Spatial Resolution Cathodoluminescence Research Articles

Germanium Fabry-Perot nanoresonators investigated by cathodoluminescence spectroscopy

We report on the experimental investigation, by means of spatially-resolved cathodoluminescence spectroscopy, of rectangular all-dielectric Ge nanoantennas sustaining Fabry-Perot resonances. The combination of spatial and spectral resolution allows us to directly image the standing-wave pattern of the local density of optical states inside the nanoantennas, which is the fingerprint of the resonant Purcell contribution to the overall emission enhancement previously reported in the literature for the same structures. Our results confirm that the emission properties of Ge nanostructures can be effectively tuned by engineering the local density of optical states and that cathodoluminescence provides valuable information to experimentally address such modulation in their emission properties.

Modulation of Cathodoluminescence by Surface Plasmons in Silver Nanowires.

The waveguide modes in chemically-grown silver nanowires on silicon nitride substrates are observed using spectrally- and spatially-resolved cathodoluminescence (CL) excited by high-energy electrons in a scanning electron microscope. The presence of a long-range, travelling surface plasmon mode modulates the coupling efficiency of the incident electron energy into the nanowires, which is observed as oscillations in the measured CL with the point of excitation by the focused electron beam. The experimental data are modeled using the theory of surface plasmon polariton modes in cylindrical metal waveguides, enabling the complex mode wavenumbers and excitation strength of the long-range surface plasmon mode to be extracted. The experiments yield insight into the energy transfer mechanisms between fast electrons and coherent oscillations in surface charge density in metal nanowires and the relative amplitudes of the radiative processes excited in the wire by theelectron.

Giant moiré trapping of excitons in twisted hBN.

Excitons in van der Waals (vdW) stacking interfaces can be trapped in ordered moiré potential arrays giving rise to the attractive phenomena of quantum optics and bosonic many-body effects. Compared to the prevalent transition metal dichalcogenides (TMDs) systems, due to the wide bandgap and low dielectric constant, excitons in twist-stacked hexagonal boron nitride (hBN) are anticipated trapped in deeper moiré potential, which enhances the strength of interactions. However, constrained by the common low detectivity of weak light-emitting in the deep-ultraviolet (DUV) bands, the moiré excitons in twist-hBN remain elusive. Here, we report that a remarkable DUV emitting band (peak located at ∼260 nm) only emerges at the twisted stacking area of hBN, which is performed by a high collection efficiency and spatially-resolved cathodoluminescence (CL) at room temperature. Significant peak red shifting contrast to defect-bound excitons of bulk hBN indicates the giant trapping effects of moiré potential for excitons. The observation of deeply trapped excitons motivates further studies of bosonic strongly correlation physics based on the twist-hBN system.

Efficient Carrier Recombination in InGaN Pyramidal µ-LEDs Obtained through Selective Area Growth

Pyramid-shaped InGaN/GaN micro-light-emitting diodes (μ-LEDs) were grown on a sapphire substrate using the selective area growth technique. A stable emission wavelength of a single μ-LED pyramid at 412 nm was observed under an injection current from 0.05 to 20 mA, despite the non-uniformity of the thickness and composition of the multiple quantum wells (MQWs) on the sidewall. An efficient carrier confinement and, thus, a high luminescence intensity were demonstrated in the middle of the sidewall through spatial-resolved cathodoluminescence (CL) characterization and were predicted by theoretical simulations. An ultra-high output power density of 1.37 kW/cm2 was obtained from the single μ-LED pyramid, illustrating its great potential for application in high-brightness micro-displays and in virtual reality and augmented reality (VR and AR) applications.

Native Point Defect Measurement and Manipulation in ZnO Nanostructures.

This review presents recent research advances in measuring native point defects in ZnO nanostructures, establishing how these defects affect nanoscale electronic properties, and developing new techniques to manipulate these defects to control nano- and micro- wire electronic properties. From spatially-resolved cathodoluminescence spectroscopy, we now know that electrically-active native point defects are present inside, as well as at the surfaces of, ZnO and other semiconductor nanostructures. These defects within nanowires and at their metal interfaces can dominate electrical contact properties, yet they are sensitive to manipulation by chemical interactions, energy beams, as well as applied electrical fields. Non-uniform defect distributions are common among semiconductors, and their effects are magnified in semiconductor nanostructures so that their electronic effects are significant. The ability to measure native point defects directly on a nanoscale and manipulate their spatial distributions by multiple techniques presents exciting possibilities for future ZnO nanoscale electronics.

Optical and spatial resolved cathodoluminescence study of phase separation in green InGaN sandwiched structure grown on GaN nanorods by MOCVD

High indium content InGaN sandwiched structure with bright green light emission was grown on GaN nanorods by MOCVD. The main emission peak is found at ˜580 nm when photoluminescence (PL) measurement was conducted at 300 K. A very stable emission energy at about 2.155 eV was found in temperature-dependent PL with a temperature range from 110 K to 275 K, which may be attributed to the strong localization of carriers in phase-separated InGaN structures. Spatial-resolved cathodoluminescence measurements were conducted to resolve the indium distributions in InGaN layer. A clear evolution of indium phase separation is found, which attributes the high emission efficiency of green InGaN sandwiched structure.

Carrier localization structure combined with current micropaths in AlGaN quantum wells grown on an AlN template with macrosteps

The microscopic structural and optical characteristics of AlGaN-based light-emitting diodes grown on AlN templates with macrosteps were evaluated. Cross-sectional transmission electron microscopy in the high-angle annular dark field scanning mode and microscopic energy dispersive X-ray spectroscopy reveal that the AlGaN cladding layer under the AlGaN quantum wells (QWs) has microscopic compositional modulations originating from the macrosteps at the AlN template surface. The Ga-rich oblique zones in the cladding layer likely behave as current micropaths. These micropaths are connected to the carrier localization structure, which is formed by the modulation of both the well widths and the compositions of the QWs. In-plane spatially-resolved cathodoluminescence (CL) spectroscopy indicated significant inhomogeneity of the CL characteristics: the brighter emission with a lower peak photon energy confirms the existence of the carrier localization structure in the QWs. Carrier localization in the QWs along with the current micropaths in the AlGaN cladding layer appears to increase the external quantum efficiency of AlGaN LEDs.

Strain-Gradient Modulated Exciton Emission in Bent ZnO Wires Probed by Cathodoluminescence.

Photoelectrical properties of semiconductor nanostructures are expected to be improved significantly by strain engineering. Besides the local strain, the strain gradient is promising to tune the luminescence properties by modifying the crystal symmetry. Here, we report the investigation of strain-gradient induced symmetry-breaking effect on excitonic states in pure bending ZnO microwires by high spatial-resolved cathodoluminescence at low temperature of 80 K. In addition to the local-strain induced light emission peak shift, the bound exciton emission photon energy shows an extraordinary jump of ∼16.6 meV at a high strain-gradient of 1.22% μm-1, which is ascribed to the strain gradient induced symmetry-breaking. Such a symmetry-breaking lifts the energy degeneracy of the electronic band structures, which significantly modifies the electron-hole interactions and the fine structures of the bound exciton states. These results provide a further understanding of the strain gradient effect on the excitonic states and possess a potential for the applications in optoelectronic devices.

Microscopic nature of crystal phase quantum dots in ultrathin GaAs nanowires by nanoscale luminescence characterization

Crystal phase quantum dots (CPQD) embedded in a nanowire (NW) geometry have recently emerged as efficient single photon emitters. In typical III–V semiconductor NWs such CPQDs are linked to the well-known zincblende (ZB)/wurtzite (WZ) polytypism that occurs mostly randomly along the NW axis, making it difficult to assess the exact position and microscopic nature of a particular emitter. Here, we employ highly spatially-resolved cathodoluminescence (CL) spectroscopy directly in a scanning transmission electron microscope to unambiguously identify type, microscopic nature, position and luminescence characteristics of single polytype defects in ultrathin GaAs–AlGaAs core–shell NWs with nanometer-scale resolution. Importantly, we find that individual twin defects (1 ML-inclusion of WZ in a ZB crystal) are the predominant source for QD emission, where the spectral position depends sensitively on the strength of radial confinement by the ultrathin GaAs NW core. By analyzing the temperature-dependent luminescence properties of a ∼1 ML thick/7 nm wide twin-defect CPQD, we determine a thermal activation energy of ∼7.4 meV for the confined excitons, as well as an evolution in linewidth that reflects phonon-mediated broadening processes, corroborating the QD-like behavior. Our findings also reveal the presence of effective carrier diffusion in-between isolated CPQDs.

Determination of the Optimal Shell Thickness for Self-Catalyzed GaAs/AlGaAs Core-Shell Nanowires on Silicon.

We present a set of experimental results showing a combination of various effects, that is, surface recombination velocity, surface charge traps, strain, and structural defects, that govern the carrier dynamics of self-catalyzed GaAs/AlGaAs core-shell nanowires (NWs) grown on a Si(111) substrate by molecular beam epitaxy. Time-resolved photoluminescence of NW ensemble and spatially resolved cathodoluminescence of single NWs reveal that emission intensity, decay time, and carrier diffusion length of the GaAs NW core strongly depend on the AlGaAs shell thickness but in a nonmonotonic fashion. Although 7 nm AlGaAs shell can efficiently suppress the surface recombination velocity of the GaAs NW core, the influence of the surface charge traps and the strain between the core and the shell that redshift the luminescence of the GaAs NW core remain observable in the whole range of the shell thickness. In addition, the band bending effect induced by the surface charge traps can alter the scattering of the excess carriers inside the GaAs NW core at the core/shell interface. If the AlGaAs shell thickness is larger than 50 nm, the luminescence efficiency of the GaAs NW cores deteriorates, ascribed to defect formation inside the AlGaAs shell evidenced by transmission electron microscopy.

Direct correlations of structural and optical properties of three-dimensional GaN/InGaN core/shell micro-light emitting diodes

Three-dimensional (3D) InGaN/GaN quantum-well (QW) core–shell light emitting diodes (LEDs) are a promising candidate for the future solid state lighting. In this contribution, we study direct correlations of structural and optical properties of the core–shell LEDs using highly spatially-resolved cathodoluminescence spectroscopy (CL) in combination with scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). Temperature-dependent resonant photoluminescence (PL) spectroscopy has been performed to understand recombination mechanisms and to estimate the internal quantum efficiency (IQE).

Influence of Cu ion implantation on the microstructure and cathodoluminescence of ZnS nanostructures

The microstructure and optical properties of as-synthesized and Cu ion implanted ZnS nanostructures with branched edges are studied by using high-resolution transmission electron microscope (TEM) and spatially-resolved cathodoluminescence measurement. Obvious crystalline deterioration has been observed in Cu-doped ZnS nanostructures due to the invasion of Cu ions into ZnS lattice. It was found that the optical emissions of ZnS nanostructures can be selectively modified through the control of Cu ion dose and subsequent heat treatment. An increase of Cu dopant content will lead to an apparent red-shift of the intrinsic band-gap emission in the UV range and the broadening of defect-related emission in visible range. The influences of Cu ion implantation on the microstructure and related optical properties were discussed.

Cathodoluminescence and Raman characteristics of CaSO4:Tm3+, Cu phosphor

The physical characterization and phosphor emission spectra are presented for CaSO4 doped with Tm and Cu. All spectral wavelengths are related to electronic transitions of Tm3+ ions. The powder X-ray diffraction pattern showed that the compound exhibits orthorhombic structure and all reflections were indexed without any other secondary impurity phases. Chemical and structural properties of the samples have been characterized by means of Raman spectroscopy and environmental scanning electron microscope (ESEM) with an attached X-ray energy dispersive system (EDS). Group frequencies concept is essential point to the interpretation of the bands due to the main SO4 vibrational units and these displayed main characteristic intensive Raman bands including typical strong intensity at 1016cm−1 that corresponds to ν1SO4 vibrational mode. From the spatially-resolved cathodoluminescence (CL) spectrum, main emission bands of Tm3+ centered at 346, 362, and 452nm, due to the respective transitions of 3P0→3H4, 1D2→3H6, 1D2→3F4 were clearly identified. The study is novel as no such CL-ESEM data are available for this doped compound.

Imaging and characterization of piezoelectric potential in a single bent ZnO microwire

We achieved direct visualization of the piezoelectric potentials in a single bent ZnO microwire (MW) using focused synchrotron radiation (soft x-ray) scanning photoelectron spectro-microscopy. Using radial-line scan across the bent section of ZnO MW, the characteristic core-level shifts were directly related to the spatial distribution of piezoelectric potentials perpendicular to the ZnO polar direction. Using piezoelectric modeling in ZnO, we delineated the band structure distortion and carrier concentration change from tensile to compressed sides by combining the spatial resolved cathodoluminescence characteristics in an individual microwire. This spectro-microscopic technique allows imaging and identification of the electric-mechanical couplings in piezoelectric micro-/nano-wire systems.

Spatially localised luminescence emission properties induced by formation of ring-shaped quasi-potential trap around V-pits in InGaN epi-layers

The correlation between surface structural and luminescence properties of InGaN epi-layers grown by metal organic chemical vapour deposition has been investigated. Ring-shaped regions with high In-content are observed around V-pits by spatial resolved cathodoluminescence and energy dispersive X-ray spectroscopy. These ring-shaped regions are found to form a potential energy well with depth about 300 meV with respect to the ordinary regions, which can attract carriers and suppress non-radiative recombination. The causes of the formation of high-In-content region are discussed. This localised characteristic of carriers is supported further by power and temperature dependent photoluminescence results. The understanding of the spatially emission characteristics might be helpful to improving the internal quantum efficiency of InGaN/GaN light emitting diodes (LEDs).

Triangular ZnO Nanosheets: Synthesis, Crystallography and Cathodoluminescence

ZnO nanosheets with triangular morphology have been synthesized on an Au-coated silicon substrate through a facile thermal evaporation process. The morphologies and microstructures of the nanosheets were studied by a scanning electron microscope (SEM) and a high-resolution transmission electron microscope (HR-TEM). These studies show that a nanosheet is commonly composed of two parts: a triangular ZnO sheet and an Au nanoparticle attached on its tip-end. Detailed crystallography analyses conclude that the formation of the highly crystalline nanostructures can be assigned to a combination of a vapor-liquid-solid (VLS) process that is believed to be responsible for its initial nucleation and subsequent crystallization along the growth direction, and a vapor-solid (VS) process that is responsible for its further radial growth. The spatially-resolved cathodoluminescence (CL) spectra exhibit a sharp strong near-band-edge (NBE) emission in the ultraviolet range and a negligible green emission.

Luminescence and micro-Raman investigations on inclusions of unusual habit in chrysoprase from Turkey

Chemical analyses performed on chrysoprase from Turkey have shown many trace elements as well as rare earth impurities. Quantitative chemical analyses of inclusions in minerals can improve our understanding of the chemistry of surface. The environmental scanning electron microscope (ESEM) with an attached X-ray energy dispersive system (EDS) is capable of producing rapid and accurate major element chemical analyses of individual inclusions in crystals larger than about 30μm in diameter. The samples were examined with lifetime-resolved and spatially-resolved cathodoluminescence (CL), and inductively coupled plasma-atomic emission spectrometry (ICP-AES). Spatially resolved CL results at room temperature were recorded for two different areas. Bulk area displays with low CL emission and pores contain iron phases such as chromite, hematite and anatase which cause the green color. For the raw data in the lifetime resolved CL spectrum, at least three broad emission bands were detected in a yellow band of the highest intensity at about 550nm, a weaker orange band at about 650nm, and a red band at 720nm. It is assumed that there are links between the CL emissions and the presence of some transition metal and REE elements, but it is obvious that all trace elements do not play a direct role. Micro-Raman measurements were performed on chrysoprase and these showed a characteristic intensive Raman band peaked at 464cm−1 which can be inferred to ν2 doubly symmetric bending mode of [SiO4/M] centers. Raman spectrum of all inclusions found in the material are also given and discussed in detail.

Spatially-Resolved Cathodoluminescence in GaN Nanorods and in AlGaN/GaN Multiquantum Discs Having Different Quantum Confinement States

The optical properties of self-assembled GaN nanorods and GaN/AlGaN multiquantum discs are investigated by using cathodoluminescence (CL) measurements at room temperature. The nanostructure has a di erent dimension along the growth direction, leading to di erent con nement energy states. The dependence of the optical properties on the di erent con nement energy states along the growth direction is clearly demonstrated using the dependence of the CL image on the photon energy (ECL).

Luminescence of Strontianite (SrCO 3) from Strontian (Scotland, UK)

An historic Strontianite-type specimen from Strontian, Scotland, UK, was characterized to broaden our knowledge on luminescence properties of common carbonates. These fibrous aggregates are Strontianite (Sr x Ca 1− x CO 3) with circa 6% of CaO, interfacial water, hydrosilicate anions and substitutional divalent cations, e.g., Ca 2+, Mn 2+, Fe 2+ in structural Sr 2+ positions. The specimen was analyzed by X-ray Fluorescence Spectrometry (XRF), Environmental Scanning Electron Microscopy coupled with an Energy Dispersive X-ray Spectroscopy (ESEM-EDS) probe, Spatially-resolved Cathodoluminescence under the Scanning Electron Microscope (SEM-CL), Differential-Thermal Analyses (DTA), Thermogravimetry (TG), Thermoluminescence (TL), Radioluminescence (RL) and High Resolution Spectra Thermoluminescence (3DTL), to gain an overview of the spectral emissions, the defect linkages were modified by heating from room temperature (RT) up to 500 °C. Substitutional transition elements are probably responsible for the spectral emission bands from 500 nm to 800 nm and hydrous molecules from 300 nm to 400 nm. DTA–TG analyses performed on little chips, to preserve the fiber interfaces coherence, exhibit minor endothermic peaks attributed to outflow of water groups in fiber interfaces. Both, CL and RL curves show common spectral positions but UV–blue and red emission intensities are counterbalanced since electron irradiation reduces the UV–blue emissions while X-irradiation increases them. The TL curves show a top thermal limit at ∼300 °C for the 300–400 nm TL emissions which become irreversibly destroyed, whereas the longer wavelength region emits at higher temperature. The non-reversible changes observed in the 320 nm and 360 nm bands during the spectra 3DTL emission could be linked with non-bridging oxygen defects, protons and hydroxyl groups and the red emissions to the 4G ( 4T 1g)– 6S Mn 2+ ion transition. Following assignations and similar spectral CL patterns of Russian Strontianite samples, the emission-defect assignments: Dy 3+ 480 nm; Tb 3+ 540 nm; Dy 3+ 580 nm and Sm 3+ 640 nm cannot be disregarded.

Strong Surface Effect on Cathodoluminescence of an Individual Tapered ZnO Nanorod

Spatial-resolved cathodoluminescence spectra are collected at spots with different diameters on individual single-crystalline tapered ZnO nanorods to study the size-dependent surface effect. A significant increase in the intensity ratio of the deep level to the near band edge emission is observed with ever-increasing nanorod surface-aspect ratio. This ratio shows notable superlinear behavior especially on the larger surface-aspect-ratio side, which can be well understood with consideration of both surface effect and self-absorption. Further discussion reveals that surface recombination dominates the deep-level emission. Our results indicate that the surface effect is crucial to modulation of the physical properties of nanostructures even before their sizes close to quantum size regime and the self-absorption effect must be taken into account for quasi-quantitative analysis at the nanoscale level, which may find its practical applications in the design of functional nanodevices.