Pulsed Electron Beam Excitation Research Articles

Relaxation of electronic excitations in K2GeF6 studied by means of time-resolved luminescence spectroscopy under VUV and pulsed electron beam excitation

Highly crystalline K2GeF6 micropowders were prepared by co-precipitation in the hydrofluoric acid-water-alcohol solution and studied by means of the cathodoluminescence and time-resolved luminescence spectroscopy under synchrotron radiation excitation at 7 K. The nature of various intrinsic emissions was revealed. The luminescence band at 510 nm detected under excitation by photons with the energy above 9 eV is related to the radiative decay of self-trapped excitons. Fast emission bands with ~400 ps decay time found in the spectral range from VUV to visible (8–2.4 eV) are assigned to the cross-luminescence transitions from K 3p hole states and intraband luminescence transitions due to the presence of the Ge 4s, 4p valence states. The creation of cation excitons was detected in the energy range 18–20 eV. Their non-radiative decay results in the formation of hole states in the Ge energy bands, which finally recombine with electrons from the above lying valence states and provide fast intraband luminescence. Important electronic band structure parameters including the energy gap width of 11 eV, cation exciton formation energy 18.1 eV and ionization energy of cation states 20.0 eV were determined from the luminescence excitation spectra.

Employing Cathodoluminescence for Nanothermometry and Thermal Transport Measurements in Semiconductor Nanowires.

Thermal properties have an outsized impact on efficiency and sensitivity of devices with nanoscale structures, such as in integrated electronic circuits. A number of thermal conductivity measurements for semiconductor nanostructures exist, but are hindered by the diffraction limit of light, the need for transducer layers, the slow scan rate of probes, ultrathin sample requirements, or extensive fabrication. Here, we overcome these limitations by extracting nanoscale temperature maps from measurements of bandgap cathodoluminescence in GaN nanowires of <300 nm diameter with spatial resolution limited by the electron cascade. We use this thermometry method in three ways to determine the thermal conductivities of the nanowires in the range of 19–68 W/m·K, well below that of bulk GaN. The electron beam acts simultaneously as a temperature probe and as a controlled delta-function-like heat source to measure thermal conductivities using steady-state methods, and we introduce a frequency-domain method using pulsed electron beam excitation. The different thermal conductivity measurements we explore agree within error in uniformly doped wires. We show feasible methods for rapid, in situ, high-resolution thermal property measurements of integrated circuits and semiconductor nanodevices and enable electron-beam-based nanoscale phonon transport studies.

Investigation of an electron-beam pumped VECSEL based on an InGaAs/AlGaAs heterostructure

We report the results of a study of an e-beam pumped vertical-external-cavity surface-emitting laser (VECSEL) based on an InGaAs/AlGaAs heterostructure. Metalorganic chemical vapour deposition (MOCVD) is employed to grow two structures of different design, which contain 10 quantum wells (QWs) and a built-in distributed Bragg reflector (DBR) mirror. Under repetitively pulsed electron-beam excitation (50 Hz, 250 ns), a peak output power of 5.5 W is achieved at a wavelength of 2.5 W and an output power of 2.5 W at 1.013 μm with a total convergence angle no larger than 20 mrad.

High-Output-Power Ultraviolet Light Source from Quasi-2D GaN Quantum Structure.

Quasi-2D GaN layers inserted in an AlGaN matrix are proposed as a novel active region to develop a high-output-power UV light source. Such a structure is successfully achieved by precise control in molecular beam epitaxy and shows an amazing output power of ≈160 mW at 285 nm with a pulsed electron-beam excitation. This device is promising and competitive in non-line-of-sight communications or the sterilization field.

Thermal and luminescent properties of M2Zn(VO3)4 (M = Rb, Cs)

We have developed processes for the synthesis of the Rb2Zn(VO3)4 and Cs2Zn(VO3)4 tetrametavanadates. Rb2Zn(VO3)4 has been prepared by solid-state reaction (350°C) between presynthesized RbVO3 and ZnV2O6 powders, and Cs2Zn(VO3)4 has been prepared by the Pechini method (sol-gel process). Both metavanadates crystallize in monoclinic symmetry (sp. gr. P21/m). Thermochemical characterization results demonstrate that the vanadates undergo complex transformations during heating to 450°C and subsequent cooling. As a result, the materials are in a nonequilibrium state at room temperature and consist of both the parent double metavanadates and their peritectic decomposition products. We believe that the formation of the structure of the M2Zn(VO3)4 compounds from their melts is a kinetically hindered process. These compounds are structurally stable only at temperatures below 369 (Rb2Zn(VO3)4) or 420°C (Cs2Zn(VO3)4). We have measured for the first time the diffuse reflectance and photoluminescence excitation spectra of the two tetrametavanadates in their emission range and their photoluminescence spectra at various excitation wavelengths and determined their chromaticity coordinates. Their X-ray luminescence and scintillation decay characteristics have been determined for the first time under pulsed electron beam excitation. The electron excitation dissipation processes in the cesium and rubidium compounds are shown to be similar. We discuss the origin of the emission bands in the mixed vanadates and their potential application areas.

Synthesis and cathodoluminescence characteristics of europium-doped Ca-sialons

Using solid-state reactions and a combination of carbothermal reduction-nitridation with solgel processing, we have synthesized a number of oxynitride phases: Eu2+-doped Ca-sialons. We have measured the pulsed cathodoluminescence spectra of the synthesized powders under pulsed electron beam excitation and determined their emission parameters, including temporal emission characteristics.

Table-top setup for investigating the scintillation properties of liquid argon

The spectral and temporal light emission properties of liquid argon have been studied in the context of its use in large liquid rare-gas detectors for detecting Dark Matter particles in astronomy. A table-top setup has been developed. Continuous and pulsed low energy electron beam excitation is used to stimulate light emission. A spectral range from 110 to 1000 nm in wavelength is covered by the detection system with a time resolution on the order of 1 ns.

Population and relaxation kinetics of the 5d[3/2]1 level upon pulsed electron-beam excitation of pure xenon

The time dependences of the spontaneous emission intensity at the 5d[3/2]1 → 6p[3/2]1 and 5d[3/2]1 → 6p[5/2]2 transitions at 2.03 and 1.73 μm, respectively, are studied in the pressure range from 0.1 to 100 Torr upon excitation by a nanosecond electron beam. It is shown that the population and relaxation of the 5d[3/2]1 level under these conditions is determined by its excitation and quenching by electrons of a recombining plasma. No plasma-chemical processes populating this level, in particular, a channel of formation of a molecular Xe2+ ion was found. The lifetime of the 5d[3/2]1 state of Xe atoms in the pressure range from 0.5 to 5 Torr is about 1 ns.

Transient and near‐edge absorption in YVO4 crystals

AbstractThe process of near‐edge absorption annealing in air was studied up to 1500C in YVO4. In this annealing process, two stages with activation energies of 0.16 eV and 0.38 eV were obtained. (Should combine to make this one paragraph)In addition to the annealing study, the transient absorption induced by pulsed electron beam excitation (270 keV, 8 ns) was completed on a set of YVO4 samples with different near‐edge absorption levels in the spectral region 3.2‐3.5 eV. The spectral range from ∼1.25 eV up to 3.0 eV is covered by strong transient absorption. Transient absorption spectra show at least three broad overlapping bands (∼1.3 eV, ∼2.0 eV and ∼3.0 eV). The 3 eV peak position is close to the near‐edge absorption band and correlation between transient absorption bands and absorption due to stable colour centers was studied. It is suggested that absorption bands at ∼1.3 eV and ∼2.0 eV are due to electron and hole polarons, correspondingly. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Cathodoluminescence of wet, dry, and hydrogen-implanted silica films

The main luminescent centers in SiO 2 films are the red luminescence R (1.85 eV) of the non-bridging oxygen hole center (NBOHC), a blue B (2.7 eV) band of the oxygen deficient centers (ODC’s), a UV (4.3 eV) band, and a yellow Y (2.2 eV) band appears especially in hydrogen-implanted silica after longer time of irradiation. By means of pulsed electron beam excitation and a new track–stop technique we have investigated the luminescence life time as well as the initial luminescence properties at the beginning of irradiation. Thus the dose dependence of the red (R) luminescence in wet and dry oxide differs, decreasing in wet oxide from an initial level and increasing in dry oxide from almost zero at room temperature. Additionally, implanted hydrogen diminishes the red luminescence in wet oxide but increases the blue and the UV bands. Thus hydrogen passivates the NBOHC and keeps the ODC’s in active emission states. A preliminary model of luminescence center transformation is based on radiolytic dissociation and re-association of mobile oxygen and hydrogen at the centers as well as formation of interstitial H 2, O 2, and H 2O molecules. A comparison with the CL of ice layers shows that the H 2O molecules should be the origin of the yellow (Y) luminescence.

Hydrothermal Synthesis and Spectroscopic Properties of Crystalline ZnO Powders and Films

Polycrystalline zinc oxide is synthesized under mild hydrothermal conditions (150–250°C). The phase composition, morphology, and pulsed cathodoluminescence (PCL) spectra of ZnO powders, polycrystalline films, and single crystals are studied as a function of synthesis conditions (the nature of the precursor, the type and concentration of the solvent, solution temperature, and the redox potential of the system). The crystallite size in the resultant materials varies from ∼100 nm to 10 μm, depending on the synthesis conditions. The PCL spectra measured under pulsed electron-beam (τ ∼ 2 ns) or laser (Nd:YAG, fourth harmonic, λ = 266 nm, τ ∼ 10 ns) excitation show two bands in the UV and visible spectral regions. The intensity ratio between these bands depends on the ZnO preparation conditions. The influence of the precursor and solvent on the PCL spectrum of ZnO is studied. The redox potential of the system is shown to have a strong effect on the luminescent properties of the resultant material.

Luminescence of ZnO nanopowders

The luminescence of ZnO nanocrystals prepared by different methods was studied under pulsed electron beam excitation. It is shown that the luminescence intensity depends on the nanocrystal sintering conditions and does not depend on the nanocrystal size within the range 10–50nm. The relative luminescence intensities for the 3.32eV (free exciton) and ∼3.20eV (bound exciton) bands showed a dependence on nanocrystal size. The role of the nanocrystal surface in excitonic luminescence is discussed.

Luminescence spectra and decay kinetics in ZnWO4 and CdWO4 crystals

AbstractThe luminescence spectra and luminescence decay kinetics in ZnWO4 (nominal pure and doped with Fe and Mo) under pulsed electron beam and pulsed nitrogen laser excitation have been investigated. It is suggested that the two‐stage intrinsic luminescence decay observed under ionizing radiation excitation in ZnWO4 and CdWO4 was due to two different self‐trapped exciton (STE) configurations in these crystals. The role of Fe and Mo impurities for scintillation efficiency in ZnWO4 and CdWO4 has been discussed. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Relaxation of electronic excitations in LiNbO3 crystals

Transient absorption both in stoichiometric and Mg doped congruent LiNbO was observed after pulsed electron beam excitation. The luminescence spectra and decay kinetics in these materials show different excitonic relaxation possibilities. The dependence on sample stoichiometry is also discussed.

The study of time-resolved absorption and luminescence in PbWO4 crystals

The transient absorption and luminescence under pulsed electron beam excitation were studied for undoped and La3+ doped PbWO4 crystals. The absorption band at ∼1.0 eV is suggested to be due to self-trapped electrons and the absorption band at 3.5 eV may be due to self-trapped holes. The formation of luminescence centers via electron–hole recombination is affected by La3+ in doped crystals. The large fraction of electrons and holes undergoes thermostimulated recombination within geminate pairs and the spatial separation of geminate pairs components is important in recombination process. The intrinsic (blue) luminescence arises in this recombination. The mechanism of La3+ influence on recombination is discussed.

Thermally stimulated current measurements induced by pulsed electron beam excitation

Association of thermally stimulated current measurements with an excitation by means of a pulsed electron beam constitutes a powerful tool for the investigation of traps in semiconductor materials. Carrier generation in wide-bandgap materials or discrimination between electron and hole traps for example can be achieved this way. Experiments performed on cadmium-zinc telluride, polycrystalline diamond and mercuric iodide illustrate the potential of this approach.

Time-resolved luminescence and absorption in CdWO4

The luminescence and short-lived absorption are studied in CdWO4 single crystals under pulsed laser beam as well as pulsed electron beam excitation. The luminescence spectrum and decay kinetics for CdWO4 powder are studied. The short-lived absorption in single crystals arises from electron transitions from the luminescence center excited state to the conduction band. From luminescence and short-lived absorption spectra the energy depths of luminescence center excited and ground states are estimated.

Luminescence of biexcitons in silver halide crystals

As it is known from Urbach rule, free excitons defining the fundamental absorption edge in AgBr 1 − xCl x(0 ⩽ x ⩽ 1). The process of primary Frenkel defect creation which involves free exciton states even in AgCl and biexciton creation in AgCl are discussed. The fast luminescence band at ≈3.0 eV observed in AgCl under high-density pulsed electron beam and laser excitation has been studied. It is shown that its intensity is a non-linear function of the excitation density. We suggest that the fast luminescence comes from biexcitons; binding energies of such biexcitons in AgBr 1 − xCl x(0 ⩽ x ⩽ 1) are estimated.

The formation of emitting CF 3 and CF 2 radical states by pulsed electron beam excitation

In electron irradiated argon/alkyl fluoride gas mixtures the quenching dynamics of argon transients and the kinetics of the UV and visible emission bands of electronically excited CF 2 and CF 3 radicals were investigated by the pulse radiolysis method. Assuming that these transient radical states are generated by a transfer of electronic excitation energy from the argon excited states to the admixed gases quenching rate constants of the argon species are obtained by numerical simulation of the experimentally obtained argon absorption time profiles as well the emission time curves of the alkyl fluoride transients. In comparison with photochemical measurements and as a result of kinetic calculations it could be concluded that the observed visible emission band around 600 nm was due to transitions of CF 3 radical states, where the UV emission spectra between 230 and 370 nm represents mainly CF 2 radical transitions. In the case of C 2F 6 the transfer from argon dimer states resulted in a contribution to the CF 3 fluorescence around 600 nm. In all other investigated systems only the atomic states Ar(4 s) were the precursers of the emitting alkyl fluoride species. The collision free lifetime of CF 2 wasestimated by studying the UV-fluorescence decay in pure alkyl fluorides as a function of pressure and the fluorescence lifetime of CF 3 was obtained as a result of the numeric fluorescence analysis.

The u.v. and visible emission bands of CF 3 radicals produced by pulsed electron beam excitation

Using time-resolved optical spectroscopy the kinetics of the u.v. and visible emission bands of electronically excited CF n ( n = 2,3) radicals were investigated by the pulse radiolysis method. In rare gases used as energy absorbing media, atomic and molecular transients which transfer their energy to foreign gases were formed by electron irradiation. In the presence of CF 3H, C 2F 6, C 3F 8 and CF 4 the decay of the absorption of the Ar(4s) as well as the Ar 2( 3Σ +) states and the growth of the Ar 2( 3Σ +) become faster. As, e.g. the argon absorption bands were influenced by the admixtures, new emission bands were obtained due to electronic transitions of excited CF 2 and CF 3 radicals in the spectral range 200–700 nm. Assuming that the emissions are generated by a transfer of electronic excitation energy from the rare gas excited states to the admixed gases, in which CF 2 and CF 3 radicals are formed by dissociation reactions, rate constants of the quenching reaction of the rare gas atomic and molecular states are obtained by numerical simulation of the experimental absorption time profiles. Collision free life-times of the emitting states were observed from simulations of the alkyl fluoride fluorescence decays as a function of the solute concentration.