Electron-paramagnetic-resonance of ZnTe:Ga and ZnTe:Sn

Photosensitive EPR measurements ofS-state centers of gallium and tin in ZnTe single crystals are reported. In addition to large hyperfine interaction with the impurity nuclei the spectra show partially resolved superhyperfine structures arising from three shells of tellurium ligands. Both HF and SHF splittings decrease with increasing temperature. The samples exhibitp-type photoconductivity with the same spectral sensitivity observed with the generation and depletion of the EPR-spectrum.     

Er:YCOB和Er:Yb:YCOB晶体的光谱分析

分别测量了Er YCOB晶体和ErYb YCOB晶体的室温吸收光谱.讨论了Yb3+离子对Er3+离子的敏化作用.     

铕取代的锆硼钨多金属氧酸盐的制备和发光特性

制备了高发射效率的稀土多金属氧酸盐BPOM:Eu及ZrPOM:Eu荧光体,并对其结构及发光性质进行了研究。结果表明样品具有Keggin结构,Eu在两类样品中均有肉眼可见的在红光区的强发射。但在BPOM:Eu中以⁵D₀→⁷F₁磁偶极跃迁发射为主,在ZrPOM:Eu中以⁵D₀→⁷F₂电偶极跃迁发射为主。铕在ZrPOM:Eu中的对称性低于在BPOM:Eu中的对称性。这种差异也使铕离子的激发光谱具有不同的特征。铕在两种多金属氧酸盐中均有浓度猝灭,在BPOM:Eu中浓度为2.0%时发射强度最大;在ZrPOM:Eu中浓度为2.4%时发射强度最大。在ZrPOM:Eu中有较弱的配体到中心铕离子的能量传递,而在BPOM:Eu中却未观测到能量传递现象。     

Fabrication and properties of ZnO:Cu and ZnO:Ag thin films

Thin films of ZnS and ZnO:Cu were grown by an original metal–organic chemical vapour deposition (MOCVD) method under atmospheric pressure onto glass substrates. Pulse photo-assisted rapid thermal annealing of ZnO:Cu films in ambient air and at the temperature of 700–800 C was used instead of the common long-duration annealing in a vacuum furnace. ZnO:Ag thin films were prepared by oxidation and Ag doping of ZnS films. At first a closed space sublimation technique was used for Ag doping of ZnO films. The oxidation and Ag doping were carried out by a new non-vacuum method at a temperature >500 C. Crystal quality and optical properties were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL). It was found that the doped films have a higher degree of crystallinity than undoped films. The spectra of as-deposited ZnO:Cu films contained the bands typical for copper, i.e. the green band and the yellow band. After pulse annealing at high temperature the 410 and 435 nm photoluminescent peaks were observed. This allows changing of the emission colour from blue to white. Flat-top ZnO:Ag films were obtained with the surface roughness of 7 nm. These samples show a strong ultraviolet (UV) emission at room temperature. The 385 nm photoluminescent peak obtained is assigned to the exciton–exciton emission.     

New Weyl theory: Geometrization of electromagnetism and gravitation: Motivations and classical results

A geometrical unified field theory of electromagnetism and gravitation is developed in a Weyl space-time. The integrability conditions of the field equations cast the laws of classical perfect fluids under electromagnetic interactions. The purely gravitational limit of the theory is Einstein's General Relativity and the purely electromagnetic case coincides with the predictions of Maxwell's theory.     

Thermoexoelectronic and thermoluminescent properties of transparent YAG:Nd and YAG:Yb nanoceramics

Preliminary investigations of thermoluminescent and thermoexoelectronic properties of transparent microstructured YAG:Nd and YAG:Yb ceramics are performed. Five peaks of thermostimulated luminescence are found for YAG:Nd in the VUV and UV ranges. Four and six peaks of thermostimulated exoemission are found for YAG:Yb and YAG:Nd, respectively. The parameters of carrier traps are calculated.     

Photo and electroluminescence of ZnO : Er and ZnO : Ag,Er electroluminors

A number of ZnO : Er and ZnO : Ag, Er electroluminors have been prepared and their photo (PL) and electroluminescent (EL) properties investigated. While the addition of Ag slightly shifts the PL spectra towards longer wavelength side, the EL spectra not only shift but consist of some new transitions. In ZnO : Er electroluminors, additional transitions also exist at higher frequencies of excitations. Brightness waves for this system consist of two secondary peaks during each half cycle of exciting field. Temperature dependence shows two broad peaks. While voltage dependence of ZnO : Er satisfies the relationB=B ₀ exp(−b/V ^1/2), the relationB=B ₀ V exp(−b/V ^1/2) is found to be suitable for ZnO : Ag, Er electroluminor. Possible mechanisms for these phenomena have been proposed. A preliminary account of this work was presented at the International Symposium on Solid State Physics held at the Indian Association for the Cultivation of Science, Calcutta in January 1977.     

Photoluminescence of crystalline and disordered BTO:Mn powder: Experimental and theoretical modeling

Disordered and crystalline Mn-doped BaTiO₃ (BTO:Mn) powders were synthesized by the polymeric precursor method. After heat treatment, the nature of visible photoluminescence (PL) at room temperature in amorphous BTO:Mn was discussed, considering results of experimental and theoretical studies. X-ray diffraction (XRD), PL, and UV-vis were used to characterize this material. Rietveld refinement of the BTO:Mn from XRD data was used to built two models, which represent the crystalline BTO:Mn (BTO:Mn_c) and disordered BTO:Mn (BTO:Mn_d) structures. Theses models were analyzed by the periodic ab initio quantum mechanical calculations using the CRYSTAL98 package within the framework of density functional theory at the B3LYP level. The experimental and theoretical results indicated that PL is related with the degree of disorder in the BTO:Mn powders and also suggests the presence of localized states in the disordered structure.     

Electronic structure of ZnS:Co and ZnSe:Co crystals

The x-ray photoelectron spectra of the valence bands and inner Co 2p levels of solid solutions of the wide-gap semiconductors ZnS, ZnS:Co, ZnSe, and ZnSe:Co are investigated. The x-ray photoelectron Co 2p spectrum of the ZnS:Co crystal indicates a 3d configuration of the ground state of cobalt and strong covalence of the cobalt-sulfur bond, but somewhat weaker than for CoS. It is established that the edge of the valence band recedes by 0.7±0.1 eV in transition from ZnSe to ZnS. Fiz. Tverd. Tela (St. Petersburg) 39, 1971–1974 (November 1997)     

Emission and thermoluminescence characteristics of CaS:Bi:Fe phosphors

The emission and thermoluminescence characteristics of CaS:Bi,Fe phosphors were studied. It is observed that addition of Fe quenches the luminescence of CaS:Bi, which may be attributed to non-radiative transitions to deep Fe levels. The trap depth (0.6 eV) obtained from glow curve maxima is attributed to the CaS host lattice.     

Emission and excitation spectra of ZnO:Ga and ZnO:Ga,N ceramics

The spectral characteristics of ZnO:Ga and ZnO:Ga,N ceramics prepared by uniaxial hot pressing have been investigated. At room temperature, the edge (exciton) band at 3.12 eV dominates in the luminescence spectra of ZnO:Ga, while a wide luminescence band at 2.37 eV, which is likely to be due to zinc vacancies, is observed in the spectra of ZnO:Ga,N. Upon heating, the edge band maximum shifts to lower energies and the bandwidth increases. The extrapolated position of the edge-band maximum at zero temperature, E _m (0) = 3.367 ± 0.005 eV, is in agreement with the data for thin zinc oxide films. The luminescence excitation spectra in the range from 3 to 6.5 eV are reported and the mechanism of energy transfer to excitons and luminescence centers is considered.     

Comparison of spectroscopic properties of Yb:YAP and Yb:YAG crystals

The Yb:YAG and Yb:YAP crystals have been grown by Czochralski method. The absorption spectra and the fluorescence spectra of Yb:YAG and Yb:YAP crystals have been investigated. It is shown that the Yb:YAG crystal has better laser properties and smaller threshold power than Yb:YAP crystal. In addition, the absorption cross-section of the Yb:YAP crystal is 2.16 times of that of the Yb:YAG crystal,so laser diode pumped Yb:YAG lasing can be easily realized. Because YAP single crystal is anisotropic, it is provided with polarization characteristics.     

Optical and luminescence properties of single crystals of LiF:Cu and LiF:Mg,Cu

Results from spectral studies of the optical and luminescent properties of single crystals of lithium fluoride grown using the Czochralski technique and doped with ions of copper and magnesium, are presented. The effect of heat treatment regimes and the dependence of the concentration of magnesium impurities on the sensitivity of crystals of LiF:Mg, Cu to ionizing radiation are discussed.     

Role of disorder in Mn:GaAs, Cr:GaAs, and Cr:GaN

We present calculations of magnetic exchange interactions and critical temperature T(c) in Ga1-xMnxAs, Ga1-xCrxAs, and Ga1-xCrxN. The local spin-density approximation is combined with a linear-response technique to map the magnetic energy onto a Heisenberg Hamiltonion, but no significant further approximations are made. We show the following: (i) configurational disorder results in large dispersions in the pairwise exchange interactions; (ii) the disorder strongly reduces T(c); (iii) clustering in the magnetic atoms, whose tendency is predicted from total-energy considerations, further reduces T(c), while ordering the dopants on a lattice increases it. With all the factors taken into account, T(c) is reasonably predicted by the local spin-density approximation in Mn:GaAs without the need to invoke compensation by donor impurities.     

Photoluminescence Spectra of CaS:Er and CaS:Tb Powder Phosphors

This paper reports the method of preparation and the emission level assignments for the light emitting powder phosphors of CaS:Er³⁺ and CaS:Tb³⁺. The recorded photoluminescence spectra of these powder phosphors have shown that the light emission would be better at lower dopant concentrations.     

Investigation and simulation of the structural and electronic properties of LiF:Mg,LiF:Cu,LiF:P nano-layer dosimeters

In this study, electronic structure of lithium fluoride thin films in pure state and doped with magnesium (Mg), copper (Cu) and phosphorus (P) impurities was studied using WIEN2K Code. The structural and electronic properties of two LiF thin films with 1.61 and 4.05?nm thicknesses were studied and compared. Results show that the distance of atoms in the surface and central layers of pure LiF are 1.975 and 2.03?nm, respectively. Electronic density of the valence band around the surface atoms is greater than that around middle atoms of the supercell. The band gap of bulk LiF is 9?eV. But, in the case of thin films, it is reduced to 2?eV. Electronic and hole-traps were not observed in composition of LiF thin films doped with Mg and P with 1.61 and 4.05?nm thickness and in fact, metallic properties were observed. When Cu atoms were doped in composition of an LiF thin film, the thin film was converted to semiconductor.     

Characteristics of surface and waveguide emitting SiGe : Er : O diodes

In this paper we demonstrate electroluminescence from erbium- and oxygen doped Si and SiGe diodes in surface emitting and waveguiding geometry. The layers were deposited by molecular beam epitaxy (MBE) on (1 0 0)Si. A series of samples with varying Er : O-doped layer thickness was grown, showing a decrease in luminescence intensity for layers thinner than 80 nm. The current–voltage characteristics and luminescence properties of another series of samples with varying Ge content were examined. They are probably influenced by strain relaxation at large Ge content. Si/SiGe waveguides were processed and their emission was analyzed by a confocal microscope setup. The spatially narrow emission and the dependence of luminescence on waveguide length proved optical guiding of the erbium emission.