Effect of Ar bombardment on the electrical and optical properties of low-density polyethylene films

The influence of low-energy Ar ion beam irradiation on both electrical and optical properties of low-density polyethylene (LDPE) films is presented. The polymer films were bombarded with 320 keV Ar ions with fuences up to 1×10¹⁵ cm^?2. Electrical properties of LDPE films were measured and the effect of ion bombardment on the DC conductivity, dielectric constant and loss was studied. Optically, the energy gap, the Urbach’s energy and the number of carbon atoms in a cluster were estimated for all polymer samples using the UV–Vis spectrophotometry technique. The obtained results showed slight enhancement in the conductivity and dielectric parameters due to the increase in ion fluence. Meanwhile, the energy gap and the Urbach’s energy values showed significant decrease by increasing the Ar ion fluence. It was found that the ion bombardment induced chain scission in the polymer chain causing some carbonization. An increase in the number of carbon atoms per cluster was also observed.     

Structural and Optical Properties of Zinc Nitride Films Prepared by Pulsed Filtered Cathodic Vacuum Arc Deposition

Polycrystalline zinc nitride films are deposited on Coming 7059 glass substrates by pulsed filtered cathodic vacuum arc deposition （PFCVAD）. The crystallographic structure is studied by means of x-ray diffraction. These measurements show that all the films are crystallized in the cubic structure, in a preferred orientation along the （332） and （631） directions. Weak XRD signal shows small crystallites distributed in an amorphous tissue. A small improvement of crystallinity is observed with annealing. Optical parameters such as absorption, energy band gap, Urbach tail, extinction coefficients have been determined. The Urbach tail energy is decreased with annealing at 500℃ for one hour. Energy band gap values are found to be increased by annealing.     

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.     

On the Urbach rule in sulphur-implanted Cu6PS5I superionic conductors

Cu₆PS₅I superionic crystals, grown using chemical vapour transport, were implanted by sulphur ions. The ion implantation effect on the phase transitions is studied by temperature isoabsorption investigation of the optical absorption edge. For the implanted crystals the optical absorption edge shape is studied in the temperature range 77-320 K, the parameters of exciton-phonon interaction, resulting in the Urbach behaviour of the optical absorption edge, are determined, the temperature dependences of the optical pseudogap and Urbach energy are obtained. The implantation effect on the ordering-disordering processes in Cu₆PS₅I superionic conductors is studied.     

Influence of holmium doping on the optical properties of quaternary InGaAsP epitaxial layers

Photoluminescence and photoconductivity measurements were used to study the influence of Ho doping on the optical properties of InGaAsP layers grown by liquid phase epitaxy (LPE). The full width at half maximum (FWHM) of the photoluminescence peak was found to decrease as the amount of Ho increases. When the amount of Ho is 0.11 wt%, the FWHM has a minimum value of 7.93 meV, about 46% lower than that of the undoped InGaAsP. The absorption tails observed in the photoconductivity were analyzed with the Urbach tail model and the Urbach energies were obtained from the fits. The Urbach energy decreases as the amount of Ho increases, indicating that Ho doping greatly reduces the amount of residual impurities in LPE-grown layers.     

Potential and restriction of high resolution,high energy photoemission in 400–8,000 eV for studying strongly correlated electron systems

InSe and InSe:Er single crystals were grown by using the Bridgman–Stockbarger method. The absorption measurements were carried out for voltage U=0 and U=30 V states of InSe and InSe:Er samples in the temperature range of 10–320 K with a step of 10 K. The absorption edge shifted towards longer wavelengths and the intensity of the absorption spectra decreased under a 5.90 kV/cm electric field. The same binding energy values for InSe and InSe:Er were calculated as 22.2 and 14.2 meV at U=0 and U=30 V, respectively. The steepness parameters and Urbach energies for InSe and InSe:Er samples increased with increasing sample temperature in the range of 10–320 K. An applied electric field caused a shift and a decrease of the intensity of the absorption spectra and an increase in the Urbach energy and steepness parameters. The shift of the absorption edge can be explained on the basis of the Franz–Keldysh effect or thermal heating of the sample under the electric field. PACS 71.20.Nr     

The Urbach rule for the PbO-SiO2 glasses

An analysis is made of the behavior of optical spectra of lead-silicate glasses, with variable lead content near the UV absorption edge, and within the 80–470-K range. A generalized formulation of the modified Urbach rule, applicable to glassy materials within a broad temperature range, is proposed for the interpretation of experimental spectral relations. Within this approach, the effective energies of the phonons responsible for the temperature-induced shift of the Urbach edge have been calculated. It is shown that the spectral and temperature parameters of the modified Urbach rule are structure-sensitive, and that their concentration behavior reflects the change of the type of short-range order in the glassy matrix.     

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.     

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.     

Absorption measurements in InSe:Ho single crystal under an electric field

InSe:Ho single crystal was grown by Bridgman-Stockberger method. Electric field effects on the absorption measurements have been investigated as a function of temperature in InSe:Ho single crystal. The absorption edge shifted towards longer wavelengths and a decrease of intensity in absorption spectra occurred under an electric field of 7.5 kV/cm. Using absorption measurements, steepness parameter and Urbach energy were calculated under electric field. Applied electric field caused an increase in the Urbach energy. At 10 K and 320 K, the first exciton energies were calculated as 1.322 and 1.301 eV for zero voltage and 1.245 and 1.232 eV for applied electric field, respectively.     

Effect of post-thermal annealing on the structural and optical properties of ZnO thin films prepared from a polymer precursor

ZnO thin films were synthesised by a new method which uses polyvinyl alcohol (PVA) as the polymer precursor. The films are annealed at different temperatures and for different annealing times. The structural parameters, like grain size, lattice constants, optical band gap, and Urbach energy, depend on the annealing temperature and time. All the films possess tensile strain, which relaxes as the annealing temperature and time increase. The photoluminescence (PL) spectra contain only ultraviolet (UV) peaks at low temperature, but as the annealing temperature and time increase, we observe peaks at the blue and green regions with a variation in the intensities of these peaks with annealing temperature and time.     

Temperature variation of electrical conductivity and absorption edge in Cu7GeSe5I advanced superionic conductor

Single crystals of Cu₇GeSe₅I superionic conductor were grown by chemical transport. Their electrical conductivity in the frequency range 1.0×10⁶–1.2×10⁹ Hz and in the temperature range 196–295 K was measured. Cu₇GeSe₅I crystal is shown to exhibit a rather high electrical conductivity (σ₂₉₅=64.0 S/m at 295 K) and a low activation energy (ΔE_a=0.125 eV). Optical absorption edge of Cu₇GeSe₅I crystals in the temperature range 77–300 K was studied, the temperature dependences of the optical pseudogap and Urbach energy being obtained. The effect of different types of disordering on the Urbach absorption edge and electron–phonon interaction parameters was investigated.     

Effect of post thermal annealing on the structural and optical properties of ZnO thin films prepared from polymer precursor

ZnO thin films were synthesised by a new method which uses polyvinyl alcohol (PVA) as polymer precursor. The films are annealed at different temperatures and for different annealing times. The structural parameters, like grain size, lattice constants, optical band gap, and Urbach energy, depend on the annealing temperature and annealing time. All the films possess tensile strain which relaxes as the annealing temperature and the annealing time increases. The photoluminescence (PL) spectra contain only ultraviolet (UV) peaks at low temperature, but as the annealing temperature and time increase we observe peaks at blue and green regions with variation of the intensities of these peaks with annealing temperature and annealing time.     

Effect of γ-irradiation on optical and chemical properties of CR-39 polymer

CR-39 polymer samples were irradiated with γ-irradiation up to dose ranging from 500 to 2000 kGy. The virgin and γ-irradiated polymer samples were investigated using UV–visible spectroscopy and Fourier transform infrared (FTIR) spectroscopy. In the present work, the Urbach energy was calculated using the Urbach edge method. Also, the direct and indirect energy band gaps in virgin and γ-irradiated CR-39 polymer samples were calculated. The values of indirect energy band gap were found to be lower than the corresponding values of direct energy band gap. The decrease in the optical energy band gap with increasing γ-irradiation dose was discussed on the basis of γ-irradiation-induced modifications in CR-39 polymer. The correlation between optical energy band gap and the number of carbon atoms in a cluster with modified Tauc's equation was also discussed. The FTIR spectra show considerable changes due to γ-irradiation, indicating that the detector is not chemically stable.     

Temperature dependence of the Urbach energy in ordered defect compounds Cu-III3-VI5 and Cu-III5-VI8

Two previous models used with success in Cu-III-VI₂ semiconductors have been employed to study the temperature dependence of the Urbach energy in ordered compounds Cu-III₃-VI₅ and Cu-III₅-VI₈. The model which contains two variable parameters seems to explain better the data over the whole temperature range studied. However, the ordered vacancy or the donor acceptor defect pair in the cation sublattice provides new features in these compounds that need further study.     

On the problem of activation energy of thermal decay of color centers and similar non-stationary processes

Thermally stimulated process is discussed as a superposition of components with equal activation energy and a distribution of frequency factors. Changes of concentrations of individual components of this process proceed according to first order rate law. A relationship between Laplace transformation of the distribution of frequency factors and the law governing the resulting changes of the concentration is studied. An application on the thermal decay of F-centers indicates that actual activation energy tends to be greater than the energy resulting e.g. from Urbach analysis. Experimental procedures are proposed allowing the actual activation energy and the distribution of frequency factors to be determined.     

Defect-induced increase in the phonon energy involved in the formation of Urbach tail in Cu-ternaries

From a combined analysis of the stoichiometric composition and Urbach tail in samples of CuInSe₂, CuInTe₂, and CuGaTe₂ of the I-III-VI₂ family of chalcopyrite semiconductors, it is found that the energy hν_p involved in the electron/exciton-phonon interaction is a linear function of a parameter Δz which is the sum of the deviations from ideal molecularity Δx and anion to cation ratio Δy. It gives evidence that in the copper ternaries hν_p is associated to the structural defects caused by cation-cation, cation-anion, and other intrinsic disorders. The high value of hν_p found in the studied samples, higher than the highest optical mode, is shown to come from the contribution of the additional phonon energy due to structural defects. This is in agreement with recently proposed models of the temperature dependence of the Urbach energy.     

First-principles calculations of the Urbach tail in the optical absorption spectra of silica glass

We present density-functional theory calculations of the optical absorption spectra of silica glass for temperatures up to 2400?K. The calculated spectra exhibit exponential tails near the fundamental absorption edge that follow the Urbach rule in good agreement with experiments. We discuss the accuracy of our results by comparing to hybrid exchange correlation functionals. We show that the Urbach rule holds in a frequency interval where optical absorption is Poisson distributed with very large statistical fluctuations. In this regime, a direct relation between the optical absorption coefficient and electronic density of states is derived, which provides a link between photoemission and absorption spectra and is used to determine the lower bound to the Urbach frequency regime.     

TeO2-TiO2-Bi2O3系统玻璃的热学特性及光学带隙研究

采用熔融淬冷法制备了系列高折射率的TeO2-TiO2-Bi2O3系统玻璃,测试了样品的密度、转变温度、析晶温度、折射率、吸收光谱,利用经典的Tauc方程计算了样品光学带隙允许的直接跃迁、允许的间接跃迁及Urbach能量.讨论了玻璃的热稳定性与组成之间的关系、研究了摩尔折射度、金属标准值、光学带隙、Urbach能量、Bi2O3和TiO2含量及光学碱度对玻璃样品折射率的影响.结果表明TeO2-TiO2-Bi2O3玻璃具有较好的热稳定性、样品的折射率随着摩尔折射度增大而增大,而光学带隙及金属标准值有减小的趋势,此外高的光学碱度对玻璃的高的三阶非线性也有一定的贡献.