Luminescent properties of diamond single crystals of pyramidal shape

The luminescence properties of needle-like crystals of diamond, obtained by selective oxidation of textured polycrystalline diamond films, are studied. Diamond films were grown by chemical vapor deposition from a methane–hydrogen mixture activated by a DC discharge. The spectra of photo- and cathodoluminescence and the spatial distribution of the intensity of radiation at different wavelengths are obtained for individual needle-like crystals. Based on the spectral characteristics, conclusions are made about the presence of optically active defects containing nitrogen and silicon impurities in their structure, as well as the significant effect of structural defects on their luminescence spectra.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Applications of cathodoluminescence in mineralogy

Cathodoluminescence microscopy and cathodoluminescence spectrometry with the electron microprobe and the scanning electron microscope are applied to the study of some natural cassiterite, quartz and sphalerite crystals. Some examples of localization and distribution of crystalline defects revealed by cathodoluminescence microscopy are given. X-ray spectrometry and secondary ion mass analysis allow to illustrate the coincidence between the spatial variations of cathodoluminescence emission and the distribution of impurities within minerals. In order to establish a relationship between the spectral distribution of the luminescence and the nature of trace elements, comparisons are made between cathodoluminescence properties of natural and artificial zinc sulphide crystals. Particular attention is given to specimen containing copper-gallium impurities. It has been possible to distinguish between the impurities which can be responsible for the luminescence and those which are not associated with cathodoluminescence emission characteristic of the crystals. The study of cathodoluminescence intensity according to the excitation conditions led to consider the contribution of the X-ray photons generated in the specimen for the production of luminescence photons. An in-depth distribution law of generated photons similar to that used to take account of X-ray emission led to introduce an absorption function in the relation giving the measured intensity of cathodoluminescence according to the incident energy and the electron beam density.     

Luminescence from Mn-doped PbI2 crystals

Luminescent properties of PbI₂ and PbI₂: 0.5 mol % MnCl₂ crystals under X-ray or N₂-laser excitation are studied experimentally. The measurements are performed at temperatures ranging from 85 to 295 K. For PbI₂ crystals under laser excitation, spectral bands with peaks near 495 and 512 nm, respectively, are observed at 85 K. With X-ray excitation at the same temperature, luminescence is observed in the 515-and 715-nm bands. The doping decreases the intensity in the 515-nm band, increases it for longer wavelengths, and shifts the highest peak to 700 nm. At 85 K, the doping has an insignificant effect on the excitation energy accumulated by trapped electrons. Certain PbI₂ crystals also exhibit a peak in a region of 580–595 nm. This peak becomes much higher if the crystal is treated with an N₂ laser at room temperature or if it is heated to 450–485 K. As the measurement temperature rises from 85 to 295 K, luminescence intensity decreases considerably. With X-ray excitation at room temperature, the yield of PbI₂: Mn luminescence peaked at 660 nm for doped crystals is about three times larger than the yield peaked at 555 nm for nondoped crystals. The spectral curves and underlying radiative processes are discussed.     

Photoluminescence of calcite crystals of different origins

We have studied the photoluminescence of calcite crystals. In the blue region of the photoluminescence spectrum of calcite crystals obtained from Siberia (Russia) and from Saaremaa Island (Estonia), three strongly overlapping luminescence bands due to intrinsic defects are observed. Luminescence due to impurities in the crystals are hardly detectable. The experimentally measured time dependence of the luminescence intensity for the indicated luminescence bands is compared with the dependences obtained as a result of a calculation based on a proposed model for the luminescence center. Better agreement between experiment and calculation is achieved if the model of the luminescence center includes a metastable level with electron ejection energy of 4 meV; the characteristic time for the radiative transition is 1.3 nsec. Studying the time dependence of the luminescence at different wavelengths within the indicated bands allows us to conclude that the photoluminescence (three bands) is due to one type of luminescence center. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 498–501, July–August, 2006.     

Oxygen-induced modification of dislocation luminescence centers in silicon

Prospects for using the long-wavelength dislocation luminescence line D1 in silicon-based light-emitting diodes are considered. The standard spectral position of this line at 807 meV, rather than being canonic, depends on the morphology of the dislocation structure and the impurity environment of an individual dislocation. Data on the spectral distribution of luminescence intensity in the region of the D1 line have been analyzed in terms of the concentration of interstitial oxygen in a sample, plastic deformation parameters, and thermal treatment. The results obtained suggest that oxygen exerts a dominant effect on the spectral position of line D1 and luminescence intensity in its vicinity. It is shown that the probable structure of recombination centers can be described in terms of the donor-acceptor pair model, in which oxygen complexes serve as donors and the acceptors are structural defects in the dislocation core.     

Roentgenoluminescence from silicon carbide

It is found that single crystals of silicon carbide exposed to soft X rays exhibit luminescence in the visible spectral range. The luminescence intensity from the single crystals produced by different methods differs by three-to fivefold. Also, the emission intensity is nonuniformly distributed over the single crystal surface, which may be related to the nonuniform distribution of impurities (activators).     

Ultraviolet luminescence of single crystals and single-crystal films of YAlO3

The nature of intrinsic emission bands of yttrium orthoaluminate in the UV spectral region at _max=220 nm (5.63 eV) and 330 nm (4.13 eV) is studied on the basis of the luminescence of single crystals and single-crystal films of YAlO₃ and Ce: YAlO₃ excited by synchrotron radiation sources with an energy of 3–25 eV at 9 and 300 K. The single crystals and single-crystal films were obtained, respectively, from solution and solution-melt by liquid-phase epitaxy and are characterized by considerably different concentrations of substitutional and vacancy defects. It is found that only the luminescence band at 300 nm, which has the decay time τ=4.1 ns and is excited in a band shifted from the range of interband transitions by 0.25 eV, has exciton-like character. The luminescence band at 220 nm with τ=0.1 µs at 9K, which is observed only for YAlO₃ single crystals and is absent in the luminescence of single-crystal films, is associated with antisite defects of the Y _Al ³⁺ type, which are a specific type of cationic isoelectronic impurities. It is shown that the phosphors based on single-crystal films of YAlO₃ have a simpler scintillation decay kinetics than their bulk analogues due to the absence of channels of excitation energy dissipation associated with the antisite defects of Y _Al ³⁺ type and vacancy defects.     

基于特征光谱参量表征发光二极管结温的方法

针对采用蓝光激发荧光粉产生白光的YAG型白光LED,通过分析其光谱波谷特性,采用常规可见光光谱仪和温控系统设计了一套基于光谱特征参量的LED结温测试系统.测量方法分为定标函数的测定和任意状态下的测量两部分.首先采用光谱仪测量在给定的多个不同结温和正常驱动电流下的相对发光光谱数据,再分析其光谱波谷处的相对光谱强度.从实用性和降低成本的角度考虑,采用正常工作电流驱动,但以正常工作电流驱动下的LED在光谱仪的固定反应时间内其自加热效应不可忽略.因此采用选定基准状态法,将各温度下的相对发光光谱强度与基准状态下的逐点作差得到相应的发光光谱强度差,同时为了减少温控系统引入的温度偏差,同样将各温度与基准温度作差得到相应的结温差.实验表明高低色温大功率LED的结温差和发光光谱强度差经过一定的函数拟合形成的定标函数其线性度都较高,R2达到0.99以上;利用定标函数,可以测量出在任意状态下的LED结温.最后将采用本方法得出的高低色温LED在不同条件下的结温数据与通过Mentor Graphics公司的T3Ster仪器的测量结果进行了比较,最大偏离度为2.82%,在可接受的误差范围内,表明此方法完全具备可行性,具有一定的实用价值.     

Pulsed cathodoluminescence of irradiated LiF-O and LiF(U)-O crystals

The spectral kinetic characteristics of the luminescence excited in previously irradiated (≤10⁶ Gy) LiF-O and LiF(U)-O crystals have been investigated in the spectral range 4–1.8 eV by pulsed spectrometry with nanosecond time resolution. The luminescence of the crystals was excited by nanosecond nitrogen-laser or electron-beam pulses at 300 K. The inertial character of uranium luminescence enhancement and the dependence of the number of enhancement stages on the excitation technique are revealed. A difference in the character of the dependences of the intensities of the pulse photo-and cathodoluminescence of uranium on the irradiation dose is found.     

Transformation of resonant Raman scattering into luminescence in CdxZn1−xTe mixed crystals: Time-resolved spectroscopy

We report an experimental study of time characteristics of secondary emission in Cd_xZn_1-xTe mixed crystals (x = 0.32) under resonant excitation with a picosecond dye laser. When the incident laser frequency is tuned on to the luminescence maximum of localized excitons, the decay curve of the intensity of “Raman-like” lines exhibits a single exponential decay. Off resonance, however, a short-lived component corresponding to Raman scattering appears in addition to the long-lived component. The intensity of the Raman component relative to that of the luminescence component increases with increase of the off-resonance frequency. From these temporal behaviors, we have found, for the first time, the transformation of resonant Raman scattering into luminescence in mixed crystals as a function of incident frequencies.     

Role of shallow impurities and lattice defects in nucleation of electron-hole droplets in Si

Contribution of lattice defects and shallow impurities to electron-hole droplet (EHD) formation has been confirmed by the study on photoluminescence due to EHD's in various kinds of silicon crystals. Sample dependence of the EHD luminescence peak energy shift in the variation of the excitation intensity is explained in terms of the surface energy contribution to EHD binding energy and concentrations of EHD's and impurities.     

Effect of a weak magnetic field on the state of structural defects and the plasticity of ionic crystals

The aim of this work is to examine the influence of a weak (on the energy scale) magnetic field on the state of dislocations and point defects in ionic crystals. It is found that complex point defects existing in a metastable state are sensitive to a magnetic field B∼1 T. The contributions are identified, and the kinetics of various types of reactions within the structural defects and between them leading to plastification of the crystals in a magnetic field are determined. The effect of light on the sensitivity of the point defects to a magnetic field is described, and the spectral characteristics of this effect are determined. A resonant effect of the combined action of a weak constant magnetic field and a high-frequency magnetic field on the dislocation mobility is found to occur when these fields satisfy the conditions of electron paramagnetic resonance. Zh. éksp. Teor. Fiz. 115, 605–623 (February 1999)     

Cathodoluminescence study of extended defects in hydrothermal ZnO crystals

Understanding the luminescence of ZnO is very important for some applications. In spite of the many studies carried out, there are still some points concerning the origin of some of the luminescence emissions in ZnO crystals that require additional study; in particular, the role of extended defects remains to be a matter of controversy. We present here a cathodoluminescence analysis of the defects generated by Vickers indentation in hydrothermal HTT crystals. Special emphasis was paid to the luminescence band peaking around 3.3 eV. The origin of this band is a matter of controversy, since it has been related to different causes, extended defects being one of the candidates for this emission. The CL images were acquired around crystal defects. It is observed that the 3.3 eV emission is enhanced around the crystal defects; though it is also observed, but weaker, out of the defect regions, which suggests that there exist two luminescence emissions peaking very close to 3.3 eV. The two emissions, one related to structural defects and the other to the LO phonon replica of the free excitonic band, appear very close each other and their relative intensity should determine the shape of the spectrum.     

Cathodoluminescence of ZnS single crystals subjected to thermal annealing in vapors of the constituents

The 77°K cathodoluminescence spectra of ZnS single crystals grown from the melt and annealed in vapors of the constituents were studied in the spectral region 360 to 550 nm. The single crystals contained oxygen whose phase state could be changed by thermal annealing. The effect of oxygen in the ZnS lattice on the appearance and intensities of various bands in the zinc sulfide spectrum was investigated. It turned out that the bands at 390–400 nm, 410–430 nm, and 500–525 nm are associated with the luminescence of a solid solution of ZnS· O in the lattice. The intensity of the green luminescence was a function of oxygen concentration in precipitates of the solid solution on dislocations. The luminescence in the 363 to 370 nm region is associated with zinc oxide which separates from ZnS containing various amounts of sulfur.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 84–88, May, 1975.     

CLAVI pulsed cathodoluminescence spectroscope

Technical parameters of a CLAVI pulsed cathodoluminescence spectroscope are presented. The device is used for the nondestructive spectral luminescence analysis of defects in solid optical and laser materials (crystals, glasses, and ceramic materials). Illustrating examples are presented. Original Text ? Astro, Ltd., 2006.     

Luminescence properties of calcium-iodide crystals

The effect of the conditions of preparation, temperature, and the action of x rays on the luminescence properties of calcium-iodide scintillation crystals is investigated. On the basis of the results of a study of the spectral characteristics of CaI₂ and CaI₂:H₂ crystals for optical and x-ray excitation in the temperature range 90–400 K, also taking into account the results of a study of the luminescence properties of CaI₂ crystals activated by Cl⁻, Br⁻, OH⁻, and Ca²⁺ impurities, it is suggested that the 236-nm band observed in the excitation spectra of crystals of calcium iodide may be caused by an uncontrollable hydrogen impurity. The luminescence of these crystals with maximum at 395 nm is ascribed to radiative recombination of excitons trapped at H⁻ ions. Zh. Tekh. Fiz. 69, 135–136 (January 1999)     

Quantum-chemical approach to defect formation processes in non-metallic crystals

Abstract

Results of the quantum-chemical simulation of the formation of structural and radiation defects are reviewed, using ice, silicon, and silicon dioxide as examples. The relationship between the structural elements of these crystals and the structural defects is analysed. Models of the main defects, their optical characteristics, and the activation energy of their migration are discussed. The relationship between the characteristics obtained by quantum-chemical calculations and the parameters of the macroscopic kinetics of the processes induced by defects in dielectric crystals is considered.     

Ultraviolet-visible spectroscopic characterization of lanthanum beryllate crystals doped with Er,Nd, or Pr ions

Spectroscopic characterization of lanthanum beryllate La₂Be₂O₅ (BLO) single crystals doped with trivalent ions of Eu, Nd or Pr, was carried out in the ultraviolet-visible spectral range using synchrotron radiation spectroscopy in combination with conventional optical absorption and luminescence spectroscopy techniques. On the basis of the obtained data, the energy level diagram for these trivalent impurity ions in BLO host lattice was developed; the optical and electronic properties of the crystals were determined; the possibility of the 4f-4f, 4f-5d and charge transfer transitions was analyzed; spectroscopic properties of the lattice defects formed during the introduction of trivalent impurity ions in the BLO host lattice, were investigated. We found that the lattice defects are responsible for a wide-band photoluminescence (PL) in the energy region of 400–600 nm. The most efficient excitation of the defect photoluminescence in the energy gap of BLO occurs in broad PL excitation-bands at 270 and 240 nm. The PL intensity of defects depends on the type of impurity ion and increases in the sequence: Pr-Nd-Er.     

Luminescence and electronic excitations in crystals K<Subscript>2</Subscript>Al<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>7</Subscript> with defects

This paper reports on the results of the comprehensive study of the dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals, obtained by low-temperature luminescence vacuum ultraviolet spectroscopy with nanosecond time resolution upon photoexcitation by synchrotron radiation. For the first time, the data have been obtained on the photoluminescence (PL) decay kinetics, PL spectra with time resolution, PL excitation spectra with time resolution, and reflection spectra at 7 K; the intrinsic nature of PL at 3.28 eV has been established; luminescence bands of defects have been separated in the visible and ultraviolet spectral regions; an intense long-wavelength PL band has been detected at 1.72 eV; channels of the formation and decay of electronic excitations in K₂Al₂B₂O₇ crystals have been discussed.