(4-dimethylamino-phenyl)-acryloyl]-pyridin-2-yl}-propanone

The photophysical properties such as singlet absorption, molar absorptivity, fluorescence spectra, fluorescence quantum yield (?_f) and transition dipole moment (μ₁₂) of 3-(4-dimethylamino-phenyl)-1-{6-[3-(4-dimethylamino-phenyl)-acryloyl]-pyridin-2-yl}-propanone (DMAPAPP) have been studied in different media. DMAPAPP exhibits a large red-shift in both absorption and emission spectra as the solvent polarity increases, indicating a large change in dipole moment of molecule upon excitation. The fluorescence quantum yield depends on the nature of the solvent. The absorption and emission spectra of DMAPAPP in dioxane–water mixture are also studied. The effect of different type of surfactants to determine their critical micellar concentration (CMC) and the microemulsion effect on the electronic absorption and emission spectra of DMAPAPP are recorded. The effect of acidity on the electronic absorption and emission spectra of DMAPAPP is studied to determine the pK_a and pK_a^? values.     

Edge absorption spectra of crystalline and glassy PbGeS<Subscript>3</Subscript>

Edge absorption spectra of crystalline and glassy lead thiogermanate (PbGeS₃) have been measured in the temperature range from 77 to 470 K. It is shown that the dependence of the absorption coefficient on the photon energy for the glassy and crystalline states in the polarization E ∥ c is described by the Urbach rule. For the crystal in the polarization E ∥ b, at T < 300 K, an almost parallel shift of the intrinsic absorption edge to lower energies occurs with an increase in temperature, whereas at T ≥ 300 K, the Urbach absorption edge is observed. The parameter σ₀, related to the electron-phonon coupling constant, and the energy ?ω_ph of the effective phonons involved in the formation of the absorption edge of crystalline PbGeS₃ are determined from the temperature dependence of the parameter of the absorption edge slope. The contributions of the dynamic and static disorders to the diffusion of the absorption edge of crystalline PbGeS₃, as well as the topological disorder of glassy PbGeS₃, have been estimated.     

Energy fluctuations in optical transitions in CuHal crystals

This paper reports on a comprehensive study of fundamental-absorption spectra of CuCl and CuBr nanocrystals measured in the temperature range extending from 80 K up to the melting point. The temperature dependences of the energy E, oscillator strength f, and temperature-broadening coefficient σ of optical-transition spectral to excitonic states were determined. A new approach to the calculation of the shape and temperature-broadening of absorption spectra, based on quantum statistics, was employed. The crystal is treated as a quantum statistical ensemble. The set of atoms involved in the electronic transition initiated by a photon absorption is taken as an element of the ensemble. Energy fluctuations of electronic states occurring in the elements of the ensemble give rise to temperature broadening of the optical-transition spectra. The temperature dependence of the χ coefficient of thermodynamic fluctuations of the Z _{1, 2} and Z ₃ exciton states was found. A theoretical substantiation of the Urbach rule was obtained in terms of the model proposed.     

Thermally and optically induced effects on sub-band gap absorption in nanocrystalline CdSe (nc-CdSe) thin films

Nanocrystalline cadmium selenide (nc-CdSe) thin films have been prepared by thermal evaporation using the inert gas condensation (IGC) technique. Transmission electron microscopy (TEM) studies show that the CdSe nanocrystals (NCs) are spherical in shape. Constant photocurrent method (CPM) is used to determine the value of absorption coefficient in low absorption region of as-deposited, annealed and light soaked nc-CdSe thin films. Values of optical band gap (E_g) have been determined by using Tauc's relation for as-deposited, annealed and light soaked nc-CdSe thin films from the α values evaluated from reflection and transmission measurements. We have used a derivative procedure to sub-gap absorption spectra in order to get information on the energetic distribution of the occupied density of states below Fermi level. Constants such as optical gap E_g, Urbach edge E_u have been obtained from CPM spectra in as-deposited, annealed and light soaked nc-CdSe thin films. Concentration of defect states has also been calculated in as-deposited, annealed and light soaked nc-CdSe thin films.     

Spectral studies on Cu ions in sodium–lead borophosphate glasses

Electron Paramagnetic Resonance (EPR) and optical absorption spectra of Cu²⁺ ions in sodium–lead borophosphate glasses doped with different concentrations of Cu²⁺ ions have been studied. EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu²⁺ ions. The values of spin-Hamiltonian parameters indicate that the Cu²⁺ ions in sodium–lead borophosphate glasses are present in octahedral sites with tetragonal distortion. The optical absorption spectra of all the glass samples show a single broad band, which has been assigned to the ²B_1g→²B_2g transition of Cu²⁺ ions. The optical band gap energy (E_opt) and Urbach energy (ΔE) are calculated from their ultraviolet absorption edges. The emission bands observed in the ultraviolet and blue region are attributed to 3d⁹4s→3d¹⁰ triplet transition in Cu⁺ ion. The FT-IR spectra show that the glass system contains BO₃, BO₄ and PO₄ structural units.     

Anomalous behavior of the absorption tail due to a phase transition in Sb2S3

Absorption spectra near the fundamental absorption edge in Sb₂S₃ are studied between 77 and 380 K. It is found that the absorption tail obeys the Urbach rule between 130 and 230 K, while an anomaly associated with the second-order diffuse phase transition is observed above 230 K.     

Urbach rule and disordering processes in Cu6P(S1−xSex)5Br1−yIy superionic conductors

Compositional behavior of Urbach absorption edge is studied as well as the effect of compositional disordering on the parameters of exciton-phonon interaction, phase transition temperatures and electric conductivity in Cu₆P(S_1−xSe_x)₅Br_1−yI_y superionic solid solutions. The effect of different types of disordering on the optical absorption processes and specific features of compositional changes in the absorption edge spectra under S→Se and Br→I anion substitution in the mixed crystals are investigated. (x, T) phase diagrams for Cu₆P(S_1−xSe_x)₅X (X=I, Br) solid solutions are studied.     

Optical study of BST films combining ellipsometry and reflectivity

Optical properties of plasma laser-deposited Ba_0.75Sr_0.25TiO₃ (BST) thin films have been investigated using variable angle spectroscopic ellipsometry (VASE) and near-normal spectroscopic reflectivity (NNSR) within a broad spectral range at room temperature. The samples prepared under various deposition conditions and the Si substrate coated with the structure SiO₂/TiO_x/Pt were measured. The X-ray diffraction, atomic force microscopy and alpha step measurement were used for characterization of the samples. A special attention was paid to study sample texture. Both sets of experimental data (VASE and NNSR) were fitted simultaneously to obtain the optical constants (e.g. complex refractive index) and thicknesses of the films. For modeling of the experimental data in the range of transparency the Cauchy and Urbach formulas were used. The direct fit procedure and the Cody-Lorentz model were applied around and below absorption edge. In the entire spectral range the reflectivity spectra were analyzed by Kramers-Kronig analysis. The data around the absorption edge were fitted using the single-wavelength method and the absorption edge features were found up about 3.5 eV. The platinum-coated Si substrate data were fitted as a semi-infinite medium using the Drude and Lorentz oscillators model. The structure model for optical characterization of the sample included not only the BST layers and substrate but also the intermix and surface roughness layers to achieve good agreement with experimental data. The substrate structure was modeled by a simple bulk with surface roughness.     

InN分凝的InGaN薄膜的光致发光与吸收谱

我们用低压MOCVD在蓝宝石衬底生长了InGaN/GaN外延层.用X射线衍射(XRD),光致发光谱(PL),光吸收谱等测量手段,研究了InGaN的辐射发光机制.In组分利用Vegard定理和XRD测量得到.我们发现随着In组分的增加,在光吸收谱上发现吸收边的红移和较宽的Urbach带尾;PL谱中低能端的发射渐渐成为主导,并且在PL激发谱中InGaN峰也变宽.我们认为压电效应改变了InGaN的能带结构,从而影响了光学吸收特性.而在InN量子点中的辐射复合则是InGaN层发光的起源.     

Letters to the editor

Abstract

Optical absorption spectra of TeO₂ crystals, irradiated at room temperature by up to 2·10¹⁸ cm^?2 10-MeV electrons and subsequently annealed to 575K, are studied. The dependence of Urbach absorption edge parameters in TeO₂ on the electron beam fluence is discussed. The irradiation-induced near-edge broad (2.5–3.5eV) absorption band is shown to be related to oxygen vacancies, annealing at 475–525K.     

Temperature effects on optical properties of InN thin films

Transmission measurements have been carried out on InN thin films grown by radio frequency magnetron sputtering on a sapphire (0001) substrate at 10–300 K. With the aid of a novel procedure developed for analyzing the transmission spectra, the effect of temperature on optical properties, such as the absorption coefficient, band-gap, Urbach bandtail characteristics, refractive index and extinction coefficient, of InN thin films has been determined. The wavelength and temperature dependence of the absorption coefficient in both the Urbach and intrinsic absorption regions has been described by a series of empirical formulae. The temperature dependence of the refractive index dispersion below the band-gap is also found to follow a Sellmeier equation. These formulae are very useful for the characterization and device design of InN films. The free-electron concentration in the InN thin film determined here is also found to be in good agreement with that obtained from infrared reflection measurements. PACS 78.66.Fd; 78.40.Fy; 78.20.Bh; 78.20.Ci     

Calculation and interpretation of the absorption and fluorescence spectra of indole in the isolated state and aqueous solution

We have calculated the vibronic absorption and fluorescence spectra of the first (¹ L _b ) and second (¹ L _a ) electronic transitions of indole in the isolated state and aqueous solution. The vibrational structure of the absorption and fluorescence spectra has been interpreted. The influence of the aqueous solution on the vibronic spectra has been shown.