Cathodoluminescence Spectroscopy Research Articles

Excitonic and deep-level emission from N- and Al-polar homoepitaxial AlN grown by molecular beam epitaxy

Using low-temperature cathodoluminescence spectroscopy, we study the properties of N- and Al-polar AlN layers grown by molecular beam epitaxy on bulk AlN{0001}. Compared with the bulk AlN substrate, layers of both polarities feature a suppression of deep-level luminescence, a total absence of the prevalent donor with an exciton binding energy of 28 meV, and a much increased intensity of the emission from free excitons. The dominant donor in these layers is characterized by an associated exciton binding energy of 13 meV. The observation of excited exciton states up to the exciton continuum allows us to directly extract the Γ5 free exciton binding energy of 57 meV.

Trace Element Identification and Quantification in Solar Cell Materials Using Energy Dispersive and Cathodoluminescence Spectroscopy.

Trace Element Identification and Quantification in Solar Cell Materials Using Energy Dispersive and Cathodoluminescence Spectroscopy.

Dislocation‐Induced Structural and Luminescence Degradation in InAs Quantum Dot Emitters on Silicon

This study probes the extent to which dislocations reduce carrier lifetimes and alter growth morphology and luminescence in InAs quantum dots (QD) grown on silicon. These heterostructures are key ingredients to achieving a highly reliable monolithically integrated light source on silicon necessary for photonic‐integrated circuits. Around 20%–30% shorter carrier lifetimes are found at spatially resolved individual dislocations at room temperature using time‐resolved cathodoluminescence spectroscopy, highlighting the strong nonradiative impact of dislocations even against the three‐dimensional confinement of QDs. Beyond these direct effects of increased nonradiative recombination, it is found that misfit dislocations in the defect filter layers employed during III–V/Si growth alter the QD growth environment to induce a crosshatch‐like variation in QD emission color and intensity when the filter layer is positioned sufficiently close to the QD emitter layer. Sessile threading dislocations generate even more egregious hillock defects that also reduce emission intensities by altering layer thicknesses, as measured by transmission electron microscopy and atom probe tomography. This work presents a more complete picture of the impacts of dislocations relevant to the development of light sources for scalable silicon photonic integrated circuits.

Cathodoluminescence as a tracing technique for quartz precipitation in low velocity shear experiments

Two simulated gouges (a pure quartz and a quartz-muscovite mixture) were experimentally deformed in a ring shear apparatus at a constant low velocity under hydrothermal conditions favourable for dissolution–precipitation processes. Microstructural analysis using scanning electron microscope cathodoluminescence imaging and cathodoluminescence spectroscopy combined with chemical analysis showed that quartz dissolution and precipitation occurred in both experiments. The starting materials and deformation conditions were chosen so that dissolution–precipitation microstructures could be unambiguously identified from their cathodoluminescence signal. Precipitated quartz was observed as blue luminescent fracture fills and overgrowths with increased Al content relative to the original quartz. In the pure quartz gouge, most of the shear deformation was localized on a boundary-parallel slip surface. Sealing of fractures in a pulverized zone directly adjacent to the slip surface may have helped keeping the deformation localized. In the quartz-muscovite mixture, some evidence was observed of shear-accommodating precipitation of quartz in strain shadows, but predominantly in fractures, elongating the original grains. Precipitation of quartz in fractures implies that the length scale of diffusive mass transfer in frictional-viscous flow is shorter than the length of the quartz domains. Additionally, fracturing might play a more important role than generally assumed. Our results show that cathodoluminescence, especially combined with chemical analysis, is a powerful tool in microstructural analyses of experimentally deformed quartz-bearing material and visualizing quartz precipitation.

Self-Assembly of Mixed-Dimensional GeS1- x Sex (1D Nanowire)-(2D Plate) Van der Waals Heterostructures.

The integration of dissimilar materials into heterostructures is a mainstay of modern materials science and technology. An alternative strategy of joining components with different electronic structure involves mixed-dimensional heterostructures, that is, architectures consisting of elements with different dimensionality, for example, 1D nanowires and 2D plates. Combining the two approaches can result in hybrid architectures in which both the dimensionality and composition vary between the components, potentially offering even larger contrast between their electronic structures. To date, realizing such heteromaterials mixed-dimensional heterostructures has required sequential multi-step growth processes. Here, it is shown that differences in precursor incorporation rates between vapor-liquid-solid growth of 1D nanowires and direct vapor-solid growth of 2D plates attached to the wires can be harnessed to synthesize heteromaterials mixed-dimensional heterostructures in a single-step growth process. Exposure to mixed GeS and GeSe vapors produces GeS1- x Sex van der Waals nanowires whose S:Se ratio is considerably larger than that of attached layered plates. Cathodoluminescence spectroscopy on single heterostructures confirms that the bandgap contrast between the components is determined by both composition and carrier confinement. These results demonstrate an avenue toward complex heteroarchitectures using single-step synthesis processes.

Electric Field Manipulation of Defects and Schottky Barrier Control inside ZnO Nanowires.

We directly measure the three-dimensional movement of intrinsic point defects driven by applied electric fields inside ZnO nano- and micro-wire metal-semiconductor-metal device structures. Using depth- and spatially resolved cathodoluminescence spectroscopy (CLS) in situ to map the spatial distributions of local defect densities with increasing applied bias, we drive the reversible conversion of metal-ZnO contacts from rectifying to Ohmic and back. These results demonstrate how defect movements systematically determine Ohmic and Schottky barriers to ZnO nano- and microwires and how they can account for the widely reported instability in nanowire transport. Exceeding a characteristic threshold voltage, in situ CLS reveals a current-induced thermal runaway that drives the radial diffusion of defects toward the nanowire free surface, causing VO defects to accumulate at the metal-semiconductor interfaces. In situ post- vs pre-breakdown CLS reveal micrometer-scale wire asperities, which X-ray photoelectron spectroscopy (XPS) finds to have highly oxygen-deficient surface layers that can be attributed to the migration of preexisting VO species. These findings show the importance of in-operando intrinsic point-defect migration during nanoscale electric field measurements in general. This work also demonstrates a novel method for ZnO nanowire refinement and processing.

Heavy-Ion-Induced Defects in Degraded SiC Power MOSFETs

Cathodoluminescence spectroscopy is used to investigate the formation of point- and extended defects in SiC power MOSFETs exposed to heavy-ions. Devices showing single event leakage current (SELC) effects are analysed and compared to pristine samples. Common luminescence peaks of defect centers localized in the thermal-SiO2 are identified, together with peaks at the characteristic wavelength of extended defects.

Cathodoluminescence and structural properties of ZnTe nanocrystals synthesized from Te/ZnO thin films

Highly mismatched alloys are of substantial interest for composition-dependent bandgap tunability for solar cell applications. Using low-energy ion implantation and subsequent thermal treatment, we have synthesized ZnTe nanocrystals from Te/ZnO bilayer thin films. However, only thermal treatment without ion implantation does not show any ZnTe nanocrystal characteristics that demonstrate the role of defects via ion implantation in nucleation growth. High-resolution transmission electron microscopy measurements confirmed the nanocrystalline growth of ZnTe and ZnTeO3 inside the mixed layer of ZnO and Te. From X-ray diffraction, it has been observed that a critical fluence of ions is required to produce a certain quantity of defects to assist in stoichiometric ZnTe formation. The absorption spectroscopy illustrates dual bandgap in the mixed alloys. Cathodoluminescence spectroscopy confirms two visible emission bands at 2.1 eV and 2.35 eV corresponding to the ZnTe and ZnO defect band along with the near band UV emission of ZnO at 3.26 eV. Such composition-dependent bandgap and light emission tunability from a mixed alloy are suitable for semiconductor device technology and energy harvesting.

Silica Polymorphs Formation in the Jänisjärvi Impact Structure: Tridymite, Cristobalite, Quartz, Trace Stishovite and Coesite

The study of silica polymorphs in impactites is important for determining the pressure and temperature of impact rock formation. Silica modifications in impact melt rocks of the Janisjärvi impact structure (Karelia, Russia) are presented by tridymite, cristobalite, quartz, trace stishovite and coesite. Silica modifications were characterized and studied by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and Raman and cathodoluminescent spectroscopy. Investigations were carried out in order to clarify polymorphs formation mechanisms and search for signs of the transition of certain structural modifications to others. For the first time, a description of tridymite with a ballen-like texture from impact melt rock is given. A sequence of silica modification and textural transformation in impact rocks after the impact event is suggested. We conclude that the pressure of 40 GPa and a temperature of more than 900 °C were achieved in the impact structure.

Interfacial interaction and intense interfacial ultraviolet light emission at an incoherent interface

Incoherent interfaces with large mismatches usually exhibit very weak interfacial interactions so that they rarely generate intriguing interfacial properties. Here we demonstrate unexpected strong interfacial interactions at the incoherent AlN/Al2O3 (0001) interface with a large mismatch by combining transmission electron microscopy, first-principles calculations, and cathodoluminescence spectroscopy. It is revealed that strong interfacial interactions have significantly tailored the interfacial atomic structure and electronic properties. Misfit dislocation networks and stacking faults are formed at this interface, which is rarely observed at other incoherent interfaces. The band gap of the interface reduces significantly to ~ 3.9 eV due to the competition between the elongated Al-N and Al-O bonds across the interface. Thus this incoherent interface can generate a very strong interfacial ultraviolet light emission. Our findings suggest that incoherent interfaces can exhibit strong interfacial interactions and unique interfacial properties, thereby opening an avenue for the development of related heterojunction materials and devices.

Lateral Integration of SnS and GeSe van der Waals Semiconductors: Interface Formation, Electronic Structure, and Nanoscale Optoelectronics

The emergence of atomically thin crystals has allowed extending materials integration to lateral heterostructures where different 2D materials are covalently connected in the plane. The concept of lateral heterostructures can be generalized to thicker layered crystals, provided that a suitably faceted seed crystal presents edges to which a compatible second van der Waals material can be attached layer by layer. Here, we examine the possibility of integrating multilayer crystals of the group IV monochalcogenides SnS and GeSe, which have the same crystal structure, small lattice mismatch, and similar bandgaps. In a two-step growth process, lateral epitaxy of GeSe on the sidewalls of multilayer SnS flakes (obtained by vapor transport of a SnS2 precursor on graphite) yields heterostructures of laterally stitched crystalline GeSe and SnS without any detectable vertical overgrowth of the SnS seeds and with sharp lateral interfaces. Combined cathodoluminescence spectroscopy and ab initio calculations show the effects of small band offsets on carrier transport and radiative recombination near the interface. The results demonstrate the possibility of forming atomically connected lateral interfaces across many van der Waals layers, which is promising for manipulating optoelectronics, photonics, and for managing charge- and thermal transport.

Spin-orbit interactions in plasmonic crystals probed by site-selective cathodoluminescence spectroscopy.

The study of spin-orbit coupling (SOC) of light is crucial to explore the light-matter interactions in sub-wavelength structures. By designing a plasmonic lattice with chiral configuration that provides parallel angular momentum and spin components, one can trigger the strength of the SOC phenomena in photonic or plasmonic crystals. Herein, we explore the SOC in a plasmonic crystal, both theoretically and experimentally. Cathodoluminescence (CL) spectroscopy combined with the numerically calculated photonic band structure reveals an energy band splitting that is ascribed to the peculiar spin-orbit interaction of light in the proposed plasmonic crystal. Moreover, we exploit angle-resolved CL and dark-field polarimetry to demonstrate circular-polarization-dependent scattering of surface plasmon waves interacting with the plasmonic crystal. This further confirms that the scattering direction of a given polarization is determined by the transverse spin angular momentum inherently carried by the SP wave, which is in turn locked to the direction of SP propagation. We further propose an interaction Hamiltonian based on axion electrodynamics that underpins the degeneracy breaking of the surface plasmons due to the spin-orbit interaction of light. Our study gives insight into the design of novel plasmonic devices with polarization-dependent directionality of the Bloch plasmons. We expect spin-orbit interactions in plasmonics will find much more scientific interests and potential applications with the continuous development of nanofabrication methodologies and uncovering new aspects of spin-orbit interactions.

Structural Peculiarities of Natural Ballas—Spheroidal Variety of Polycrystalline Diamond

Ballas is a rare polycrystalline diamond variety characterized by a radially oriented internal structure and spheroidal outer shape. The origin of natural ballases remains poorly constrained. We present the results of a comprehensive investigation of two classic ballas diamonds from Brazil. External morphology was studied using SEM, high-resolution 3D optical microscopy, and X-ray tomography. Point and extended defects were examined on polished central plates using infra-red, photo- and cathodoluminescence spectroscopies, and electron back-scattering diffraction; information about nanosized inclusions was inferred from Transmission Electron Microscopy. The results suggest that fibrous diamond crystallites comprising ballas are split with pronounced rotation, causing concentric zoning of the samples. Pervasive feather-like luminescing structural features envelop single crystalline domains and most likely represent fibers with non-crystallographic branching. These features are enriched in N3 point defects. Twinning is not common. The nitrogen content of the studied samples reaches 700 at.ppm; its concentration gradually increases from the center to the rim. Annealing of the ballases took place at relatively high temperatures of 1125–1250 °C; the annealing continued even when the samples were fully grown, as suggested by the presence of the H4 nitrogen-related defects in the outer rim. Presumably, the ballas diamond variety was formed at high supersaturation but in conditions favoring a small growth kinetic coefficient. The carbon isotopic composition of the studied ballases (δ13C = −5.42, −7.11‰) belongs to the main mode of mantle-derived diamonds.

A Combination of Ion Implantation and High‐Temperature Annealing: Donor–Acceptor Pairs in Carbon‐Implanted AlN

Herein, carbon‐implanted high‐temperature annealed (HTA) AlN layers are analyzed and donor–acceptor pair (DAP) transitions probably between the two most abundant impurities, carbon and oxygen, are identified. Both are regarded as the main, hard‐to‐avoid impurities in crystal growth. Oxygen is believed to lead to absorption in the deep UV below a wavelength of 250 nm. In contrast, carbon is the most likely candidate to be responsible for a distinct absorption band around 265 nm. This interpretation has recently been challenged. In this study, carbon‐implanted and HTA AlN layers with ion fluences above 8.1 × 1015 cm−2 are analyzed using low‐temperature and time‐resolved cathodoluminescence spectroscopy. Due to the high concentration of oxygen inside the AlN, as a result of the HTA process, a DAP transition between a most likely carbon‐related acceptor and ON is observed. The measured temperature‐ and power‐dependent blueshift of the peak emission energy as well as the luminescence transients can be clearly explained by a continuous change from a DAP transition at low temperature to a free electron to acceptor transition with increasing temperature. The findings are supported by a configurational coordinate model that describes the measured behavior qualitatively.

Porous Semiconductor Compounds with Engineered Morphology as a Platform for Various Applications

Porous semiconductor compounds represent a class of materials which is under an intense research focus over the last years. Herein, morphologies and topologies produced by anodization in binary semiconductor compounds having various bandgaps and crystallographic orientations are demonstrated with a focus on technological procedures applied for generating arrays of pores with a controlled design. The mechanism of pores growth under the photoresist masks is discussed and the connection between the design of the mask and the architecture of the produced porous structure is disclosed. The evolution of physical characteristics of the materials such as luminescence, optical, photonic, vibrational, hydrophilic, and hydrophobic properties as a result of anodization is investigated. Investigations are performed by means of scanning electron microscopy, photoluminescence and cathodoluminescence spectroscopy, and contact angle measurements. Some possible practical applications of the proposed technological approaches and the produced porous structures are briefly discussed.

Cathodoluminescence Properties of Ni-Decorated Hexagonal Cr Microrods for Magneto-Plasmonic Applications

Multifunctional low-dimensional non-noble metal nanostructures exhibiting a broad range of plasmonic resonances and magnetic properties are expected to be innovative materials suitable for advanced device applications. In this work, we synthesized Ni-nanoparticle-decorated Cr microrods from Cr/Ni thin films driven by solid-state dewetting. The Cr microrod exhibits the phase evolution of the Cr(FCC) to Cr(HCP) with temperature confirmed by X-ray diffraction and high-resolution transmission microscopy. Cathodoluminescence (CL) imaging and spectroscopy of a single Cr microrod reveal plasmonic modes in UV (315 nm), vis (563 nm), and IR (705 nm) ranges. With a dewetting temperature, the red-shifted CL emission peaks indicate tunable plasmonic modes. The origin of the three plasmonic modes can be understood from the orbital electron density of states for d–d intraband and s–d interband transitions. The Ni nanoparticles decorated on Cr microrods also show significant room temperature ferromagnetism. The Ni-nanoparticle-designed Cr microrods may expand the range of magneto-plasmonic materials beyond noble metals. The combination of plasmonic and magnetic properties could facilitate emission-sensitive detection and therapy in plasmonic-based optoelectronic technology.

Photon superbunching in cathodoluminescence of excitons in WS2 monolayer

Cathodoluminescence spectroscopy in conjunction with second-order auto-correlation measurements of allows to extensively study the synchronization of photon emitters in low-dimensional structures. Co-existing excitons in two-dimensional transition metal dichalcogenide monolayers provide a great source of identical photon emitters which can be simultaneously excited by an electron. Here, we demonstrate large photon bunching with up to of a tungsten disulfide monolayer (WS2), exhibiting a strong dependence on the electron-beam current. To further improve the excitation synchronization and the electron-emitter interaction, we show exemplary that the careful selection of a simple and compact geometry—a thin, monocrystalline gold nanodisk—can be used to realize a record-high bunching of up to . This approach to control the electron excitation of excitons in a WS2 monolayer allows for the synchronization of photon emitters in an ensemble, which is important to further advance light information and computing technologies.

Impact of neutron irradiation on electronic carrier transport properties in Ga2O3 and comparison with proton irradiation effects

The minority charge carrier transport in beta gallium oxide (-Ga2O3) before and after exposure to neutrons from a 241Am-Be source was studied. Cathodoluminescence (CL) spectroscopy and current–voltage (I-V) characteristics were used to study minority carrier behavior. High energy radiation affects minority carrier properties through the production of carrier traps that reduce the conductivity and mobility of the material. In this paper, we report the effects of neutron radiation in silicon-doped -Ga2O3, using rectifiers or transistors as the measurement platform. The thermal activation energy of the reference (non-irradiated) sample was 40.9 meV, which is ascribed to silicon-donors. CL measurements indicate a slightly indirect bandgap energy of 4.9 eV. Neutrons create disordered regions in -Ga2O3 rather than just point defects, resulting in the lowest carrier removal rate because of the lowest average non-ionizing energy loss. The neutron irradiation caused an increase in reverse bias leakage current in rectifiers and a carrier removal rate of 480 cm−1 but had little effect on reverse recovery switching time. Differences in CL spectra were observed between vertical and lateral device structures, emphasizing the importance of impurities and defects in the starting material. The neutron energies in the Am-Be source range up to 11 MeV, with an average energy between 4 and 5 MeV (Figure 1). We also contrasted neutron damage with the effects of irradiation with 10 MeV protons. Under 10 MeV proton irradiation, the thermal activation energies increase. This is attributable to high order defects and their influence on carrier lifetimes. The measurements show that -Ga2O3 is more resistant to radiation damage than other wide bandgap semiconductors due to its higher displacement threshold energy, which is inversely proportional to the lattice constant.

Phase-locked photon–electron interaction without a laser

Ultrafast photon–electron spectroscopy in electron microscopes commonly requires ultrafast laser setups. Photoemission from an engineered electron source is used to generate pulsed electrons, interacting with a sample excited by the laser pulse at a known time delay. Thus, developing an ultrafast electron microscope demands the exploitation of extrinsic laser excitations and complex synchronization schemes. Here we present an inverse approach to introduce internal radiation sources in an electron microscope based on cathodoluminescence spectroscopy. Our compact method is based on a sequential interaction of the electron beam with an electron-driven photon source and the investigated sample. Such a source in an electron microscope generates phase-locked photons that are mutually coherent with the near-field distribution of the swift electron. We confirm the mutual frequency and momentum-dependent correlation of the electron-driven photon source and sample radiation and determine a degree of mutual coherence of up to 27%. With this level of mutual coherence, we were able to perform spectral interferometry with an electron microscope. Our method has the advantage of being simple, compact and operating with continuous electron beams. It will open the door to local photon–electron correlation spectroscopy of quantum materials, single-photon systems and coherent exciton–polaritonic samples with nanometre resolution.

Provenance Analysis of Marbles by Combination of Cathodoluminescence Spectroscopy and Electron Microprobe Analyses—Methodological Comments

Various marbles from historic quarries of the Czech Republic were examined by means of cathodoluminescence (CL) spectroscopy (quantitative data) to determine the possible inclusion of the method in marble provenance studies. The methodology used was based on a combination of electron microprobe analysis (Ca, Mg, Fe and Mn composition) and CL spectroscopy (intensity) of calcite and dolomite grains of the marbles studied. Several statistical techniques were applied to the CL-spectra to find the most effective way of characterization of the CL-spectra for provenance discrimination. The combination of Mg-admixture of calcite and position of the maximum (i.e., centre) of a single Gaussian curve was revealed to be the most discriminative dependence of the marbles studied.