Cathodoluminescence Detection Research Articles

APPLICATION OF CATHODOLUMINESCENCE FOR ON-LINE MONITORING OF REGIME OF AN INDUSTRIAL ELECTRON ACCELERATOR

Continuous monitoring of critical parameters of an industrial electron accelerator provides quality of product processing. For that purpose, the methods of contact-free diagnostics of processing regime are developed. One of them is based on application of a wide-aperture stack-monitor for on-line measurement of beam current, electron energy, and also the mean absorbed dose over the plain of beam scanning in an irradiated object [1]. In the work, the conditions of application of cathodoluminescence (CL), accompanied action of accelerated electrons on amorphous dielectrics, for adjustment of the stack-monitor, and also for measuring in on-line mode the dose in a point of control as well as of distribution of the electron flux density on the surface of the object, is studied. It is shown, that titanium dioxide, keeping the radiation-optical yield at an accumulated dose of up to 4 MGy, can be considered as a promising material for manufacturing of CL detector.

Luminescence from AlGaN/GaN HEMT structures by very-low-energy (100 eV) electron beams using beam deceleration technique

A low-beam-energy cathodoluminescence (CL) system has been developed. This CL system consists of a scanning electron microscope equipped with a beam deceleration (also known as “retarding”) tool and a CL detection system. We employed the system to study an AlGaN/GaN-based high electron mobility transistor structure and observed band-edge emissions even at 100 eV. The band-edge emissions significantly decreased after the plasma irradiation for a few seconds. The low-beam-energy CL is highly sensitive to the surface and can be used for the optimization of the dry process used in the device fabrication.

Cathodoluminescence spectra recorded from surfaces of solids with hydrous molecules

Modern cathodoluminescence detectors coupled to Environmental Scanning Electron Microscopes (ESEM) with X-ray microanalysis are increasingly sensitive and, nowadays, low intense luminescence spectra may be recorded from nominally non-luminescent solid materials. Structural or attached hydroxyl groups are universally present in solid samples kept in environmental conditions being potentially active luminescence emitters. Hence, we collected cathodoluminescence (CL) spectra of sixty solid materials of minerals and synthetic compounds, organics and inorganics, metallic and non-metallic, crystalline and amorphous, including widespread CL bands at circa 315 nm and 620 and 650 nm from surfaces of solids with hydroxyl and hydrous molecules. We observed as iron bearing materials, such as biotite, olivine or native iron, show appreciable cathodoluminescence spectral bands associated with hydroxyl groups (315 nm) and water molecules (620 and 650 nm), being probable precursors of non-bridging oxygen hole centers. The wide set of very different samples analyzed shows spectral similarities among separated sample types but with the only common factor of the hydrous molecules. These conclusions seem crucial for remote sensing detection of water; environmental dosimetry; degradation of phosphor materials, etc. Analyzing spectral luminescence emissions of hydrated materials it is frequent to observe erroneous interpretations of their defect-emission relationships which should be avoided.

Nano-Cathodoluminescence Measurement of Asymmetric Carrier Trapping and Radiative Recombination in GaN and InGaN Quantum Disks.

The ability to characterize recombination and carrier trapping processes in group-III nitride-based nanowires is vital to further improvements in their overall efficiencies. While advances in scanning transmission electron microscope (STEM)-based cathodoluminescence (CL) have offered some insight into nanowire behavior, inconsistencies in nanowire emission along with CL detector limitations have resulted in the incomplete understanding in nanowire emission processes. Here, two nanowire heterostructures were explored with STEM-CL: a polarization-graded AlGaN nanowire light-emitting diode (LED) with a GaN quantum disk and a polarization-graded AlGaN nanowire with three different InGaN quantum disks. Most nanowires explored in this study did not emit. For the wires that did emit in both structures, they exhibited asymmetrical emission consistent with the polarization-induced electric fields in the barrier regions of the nano-LEDs. In the AlGaN/InGaN sample, two of the quantum disks exhibited no emission potentially due to the three-dimensional landscape of the sample or due to limitations in the CL detection.

Research progress of plasmonic cathodoluminesecence characterization

Surface plasmons as the collective electrons oscillation at the interface of metal and dielectric materials, have induced tremendous applications for the nanoscale light focusing, waveguiding, coupling, and photodetection. As the development of the modern technology, cathodoluminescence (CL) has been successfully applied to describe the plasmon resonance within the nanoscale. Usually, the CL detection system is combined with a high resolution scanning electron microscope (SEM). The fabricated plasmonic nanostructure is directly excited by the electron beam, and detected by an ultra-sensitive spectrometer and photodetector. Under the high energy electron stimulation, all of the plasmon resonances of the metallic nanostructure can be excited. Because of the high spatial resolution of the SEM, the detected CL can be used to analyze the details of plasmon resonance modes. In this review, we first briefly introduced the physical mechanism for the CL generation, and then discussed the CL emission of single plasmonic nanostructures such as different nanowires, nanoantennas, nanodisks and nanocavities, where the CL only describes the individual plasmon resonance modes. Second, the plasmon coupling behavior for the ensemble measurement was compared and analyzed for the CL detection. Finally, the CL detection with other advanced technologies were concluded. We believe with the development of the nanophotonics community, CL detection as a unique technique with ultra-high energy and spatial resolution has potential applications for the future plasmonic structure design and characterization.

Novel Cathodoluminescence Detector with Extremely Large Field of View

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Nanometric Resolved Cathodoluminescence on Few-Layer h-BN Flakes

Within the latest years number of layered materials at reduced dimensions have demonstrated remarkable optical properties. However most studies focused on perfect system and the role of defects as optical active centers remain still largely unexplored. Hexagonal boron nitride (h-BN) is one of the most promising candidates for light emitting devices in the far UV region, presenting a single strong excitonic emission at 5.8 eV. However, a single line appears only in extremely pure monocrystals that can hardly be obtained only though complex synthesis processes. Common h-BN samples present more complex emission spectra that have been generally attributed to the presence of structural defects. Despite a large number of experimental studies up to now it was not possible to attribute specific emission features to well identify defective structures. Here we address this fundamental questions by adopting a theoretical and experimental approach combining few nanometer resolved cathodoluminescence techniques with high resolution transmission electron microscopy images and state of the art quantum mechanical simulations. Very recently, the Orsay team has developed a cathodoluminescence detection system integrated within a scanning transmission electron microscope. This unique experimental set up is now able to provide full emission spectra with a resolution as low as few tens of meV associated with an electron probe size of one nanometer. A cathodoluminescence spectrum-image can thus be recorded in parallel with an HAADF image. Nanometric resolved cathodoluminescence on few-layer chemically exfoliated h-BN crystals have shown that emission spectra are strongly inhomogeneous within individual flakes. Emission peaks close to the free exciton appear in extended regions. Complementary investigations through high resolution transmission electron microscopy allow to associate these emission lines with extended crystal deformation such as stacking faults and folds of the planes. By means of ab-initio calculations in the framework of Many Body Perturbation Theory (GW approximation and Bethe-Salpeter equation) we provide an in-depth description of the electronic structure and spectroscopic response of bulk hexagonal boron nitride in the presence of extended morphological modifications. In particular we show that, in a good agreement with the experimental results, additional excitons are associated to local symmetry changes occurring at crystal stacking faults. Additional features appearing within the band gap present a high spatial localization, typically less than 100 nm, and thus they can be related to individual point defects. When addressed individually through a highly focused electron probe they might have a single photon emitter quantum character. This hypothesis has been recently confirmed by experiments combining our cathodoluminescence system with an Hanbury Brown and Twiss (HBT) interferometer [4].

Confocal filtering in cathodoluminescence microscopy of nanostructures

Cathodoluminescence (CL) microscopy allows optical characterization of nanostructures at high spatial resolution. At the nanoscale, a main challenge of the technique is related to the background CL generated within the sample substrate. Here, we implement confocal detection of the CL signal to minimize the background contribution to the measurement. Nano-phosphors were used as point sources to evaluate the filtering capabilities of our confocal CL system, obtaining an axial intensity profile with 2.7 μm full width at half maximum for the central peak, in good correspondence with theoretical expectations. Considering the electron interaction volume, we found that the confocal filter becomes effective for electron energies above 20 keV, when using a 25 μm pinhole (0.86 Airy units). To illustrate our approach, we present confocal CL imaging of gold nanowires and triangular shaped plates deposited on an indium-tin oxide covered glass substrate, comparing the images with those obtained in standard unfiltered CL detection. The results show that confocal CL microscopy is a valuable tool for the investigation of nanostructures on highly cathodoluminescent substrates, widely used in biological and optical applications.

Focus on photonics and biomedical optics

Focus on photonics and biomedical optics

Cathodoluminescence Microscopy of nanostructures on glass substrates

Cathodoluminescence (CL) microscopy is an emerging analysis technique in the fields of biology and photonics, where it is used for the characterization of nanometer sized structures. For these applications, the use of transparent substrates might be highly preferred, but the detection of CL from nanostructures on glass is challenging because of the strong background generated in these substrates and the relatively weak CL signal from the nanostructures. We present an imaging system for highly efficient CL detection through the substrate using a high numerical aperture objective lens. This system allows for detection of individual nano-phosphors down to thirty nanometer in size as well as the up to ninth order plasmon resonance modes of a gold nanowire on ITO coated glass. We analyze the CL signal-to-background dependence on the primary electron beam energy and discuss different approaches to minimize its influence on the measurement.

Spatial modulation of above-the-gap cathodoluminescence in InP nanowires

We report the observation of light emission on wurtzite InP nanowires excited by fast electrons. The experiments were performed in a scanning transmission electron microscope using an in-house-built cathodoluminescence detector. Besides the exciton emission, at 850 nm, emission above the band gap from 400 to 800 nm was observed. In particular, this broad emission presented systematic periodic modulations indicating variations in the local excitation probability. The physical origin of the detected emission is not clear. Measurements of the spatial variation of the above-the-gap emission points to the formation of leaky cavity modes of a plasmonic nature along the nanowire length, indicating the wave nature of the excitation. We propose a phenomenological model, which fits closely the observed spatial variations.

Time resolved microscopic cathodoluminescence spectroscopy for phosphor research

In impurity doped inorganic luminescent materials, there is often a close relation between the structural and optical properties. When a phosphor shows a distribution of structural properties (such as local variations in composition, dopant concentration, crystalline phase or particle size), the luminescence properties can show a similar distribution. When evaluating the synthesized phosphors at the macroscopic scale only, these properties are averaged out, which can lead to a broadened emission spectrum or a luminescence decay curve consisting of several exponentials.This paper discusses cathodoluminescence detection inside a scanning electron microscope (SEM-CL) for the study of local emission characteristics in phosphors. In this type of instrument, imaging, local element identification as well as cathodoluminescent spectral mapping can be performed. It is shown that with the aid of a beam blanker, local decay time analysis and spectrally resolved decay time mapping can be performed. By correlating simultaneously obtained structural, compositional and luminescence properties, a profound understanding of a phosphor's behavior can be obtained. This is illustrated on Ca2SiS4:Eu, a red-emitting phosphor material of current interest for white light emitting diode (LED) wavelength conversion.

Cathodoluminescence Detectors Suitable for Easy and Fast Analysis in Geosciences

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Development of a Cathodoluminescence Detection System for the (S)TEM Demonstrating Sub-nm Spatial and Sub-meV Spectral Resolution

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Leaching of uranyl–silica complexes from the host metapelite rock favoring high radon activity of subsoil air: case of Castañar cave (Spain)

Castanar cave is a subterranean site with an outstanding natural environmental radioactivity. The maintaining of high radon activity of cave air and the detection of spatially anomalies of this gas in some cave emplacements, suggests that some natural geochemical processes are involved on the mobilization of radioactivity sources to cave environment, other than a simple exhalation of radon from the host-rock. The host rocks are interlaid dolostone–metapelite beds with radioactive nuclides of the three actinium, thorium and uranium decay series. In situ measurements on the spatial distribution of radioactivity, uranyl group’s luminescence and radon gas concentrations inside cave were main focus of this work to model lixiviation and deposition mechanisms of radioactive elements from the host rock to the karstic system. In addition, collected micro-samples were also analyzed by a multi-approach suite of analytical techniques: inductively coupled plasma mass spectrometry, environmental scanning electron microscopy with an attached X-ray energy dispersive system and spectral cathodoluminescence detector, thermoluminescence, Raman spectroscopy, X-ray diffraction, differential-thermal and thermogravimetry analysis, Alpha-spectrometry and Gamma-spectrometry techniques. The host metapelitic beds contain Zr(Hf)–Th(U)–Ti–P–REE phases such as zircon, xenotime-(Y), monazite-(Ce, La) and poly-metallic mineralization veins of hydrothermal origin. Carbonated host beds and speleothems show frequently chemical elements leaked from the upper host rock masses. The weathering leakage processes are favored by the existence of pyrite and limonite in the dolostone masses. The cave exhibits under UV lamps abundant hydrous silica–uranyl coatings covering carbonated speleothems with radionuclides of 238U natural decay series. The long-lived radio-nuclides of the radium radioactive decay chain are responsible of the continuous regeneration of radon gas inside cave. The experimental work was focused to identify origin and remobilization processes of radio-nuclides and their latte settlement into the cave environment associated to mineral phases of speleothems and cave deposits.

Development of Novel Optical Fiber System for Cathodoluminescence Detection in High Voltage Transmission Electron Microscope

We have installed a new system to observe cathodoluminescence (CL) in a high voltage transmission electron microscope (HVTEM). The system is constructed of a ball lens for collection of CL, an optical fiber and a multi-channel optical detector. The system can be operated without any disturbance of TEM observation. The system was proved to be very useful to observe CL in HVTEM and will be a powerful tool to investigate production mechanisms of luminescence centers by observing CL changes with incident electron energy and flux (displacement damage effect), and temperature, together with simultaneous TEM observations of microstructure changes. [doi:10.2320/matertrans.M2013025]

Defect detection in semiconductor layers with built-in electric field with the use of cathodoluminescence

Cathodoluminescence (CL) in scanning electron microscopy (SEM) is commonly accepted as revealing local properties of a specimen region illuminated by an electron beam. CL is widely used to visualize defects in semiconductor structures. However, the presence of a strong electric field in, for example, heterojunctions or p–n junctions causes a separation of generated electron–hole (e–h) pairs and suppresses recombination in the specimen region excited by the beam. As a result CL – a radiative recombination – becomes quenched. At the same time, electron beam-induced current (EBIC) flows throughout the structure, which may produce secondary electroluminescence that is registered by the CL detector. Consequently, the CL measurement is distorted and if there are defects in the structure, they remain unrevealed. The current study shows that registration of the CL signal for different values of electron beam current (including high ones) enables true defect detection in semiconductor layers with built-in electric field. Results for a special test structure prepared with focused ion beam on AlGaAs/GaAs laser heterostructures with an 8nm InGaAs quantum well are presented.

Optimization of decay kinetics of YAG:Ce single crystal scintillators for S(T)EM electron detectors

To enhance the detective quantum efficiency (DQE) of scintillation electron detectors for scanning electron microscopes or scanning transmission electron microscopes (S(T)EM), the cathodoluminescence (CL) decay kinetics of YAG:Ce single crystal scintillators is studied in this paper. Some possibilities for reductions of decay time and afterglow of the mentioned scintillators are presented. The pulse mode utilizing a blanking system and 10 keV electrons for the excitation and a sampling oscilloscope for the CL detection were used for the measurement of decay characteristics. The CL decay characteristics of YAG:Ce single crystals of different Ce concentrations, pulled by the Czochralski method and cleaned and annealed under specific conditions, were measured in dependence on excitation pulse duration and on the specimen temperature. To interpret the presented results, a kinetic model of cathodoluminescence of the YAG:Ce single crystals was created. The effects of quenching impurities and of defect centers in YAG:Ce were specified.

FIB-SEM cathodoluminescence tomography: practical and theoretical considerations

Focused ion beam-scanning electron microscope (FIB-SEM) tomography is a powerful application in obtaining three-dimensional (3D) information. The FIB creates a cross section and subsequently removes thin slices. The SEM takes images using secondary or backscattered electrons, or maps every slice using X-rays and/or electron backscatter diffraction patterns. The objective of this study is to assess the possibilities of combining FIB-SEM tomography with cathodoluminescence (CL) imaging. The intensity of CL emission is related to variations in defect or impurity concentrations. A potential problem with FIB-SEM CL tomography is that ion milling may change the defect state of the material and the CL emission. In addition the conventional tilted sample geometry used in FIB-SEM tomography is not compatible with conventional CL detectors. Here we examine the influence of the FIB on CL emission in natural diamond and the feasibility of FIB-SEM CL tomography. A systematic investigation establishes that the ion beam influences CL emission of diamond, with a dependency on both the ion beam and electron beam acceleration voltage. CL emission in natural diamond is enhanced particularly at low ion beam and electron beam voltages. This enhancement of the CL emission can be partly explained by an increase in surface defects induced by ion milling. CL emission enhancement could be used to improve the CL image quality. To conduct FIB-SEM CL tomography, a recently developed novel specimen geometry is adopted to enable sequential ion milling and CL imaging on an untilted sample. We show that CL imaging can be manually combined with FIB-SEM tomography with a modified protocol for 3D microstructure reconstruction. In principle, automated FIB-SEM CL tomography should be feasible, provided that dedicated CL detectors are developed that allow subsequent milling and CL imaging without manual intervention, as the current CL detector needs to be manually retracted before a slice can be milled. Due to the required high electron beam acceleration voltage for CL emission, the resolution for FIB-SEM CL tomography is currently limited to several hundreds of nm in XY and up to 650 nm in Z for diamonds. Opaque materials are likely to have an improved Z resolution, as CL emission generated deeper in the material is not able to escape from it.

Y-rich oxide distribution in plasma sprayed MCrAlY-coatings studied by SEM with a cathodoluminescence detector and Raman spectroscopy

Abstract SEM cathodoluminescence (CL) imaging and Raman spectroscopy were used to study the compound type, distribution and morphology of Y-rich oxide particles in MCrAlY-coatings at various stages of the manufacturing and oxidation exposure. Compared to conventional SEM/EDX, CL allows easy detection of Y-rich oxides at a high, sub-µm resolution with a fast image acquisition. Furthermore, phase analysis of the Y-rich oxide compounds based on the wavelength resolved Raman spectra and in some cases also using CL-spectra is possible. The presented results of the cathodoluminescence and Raman spectroscopy studies indicate that the type of the Y-rich oxide phases after heat-treatment can be correlated with the content and/or reservoir of metallic Y in the coating. Hence, these analytical methods have potential to be applied as (non-destructive) quality control methods in the manufacturing of MCrAlY-coatings. The distribution of the Y-rich oxide precipitates formed during manufacturing has been shown to affect the growth rate and mechanical stability of the alumina scales during service. Y-oxide distribution in the MCrAlY-coatings, therefore, represents a factor, which influences the lifetime of overlay and TBC-coatings.