Cathodoluminescence Mapping Research Articles

Soft X-Ray and Cathodoluminescence Examination of a Tanzanian Graphite Deposit.

Hyperspectral soft X-ray emission (SXE) and cathodoluminescence (CL) spectrometry have been used to investigate a carbonaceous-rich geological deposit to understand the crystallinity and morphology of the carbon and the associated quartz. Panchromatic CL maps show both the growth of the quartz and the evidence of recrystallization. A fitted CL map reveals the distribution of Ti4+ within the grains and shows subtle growth zoning, together with radiation halos from 238U decay. The sensitivity of the SXE spectrometer to carbon, together with the anisotropic X-ray emission from highly orientated pyrolytic graphite, has enabled the C Kα peak shape to be used to measure the crystal orientation of individual graphite regions. Mapping has revealed that most grains are predominantly of a single orientation, and a number of graphite grains have been investigated to demonstrate the application of this new SXE technique. A peak fitting approach to analyzing the SXE spectra was developed to project the C Kα 2pz and 2p(x+y) orbital components of the graphite. The shape of these two end-member components is comparable to those produced by electron density of states calculations. The angular sensitivity of the SXE spectrometer has been shown to be comparable to that of electron backscatter diffraction.

Revealing multiple classes of stable quantum emitters in hexagonal boron nitride with correlated optical and electron microscopy.

Defects in hexagonal boron nitride (hBN) exhibit high-brightness, room-temperature quantum emission, but their large spectral variability and unknown local structure challenge their technological utility. Here, we directly correlate hBN quantum emission with local strain using a combination of photoluminescence (PL), cathodoluminescence (CL) and nanobeam electron diffraction. Across 40 emitters, we observe zero phonon lines (ZPLs) in PL and CL ranging from 540 to 720 nm. CL mapping reveals that multiple defects and distinct defect species located within an optically diffraction-limited region can each contribute to the observed PL spectra. Local strain maps indicate that strain is not required to activate the emitters and is not solely responsible for the observed ZPL spectral range. Instead, at least four distinct defect classes are responsible for the observed emission range, and all four classes are stable upon both optical and electron illumination. Our results provide a foundation for future atomic-scale optical characterization of colour centres.

Optical properties evolution of GaN film grown via lateral epitaxial overgrowth

Spatially resolved Raman scattering mapping and room-temperature cathodoluminescence (CL) mapping of GaN via epitaxial lateral overgrowth (ELOG) on a SiNx-masked GaN template grown on a sapphire substrate were studied. Scanning electron microscope (SEM) measurements show that the ELOG layer is coalesced. Plane view Raman mapping shows that the E2 (high) phonon intensity is stronger in the wing regions than in the window regions. Transmission electron microscopy (TEM) of a cross-section indicates the dislocation annihilation process. Furthermore, the intensity of Raman mapping on a cross-section of A1 (TO) phonon in the coalesced area is more increased than the GaN template. Interestingly, the cross-sectional CL image shows a butterfly shaped pattern that provides a comprehensive understanding of the variations in the point defect distribution during the ELOG GaN regrowth process.

Electron‐Beam‐Related Studies of Halide Perovskites: Challenges and Opportunities

AbstractElectron beam microscopy and related characterization techniques play an important role in revealing the microstructural, morphological, physical, and chemical information of halide perovskites and their impact on associated optoelectronic devices. However, electron beam irradiation usually causes damage to these beam‐sensitive materials, negatively impacting their device performance, and complicating this interpretation. In this article, the electron microscopy and spectroscopy techniques are reviewed that are crucial for the understanding of the crystallization and microstructure of halide perovskites. In addition, special attention is paid to assessing and mitigating the electron beam‐induced damage caused by these techniques. Since the halide perovskites are fragile, a protocol involving delicate control of both electron beam dose and dose rate, coupled with careful data analysis, is key to enable the acquisition of reliable structural and compositional information such as atomic‐resolution images, chemical elemental mapping and electron diffraction patterns. Limiting the electron beam dose is critical parameter enabling the characterization of various halide perovskites. Novel methods to unveil the mechanisms of device operation, including charge carrier generation, diffusion, and extraction are presented in scanning electron microscopy studies combined with electron‐beam‐induced current and cathodoluminescence mapping. Future opportunities for electron‐beam‐related characterizations of halide perovskites are also discussed.

Microstructural controls on the chemical heterogeneity of cassiterite revealed by cathodoluminescence and elemental X-ray mapping

Abstract Quantitative X-ray element maps of cassiterite crystals from four localities show that Ti, Fe, Nb, Ta, and W define oscillatory zonation patterns and that the cathodoluminescent response is due to a complex interplay between Ti activated emission paired with quenching effects from Fe, Nb, Ta, and W. Sector zonation is commonly highlighted by domains of high Fe, incorporated via a substitution mechanism independent of Nb and Ta. A second form of sector zonation is highlighted by distributions of W separate to the Fe-dominant sector zone. Both sector zones show quenched cathodoluminescence and are indistinguishable under routine SEM CL imaging. For cassiterite already high in Fe (and Nb or Ta), such as in pegmatitic or granitic samples, the internal structure of the grain may remain obscured when imaged by cathodoluminescence techniques, regardless of the presence of sector zonation. Careful petrogenetic assessments using a combination of panchromatic and hyperspectral CL, aided by quantitative elemental X-ray mapping, is a prerequisite step to elucidate cassiterite petrogenetic history and properly characterize these grains for in situ microanalysis. The absence of a clear petrogenetic framework may lead to unknowingly poor spot selection during in situ analyses for geochronology and trace element geochemistry, and/or erroneous interpretations of U-Pb and O isotopic data.

A combined Raman spectroscopy, cathodoluminescence, and electron backscatter diffraction study of kyanite porphyroblasts from diamondiferous and diamond‐free metamorphic rocks (Kokchetav massif)

AbstractA series of precise nondestructive analytical methods (Raman spectroscopy, cathodoluminescence, and EBSD—electron backscatter diffraction) has been employed to investigate the internal textures of kyanite porphyroblasts from diamondiferous and diamond‐free ultrahigh‐pressure metamorphic rocks (Kokchetav massif, Northern Kazakhstan). Such internal kyanite characteristics as twinning, radial fibrous pattern, and spotty zoning were identified by means of Raman and cathodoluminescence imaging, whereas an intergrowth of two kyanite crystals was distinguished only by Raman imaging. The EBSD analysis recorded an ~10–25° changing of orientations along the elongation in the investigated kyanite porphyroblasts. The absence of a radial fibrous pattern and a spotty zoning on the EBSD maps indicates that these textures are not related to variations in crystallographic orientation. The absence of clear zoning patterns (cores, mantles, and rims) on the Raman, cathodoluminescence, or EBSD maps of the kyanite porphyroblasts indicates the rapid single‐stage formation of these porphyroblasts near the peak metamorphic conditions and the lack of recrystallization processes. The obtained results provide important implications for deciphering of mineral internal textures, showing that the data obtained by cathodoluminescence mapping can be clearly reproduced by Raman imaging, with the latter method occasionally being even more informative. This observation is of significant importance for the study of minerals that are unexposed on a thin section surface or Fe‐ and Ni‐rich minerals that do not show luminescence emission. The combination of the Raman spectroscopic, cathodoluminescence, and EBSD techniques may provide better spatial resolution for distinguishing different domains and textural peculiarities of mineral than the selective application of individual approaches.

Two-dimensional analysis of the nonuniform quantum yields of multiple quantum wells for AlGaN-based deep-ultraviolet LEDs grown on AlN templates with dense macrosteps using cathodoluminescence spectroscopy

AlGaN-based deep-ultraviolet light-emitting diodes (LEDs) incorporating uneven multiple quantum wells (MQWs) with inclined and terrace zones, which were fabricated on an AlN template with dense macrosteps, have exhibited a high internal quantum efficiency (IQE). To investigate the microscopic structure of uneven MQWs, cathodoluminescence (CL) mapping characterization was carried out, and the maps of the CL intensity at 300 K relative to that at 38 K were obtained for uneven MQWs that targeted 265 and 285 nm LEDs. At an electron beam current of less than 1.0 nA, the signals from inclined and terrace zones of the uneven MQWs were confirmed to satisfy the nonsaturated excitation condition at 300 K. Nonradiative recombination (NR) was insufficiently frozen even at 38 K, specifically on the terraces in the 265 nm MQW, suggesting high concentrations of NR centers due to point defects (PDs). In contrast, NR in the 285 nm MQW at 38 K was closer to freeze-out. The concentration of PDs in the 285 nm MQW was likely to be lower than that in the 265 nm MQW. Finally, the ratios of the CL intensity at 300 K to those at 38 K were mapped, demonstrating an approach to creating an approximate map of IQE. The values in the CL intensity ratio maps are discussed by considering the analytical error factors. The results support the model of localized current injection through Ga-rich stripe zones in the n-AlGaN cladding layer.

Mechanism of Improved Luminescence Intensity of Ultraviolet Light Emitting Diodes (UV-LEDs) Under Thermal and Chemical Treatments

In this work, the influences of thermal annealing and chemical passivation on the optical and electrical properties of ultraviolet light-emitting-diode (UV-LED) were investigated. The electroluminescence (EL) intensities of the LEDs under KOH treatment and thermal annealing increased by 48% and 81%, respectively compared to as-fabricated LED under current level of 10 mA. Cathodoluminescence (CL) mapping of UV-LEDs confirmed no variation of the density of the non-radiative recombination centers after surface treatments, and no obvious change in surface morphology was identified due to lacking of energy for surface atom migration. However, Raman spectroscopy indicates a relaxation of compressive strains inside the thin film after both thermal and chemical treatments, and conductive atomic force microscopy (c-AFM) also illustrated reduced leakage current after KOH passivation, which are responsible for the improved luminescence properties of UV-LEDs.

Non-radiative recombination at dislocations in InAs quantum dots grown on silicon

We study the impact of misfit dislocations on the luminescence from InAs quantum dots (QDs) grown on Si substrates. Electron channeling contrast imaging is used together with cathodoluminescence mapping to locate misfit dislocations and characterize the resulting nonradiative recombination of carriers via near-infrared light emission profiles. With a 5 kV electron beam probe, the dark line defect width due to a typical misfit dislocation in a shallow QD active layer is found to be approximately 1 μm, with a 40%–50% peak emission intensity loss at room temperature. Importantly, we find that at cryogenic temperatures, the dislocations affect the QD ground state and the first excited state emission significantly less than the second excited state emission. At the same time, the dark line defect width, which partially relates to carrier diffusion in the system, is relatively constant across the temperature range of 10 K–300 K. Our results suggest that carrier dynamics in the QD wetting layer control emission intensity loss at dislocations, and that these defects reduce luminescence only at those temperatures where the probability of carriers thermalizing from the dots into the wetting layer becomes significant. We discuss the implications of these findings toward growing dislocation-tolerant, reliable quantum dot lasers on silicon.

Nanoscale spectroscopic imaging of GaAs-AlGaAs quantum well tube nanowires: correlating luminescence with nanowire size and inner multishell structure

Abstract The luminescence and inner structure of GaAs-AlGaAs quantum well tube (QWT) nanowires were studied using low-temperature cathodoluminescence (CL) spectroscopic imaging, in combination with scanning transmission electron microscopy (STEM) tomography, allowing for the first time a robust correlation between the luminescence properties of these nanowires and their size and inner 3D structure down to the nanoscale. Besides the core luminescence and minor defects-related contributions, each nanowire showed one or more QWT peaks associated with nanowire regions of different diameters. The values of the GaAs shell thickness corresponding to each QWT peak were then determined from the nanowire diameters by employing a multishell growth model upon validation against experimental data (core diameter and GaAs and AlGaAs shell thickness) obtained from the analysis of the 3D reconstructed STEM tomogram of a GaAs-AlGaAs QWT nanowire. We found that QWT peak energies as a function of thus-estimated (3–7 nm) GaAs shell thickness are 40–120 meV below the theoretical values of exciton recombination for uniform QWTs symmetrically wrapped around a central core. However, the analysis of the 3D tomogram further evidenced azimuthal asymmetries as well as (azimuthal and axial) random fluctuations of the GaAs shell thickness, suggesting that the red-shift of QWT emissions is prominently due to carrier localization. The CL mapping of QWT emission intensities along the nanowire axis allowed to directly image the nanoscale localization of the emission, supporting the above picture. Our findings contribute to a deeper understanding of the luminescence-structure relationship in QWT nanowires and will foster their applications as efficient nanolaser sources for future monolithic integration onto silicon.

Temperature-dependent cathodoluminescence mapping of InGaN epitaxial layers with different In compositions

We performed temperature-dependent cathodoluminescence (CL) mapping of InGaN epitaxial layers, for which the presence of potential barriers around threading dislocations was previously confirmed by photoluminescence (PL) mapping using scanning near-field optical microscopy. The dark spot contrast in the panchromatic CL intensity images decreased with increasing temperature owing to the activation of nonradiative recombination processes. The change in the distribution of monochromatic CL (MCL) intensity with increasing temperature was ascribed to a carrier diffusion process in which thermally activated carriers diffuse to the potential minima. The change in the MCL intensity images of the InGaN epitaxial layer with higher In composition at higher temperatures was remarkable, indicating that the carriers cannot readily move to the potential minima owing to the higher potential barrier. This tendency is consistent with the temperature dependence of the intensity ratio of the two emission components previously observed in macroscopic PL measurements.

Investigation of GaN nanowires containing AlN/GaN multiple quantum discs by EBIC and CL techniques

In this work, nanoscale electrical and optical properties of n-GaN nanowires (NWs) containing GaN/AlN multiple quantum discs (MQDs) grown by molecular beam epitaxy are investigated by means of single wire I(V) measurements, electron beam induced current microscopy (EBIC) and cathodoluminescence (CL) analysis. A strong impact of non-intentional AlN and GaN shells on the electrical resistance of individual NWs is put in evidence. The EBIC mappings reveal the presence of two regions with internal electric fields oriented in opposite directions: one in the MQDs region and the other in the adjacent bottom GaN segment. These fields are found to co-exist under zero bias, while under an external bias either one or the other dominates the current collection. In this way EBIC maps allow us to locate the current generation within the wire under different bias conditions and to give the first direct evidence of carrier collection from AlN/GaN MQDs. The NWs have been further investigated by photoluminescence and CL analyses at low temperature. CL mappings show that the near band edge emission of GaN from the bottom part of the NW is blue-shifted due to the presence of the radial shell. In addition, it is observed that CL intensity drops in the central part of the NWs. Comparing the CL and EBIC maps, this decrease of the luminescence intensity is attributed to an efficient charge splitting effect due to the electric fields in the MQDs region and in the GaN base.

Mapping the Origins of Luminescence in ZnO Nanowires by STEM-CL.

In semiconductor nanowires, understanding both the sources of luminescence (excitonic recombination, defects, etc.) and the distribution of luminescent centers (be they uniformly distributed, or concentrated at structural defects or at the surface) is important for synthesis and applications. We develop scanning transmission electron microscopy-cathodoluminescence (STEM-CL) measurements, allowing the structure and cathodoluminescence (CL) of single ZnO nanowires to be mapped at high resolution. Using a CL pixel resolution of 10 nm, variations of the CL spectra within such nanowires in the direction perpendicular to the nanowire growth axis are identified for the first time. By comparing the local CL spectra with the bulk photoluminescence spectra, the CL spectral features are assigned to internal and surface defect structures. Hyperspectral CL maps are deconvolved to enable characteristic spectral features to be spatially correlated with structural features within single nanowires. We have used these maps to show that the spatial distribution of these defects correlates well with regions that show an increased rate of nonradiative transitions.

Spatial Distribution of Defect Luminescence in ZnO Nanorods: An Investigation by Spectral Cathodoluminescence Imaging

The spatial distribution of ubiquitous green luminescence (GL) in ZnO nanorods is investigated using cathodoluminescence (CL) spectral imaging. The vertically aligned, single‐crystal nanorods exhibit a strong GL emission at 2.42 eV at 80 K, attributable to oxygen vacancies. The spectral imaging reveals the GL emission is predominantly located in the surface layer of nanorods; the thickness and intensity of this layer decreases rapidly at elevated temperatures. On the other hand, the near‐band‐edge emission is weakest near the nanorod edges. The temperature‐dependent CL maps are consistent with the properties of a model in which singly ionized oxygen vacancies are stabilized by the surface band bending, which leads to the GL enhancement at the expense of near‐band‐edge emission. These results demonstrate the utility of spectral CL imaging to map the spatial distribution of defect luminescence in nanostructured materials.

The Ti Record of Quartz in Anatectic Aluminous Granulites

The distribution and concentration of Ti in quartz was assessed in five rutile-bearing anatectic aluminous granulites from the Grenville Province, Canada, each with previously constrained P–T conditions of metamorphism. Characterisation of quartz in these samples with the aid of cathodoluminescence (CL) mapping revealed two distinct types in each sample, resorbed quartz partly consumed by melting reactions during the prograde portion of metamorphism and quartz grown from crystallisation of partial melt during retrogression. In two of the samples, pseudomorphs after former melt in the form of quartz overgrowths on existing quartz were discovered using CL which are not visible by other methods. The P–T conditions recorded by each type based on Ti-in-quartz thermobarometry are in poor agreement with microstructures and P–T results inferred from phase equilibria modelling. The negative correlation between Ti incorporation into quartz and P suggests that prograde quartz, which is expected to record near-peak P–T, should be Ti-poor relative to retrograde quartz, which crystallised at conditions several kbar lower than peak P. The opposite trend was found; retrograde quartz is typically relatively Ti-poor and quartz overgrowths may be nearly Ti-free. Additionally, prograde quartz has variable Ti contents and fails to systematically record near peak P–T. Despite the presence of rutile in the samples, which is generally considered to ensure a Ti-saturated system, these results are strong evidence for Ti-undersaturation of quartz caused by disequilibrium between quartz and rutile. In addition, Zr-in-rutile thermometry generally gives lower temperatures than expected for these rocks, which can be explained by retrograde resetting of Zr in rutile and potentially by disequilibrium between rutile and zircon. Based on these results, the degree of Ti (or Zr) saturation achieved at any stage of metamorphism should be assessed with caution, as co-existence of quartz and rutile (or rutile and zircon) within a thin section or sample may not be a sufficient criterion for equilibrium.

Cathodoluminescence study on local high-energy emissions at dark spots in AlGaN/AlGaN multiple quantum wells

We investigated the spatial distribution of luminescence near threading dislocations in AlGaN/AlGaN multiple quantum wells (MQWs) by cathodoluminescence mapping. Emission at the higher-energy side of the AlGaN MQW peak was locally observed near the threading dislocations, which were not accompanied by any surface V-pits. Such higher-energy emission was not observed in the AlGaN epilayers. The energy difference between the AlGaN MQW peak and the higher-energy emission peak increased with increasing barrier-layer Al composition. These results suggest that the origin of the higher-energy emission is likely local thickness fluctuation around dislocations in very thin AlGaN MQWs.

Morphologies and optical and electrical properties of InGaN/GaN micro-square array light-emitting diode chips.

InGaN/GaN micro-square array light-emitting diode (LED) chips (micro-chips) have been prepared via the focused ion beam (FIB) etching technique, which can not only reduce ohmic contact degradation but also control the aspect ratio precisely in three-dimensional (3D) structure LED (3D-LED) device fabrication. The effects of FIB beam current and micro-square array depth on morphologies and optical and electrical properties of the micro-chips have been studied. Our results show that sidewall surface morphology and optical and electrical properties of the micro-chips degrade with increased beam current. After potassium hydroxide etching with different times, an optimal current-voltage and luminescence performance can be obtained. Combining the results of cathodoluminescence mappings and light output-current characteristics, the light extraction efficiency of the micro-chips is reduced as FIB etch depth increases. The mechanisms of micro-square depth on light extraction have been revealed by 3D finite difference time domain.

Mechanical Stress in InP Structures Etched in an Inductively Coupled Plasma Reactor with Ar/Cl2/CH4 Plasma Chemistry

We investigated the crystal lattice deformation that can occur during the etching of structures in bulk InP using SiNx hard masks with Ar/Cl2/CH4 chemistries in an inductively coupled plasma reactor. Two techniques were used: degree of polarization (DOP) of the photo-luminescence, which gives information on the state of mechanical stress present in the structures, and spectrally resolved cathodo-luminescence (CL) mapping. This second technique also provides elements on the mechanical stress in the samples through analysis of the spectral shift of the CL intrinsic emission lines. Preliminary DOP mapping experiments have been conducted on the SiNx hard mask patterns without etching the underlying InP. This preliminary study demonstrated the potential of DOP to map mechanical stress quantitatively in the structures. In a second step, InP patterns with various widths between 1 μm and 20 μm, and various depths between 1 μm and 6 μm, were analyzed by the 2 techniques. DOP measurements were made both on the (100) top surface of the samples and on the (110) cleaved cross section. CL measurements were made only from the (100) surface. We observed that inside the etched features, close to the vertical etched walls, there is always some compressive deformation, while it is tensile just outside the etched features. The magnitude of these effects depends on the lateral and depth dimensions of the etched structures, and on the separation between them (the tensile deformation increases between them due to some kind of proximity effect when separation decreases).

Generated Carrier Dynamics in V-Pit-Enhanced InGaN/GaN Light-Emitting Diode

We investigate the effects of V-pits on the optical properties of a state-of-the-art, highly efficient, blue InGaN/GaN multi-quantum-well (MQW) light-emitting diode (LED) with a high internal quantum efficiency (IQE) of >80%. The LED is structurally enhanced by incorporating a pre-MQW InGaN strain-relief layer with low InN content and a patterned sapphire substrate. For comparison, a conventional (unenhanced) InGaN/GaN MQW LED (with an IQE of 46%) grown under similar conditions was subjected to the same measurements. Scanning transmission electron microscopy reveals the absence of V-pits in the unenhanced LED, whereas in the enhanced LED, V-pits with {10-11} facets, emerging from threading dislocations (TDs), were prominent. Cathodoluminescence mapping reveals the luminescence properties near the V-pits, showing that the formation of V-pit defects can encourage the growth of defect-neutralizing barriers around TD defect states. The diminished contribution of TDs in the MQWs allows indium-rich localization...

Hyperspectral cathodoluminescence study of indium-bearing sphalerite from the Mt Carlton high-sulphidation epithermal deposit, Queensland, Australia

Sphalerite is the most important host mineral for the recovery of indium. New techniques to study the presence and distribution of this critical metal in sphalerite can therefore be of interest for scientific and technological purposes alike. In this study we use the emerging tool of hyperspectral cathodoluminescence (CL) mapping, combined with X-ray element mapping and spot analyses, to characterise the composition and CL properties of indium-bearing sphalerite from the Mt Carlton high-sulphidation epithermal deposit (NE Queensland, Australia). Mt Carlton sphalerite contains highly elevated concentrations of indium (up to 19.59 wt%) occurring within similar to 1 mu m thick colloform bands, which show an average composition of (Zn0.63Cu0.20In0.15Ga0.01) S-0.96. Indium, Cu and Ga are interpreted to have been incorporated via the coupled substitution 2Zn(2+) Cu+(In, Ga) (3+). Hyperspectral CL mapping reveals a high-intensity CL emission directly related to In-Cu-(Ga)-rich sphalerite, centred at wavelengths between similar to 500 and similar to 600 nm. The CL peak is shifted to the higher-wavelength positions when the proportion of In relative to Cu increases. Our study shows that hyperspectral CL mapping is a powerful and efficient technique to study the distribution of In in sphalerite.