Cathodoluminescence Studies Research Articles

Interlayer Bound Wannier Excitons in Germanium Sulfide.

We report a cathodoluminescence (CL) study of layered germanium sulfide (GeS) where we observe a sharp emission peak from flakes covered with a thin hexagonal boron nitride film. GeS is a material that has recently attracted considerable interest due to its emission in the visible region and its strong anisotropy. The measured CL peak is at ~1.69 eV for samples ranging in thickness from 97 nm to 45 nm, where quantum-confinement effects can be excluded. By performing ab initio ground- and excited-state simulations for the bulk compound, we show that the measured optical peak can be unambiguously explained by radiative recombination of the first free bright bound exciton, which is due to a mixing of direct transitions near the -point of the Brillouin Zone and it is associated to a very large optical anisotropy. The analysis of the corresponding excitonic wave function shows a Wannier–Mott interlayer character, being spread not only in-plane but also out-of-plane.

Highly regular rosette-shaped cathodoluminescence in GaN self-assembled nanodisks and nanorods

Self-assembled GaN nanorods were grown by metal-organic chemical vapor deposition. A highly regular rosette-shaped cathodoluminescence pattern in the GaN nanorods is observed, where its origin is helpful to deepen the understanding of GaN nanorod growth. The pattern forms at the very early stages of nanorod growth, which consists of yellow luminescence at the edges and the non-luminous region at six vertices of the hexagon. To clarify its origin, we carried out detailed cathodoluminescence studies, electron microscopy studies and nanoscale secondary ion mass spectrometry at both the nanorod surface and cross-section. We found the pattern is not related to optical resonance modes or polarity inversion, which are commonly reported in GaN nanostructures. After chemical composition and strain analysis, we found higher carbon and nitrogen cluster concentration and large compressive strain at the pattern area. The pattern formation may relate to facet preferential distribution of non-radiative recombination centers related to excess carbon/nitrogen. This work provides an insight into strain distribution and defect-related emission in GaN nanorod, which is critical for future optoelectronic applications.

Cathodoluminescence Study of Dislocations in Step-Graded InGaP Buffer Layers of Metamorphic Single-Junction InGaAs Solar Cells

Epitaxial growth of lattice-mismatched materials is useful for solar cells, but lattice dislocations must be controlled for best device performance. It has been shown that metamorphic growth enables fabrication of InGaAs p–n junctions with good performances on GaAs substrates due to the insertion of buffer layers. Here, we investigate misfit and threading dislocations inside the step-graded InGaP buffer layers of a single-junction InGaAs solar cell by cathodoluminescence microscopy. Prior to measurement, the device edges were polished at various angles (less than 10° with respect to the substrate surface). By using this technique, cross sections of very thin layers can be directly imaged with a resolution that allows us to observe misfit and threading dislocations. In the present device, the densities of the two types of dark lines depend on the position in the buffer structure. In particular, near the InGaAs base layer, the density of the dark lines extending in the [110] direction is higher than that of the dark lines extending in the [1-10] direction. We believe that this difference in the dark line density is related to the surface morphology.

Structural, opto-electronic characteristics and second harmonic generation of solution processed CH3NH3PbI3-xClx thin film prepared by spray pyrolysis

Nanorod like crystalline CH3NH3PbI3-xClx thin film was prepared by spray pyrolysis at an ambient atmosphere. X-ray diffraction (XRD) and Raman studies reveal the tetragonal perovskite phase of spray deposited film. The optical band gap of CH3NH3PbI3-xClx is estimated to ~1.64 eV which is consistent with vacuum based deposited perovskites. The CH3NH3PbI3-xClx film exhibits a high absorption coefficient of the order of 106 m−1. Photoluminescence (PL) study suggests that band-to-band radiative recombination occurs in CH3NH3PbI3-xClx film. Cathodoluminescence study confirms the emission of 1.67 eV at 80 K and 1.80 eV at 300 K. In this study, the second harmonic generation at photon energy 3.6 eV is reported for the first time at room temperature (RT). It is noted that the band edge peak position at 1.67 eV is being diffused and shifted to 1.80 eV at RT while the second harmonic peak position at 3.6 eV remains unchanged.

Cathodoluminescence study of electric field induced migration of defects in single crystal m-plane ZnO

Internal electric fields can have a significant effect on the behavior of charged defects, dopants, and impurities in operating electronic devices that can adversely impact on their long-term performance and reliability. In this paper, we investigate the redistribution of charged centers in single crystal m-plane ZnO under the action of a DC electric field at 873 K using in-plane and in-depth spatially resolved cathodoluminescence (CL) spectroscopy. The CL intensities of the ultra-violet near band edge (NBE) emission at 3.28 eV and green luminescence (GL) at 2.39 eV were observed to both uniformly increase on the anode side of the electrode gap. Conversely, toward the cathode, the NBE and GL steadily decrease and increase, respectively. The GL quenched after hydrogen donor doping, confirming that the emission is related to acceptor-like centers. Based on the electro-migration and hydrogen doping results, the GL is attributed to radiative recombination involving ZniandVZn pairs. The intensity of an orange luminescence centered at 2.01 eV was unaffected by the electric field and is assigned to substitutional Li acceptors.

Comprehensive study of photoluminescence and cathodoluminescence of Eu and Tb doped Mg2SiO4 prepared via a solid-state reaction technique

We report narrow-band green-red emitting Mg2SiO4 phosphors successfully synthesized through solid-state reaction method, and the cathodoluminescence (CL) and photoluminescence (PL) properties of the samples were investigated in detail. Under electron beam and 275 nm excitation, Mg2SiO4 phosphors doped with various Eu3+ and Tb3+ concentrations in the range of 1 mol % up to 10 mol % exhibit typical green and red emissions, respectively. Tb doped samples were efficiently excited by a low voltage electron beam and UV light, yielding several emission peaks between 370 and 760 nm, and produced a bright green light peaking at 541 nm due to the 5D4 →7F5 transition. Eu3+ doped samples exhibited CL and PL emission spectra from 5D0 to 7Fj manifold transitions of Eu3+. A strong red-light emission peaking at 610 nm also supports the incorporation of Eu3+ ions. A concentration quenching effect was observed and discussed for both phosphors. The optimal doping concentration of Eu3+ and Tb3+ doped phosphors was 7 mol %. In view of the outstanding performance in the PL and CL, the Mg2SiO4:Eu3+, Tb3+ can be considered as a promising green and red phosphor in solid-state lighting applications.

Исследование импульсной катодолюминесценции фторидов кальция, бария, лития и магния

Исследование импульсной катодолюминесценции фторидов кальция, бария, лития и магния

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.

Morphological, cathodoluminescence and thermoluminescence studies of defects in diamond films grown by HF CVD technique

A set of diamond layers was synthesized by Hot Filament Chemical Vapor deposition (HF CVD) from methanol-hydrogen (CH3OH–H2) gas mixture. By changing feed gas composition the obtained diamond films show different morphologies and microcrystal sizes as indicate the Scanning Electron Microscopy (SEM) pictures.The obtained diamond films were analysed by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), cathodoluminescence (CL) and thermoluminescence (TL) spectroscopy.The diamond's microcrystallities preferred orientation in diamond layers was assessed from X-ray diffraction measurements and described by texture coefficient TC(hkl). The cathodoluminescence study clearly shows that relative intensities of the band at 430 nm and at 605 nm changes as the preferred orientation changes from <400> to <220>. It was also shown that the morphology and the microcrystallities preferred orientation has an essential influence on the thermoluminescence properties of polycrystalline diamond layers.

The role of surface depletion layer effects on the enhancement of the UV emission in ZnO induced by a nanostructured Al surface coating

The UV enhancement of Al-coated ZnO single crystals with a wide range of carrier densities is systematically studied using depth-resolved cathodoluminescence and photoluminescence as well as valence band X-ray photoemission spectroscopy (VB-XPS). An up to 17-fold enhanced PL UV emission for Al-coated ZnO with the highest carrier density was measured, which falls to a 12-fold increase for the lowest carrier density. Depth-resolved cathodoluminescence measurements confirm that the enhancement is strongest near the Al-ZnO interface consistent with an increased UV emission due to an exciton-localized surface plasmon coupling mechanism. Correlative cathodoluminescence, photoluminescence and VB-XPS studies reveal that a number of additional effects related to the presence of the Al surface coating also contribute to the UV enhancement factor. These include increased UV enhancement due to the formation of a surface depletion layer induced by the Al coating, which also passivates competitive non-radiative surface recombination channels found in uncoated ZnO. Significantly, it was established that the magnitude of the emission enhancement factor can be raised in a controlled way by reducing the thickness of the depletion layer by increasing the carrier density. The contribution of these effects collectively provides an explanation for the large span of enhancement factors reported in the literature.

Low temperature cathodoluminescence study of Fe-doped β-Ga2O3

Abstract Optical and electrical properties along the b-axis of Fe-doped β-Ga2O3 were studied using low temperature cathodoluminescence (CL) spectroscopy, optical absorption spectroscopy and current-voltage (IV) measurements. The optical absorption spectroscopy showed an absorption edge without near edge shoulder and the corresponding optical bandgap was calculated to be 4.45 eV using direct band gap treatment. The temperature dependent CL measurements exhibited a strong blue to ultraviolet (UV) band composed of multiple low intensity peaks in the blue range, a main blue peak, a main UV peak, and a weak UV band from the as-grown Fe-doped β-Ga2O3. After a controlled annealing in air, the emissions changed to a red to near infrared (R-NIR) band with two sharp peaks and an UV band that is resolved at room temperature to three UV broad peaks. The R-NIR sharp peaks from the air-annealed sample were ascribed to incorporation of nitrogen during air annealing.

Conduction mechanisms in ZnO nanowires based Schottky diode grown under an electric field

We present a cathodoluminescence (CL) and electrical study of aligned ZnO nanowires based Schottky diodes synthesized by applying an AC electric field between two Au microcontacts. Our results reveal that the applied electric field aligns the ZnO nanowires between the electrodes and inhibits the formation of ZnO oxygen vacancies (VO). Local CL measurements of ZnO nanowires grown at different zones of the device show that the applied electric field inhibited the formation of oxygen vacancies (VO). Furthermore, CL spectra display an energy shift of the ZnO band edge emission, generated by changes in the relative intensity of two CL bands centered at 3.23 and 3.27 eV that correspond to the donor-acceptor pair and free electrons-acceptor transitions, respectively. We propose the formation of zinc vacancies (VZn) in nanowires that act as acceptor centers in the generation of these two electronic transitions. I-V curves acquired at room temperature reveal the photoresponse of the ZnO nanowires based Schottky diode exposed under UV (365 nm) illumination, exhibiting photocurrent intensities several times higher than that observed under dark conditions for applied bias lower than 1 V. The electrical conduction mechanisms in aligned ZnO nanowires of the device were tunneling and thermionic-emission for applied bias lower than 400 and 700 mV under dark and UV (365 nm) illumination conditions, respectively. For higher bias values, the device showed a conduction mechanism type field-emission.

Toward High‐Performance Diamond Electronics: Control and Annihilation of Dislocation Propagation by Metal‐Assisted Termination

A major obstacle limiting diamond electronics is dislocations, which deteriorate device properties. As threading dislocations (TDs) are normally inherited from the substrate to the epitaxial layer, control and annihilation of their propagation are important. Herein, metal‐assisted termination (MAT), in which the propagation of dislocations is suppressed by in situ metal doping, is proposed. Heavy W doping is realized by a hot‐filament (HF) chemical vapor deposition (CVD) using heated wires at a high temperature of &gt;2400‐K. A large reduction of TD density is confirmed by cathodoluminescence studies and etch‐pit analysis. The impact of dislocation reduction is investigated electrically. After insertion of the MAT buffer layer, Schottky barrier diodes (SBDs) show improved rectifying action and highly uniform characteristics even when substrates with high dislocation densities (mosaic and heteroepitaxial wafers) are used. The 3D structure of dislocation propagation is successfully captured by two‐photon‐excited photoluminescence (2PPL) imaging of Band‐A luminescence. The 2PPL Band‐A luminescence (without band‐edge excitation) shows high spatial resolution in the depth direction. An abrupt decrease in TD density is captured for heteroepitaxial substrates with an inserted MAT buffer layer. The MAT technique provides an effective approach to realize high‐performance diamond electronics.

Glendonite-Like Carbonate Aggregates from the Lower Ordovician Koporye Formation (Russian Part of the Baltic Klint): Detailed Mineralogical and Geochemical Data and Paleogeographic Implications

Stellate and plate-like carbonate bodies, traditionally called anthraconites, are found throughout the Baltic-Ladoga Klint in bituminous shale of the Koporye Formation (Tremadocian, Lower Ordovician). Although this time interval is usually considered as a greenhouse, there is some evidence for the existence of at least temporary cold conditions during the Cambrian–Ordovician. However, the origin of anthraconites is still strongly debated. We studied the mineralogical, petrographic, cathodoluminescence, geochemical, and isotopic characteristics of anthraconites from five sections of the Russian part of the Baltic paleobasin. A close similarity between the morphological, petrographic, cathodoluminescence, and isotopic characteristics of the studied anthraconites with those of glendonites allow us to suggest that these bodies formed in a similar paleo-environment and should be considered as pseudomorphs of the mineral ikaite. The oxygen and carbon isotope ratios reveal that ikaite precipitation occurred in low-temperature conditions on the seafloor. The carbon isotopic values reveal influence of inorganic seawater carbon along with organic matter decomposition and/or methane oxidation during ikaite-glendonite transformations. The oxygen isotopic composition significantly changed after deposition due to meteoric diagenesis. We propose that the studied Tremadocian anthraconites formed under a region of upwelling, where cold phosphate-rich deep waters rose to the relatively shallow part of the Baltic paleobasin, providing favorable conditions for ikaite precipitation. Based on our cathodoluminescence study, we suggest that ikaite was transformed to calcite over several stages during diagenesis. Mineralogical studies also reveal that primary calcite was transformed to sulfate (gypsum) or dolomite during late superimposed processes.

A Combined WDS, EDS and Cathodoluminescence Study of Carbonate Grains in Water-Rich Meteorites

A Combined WDS, EDS and Cathodoluminescence Study of Carbonate Grains in Water-Rich Meteorites

Reef-carbonate diagenesis in the Pleistocene–Holocene of the well Xike#1, Xisha Islands, South China Sea: implications on sea-level changes

The Pleistocene was characterized by an unstable meteoric–marine diagenetic environment, influenced by high-amplitude global and regional eustatic variations. This paper focuses on the diagenetic characteristics of the Pleistocene–Holocene reef-carbonate rocks in the well Xike#1, drilled on the Stone Island (part of the Xisha Islands) in the South China Sea. A petrographic, cathodoluminescence and stable isotope study allowed the evaluation of the diagenetic evolution of the reef-carbonate rocks. The rock types present include framestones, packstones, wackestones and grainstones. Rock fabric analysis reveals the existence of skeletal grains, biodetritus, intraclasts, matrix and calcite cements. The upper part of the cored interval (0–180 m) has been heavily altered by meteoric fluids. Unequivocal evidence of meteoric diagenesis includes consistently negative δ18O values, well-developed subaerial exposure horizons and typical vadose cements. The deepest exposure surface is ~ 80 m above the base of meteoric diagenesis, which means that the paleowater table could be extended to as deep as 80 m. The grainstone present at 7.74 m represents the boundary between the vadose and phreatic zones, which is marked by the first appearance of isopachous dogtooth cements and well-developed moldic pores. The reef-carbonate rocks between depths of 21.66 m and 180 m have undergone diagenesis in both meteoric and marine environments, in which the early-stage fibrous marine cements are surrounded by later meteoric drusy calcite cements. The end products of phreatic meteoric diagenesis are the limestones composed of low-Mg calcite with micrite envelopes, moldic pores, blocky spar calcite cementation, and aragonite neomorphism. The interval from 180 to 216 m has undergone a marine diagenesis, which is marked by cements with non-luminescent CL characteristic and a limited, relatively enriched C and O isotope composition. The study reveals that the diagenetic evolution of reef carbonate is mainly controlled by high-frequency eustatic sea-level changes, while the sedimentation rate variations, which were induced by regional subsidence and nutrient variations could also have had an impact on the diagenetic evolution.

Megacircones ediacáricos de la sienita nefelínica de San José del Guaviare y su potencial como material de referencia para datación U/Pb mediante LA-ICP-MS

With the aim of expanding the reference materials of the Geochronology Laboratory of the Colombian&nbsp;Geological Survey (LG-SGC, in Spanish), zircons were collected from the nepheline syenite of&nbsp;San José del Guaviare, Jordán locality (Colombia). The SNG-1 sample was composed of 24 zircons&nbsp;selected from the fraction between 2 and 5 millimeters long, which were mounted and polished in&nbsp;an epoxy resin block. The zircons were dated by analyzing the isotopic ratios of uranium and lead&nbsp;using the LA-ICPMS method. In the LG-SGC, 92 points on the zircons were selected for analysis,&nbsp;and at the University of Rennes in France (LG-URF), 24 points from crystals found nearby were&nbsp;selected for analysis from a previous cathodoluminescence study. Concordant ages for the zircons&nbsp;for both laboratories corresponded to a crystallization age of 608 ± 1.2 Ma and 602 ± 3 Ma,&nbsp;respectively, which differ by less than 1% despite the difference in instrumentation and analytical&nbsp;procedures. For systematic error correction and quality control, international reference material,&nbsp;such as Plešovice, GJ-1, FC-5 Duluth and Mount Dromedary zircon, was used. The uncertainties&nbsp;generated during the analyses are similar (1.2 and 3.2 Ma), and the mean square-weighted deviation (MSWD) of the concordia diagrams was &lt; 1 in both cases, indicating low dispersion of the&nbsp;results. Some discordant ages show that the U/Pb ratios are not uniform, as expected for a primary&nbsp;reference material. However, we believe that the zircons of the Jordán syenite have potential use as&nbsp;a secondary reference material for the LG-SGC provided they are carefully selected and prepared,&nbsp;their geochemical content (including rare earth elements, REE) is characterized, and quality monitoring&nbsp;of U/Pb dating is performed, as these zircons are large, reaching up to 4 cm, abundant, and&nbsp;possible to collect in the Colombian territory.

Cathodoluminescence and scanning transmission electron microscopy study of InGaN/GaN quantum wells in core-shell GaN nanowires

InGaN/GaN quantum wells (QWs) on GaN nanowire are expected for applications in high efficiency optoelectronic devices. To improve efficiency, it is important to clarify the relation between optical and structural properties. A systematical optical and structural characterization of QWs in core–shell GaN nanowires is performed by cathodoluminescence and scanning transmission electron microscopy. The QWs grown on the m-facet show varied optical behavior with respect to height position: main-body region (2.82 ∼ 3.10 eV), upper region (3.26 eV), and corner region (2.88 ∼ 3.20 eV). The variant luminescence behaviors are attributed to interface quality and In diffusion.

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.

Solution-processed mixed halide CH3NH3PbI3−xClx thin films prepared by repeated dip coating

The mixed halide CH3NH3PbI3−xClx crystalline thin film has been prepared by two-step solution-processed repeated dip coating method at an ambient atmosphere. X-ray diffraction study reveals the presence of tetragonal and cubic phases in deposited film. Raman study confirms the metal halide bond in the inorganic framework and organic CH3 stretching/bending of C–H bond in CH3NH3PbI3−xClx perovskite. Scanning electron microscopy shows that cuboid and polyhedral-like crystal grains of 100 nm to 2 μm may find applications in optoelectronics. The perovskite CH3NH3PbI3−xClx thin film shows high spectral absorption coefficient of the order of 106 m−1. In optical band gap study, we found the coexistence of cubic and tetragonal perovskite phases. The energy band gap is dominated by cubic phase having Eg = 2.50 eV over tetragonal phase with band gap Eg = 1.67 eV. The room-temperature photoluminescence study confirms band edge, shallow and deep-level emissions. The temperature-dependent cathodoluminescence study shows red, green and ultraviolet emissions. The dominating green luminescence evolved for cubic phase at 2.51 eV. The red and ultraviolet emissions are also found for mixed-phase CH3NH3PbI3−xClx thin film, suitable for preparation of light-emitting devices.