Thermoluminescence of (ZnS∶CdS∶Ag∶Ni∶Co) phosphors

The doping of (ZnSCdSAgNi) phosphors with cobalt impurity results in new phosphors with thermoluminescence curves showing five distinct peaks. Four of these peaks are associated with chlorine, cadmium, cobalt and nickel impurities and have energy depths of 0·26 eV, 0·52 eV, 0·53 eV and 0·78 eV respectively. The fifth peak, appearing at high cobalt concentrations, has an energy depth of 1·09 eV and is attributed to the formation of (Co⁺⁺ - Ag^–) associates. The emitted thermoluminescence consists of two bands: one in the yellow and the other in the red spectral region. The yellow band subsumes the chlorine, cadmium and cobalt peaks and is due to recombination processes occurring through silver centres. The red band, on the other hand, includes the nickel and the (Co⁺⁺-Ag^–) peaks. The appearance of the nickel peak in the red band is explained by recombination processes at energy levels created by the nickel impurity centres.     

Fe-N共掺杂锐钛矿相TiO2电子性质与光学性质的第一性原理研究

近年来,Fe和N掺杂锐钛矿相TiO2半导体在实验中发现许多优异性能,本文采用基于密度泛函理论的平面波超软赝势方法研究了纯锐钛矿相TiO2、Fe和N单掺杂及Fe和N共掺杂TiO2的能带结构、电荷布居、态密度和光学性质.分析发现:Fe掺杂引起杂质能带位于禁带中央,杂质能带最高点与导带相距大约0.6 eV而最低点与价带相距大约0.2 eV;N掺杂引起的杂质能带位于价带顶部附近. Fe和N共掺杂后杂质能带由两部分组成,位于价带顶上方0.62 eV和导带底下方0.22 eV处,其中一层杂质能带主要由N原子的2p轨道和Fe原子的3d轨道杂化形成,而另一条杂质能带主要由Fe原子的3d轨道形成,由于杂质能级的出现,使锐钛矿TiO2的禁带宽度变小.对光学性质分析发现:Fe和N共掺杂会使锐钛矿TiO2光学吸收带边红移,可见光区的光吸收系数明显增大,在低能区出现了新的吸收峰,对应能量为1.82 eV,与实验结果相符.     

Electrical and optical properties of titanium dioxide films

A method is described for producing insulating films of titanium dioxide by vacuum evaporation. Photomicrographs showing the growth of crystallites in the layer are presented, and X-ray and electron diffraction data are given. Sputtering regimes by means of which it is possible to produce compact layers of specific resistance 10¹³ ohm · · cm, dielectric strength 10⁶V/cm, and dielectric constant of 30 are given.By investigating optical density spectra and temperature variation of electrical conductivity for these specimens values were obtained for the optical and thermal forbidden band widths in titanium dioxide; these were 4.2 and 3.9 eV respectively. It was found that insulating films of titanium dioxide were completely transparent over a wide range of wavelengths from 300 to 1000 m. Using the same investigations in layers with electrical conductivity greater than 10^–13/ohm · · cm defects were discovered, which were connected with oxygen deficiency and gave three impurity energy levels in the forbidden band of titanium dioxide.In conclusion the authors would like to express their gratitude to Prof. K. V. Shalimova under whose guidance the work was executed.     

Electrical properties of Al-SiO2Si structure with plasmochemical insulator: Potential barrier heights and electron capture in SiO2

It is shown that in the forbidden band of the plasmochemical SiO₂ there are electron capture centers with capture cross section 2·10^–13cm², density 2·10¹⁹ cm^–3, and thermal activation energy 0.41 eV for the process of trap emptying. It is noted that the charge state of the insulator in an external electric field is unstable, this being due to the exchange of electrons between the insulator and the aluminum electrode. An energy band diagram of the structure is deduced from the results of the investigation. The potential barrier heights were found to be 2.78 eV at the Al-SiO₂ interface and 4.36 eV at the SiO₂-Si interface. The dynamical current-voltage characteristic of the structure is used to determine the density of mobile ions in the insulator, which is found to be S·10¹³ cm^–2 at T=230°C and rate of change 5 mV/sec of the voltage.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 61–66, January, 1981.     

W掺杂ZnO透明导电薄膜的理论及实验研究

本文从理论与实验两方面入手, 对高价态差金属W掺杂ZnO (WZO) 薄膜材料的特性进行分析讨论. 采用基于密度泛函理论的平面波赝势方法对WZO材料特性进行理论分析, 计算结果表明: W以替位形式掺入ZnO六角纤锌矿晶格结构中, 由于W-O键的键长较长引起晶格常数增加, 产生晶格畸变; 掺杂后费米能级进入导带, 其附近的导电电子主要由W 5d, O 2p及Zn 3d电子轨道提供, 材料表现出n型半导体的特性; 同时能带简并效应使其光学带隙展宽. 为进一步验证该理论分析结果的适用性, 本文采用脉冲直流磁控溅射技术进行了本征ZnO及WZO薄膜的实验研究, 结果表明: W掺入未改变ZnO的生长方式, 但引起薄膜的晶格常数增加, 电阻率由本征ZnO的1.35× 10^-2 Ω·cm减小到1.55× 10^-3 Ω·cm, 光学带隙由3.27 eV展宽到3.48 eV. 制备的WZO薄膜在400-1100 nm的平均透过率大于83%. 实验结果对理论计算结果进行了验证, 表明WZO薄膜作为透明导电薄膜的应用潜力.     

The electrical, optical and magnetic properties of Si-doped ZnO films

In this paper, the influences of Si-doping on electrical, optical and magnetic properties of ZnO films have been systematically investigated. It is found that the resistivity of the films decreases from 3.0 × 10³ to 6.2 × 10^-2 Ωcm with Si-doping due to the increase of carrier concentration. The bandgap of ZnO films increases from 3.28 to 3.52 eV with increasing of Si concentration, which is found to be due to the collective effects of bandgap narrowing and Burstein-Moss effect induced by high carrier concentration. With increase of Si concentration, the near band edge (NBE) emission decreases due to the deterioration of crystal quality, while the yellow emission enhances due to the increase of extrinsic impurity or defects. The additional Si-doping has a profound influence on the enhancement of magnetic property and the maximum magnetic moment of 2.6 μ_B/Si is obtained. The ferromagnetic ordering is seen to be correlated with carrier concentration and structural defects.     

Space-charge-limited currents and photoconductive properties of Tl2InGaSe4 layered crystals

The extrinsic electronic parameters of Tl₂InGaSe₄ layered crystals were investigated through measurement of the temperature-dependent dark conductivity, space-charge-limited currents and photoconductivity. Analysis of the dark conductivity reveals the existence of two extrinsic energy levels at 0.40 and 0.51 eV below the conduction band edge, which are dominant above and below 260 K, respectively. Current–voltage characteristics show that the one at 0.51 eV is a trapping energy level with a concentration of (4.8–7.7) × 10¹⁰ cm^?3. Photoconductivity measurements reveal the existence of another energy level located at 0.16 eV. In the studied temperature range, the photocurrent increases with increasing temperature. The dependence of the photoconductivity on the incident light intensity exhibits a linear recombination character near room temperature and a supralinear character as the temperature decreases. The change in recombination mechanism is attributed to an exchange in the behavior of sensitizing and recombination centres.     

Investigations on structural, vibrational, morphological and optical properties of CdS and CdS/Co films by ultrasonic spray pyrolysis

CdS and CdS/Co films have been deposited on glass substrates by an ultrasonic spray pyrolysis method. The effects of Co incorporation on the structural, optical, morphological, elemental and vibrational properties of these films were investigated. XRD analysis confirmed the hexagonal wurtzite structure of all films and had no impurity phase. While CdS film has (0 0 2) as the preferred orientation, CdS/Co films have (1 1 0) as the preferred orientation. The direct optical band gap was found to decrease from 2.42 to 2.39 eV by Co incorporation. The decrease of the direct energy gaps by increasing Co contents is mainly due to the sp-d exchange interaction between the localized d-electrons of Co²⁺ ions and band electrons of CdS. After the optical investigations, it was seen that the transmittance of CdS films decreased by Co content. The Raman measurements revealed two peaks corresponding to the 1LO and 2LO modes of hexagonal CdS. The vibrational modes of Cd-S were obtained in the wavenumber range (590-715 cm⁻¹) using Fourier transform infrared spectroscopy (FTIR). The elemental analysis of the film was done by energy dispersive X-ray spectrometry.     

The optical gap of NiO

The nature of the optical absorption gap in NiO at 4.0 eV is investigated. It is found that this gap is due to a band to band transition, where an electron is taken out of the valence band and placed into the conduction band. The optical gap of 6.0 eV found in NiMgO is of a nature, where an electron is taken out of the oxygen 2p band and placed into the first affinity level of the Ni²⁺ ion (3d ⁸L»3d ⁹L^–1). The impurity band created in Ni_1–x Li _x O by the Li ions is found 2.3 eV below the bottom of the conduction band in agreement with model predictions.     

掺铈氧化钛薄膜光吸收性能的研究

利用射频磁控溅射法制备出TiO2 薄膜及不同掺 Ce 比的 TiO2 薄膜.用紫外-可见分光光度计对薄膜的透过率做了测定,结果显示在一定掺杂范围内随着掺Ce 量的增加,薄膜的光学吸收边出现红移.计算发现通过CeO2 的掺杂,氧化钛薄膜禁带宽度 Eg 由 3.40 eV 减小至2.73 eV,从而使吸收边由366 nm 红移至455 nm 处,增强了对可见光的吸收,这与CeO2 在TiO2 导带与价带间引入杂质能级有关.     

Electron trapping levels in silicon dioxide thermally grown on silicon

Photocurrents associated with optical release of photoinjected electrons trapped in thin films of amorphous silicon dioxide have been studied. Temporal and spectral variation of the photocurrents were examined in detail: the effects on spectrally resolved response caused by variations in applied electrical field, wavelength sweep rate, and optical belaching are reported. All measurements were made on metal-oxide-semiconductor capacitors. The experiments were interpreted in terms of a straightforward model of optical excitation and transport of electrons out of localized energy levels in the silicon dioxide band gap. Semi-quantitative analysis indicated that a distribution of states peaked approximately 2·1 eV below the conduction band edge was associated with an electron trapping center distributed rather uniformly throughout the oxide film. In the wet thermal oxide specimens examined, the average density of trapping centers was greater than 10¹⁴ cm⁻³. A time-stable spread in energy of approximately 0·5 eV was measured, and was attributed to local disorder in the amorphous insulator. The existence of an optically inactive charge distribution in the oxide films, with bulk average density greater than 10¹⁵ cm⁻³, was indicated by collected charge vs. applied field data.     

Double source optical absorption and spectral photoconductivity measurements in the extrinsic region of semiconductors

The two-light-sources method of optical absorption and photoconductivity measurements of semiconductors in extrinsic region is discussed with emphasis on problems of steady-state population of impurity levels and effects accompanying the redistribution of charges under illumination. The equivalence of information on absorption and photoionization cross section of deep impurities obtained from both kinds of experiments is estimated and the flexibility and sensitivity of these methods are compared.Formulae for the steady-state population under extrinsic illumination are derived for the case when one or two levels are involved simultaneously and expressed as a function of optical cross sections for individual transitions for the high light intensity limit.The autor is indebted to M. Nekvinda and F. Karel for valuable discussions.     

Positron scattering from alkaline-earth elements

The total (elastic + inelastic) cross sections fore ⁺ impact on alkaline-earth elements from Be to Ra are calculated by employing a complex spherical optical potential. This potential has static, polarization and absorption components. The positron energy range is from a few eV to several thousand eV. We have compared our elastic cross sections for Mg and Ca with the other available results and the agreement is good for energies above 100eV. We have also compared our absorption cross sections withe ⁻ ionization cross sections at high energies where our absorption cross sections are in good accord. We have made Bethe plots fore ⁺ scattering on these elements.     

Optical properties of Tl2In2Se3S layered single crystals

The optical properties of Tl₂In₂Se₃S layered single crystals have been analyzed using transmission and reflection measurements in the wavelength region between 500 and 1100 nm. The optical indirect transitions with a band gap energy of 1.96 eV and direct transitions with a band gap energy of 2.16 eV were determined from analysis of absorption data at room temperature. Dispersion of the refractive index is discussed in terms of the Wemple–DiDomenico single-effective-oscillator model. The refractive index dispersion parameters – oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index – were found to be 4.67 eV, 45.35 eV, 1.38 × 10¹⁴ m ^{? 2} and 3.27, respectively. Transmission measurements were also performed in the temperature range 10–300 K. As a result of temperature-dependent transmission measurements, the rate of change in the indirect band gap with temperature, i.e. γ = ?5.6 × 10^?4 eV/K, and the absolute zero value of the band gap energy, E _gi(0) = 2.09 eV, were obtained.     

Ellipsometric characterization of PbI2 thin film on glass

The optical properties of polycrystalline lead iodide thin film grown on Corning glass substrate have been investigated by spectroscopic ellipsometry. A structural model is proposed to account for the optical constants of the film and its thickness. The optical properties of the PbI₂ layer were modeled using a modified Cauchy dispersion formula. The optical band gap E_g has been calculated based on the absorption coefficient (α) data above the band edge and from the incident photon energy at the maximum index of refraction. The band gap was also measured directly from the plot of the first derivative of the experimental transmission data with respect to the light wavelength around the transition band edge. The band gap was found to be in the range of 2.385±0.010 eV which agrees with the reported experimental values. Urbach's energy tail was observed in the absorption trend below the band edge and was found to be related to Urbach's energy of 0.08 eV.     

Calculations of physical properties of pure and doped crystals: Ab initio and semi-empirical methods in application to YAlO3:Ce and TiO2

In this paper we demonstrate that two independent methods of calculations (DFT based ab initio and semi-empirical crystal field theory) can be used to form a complementary picture of the optical and electronic properties of the doped host and impurity ion. The crystals considered in the present paper are: (i) YAlO₃:Ce³⁺ and (ii) two dominant phases of TiO₂—rutile and anatase. As an example, detailed calculations of the band structure and crystal field energy level scheme of YAlO₃:Ce³⁺ are reported. From the analysis of the band structure and density of states, the character of the YAlO₃ energetic bands and positions of the Ce impurity energy levels were established. It was also shown how the ab initio methods can be used for calculations of the structural properties of solids under elevated pressure. Taking the two dominant phases of TiO₂ as an example, it was demonstrated how the elastic properties can be extracted from the calculated unit cell’s volume at different pressures. Particular attention was paid to the microscopic effects of crystal field, which were evidenced by the pressure-induced changes of the structure and shape of distribution of the Ti 3d electrons density of states. It was demonstrated how the difference in crystal structure of the anatase and rutile phases leads to remarkable difference in microscopic crystal field effects, which was explained by different Ti-O distances in both phases. In addition, the pressure dependence of the band gaps for anatase and rutile was investigated. It was shown that the hydrostatic pressure leads to the band gap narrowing in anatase and band gap widening in rutile, with pressure coefficients +0.00681 eV/GPa for rutile and −0.0088 eV/GPa for anatase.     

过渡金属W、Mn、V、Ti掺杂二维材料MoSi2N4的第一性原理计算

本文基于密度泛函理论(DFT)的第一性原理计算了W、Mn、V、Ti替位掺杂二维MoSi₂N₄后的几何结构、电子结构以及光学性质的变化.电子结构分析表明W、Mn、W、Ti替位掺杂二维MoSi₂N₄后的禁带宽度分别为1.806 e V、1.003 e V、1.218 e V和1.373 e V;四种过渡金属掺杂后MoSi₂N₄的带隙类型没有发生改变,均为间接带隙半导体;W掺杂后的杂质能级靠近价带顶,费米能级靠近价带顶,为p型半导体,杂质能级为受主能级;Mn掺杂后的杂质能级靠近导带底,费米能级靠近导带底,为n型半导体;V和Ti掺杂后杂质能级位于费米能级附近,为复合中心;光学性质分析表明,在2 e V～4 e V的能量区间内,W掺杂结构的吸收波长为336 nm,体系发生红移;Mn、V和Ti替位掺杂后的吸收波长分别为320 nm、358 nm和338 nm,且掺杂体系均发生蓝移.     

Optical and Photoelectric Inhomogeneity of CdTe:V Crystals

The optical and photoelectric properties of CdTe:V crystals with the doping impurity concentration N _V = 5·10¹⁸–5·10¹⁹ cm^–3 are investigated and the possibility of their use as a photorefractive material is considered. As is seen from the spectra of optical transmission, the crystals of both types possess high transparency (50–65%), which for CdTe:V specimens with N _V = 5·10¹⁹ cm^–3 decreases sharply and in the range 12–14 m does not exceed 5%, whereas for CdTe:V crystals with vanadium concentration of 5·10¹⁸ cm^–3 such a value of transmission remains unchanged up to 25 m, implying a good optical quality of the latter crystals and their possible application in the spectral range 1.06–1.25 m in modern fiber-optic communication lines.     

掺锰铌酸锂晶体第一性原理研究

利用第一性原理研究了Mn:LiNbO3晶体的电子结构和光学性质。结果表明,Mn掺杂产生了杂质能级,主要由Mn的d态轨道贡献。杂质能级与导带之间的带隙小于理想LiNbO3晶体导带与价带之间宽度,降低了电子跃迁所需能量。同时,掺杂也降低了各原子的电子轨道能量。晶体中最高占据轨道不再是O的2p轨道,而是Mn的d 轨道。掺杂离子在晶体中同时充当电子的施主与受主。静态介电常数在掺杂后有明显的增大。Mn的掺杂使晶体在可见光区域的光学性质产生变化,并使晶体的吸收谱在可见光区域产生吸收峰。     

掺铁铌酸锂晶体第一性原理研究

利用第一性原理研究了Mn:LiNbO3晶体的电子结构和光学性质。结果表明,Mn掺杂产生了杂质能级,主要由Mn的d态轨道贡献。杂质能级与导带之间的带隙小于理想LiNbO3晶体导带与价带之间宽度,降低了电子跃迁所需能量。同时,掺杂也降低了各原子的电子轨道能量。晶体中最高占据轨道不再是O的2p轨道,而是Mn的d 轨道。掺杂离子在晶体中同时充当电子的施主与受主。静态介电常数在掺杂后有明显的增大。Mn的掺杂使晶体在可见光区域的光学性质产生变化,并使晶体的吸收谱在可见光区域产生吸收峰。