Showing papers on "Cathodoluminescence published in 2010"

Optical excitations in electron microscopy

Abstract: This review discusses how low-energy, valence excitations created by swift electrons can render information on the optical response of structured materials with unmatched spatial resolution. Electron microscopes are capable of focusing electron beams on sub-nanometer spots and probing the target response either by analyzing electron energy losses or by detecting emitted radiation. Theoretical frameworks suited to calculate the probability of energy loss and light emission (cathodoluminescence) are revisited and compared with experimental results. More precisely, a quantum-mechanical description of the interaction between the electrons and the sample is discussed, followed by a powerful classical dielectric approach that can be in practice applied to more complex systems. We assess the conditions under which classical and quantum-mechanical formulations are equivalent. The excitation of collective modes such as plasmons is studied in bulk materials, planar surfaces, and nanoparticles. Light emission induced by the electrons is shown to constitute an excellent probe of plasmons, combining sub-nanometer resolution in the position of the electron beam with nanometer resolution in the emitted wavelength. Both electron energy-loss and cathodoluminescence spectroscopies performed in a scanning mode of operation yield snap shots of plasmon modes in nanostructures with fine spatial detail as compared to other existing imaging techniques, thus providing an ideal tool for nanophotonics studies.

Near UV LEDs made with in situ doped p-n homojunction ZnO nanowire arrays

Abstract: Catalyst-free p-n homojunction ZnO nanowire (NW) arrays in which the phosphorus (P) and zinc (Zn) served as p- and n-type dopants, respectively, have been synthesized for the first time by a controlled in situ doping process for fabricating efficient ultraviolet light-emitting devices. The doping transition region defined as the width for P atoms gradually occupying Zn sites along the growth direction can be narrowed down to sub-50 nm. The cathodoluminescence emission peak at 340 nm emitted from n-type ZnO:Zn NW arrays is likely due to the Burstein-Moss effect in the high electron carrier concentration regime. Further, the electroluminescence spectra from the p-n ZnO NW arrays distinctively exhibit the short-wavelength emission at 342 nm and the blue shift from 342 to 325 nm is observed as the operating voltage further increasing. The ZnO NW p-n homojunctions comprising p-type segment with high electron concentration are promising building blocks for short-wavelength lighting device and photoelectronics.

Down- and up-conversion photoluminescence, cathodoluminescence and paramagnetic properties of NaGdF4 : Yb3+,Er3+ submicron disks assembled from primary nanocrystals

Abstract: NaGdF4 : Yb3+,Er3+ submicron disks have been synthesized at a relatively low temperature in aqueous solution with citric acid as the structure-directing agent. The structure, luminescence, and magnetic properties of the synthesized materials have been characterized by a variety of techniques. The as-prepared NaGdF4 : Yb3+,Er3+ has hexagonal structure, and is mainly composed of submicron disks with a diameter of around 870 nm and a thickness of about 420 nm. Citrate groups selectively bonded to a certain crystal surface of the nanocrystals probably provide the driving force that makes primary particles assemble into submicron disks. The phase structure of these submicron disks is affected by the annealing temperature, while the disk morphology could be essentially preserved even when annealed at high temperatures. The annealed submicron disks exhibit prominent visible emission with different excitation sources, including ultraviolet light, low-voltage electron beam and near-infrared laser. In addition, these disks exhibit paramagnetic features with the mass magnetic susceptibility value of 9.82 × 10−5 emu/g·Oe at room temperature. These multifunctional disks would have potential in applications as building blocks for many functional devices such as solid-state lasers, lighting and displays, magnetic resonance imaging and so on.

Continuous-flux MOVPE growth of position-controlled N-face GaN nanorods and embedded InGaN quantum wells

Abstract: We demonstrate the fabrication of N-face GaN nanorods by metal organic vapour phase epitaxy (MOVPE), using continuous-flux conditions. This is in contrast to other approaches reported so far, which have been based on growth modes far off the conventional growth regimes. For position control of nanorods an SiO2 masking layer with a dense hole pattern on a c-plane sapphire substrate was used. Nanorods with InGaN/GaN heterostructures have been grown catalyst-free. High growth rates up to 25??m?h ? 1 were observed and a well-adjusted carrier gas mixture between hydrogen and nitrogen enabled homogeneous nanorod diameters down to 220?nm with aspect ratios of approximately 8:1. The structural quality and defect progression within nanorods were determined by transmission electron microscopy (TEM). Different emission energies for InGaN quantum wells (QWs) could be assigned to different side facets by room temperature cathodoluminescence (CL) measurements.

Vacancy defect and defect cluster energetics in ion-implanted ZnO

Abstract: We have used depth-resolved cathodoluminescence, positron annihilation, and surface photovoltage spectroscopies to determine the energy levels of Zn vacancies and vacancy clusters in bulk ZnO crystals. Doppler broadening-measured transformation of Zn vacancies to vacancy clusters with annealing shifts defect energies significantly lower in the ZnO band gap. Zn and corresponding O vacancy-related depth distributions provide a consistent explanation of depth-dependent resistivity and carrier-concentration changes induced by ion implantation.

Spatial distribution of defect luminescence in GaN nanowires.

Abstract: The spatial distribution of defect-related and band-edge luminescence from GaN nanowires grown by metal−organic chemical vapor deposition was studied by spatially resolved cathodoluminescence imaging and spectroscopy. A surface layer exhibiting strong yellow luminescence (YL) near 566 nm in the nanowires was revealed, compared to weak YL in the bulk. In contrast, other defect-related luminescence near 428 nm (blue luminescence) and 734 nm (red luminescence), in addition to band-edge luminescence (BEL) at 366 nm, were observed in the bulk of the nanowires but were largely absent at the surface. As the nanowire width approaches a critical dimension, the surface YL layer completely quenches the BEL. The surface YL is attributed to the diffusion and piling up of mobile point defects, likely isolated gallium vacancies, at the surface during growth.

Shape-Controllable Synthesis and Morphology-Dependent Luminescence Properties of GaOOH:Dy3+ and β-Ga2O3:Dy3+

Abstract: Dy3+-doped gallium oxide hydroxides (GaOOH:Dy3+) with various morphologies (submicrospindles, submicroellipsoids, 3D hierarchical microspheres) were synthesized by a facile soft-chemical method. After annealing at 1000 °C, the GaOOH:Dy3+ precursor was easily converted to β-Ga2O3:Dy3+ phosphors which kept their original morphologies. The as-prepared GaOOH:Dy3+ and β-Ga2O3:Dy3+ products were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), low- to high-resolution transmission electron microscopy (TEM), selected area electron diffraction (SAED), photoluminescence (PL) spectra, cathodoluminescence (CL) spectra, and quantum yield (QY). With an increase in pH from 4 to 9, the morphology of GaOOH:Dy3+ varied from submicrospindles to 3D hierarchical microspheres of self-assembled nanoparticles. A possible mechanism for the formation of various morphologies of GaOOH:Dy3+ and β-Ga2O3:Dy3+ was proposed. Under ultraviolet and low-voltage electron beam excitation, the p...

Facile Fabrication and Optical Property of Hollow SnO2 Spheres and Their Application in Water Treatment

Abstract: Hollow SnO2 spheres with smooth surface have been fabricated by a low temperature template-free solution phase route via self-assembly of small nanocrystalline particles. These hollow spheres have a very thin shell thickness of about 10 nm and are built from SnO2 nanocrystals of an average size of 5.3 nm. The evacuation behavior of inside-out Ostwald ripening can be used to explain the formation of hollow spheres according to results of time-dependent reactions. The cathodoluminescence spectrum indicates a blue shift of the band gap emission peak of SnO2, originating from quantum confinement effect due to the nanoscle size of SnO2 particles. The as-prepared SnO2 hollow spheres were also found to exhibit excellent performance in wastewater treatment.

Strain influenced indium composition distribution in GaN/InGaN core-shell nanowires

Abstract: The optical properties, indium concentration and distribution, defect morphology, and strain distribution of GaN/InGaN coaxial nanowires grown by metal organic chemical vapor deposition were investigated using spatially resolved cathodoluminescence, scanning transmission electron microscopy, and finite element analysis The results indicate that InGaN layers with 40% or greater indium incorporation and low defect density can be achieved The indium distribution in the InGaN shell layer was measured and qualitatively correlated with the calculated strain distribution The three-dimensional compliance of the GaN nanowire leads to facile strain relaxation in the InGaN heteroepitaxial layer, enabling high indium incorporation and high crystalline quality

Room-Temperature Synthesis of Single-Crystalline Anatase TiO2 Nanowires

Abstract: Single-crystalline anatase TiO2 nanowires have been synthesized using a novel, room-temperature method involving the use of seed particles and a radio frequency magnetron sputter deposition technique. Also, the growth time required was less than 60 min. Transmission electron microscopy and X-ray absorption near-edge structure analyses show that the obtained nanowires are single-crystalline anatase TiO2. Cathodoluminescence spectroscopy spectrum shows that the nanowires’ band-to-band emission exhibits a blue shift from to 3.20 (387) to 3.46 eV (358 nm). Conventional vapor−liquid−solid and vapor−solid−solid growth modes are not applicable to explain the growth of the obtained single-crystalline anatase TiO2 nanowires at room-temperature. A folded-growth mode is therefore suggested.

Surfaces and Interfaces of Electronic Materials

Abstract: Preface 1. Introduction 2. Historical Background 3. Electrical Measurements 4. Interface states 5. Ultrahigh vacuum technology 6. Surface and interface analysis 7. Photoemission spectroscopy 8. Photoemission with soft X-rays 9. Particle-solid scattering 10. Electron energy loss spectroscopy 11. Rutherford backscattering spectrometry 12. Secondary ion mass spectrometry 13. Electron diffraction 14. Scanning tunneling microscopy 15. Optical spectroscopies 16. Cathodoluminescence spectroscopy 17. Electronic Materials' Surfaces 18. Adsorbates on Electronic Materials' Surfaces 19. Adsorbate-Semiconductor Sensors 20. Heterojunctions 21. Metals on semiconductors 22. The future of interfaces Appendices

Carrier localization and nonradiative recombination in yellow emitting InGaN quantum wells

Abstract: InGaN quantum wells, with luminescence in the yellow region of the visible spectrum, have been studied using conventional and time-resolved cathodoluminescence. We observe the absence of strong localization effects and a relatively high internal quantum efficiency of ∼12%, which are unexpected for InGaN in this-long wavelength emission range. We have also observed a steady decrease of the peak emission energy, and a continuous increase in the radiative recombination lifetime with temperature up to 100 K. These two features are manifestations of recombination due to nonlocalized excitons. Nonradiative recombination centers, with activation energy of ∼6 meV, appear to constitute the main mechanism limiting the internal quantum efficiency of these films.

Optical and magnetic properties of CuO nanowires grown by thermal oxidation

Abstract: CuO nanostructures with different morphologies, such as single-crystal nanowires, nanoribbons and nanorods, have been grown by thermal oxidation of copper in the 380–900 °C temperature range. Cathodoluminescence spectra of the nanostructures show a band peaked at 1.31 eV which is associated with near band gap transitions of CuO. Two additional bands centred at about 1.23 and 1.11 eV, suggested to be due to defects, are observed for nanostructures grown at high temperatures. The magnetic behaviour of nanowires with lengths in the range of several micrometres and diameters of 50–120 nm has been investigated. Hysteresis loops of the nanowires show ferromagnetic behaviour from 5 K to room temperature.

Probing the strain effect on near band edge emission of a curved ZnO nanowire via spatially resolved cathodoluminescence.

Abstract: Curved ZnO nanowires were deliberately prepared on a Si substrate and the strain effect on their near band edge (NBE) emission was investigated by spatially resolved cathodoluminescence (CL). By moving the electron beam step-by-step across individual curved nanowires and acquiring the CL spectra simultaneously, we found that the NBE emissions from the inner region of the curved nanowires with compressive strain show blueshift, while those from the outer region with tensile strain show redshift. Both the strains have been estimated from the local curvature by a geometrical model and have been further examined by high-resolution transmission electron microscopy. A nearly linear relation between the strain and the peak energy shift in NBE emission was obtained. The result indicates that the optical band gap of ZnO nanowire is quite sensitive to and can be readily modulated by the induced strain via simply curving the nanowire, which has potential applications for designing new optical-electromechanical (OEM) and flexible optoelectronic nanodevices.

Semipolar GaN films on patterned r-plane sapphire obtained by wet chemical etching

Abstract: It is shown that (112¯2)-oriented GaN films can be achieved from r-sapphire patterned by chemical etching. Growth first occurs selectively from the inclined c-facet of sapphire, leading finally to a fully coalesced layer with (112¯2) orientation. The structural and optical quality of these layers was assessed by x-ray diffraction, cathodoluminescence and photoluminescence measurements. The results clearly show that the quality of (112¯2) GaN on patterned r-sapphire is markedly improved in comparison with (112¯2) GaN on m-sapphire.

Large internal dipole moment in InGaN/GaN quantum dots

Abstract: Direct observation of large permanent dipole moments of excitonic complexes in InGaN/GaN quantum dots is reported. Characteristic traces of spectral diffusion, observed in cathodoluminescence of InGaN/GaN quantum dots, allow deducing the magnitude of the intrinsic dipole moment. Our experimental results are in good agreement with realistic calculations of quantum dot transition energies for position-dependent external electric fields.

Synthesis and luminescent properties of CaTiO3: Pr3+ microfibers prepared by electrospinning method

Abstract: One-dimensional Pr3+-doped CaTiO3 microfibers were fabricated by a simple and cost-effective electronspinning process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential analysis (TG-DTA), scanning electron microscopy (SEM), energy-dispersive X-ray spectrum (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), quantum efficiency (QE), and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. Under ultraviolet excitation and low-voltage electron beams (1-3 kV) excitation, the CaTiO3:x Pr3+ samples show the red emission at 612 nm, corresponding to 1D2-3H4 transition of Pr3+. The luminescence intensity, quantum efficiency, and the lifetime have been studied as a function of the doping concentration of Pr3+ in the CaTiO3 samples.

The effect of Al-doping on the structural, optical, electrical and cathodoluminescence properties of ZnO thin films prepared by spray pyrolysis

Abstract: Aluminum doped zinc oxide (Al-doped ZnO) thin films were deposited by the spray pyrolysis technique onto the glass substrates at 450 °C using anhydrous zinc chloride (ZnCl 2 ) and aluminum chloride (AlCl 3 ) as sources of zinc and aluminum ions, respectively. The effect of [Al]/[Zn] ratio in the solution on the structural, optical, electrical and cathodoluminescence properties of these films were investigated. XRD study revealed that both undoped and Al-doped ZnO films were polycrystalline with hexagonal structure and exhibited (0 0 2) preferential orientation. The optical and electrical studies showed that the film deposited with the [Al]/[Zn] ratio equal to 0.05 had high transmittance (of about 80% and 95% in the visible and near infra-red regions, respectively) and minimum resistivity of 1.4×10 −3 Ω cm, respectively. This resistivity value decreased with increase in temperature indicating the semiconducting nature of Al-doped ZnO films. The chemical composition analysis (EPMA) showed that this film was nearly stochiometric with a slight oxygen deficiency.

Extensive analysis of the luminescence properties of AlGaN/GaN high electron mobility transistors

Abstract: This paper reports on an extensive analysis of the electroluminescence spectra of GaN-based high-electron mobility transistors (HEMT) submitted to different bias regimes. The results described within this paper indicate that: (i) under ON-state bias conditions, HEMT can emit a weak luminescence signal, localized at the edge of the gate toward the drain side; (ii) for low drain voltage levels, the electroluminescence spectrum has a Maxwellian shape, which is typical for hot carrier luminescence; (iii) for high drain voltage levels, parasitic emission bands are generated, possibly due to the recombination of hot electrons through defect-related sites. Electroluminescence data are compared with results of cathodoluminescence measurements, to provide an interpretation for the experimental results.

GaN-based nanowires: From nanometric-scale characterization to light emitting diodes

Abstract: We studied and improved gallium nitride (GaN) nanowire (NW) based light emitting diodes (LEDs). PIN nanodiodes with and without InGaN/GaN multiple quantum wells (MQWs) were grown by molecular beam epitaxy (MBE) under N-rich conditions on n-doped Si(111) substrates. Thanks to the coalescence of the p-type region of the NWs grown at low temperature, an autoplanarization process has been performed to obtain LEDs. Ni/Au top contacts have been deposited and patterned in order to bias the devices. A multiple-scale characterization approach has been carried out through the comparison of localized cathodoluminescence (CL) and macroscopic electroluminescence (EL) spectra. It shows that the EL emission of PIN-based LED at room temperature is related to defects in the p-type region of the NWs. In order to enhance the radiative recombinations of NW-based LEDs, we have first added InGaN/GaN MQWs, and secondly an electron blocking layer (EBL) has been inserted between the MQWs and the p-type zone of the NWs. The LED with EBL exhibited an emission band at 420 nm. The blue-shift of this emission band with increasing injected current is attributed to quantum confined Stark effect (QCSE) and evidences the radiative emission of InGaN/GaN MQWs. At 50 mA dc current, this improved NW-based LED emits about 500 times more light than the heterostructure without EBL. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Characteristics of emission centers in alkali feldspar: A new approach by using cathodoluminescence spectral deconvolution

Abstract: Alkali feldspars in syenite from the Cerro Balmaceda pluton in the Patagonian Andes, Chile, show various petrographic microtextures formed during the magmatic to high- and low-temperature hydrothermal stages in which cathodoluminescence (CL) shows a wide range of blue, violet, and pink to red colors with variable brightness. Their CL spectra exhibit two emission bands: one at 405–420 nm in the blue region and the other at 700–760 nm in the red-infrared (IR) region. Asymmetrically shaped spectral peaks in energy units suggest overlapping of each individual emission, which corresponds to various luminescence centers. Blue emission bands were separated into two spectral peaks fitted by Gaussian curves centered at 3.055–3.076 and 2.815–2.845 eV. A positive correlation is found between emission intensities at 3.055–3.076 eV and TiO 2 contents, suggesting the activation of a Ti 4+ impurity as an emission center. The intensities at 2.815–2.845 eV, where clear and featureless feldspar (CF; not affected by hydrothermal metasomatism) is shown under optical microscopy, which have intensities appreciably higher than those showing patched microperthite (PMP), formed during low-temperature hydrothermal reactions, correlate reciprocally with the intensities of red-IR emission caused by a Fe 3+ impurity center. The peak at 2.815–2.845 eV can be attributed to oxygen defects associated with Al-O-Al and Al-O-Ti bridges. Most of the areas show CL emissions at 700–760 nm in the red-IR region, in which intensities increase with an increase in Fe 2 O 3 contents as impurities. The Fe 3+ ion acts as an activator for the red-IR emission. The Ab-rich and Or-rich phases of PMP have emission components at 1.644 eV (754 nm) and 1.727 eV (717 nm), respectively. The red-IR emission from CF consists of emission components at 1.677 eV (739 nm) and 1.557 eV (796 nm), according to an Fe 3+ impurity center in the Or-rich phase and in the Ab-rich phase as cryptoperthite, respectively. Both components are centered at a wavelength longer than the emission band of Ab-rich and Or-rich phases of PMP, suggesting a change in configurational state around the Fe 3+ ion from the T2 to the T1 site by low-temperature hydrothermal metasomatic reactions. Accordingly, the peak positions of the red-IR emission are controlled by the ordering state of Fe 3+ ion into the T1 site, the existence of multiphase perthite and chemical composition.

Confined three-dimensional plasmon modes inside a ring-shaped nanocavity on a silver film imaged by cathodoluminescence microscopy.

Abstract: The confined modes of surface plasmon polaritons in boxing ring-shaped nanocavities have been investigated and imaged by using cathodoluminescence spectroscopy. The mode of the out-of-plane field components of surface plasmon polaritons dominates the experimental mode patterns, indicating that the electron beam locally excites the out-of-plane field component of surface plasmon polaritons. Quality factors can be directly acquired from the spectra induced by the ultrasmooth surface of the cavity and the high reflectivity of the silver (Ag) reflectors. Because of its three-dimensional confined characteristics and the omnidirectional reflectors, the nanocavity exhibits a small modal volume, small total volume, rich resonant modes, and flexibility in mode control.

Tunable Cathodoluminescence Properties of Tb3 + -Doped La2O3 Nanocrystalline Phosphors

Abstract: Nanocrystalline Tb 3+ -doped La 2 0 3 phosphors were prepared through a Pechini-type sol-gel process. X-ray diffraction (XRD), field-emission-scanning electron microscopy (FESEM), photoluminescence, cathodoluminescence (CL) spectra, and lifetimes were utilized to characterize the synthesized phosphors. The XRD results revealed that a pure La 2 0 3 phase can be obtained at 700°C. FESEM images indicated that the La 2 O 3 :Tb 3+ phosphors are composed of aggregated spherical particles with sizes ranging from 60 to 100 nm. Under the excitation of UV light and low voltage electron beams (0.5-3 kV), the La 2 O 3 :Tb 3+ phosphors showed the characteristic emissions of Tb 3+ ( 5 D 3,4 - 7 F 6,5,4,3 transitions). The CL colors of La 2 O 3 :Tb 3+ phosphors was tuned from blue to green by changing the doping concentration of Tb 3+ to some extent. The optimum blue CL of La 2 O 3 :Tb 3+ phosphors had better Commission International l'Eclairage (CIE) coordinates and higher emission intensity than those of the commercial product Y 2 SiO 5 :Ce 3+ (blue, product no. 1047, Nichia Kagaku Kogyo Kabushiki, Japan).

Solution Growth and Cathodoluminescence of Novel SnO2 Core-Shell Homogeneous Microspheres

Abstract: Large-scale novel core−shell microspheres of SnO2 have been synthesized through a simple solution method at 200 °C. Morphologies, microstructures, and compositions of the products are investigated by X-ray powder diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. Results indicate that the SnO2 microspheres are core−shell structures with an average diameter of 6 μm and an average shell thickness of 200 nm. A growth mechanism and the cathodoluminescence properties of these novel structures are presented.

Patterning of Light-Emitting YVO4:Eu3+ Thin Films via Inkjet Printing

Abstract: Printed electronics is expected to be used for fabricating the next-generation displays. However, this technique is still in the laboratory scale and mainly limited to organic luminescent materials and inorganic quantum dots. In this article, a new attempt was made by combining the Pechini-type sol−gel process and inkjet printing for patterning an inorganic YVO4:Eu3+ thin film phosphor. The mixed solution of metal salts precursors, citric acid, and poly(ethylene glycol) was directly used as ink to deposit patterns on ITO-coated glass substrate. After calcination at 600 °C in air, the YVO4:Eu3+ patterns in micrometer-scale were formed on the substrates, and the photoluminescence (PL) and cathodoluminescence (CL) spectra were employed to characterize the obtained samples. A dominating red emission coming from 5D0-7F2 transition of Eu3+ was observed under excitation of UV light or electronic beam. These results demonstrate that the Pechini-type sol−gel process has good compatibility with the inkjet printing ...

Correlation between kink and cathodoluminescence spectra in AlGaN/GaN high electron mobility transistors

Abstract: The “kink” effect in AlGaN/GaN high electron mobility transistor current-voltage characteristics is shown to be associated with the epitaxial growth and is unaffected by fabrication process or growth substrate in device wafers from two epitaxy sources and three foundries. We demonstrate that there is a direct correlation between the presence of the “kink” and the presence of a broad yellow cathodoluminescence band. On the basis of generally accepted models for yellow luminescence, we propose that the kink is due to the presence of deep levels in the GaN buffer layer which decrease the drain current when negatively charged.