Optical depth of traps in AL2O3:C determined by the variable energy of stimulation OSL (VES-OSL) method

Standard methods of OSL measurements (CW-OSL or LM-OSL) do not allow for the direct determination of optical depth of traps. The variable energy of stimulation optically stimulated luminescence (VES-OSL) method gives such possibility. It consists in optical stimulation with the continuous increase of stimulation light energy and is analogous to the glow curve method in TL measurements. The VES-OSL curve shape and maximum position can be regulated by the stimulation photon flux, the rate of stimulation energy increase and by measurement temperature. This allows for detecting the OSL from very deep traps that give the TL signal overlapping with strong incandescence. The VES-OSL measurements carried out for Al₂O₃:C showed that traps having the optical depth between 2.0 and 2.8 eV are responsible for the OSL signal related to TL peak at about 200 °C. The OSL signal from the much deeper traps from the range 2.8–3.3 eV was also detected. The TL signal related do these traps cannot be detected below 500 °C.     

Nature of the anomalies of the thermolumenescence properties of dosimetric α-Al2O3 crystals

The results of investigations into the anomalies of the thermoluminescence properties of dosimetric corundum crystals are presented. The decisive role of deep-lying traps in the quenching of luminescence in anion-defect Al₂O₃ monocrystals is shown. The existence of deep-lying traps is demonstrated by the method of direct observations of thermoluminescence (TL) peaks associated with them. Experimental evidence for the influence of the degree of occupation of deep-lying traps on the main features of the TL dosimetric peak at 450 K is given. The results obtained are interpreted for a model of the interactive system of traps, which differs radically from the models described in the literature by a consideration of the temperature dependence of the probability of trapping of charge carriers on deep-lying traps. We believe that the heat quenching of luminescence is due to the thermal ionization of excited F-center states. Ural State Technical University. Translated from Izvestiya Vysshikh Uchebhykh Zavedenii, Fizika, No. 3, pp. 55–65, March, 2000.     

Modelling the optical bleaching of the thermoluminescence of K2YF5:Pr3+

Optical bleaching of the thermoluminescence (TL) curve of K₂YF₅:Pr³⁺ has been observed after optically stimulated luminescence (OSL) readout of pre-irradiated crystals. The traps being responsible for the TL signal are not emptied completely by the optical stimulation. Furthermore, if the illumination time is increased a constant intensity level of the residual TL glow curve is eventually achieved. On the other hand, if the low temperature peak of the glow curve is thermally cleaned, no subsequent OSL is measured. This behavior has been successfully explained by assuming that part of the electrons in the trap being responsible for the low temperature glow peak of K₂YF₅:Pr³⁺ recombine with holes via localized transitions during optical stimulation. During TL all trapped electrons recombine via delocalized transitions. Simulations have been carried out in order to demonstrate the feasibility of the model.     

The origin of two quenching concentrations and unusual afterglow behaviors of Ba2SnO4:Sm3+ phosphor

In order to interpret the origin of two emission quenching concentrations and a sharp increase of afterglow at certain Sm³⁺ doping concentration in Ba₂SnO₄:Sm³⁺ phosphor, the Eu³⁺ is used as a probe and it reveals that the Sm³⁺ ions occupy Sn⁴⁺ sites at low Sm³⁺ concentration and then the Ba²⁺ sites at a high concentration. The Sm³⁺ centers at different sites lead to two quenching processes and the two quenching concentrations effect. The thermoluminescence reveals that the substitution of Sm³⁺ for Ba²⁺ sites will induce a large amount of shallow hole traps and it should be mainly responsible for the sharp enhancement of the afterglow.     

Electronic states and the resonant optical non-linearity of exciton in a narrow band InSb quantum dot

Binding energy, interband emission energy and the non-linear optical properties of exciton in an InSb/InGa_xSb_1−x quantum dot are computed as functions of dot radius and the Ga content. Optical properties are obtained using the compact density matrix approach. The dependence of non-linear optical processes on the dot sizes is investigated for different Ga concentrations. The linear, third order non-linear optical absorption coefficients, susceptibility values and the refractive index changes of the exciton are calculated for different concentrations of gallium content. It is found that gallium concentration has great influence on the optical properties of InSb/InGa_xSb_1−x dots.     

Application of optical and luminescent techniques to the characterization of oxide thin films

The interaction between light and electrons in oxide compounds forms the basis for many interesting and practical effects, which are related to microstructure, energy band, traps, carrier transport and others. Thin films of oxides like WO₃, Ga₂O₃, Y₂O₃ and SrTiO₃ were investigated using various improved optical and luminescent techniques. The home-made systems for optical and luminescent measurements were described in detail. The facilities of photo-Hall and photoconductivity transients have been proven to be powerful tools in the studies, which allow us to perform photoinduced process and relaxation measurements over a wide time range from 10⁻⁸ to 10⁴ s. Furthermore, we extended the measurement capabilities of the commercial luminoscope by using an interferometer system with optical fiber and illuminance meter instead of an optical microscope. The cathodoluminescent measurements can be performed at a relative high pressure (20-60 mTorr) compared to ultra-high-vacuum condition of most commercial products. Luminescent characterization was employed as a probe to study doping ions, oxygen vacancies, trap and/or exciton levels in oxide thin films. Our results suggest that various traps and/or excitons in thin films of WO₃, Ga₂O₃ and SrTiO₃ involve in the process of photoconductivity relaxation and emission.     

Effects of rapid thermal annealing on the morphology and optical properity of ultrathin InSb film deposited on SiO2/Si substrate

Ultrathin InSb thin films on SiO₂/Si substrates are prepared by radio frequency (RF) magnetron sputtering and rapid thermal annealing (RTA) at 300, 400, and 500℃, respectively. X-ray diffraction (XRD) indicates that InSb film treated by RTA at 500℃, which is higher than its melting temperature (about 485℃), shows a monocrystalline-like feature. High-resolution transmission electron microscopy (HRTEM) micrograph shows that melt recrystallization of InSb film on SiO₂/Si(111) substrate is along the (111) planes. The transmittances of InSb films decrease and the optical band gaps redshift from 0.24 eV to 0.19 eV with annealing temperature increasing from 300℃ to 500℃, which is indicated by Fourier transform infrared spectroscopy (FTIR) measurement. The observed changes demonstrate that RAT is a viable technique for improving characteristics of InSb films, especially the melt-recrystallized film treated by RTA at 500℃.     

Effect of high-temperature annealing on the dielectric properties of PbTiO<Subscript>3</Subscript> crystals

The permittivity anomalies observed in PbTiO₃ in the range 20–300°C are assumed to be initiated by electron-induced polarization of the trap—electron dipoles with relaxation. The traps are lead and oxygen vacancies. Thermal quenching of the traps suppresses the anomalies.     

Calcination temperature optimization,energy transfer mechanism and fluorescence temperature dependence of KLa(MoO4)2:Eu3+ phosphors

The optimum calcination temperature for KLa(MoO₄)₂:Eu³⁺ phosphor was confirmed to be 1000 °C via checking the XRD patterns, SEM images and fluorescence spectra for the samples derived from solid state reaction. The energy transfer behavior between Eu³⁺ ions was studied. It was found that electric dipole–dipole interaction is responsible for the fluorescence quenching of ⁵D₂ and ⁵D₁ levels, but exchange interaction is in charge of ⁵D₀ fluorescence quenching. It was also observed that color coordinates of the studied phosphor can be tuned when the doping concentration is relatively low. The fluorescence thermal quenching process was investigated. It was found the thermal quenching followed well crossover model. Judd–Ofelt parameters of Eu³⁺ in KLa(MoO₄)₂ were obtained, and the optical transition properties were further discussed.     

Quenching of electroluminescence and charge trapping in high-efficiency Ge-implanted MOS light-emitting silicon diodes

Combined measurements of charge trapping and electroluminescence intensity as a function of injected charge and current have been carried out with the aim of clarifying the mechanisms of electroluminescence (EL) quenching in Ge-implanted ITO-SiO₂-Si light-emitting silicon diodes. Good correlation between the negative charge capture in traps of small effective capture cross-sections (σ_t1 ^e=1.7×10^-19 cm² and σ_t2 ^e=4.8×10^-20 cm²) located in SiO₂, and the quenching of the asymmetrical EL line with a maximum intensity at 400 nm has been observed. Similar correlation between the electron capture in traps with extremely small effective capture cross-section (σ_t3 ^e=5×10^-21 cm²) and the quenching of the EL line at 637 nm has been established. A quantitative model for the EL quenching has been developed, which takes into account the modification of the luminescent centers with subsequent electron capture at the newly generated traps. The model shows good agreement between simulation and experimental data. It also demonstrates that small effective capture cross-sections for electron charging during the EL quenching are determined by the probability of the luminescence centers (LCs) being disrupted, and enables one to estimate the Ge concentration associated with the EL at 400 nm. PACS 72.20.Jv; 73.40.Qv; 73.50.Gr     

Cu addition and its role in thermoelectric properties and nanostructuring in the series compounds (InSb)nCum

We have performed a comparative investigation of the series compounds (InSb)_nCu_m to assess the roles of Cu addition on the thermoelectric properties and nanostructuring in bulk InSb. Detailed temperature dependent transport properties including electrical conductivity, the Seebeck coefficient, and thermal conductivity are presented. The Seebeck coefficients of In₂₀Sb₂₀Cu (m:n = 1:20) are increased by 13 percent in magnitude if compared to those of InSb, which is responsible for the 22 percent enhancement in the highest ZT value at 687 K. Although the magnitudes of κ_L are larger than those of InSb over the entire temperature range, a remarkable reduction in lattice thermal conductivities (κ_L) was observed with measuring temperature elevation. Such changes are mainly due to the precipitation of a large number of Cu₉In₄ nanoparticles with the size of smaller than 5 nm, dispersed in the matrix observed using high resolution transmission electron microscopy (HRTEM) images.     

Luminescence and energy transfer in TbAl3B4O12

An investigation of the luminescence properties of TbAl₃B₄O₁₂ in the temperature region 1.4–300 K is reported. Laser site-selection and time-resolution techniques were used.The results show that energy migration among Tb³⁺ ions on the regular crystallographic sites occurs. The transfer characteristics at room temperature are consistent with a diffusion-limited transfer process to Mo³⁺ quenching centres. The diffusion constant and the critical transfer distance for Tb³⁺ → Mo³⁺ transfer are derived. The rate of diffusion increases for decreasing temperature. At about 60 K a transition from diffusion-limited transfer to trapping-limited transfer occurs. This behaviour is due to the variation in the diffusion constant with temperature. In the temperature region below 60 K transfer to Tb³⁺ traps is observed. The intensity of the emission from the traps increases exponentially with decreasing temperature. However, the overall transfer rate to Mo³⁺ and traps remains roughly constant. A simple model including time-independent transfer rates and back transfer from traps to intrinsic Tb³⁺ ions is proposed to explain the results. The rates of transfer to Mo³⁺ and traps are obtained.     

Real-time luminescence from Al2O3 fiber dosimeters

The real-time luminescence signal from Al₂O₃ single crystal fibers, monitored during simultaneous irradiation and optical stimulation, was investigated using computer simulations and experimental measurements. Both radioluminescence (RL) and optically stimulated luminescence (OSL) signals were studied. The simulations were performed initially using a simple one-trap/one-recombination-center energy band model, and then extended to include shallow and deep electron traps as well. Real-time luminescence experiments were performed for different radiation dose rates and optical stimulation powers using periodic laser stimulation of the samples through a fiber optic cable, and the experimental results were compared with the predictions from the computer simulations. The luminescence signal was observed, both theoretically and experimentally, to increase from its initial value to a steady-state level. The steady-state RL and OSL levels were found to be dependent on dose rate, the steady-state level of the real-time OSL being independent of laser power. It was also shown that the total integrated absorbed dose throughout the irradiation period can be determined by correcting the real-time OSL signal for depletion caused by each laser stimulation pulse. The effects of the shallow and deep traps on the time-dependence of the real-time luminescence signal were studied comparing the experimental data from several Al₂O₃ fibers known to have different trapping state concentrations. The additional traps were found to slow the response of the real-time luminescence such that the time to reach steady state was increased as the additional traps were added.     

用错位相位光栅产生的可调光学双阱

提出了用相位型错位光栅产生光学双阱的新方案.用平面光波(或TEM₀₀模式高斯光波)照射、正透镜聚焦,在透镜焦平面上产生的适用于冷原子或冷分子囚禁的多对可调光学双阱.计算和推导了双阱的光强分布、强度梯度以及光阱的几何参数与光学系统参数间的解析关系,研究了双阱到单阱三种不同的演化过程.同时还计算了光学双阱囚禁冷原子的光学偶极势和光子散射速率.研究发现,该方案不仅简单可行、操作方便,而且在原子物理、原子光学、分子光学和量子光学领域中有着广阔的应用前景. 关键词： 原子光学 相位光栅 光学双阱 冷原子囚禁     

The role of deep traps in luminescence of anion-defective α-Al2O3: C crystals

This paper presents the results of a study of deep traps responsible for thermally stimulated luminescence in single crystals of the anion-defective corundum α-Al₂O₃: C observed at temperatures of about 703, 778, 830, and 903 K. The effect of the filling of these traps by carriers of both signs on the sensitivity to radiation of the main thermoluminescence peak at 450 K, optically stimulated luminescence, and phototransferred thermoluminescence has been investigated. The results obtained are treated in terms of a model involving interactive competition of traps of different depth.     

Electric field induced exciton binding energy and its non-linear optical properties in a narrow InSb/InGaxSb1−x quantum dot

The effect of electric field on the binding energy, interband emission energy and the non-linear optical properties of exciton as a function of dot radius in an InSb/InGa_xSb_1?x quantum dot are investigated. Numerical calculations are carried out using single band effective mass approximation variationally to compute the exciton binding energy and optical properties are obtained using the compact density matrix approach. The dependence of the nonlinear optical processes on the dot sizes is investigated for various electric field strength. The linear, third order non-linear optical absorption coefficients, susceptibility values and the refractive index changes of electric field induced exciton as a function of photon energy are obtained. It is found that electric field and the geometrical confinement have great influence on the optical properties of dots.     

Enhanced optical nonlinearities of superlattices within the Kronig-Penney model incorporating inherent bulk nonlinearities

Third order nonlinear optical susceptibilities χ⁽³⁾ of GaAs/Ga_1?xAlAs superlattices have been predicted which are two orders of magnitude larger than those of bulk GaAs. This enhancement is due to the band nonparabolicity arising from the additional periodicity of the superlattice. These predictions, based on a tight-binding model of the superlattice dispersion, are here extended to the more realistic Kronig-Penney (KP) model. Corrections to tight-binding are non-negligible; however, enhancements of χ⁽³⁾ are still large but reduced approximately 30%–50% over previous estimates. The KP model is also here applied to superlattices employing InSb as the quantum well material. Because of the smaller effective mass of InSb, and taking account of its bulk nonparabolicity, the minibands move to higher energy, enhancing the interwell overlap and increasing χ⁽³⁾ by about one order of magnitude over that of bulk InSb. The role of the barrier material in this case is important and is discussed. The interplay between the bulk nonparabolicity and that arising from the superlattice is also addressed.     

新型光存储材料Sr2SnO4: Tb3+, Li+ 的合成及其红外上转换光激励发光性能的研究

采用高温固相法获得了一种只具有 微弱余辉的新型电子俘获型光存储材料Sr₂SnO₄:Tb^{3 +}, Li ⁺. 发光性能研究结果表明: 该材料对980 nm的红外激光具有很好的上转换光激励信息读出响应, 同时292 nm紫外光为其最佳信息写入光源. 光存储性能研究结果表明: 该材料的浅陷阱较少, 因此其余辉发光很弱, 不到500 s; 另一方面, 该材料中存在大量的深蓄能陷阱. 因此, Sr₂SnO₄: Tb^{3 +}, Li⁺是一种具有较好实际应用价值的新型电子俘获型光存储材料. 此外, 还讨论了Sr₂SnO₄: Tb^{3 +}, Li⁺的光存储发光机理.     

Classical in-plane negative magnetoresistance and quantum positive magnetoresistance in undoped InSb thin films on GaAs (1 0 0) substrates

Magnetoresistance (MR) effects have been investigated in perpendicular and parallel magnetic fields at 300, 80 K and liquid He temperatures for undoped InSb thin films 0.1–2.3 μm thick grown on GaAs(1 0 0) substrates by MBE. At high temperatures, the intrinsic carriers show the parabolic negative MR observable only in magnetic fields parallel to the film. The skipping-orbit effect due to surface boundary scattering in the classical orbits in the plane vertical to the film has been argued to be responsible for the negative MR. At low temperatures (T=80 K), the transport is dominated by the two-dimensional (2D) electrons in the accumulation layers at the InSb/GaAs(1 0 0) hetero interface; MR is positive and shows a logarithmic increase with anisotropy between parallel and perpendicular field orientation, arising from the 2D weak anti-localization (WAL) that reflects the interplay between the spin-Zeeman effect and strong spin–orbit interaction caused by the asymmetric potential at the interface (Rashba term). The zero-field spin splitting energy of Δ₀13 meV, the electron effective mass of m^*0.10m₀ seven times of the band edge mass in bulk InSb and the effective g-factor of |g^*|15 in the accumulation layer have been inferred from fits of MR for the 0.1 μm thick film to the 2D WL theory.     

Electric dipole mechanism of the generation of electromagnetic radiation due to the spin transport in an InSb semiconductor

Conditions for the generation of electromagnetic radiation by spin-polarized electron transport through a junction made on the basis of an InSb semiconductor and an HgCr₂Se₄ or Co₂MnSb ferromagnetic material are investigated. It is shown that electromagnetic radiation from the junction appears only when the electron flow passing from the ferromagnet to the InSb semiconductor is polarized. The radiation intensity is found to depend on the direction of the external magnetic field with respect to the InSb crystal axes. Maximum intensity values are observed for the field directions corresponding to the highest probability of electric dipole spin transitions between Zeeman levels.