Cathodoluminescence Studies Research Articles

Enigmatic cathodoluminescent objects in the Dhofar 025 lunar meteorite: Origin and sources

Dhofar 025 is a lunar highland breccia consisting mainly of anorthositic, with less common noritic, gabbronoritic, and troctolitic material. Rare fragments of low-Ti basalts are present as well, but no KREEP (component enriched in incompatible elements) was found in the meteorite. The cathodoluminescence study of this meteorite showed that its impact–melt matrix contains unusual cathodoluminescent (CL) objects of feldspathic composition, which frequently contain microlites of Fe-Mg spinel (pleonaste). They were presumably formed by impact mixing and melting of olivine and plagioclase with subsequent rapid quenching of the impact melts. Such mixing could happen either during assimilation of anorthosites by picritic/troctolitic magmas or during impact melting of troctolites. The enrichment of CL objects of Dhofar 025 in incompatible trace elements suggests that the mafic component of the impact mixture may be related to the high-magnesium suite rocks, which are frequently enriched in KREEP component. The depletion of CL objects in alkalis indicates their possible relation with residual glasses formed by evaporation. However, the presence of FeO in most objects points to the insignificant extent of evaporation. Thus, evaporation cannot explain the enrichment of the CL objects in Al2O3 and other refractory components, although this process definitely took place in their formation. Their similarity to the lunar pink spinel anorthosites, whose existence was predicted from orbital data, serves as an argument in support of the possible formation of the latters by impact mixing.

Cathodoluminescence of YAG:Nd optical nanoceramics in the visible and UV ranges

Results of cathodoluminescence studies of optical YAG:Nd nanoceramics with different microstructure are presented. The composition of the samples was studied by the electron-probe microanalysis. The estimation of the average grain size was performed with the use of scanning electron microscopy and based on X-ray diffraction analysis. Local cathodoluminescence technique was used to collect the emission spectra and decays of emission in the UV and visible range. The “memory effect” in YAG:Nd nanoceramics luminescence was noticed and discussed. In particular, it was observed that the initial electron beam irradiation of sample leads to the increase of luminescence efficiency in the visible and UV ranges. It was confirmed also that the observed effect is long-lasting and remains for years. To understand the mechanism of the effect, the estimation of temperature annealing under continuous irradiation by electron beam was done. The model of memory effect involving trap states was proposed and discussed.

Growth and characterization of wurtzite InP/AlGaP core–multishell nanowires with AlGaP quantum well structures

We report on the selective-area growth and characterization of wurtzite (WZ) InP/AlGaP core–multishell nanowires. Quantum well (QW) structures were fabricated in AlGaP multishells by changing the alloy composition. Transmission electron microscopy revealed that the AlGaP multishells were grown with a WZ structure on the side of the WZ InP core. The lattice constants of the WZ InP core and WZ AlGaP shell were determined by X-ray diffraction. Cathodoluminescence studies showed that the WZ AlGaP QW with an Al composition of 20% exhibited green emissions at 2.37 eV. These results open the possibility of fabricating green light-emitting diodes using WZ AlGaP-based materials.

Local carrier recombination and associated dynamics in m-plane InGaN/GaN quantum wells probed by picosecond cathodoluminescence

We report on spatially resolved and time-resolved cathodoluminescence (CL) studies of the recombination mechanisms of InGaN/GaN quantum wells (QWs) grown by metal-organic vapour phase epitaxy on bulk m-plane Ammono GaN substrates. As a result of the 2° miscut of the GaN substrate, the sample surface exhibits step bunches, where semi-polar QWs with a higher indium concentration than the planar m-plane QWs form during the QW growth. Spatially resolved time-integrated CL maps under both continuous and pulsed excitation show a broad emission band originating from the m-plane QWs and a distinct low energy emission originating from the semi-polar QWs at the step bunches. High resolution time-resolved CL maps reveal that when the m-QWs are excited well away from the step bunches the emission from the m-plane QWs decays with a time constant of 350 ps, whereas the emission originating semi-polar QWs decays with a longer time constant of 489 ps. The time constant of the decay from the semi-polar QWs is longer due to the separation of the carrier wavefunctions caused by the electric field across the semi-polar QWs.

Optical properties of ZnO microrods grown by a hydrothermal method – a cathodoluminescence study

Optical properties of individual zinc oxide microrods grown by a microwave-assisted hydrothermal method are investigated by spatially and spectrally resolved cathodoluminescence spectroscopy and imaging at liquid-helium temperature. For optimized growth conditions the strong localization of cathodoluminescence emission at the corners of the individual hexagonal ZnO microrods has been revealed. Locally distributed luminescence and fine structure of near-band-edge emission are discussed as a manifestation of whispering gallery modes of the hexagonal resonator occurring for the near band-gap luminescence in the investigated microrods.

Optical Signature of the Electron Injection in Ga2O3

Electron injection-induced effect and its impact on the optical properties of β-Ga2O3 ultra-wide bandgap semiconductor have not yet been studied. At the same time, this information is critical for β-Ga2O3 applications in photovoltaic devices such as, photodetectors. In this work, we report the variable temperature cathodoluminescence studies of silicon-implanted β-Ga2O3 and identify a possible mechanism for electron injection-induced luminescence decay observed in this material.

Correlation of carbon doping variations with the vertical breakdown of GaN-on-Si for power electronics

In this work gallium nitride (GaN) grown on silicon substrates was investigated in order to determine critical defects responsible for differences in the vertical breakdown of HEMT structures. Cathodoluminescence studies at the SEM revealed a direct correlation between the intensity of the blue luminescence (BL) band and the carbon doping concentration. Observing this, carbon depletion zones were found around threading dislocations in the active GaN layer, as well as a deep depletion in growth columns concluded from a reduction of the BL intensity. Using the given results a model of a defect, here called the deep carbon depletion (DCD), is proposed to explain the correlation between carbon variations and the vertical breakdown.

Geometry dependent photoconductivity of In2S3 kinks synthesized by kinetically controlled thermal deposition

High quality In2S3 kinks were synthesized via a kinetically controlled thermal deposition process and their optoelectronic characteristics were systematically explored. The growth mechanism was attributed to the combination of kinetic dynamic, crystal facial energy, and surface roughness. Two trap induced emission bands were evidenced via a low temperature cathodoluminescence (CL) study. Furthermore, the nanowire junctions demonstrated a degenerative photodetection performance, as compared to the straight arms, attributed to a stress-induced extra series resistance measured from the kinked area. The well-controllable shape of the inorganic nanostructures and the detailed exploration of their optoelectronic properties are particularly valuable for their further practical applications.

Photo- and cathodo-luminescence of needle-like single crystal diamonds

Comparative study of photoluminescence (PL) and cathodoluminescence (CL) is performed for the needle-like diamond crystallites. These individual diamond crystallites were obtained by selective oxidation of polycrystalline films produced by plasma enhanced chemical vapor deposition (PE CVD). Two types of diamond films were produced with different composition of gaseous medium. For the first type, gas mixture consisted only of hydrogen and methane during the whole process. And for the second type, nitrogen was injected into the same hydrogen-methane gas mixture during a short time in the middle part of the deposition process. The samples of the needle-like diamonds, produced in these two types of CVD, were studied to determine their PL and CL spectra and spatial distribution of the luminescence intensity at various wavelengths. The characteristics of obtained PL and CL spectra indicate presence of luminescence centers related to nitrogen and silicon impurities in the diamond crystallites. The detected spatial distribution and spectral features of the luminescence are explained by variation of structural perfection at near-surface region of the crystallites.

Cathodoluminescence study of luminescence centers in hexagonal and cubic phase GaN hetero-integrated on Si(100)

Hexagonal and cubic GaN—integrated on on-axis Si(100) substrate by metalorganic chemical vapor deposition via selective epitaxy and hexagonal-to-cubic-phase transition, respectively—are studied by temperature- and injection-intensity-dependent cathodoluminescence to explore the origins of their respective luminescence centers. In hexagonal (cubic) GaN integrated on Si, we identify at room temperature the near band edge luminescence at 3.43 eV (3.22 eV), and a defect peak at 2.21 eV (2.72 eV). At low temperature, we report additional hexagonal (cubic) GaN bound exciton transition at 3.49 eV (3.28 eV), and a donor-to-acceptor transition at 3.31 eV (3.18 eV and 2.95 eV). In cubic GaN, two defect-related acceptor energies are identified as 110 and 360 meV. For hexagonal (cubic) GaN (using Debye Temperature (β) of 600 K), Varshni coefficients of α=7.37±0.13×10−4 (6.83±0.22×10−4)eV/K and E0=3.51±0.01 (3.31±0.01) eV are extracted. Hexagonal and cubic GaN integrated on CMOS compatible on-axis Si(100) are shown to be promising materials for next generation devices.

ZnO-Based Microcavities Sculpted by Focus Ion Beam Milling.

We reported an easy fabrication method to realize ZnO-based microcavities with various cavity shapes by focused ion beam (FIB) milling. The optical characteristics of different shaped microcavities have been systematically carried out and analyzed. Through comprehensive studies of cathodoluminescence and photoluminescence spectra, the whispering gallery mode (WGM) was observed in different shaped microcavities. Up further increasing excitation, the lasing action was dominated by these WGMs and matched very well to the simulated results. Our experiment shows that ZnO microcavities with different shapes can be made with high quality by FIB milling for specific applications of microlight sources and optical devices.

Cathodoluminescence study on the impurity behaviors at threading dislocations in GaN

The threading dislocations in GaN provide diffusion routes for impurity incorporation, which substantially influences GaN based optoelectronic devices. Here, we investigated the characteristics of the impurity incorporation at threading dislocations by HCl vapor phase etching and cathodoluminescence (CL) measurements. The distinctive etch pits induced by HCl vapor phase etching correspond definitely to different types of dislocations, and the etching process causes simultaneously H incorporation at a high temperature. CL spectra of the etch pits were analyzed, and the observed difference in the redshift of near band emission peak indicates that mix dislocations are more favorable for impurity incorporations than that of edge dislocations. The understanding on the impurity incorporation behavior at threading dislocations helps to obtain high quality GaN nanostructures.

Cathodoluminescence studies of exciton diffusion in gallium nitride

Gallium nitride is used as an example to show the possibilities of the application of a developed threedimensional mathematical model in the cathodoluminescence studies of direct-gap semiconductor materials using the time-of-flight procedure for obtaining quantitative information on their properties. The values of the diffusion coefficient in gallium nitride are determined from the results of its experimental studies.

Influence of oxygen incorporation on the defect structure of GaN microrods and nanowires. An XPS and CL study

We report a cathodoluminescence (CL) and x-ray photoelectron spectroscopy (XPS) study of the influence of oxygen incorporation on the defect structure of GaN microrods and nanowires. The micro- and nanostructures were synthesized by a thermal evaporation method, which enables us to incorporate oxygen at different concentrations by varying the growth temperature. HR-TEM measurements revealed that oxygen generates stacking fault defects and edge dislocations along the GaN nanowires. Amorphous GaOxNy compounds were segregated on the surface of the nanowires. XPS, XRD and CL measurements suggests that the microrods and nanowires were composed of amorphous oxynitride compounds at their surface and GaN at their inner region. CL measurements revealed that the nanostructures generated an emission of 2.68 eV that increased in intensity proportionally to their oxygen content. We have attributed this emission to electronic transitions between donor substitutional-oxygen (ON) and acceptor interstitial-oxygen (Oi) state levels.

Cathodoluminescence study of Mg activation in non-polar and semi-polar faces of undoped/Mg-doped GaN core–shell nanorods

Spectrally and spatially resolved cathodoluminescence (CL) measurements were carried out at 80 K on undoped/Mg-doped GaN core–shell nanorods grown by selective area growth metalorganic vapor phase epitaxy in order to investigate locally the optical activity of the Mg dopants. A study of the luminescence emission distribution over the different regions of the nanorods is presented. We have investigated the CL fingerprints of the Mg incorporation into the non-polar lateral prismatic facets and the semi-polar facets of the pyramidal tips. The amount of Mg incorporation/activation was varied by using several Mg/Ga flow ratios and post-growth annealing treatment. For lower Mg/Ga flow ratios, the annealed nanorods clearly display a donor–acceptor pair band emission peaking at 3.26–3.27 eV and up to 4 LO phonon replicas, which can be considered as a reliable indicator of effective p-type Mg doping in the nanorod shell. For higher Mg/Ga flow ratios, a substantial enhancement of the yellow luminescence emission as well as several emission subbands are observed, which suggests an increase of disorder and the presence of defects as a consequence of the excess Mg doping.

Cathodoluminescence of N-doped SnO<sub>2</sub> nanowires and microcrystals

We present a cathodoluminescence (CL) study of the point defects in N-doped SnO2 nanowires and microcrystals synthesized by thermal evaporation at different growth temperatures and N concentrations. SnO2:N nanowires were grown at temperatures higher than 1150 °C with N concentrations below of about 3 at.%, while irregular microcrystals were obtained at lower temperatures increasing their N concentration gradually with the growth temperature. EELS and XPS measurements confirmed that N atoms were incorporated into the SnO2 lattice as substitutional impurities (NO). TEM and EDS measurements revealed that the nanowires grew along the [001] direction by a self-catalyzed growth mechanism. CL measurements showed that the nanowires and microcrystals generated a broad emission composed by three components centered at about 2.05, 2.47 and 2.75 eV. CL spectra obtained at 300 and 100 K showed that the component of 2.05 eV decreased in intensity proportionally to the nitrogen content of samples. We attribute this effect to a decrease of oxygen vacancies in the SnO2 nanowires and microcrystals, generated by the incorporation of nitrogen in their lattice.

Distribution of Eu2+ and Eu3+ Ions in Hydroxyapatite: A Cathodoluminescence and Raman Study

We present a cathodoluminescence study of the spatial distribution of Eu2+ and Eu3+ dopants in hydroxyapatite powders. The results demonstrate that the distribution of europium ions in the hydroxyapatite lattice depends on their valence state. Monochromatic cathodoluminescence images from prismatic powders show that although the Eu2+ is distributed homogeneously in the entire powder volume, the Eu3+ is present mainly at the powder edges. The luminescence spectrum of the Eu2+ ions displayed a wide and strong blue emission centered at 420 nm, while the luminescence spectrum of the Eu3+ ions displayed several orange-red emissions covering the range from 575 to 725 nm. These emissions correspond to transitions between levels 4f65d1-4f7 (8S7/2) of the Eu2+ ions and 5D0-7FJ levels of the Eu3+ ions. Micro Raman measurements reveal that europium doping generates two phonon signals with frequencies of 555 and 660 cm-1, both of which have not been reported earlier. The powders were synthesized by the combustion synthesis method, maintaining constant the concentration of the europium salt used, and varying the pH of the precursor solutions to modify the concentration ratio of Eu2+ with respect to Eu3+. X-ray photoelectron spectroscopy measurements were used to determine values of 0.32 and 0.55 for the ratio Eu2+/Eu3+ in samples synthesized at pH values of 6 and 4, respectively. Thermal treatments of the samples, at 873 K in an oxygen atmosphere, resulted in a strong quenching of the Eu2+ luminescence due to oxidation of the Eu2+ ions into Eu3+, as well as probable elimination of calcium vacancy defects by annealing.

Cathodoluminescence and depth profiling studies of unintentionally doped GaN films grown by MOVPE

GaN layers are grown by metalorganic vapor phase epitaxy at 1050 °C on porous silicon and (111) oriented silicon substrates. AlN buffer layers of about 100 nm thickness were previously deposited on Si substrates. The effect of substrates on optical properties is revealed by Cathodoluminescence measurements (CL), recorded at room temperature and liquid nitrogen temperature. Various excitonic transitions are depicted. Spectral features associated with F°X energy around 3.4 eV and bound excitons (D°X and A°X in the range 3.29–3.35 eV) related to wurtzite GaN excitons are observed. Yellow band is located around 2.15 eV. CL depth profiling is also investigated at various e-beam energies (3–25 keV). The low-energy electron beam irradiation reveals an inhomogeneous distribution of point defects in depth, and high non-radiative recombination beyond a threshold energy. Good agreement between our experimental data and literature is obtained. Moreover, CL investigations prove that growth of GaN on (111) oriented Si substrate improve the crystalline quality of the layer.

Direct Synthesis of Alumina Nanowires on Ceramic Substrates Using Sn Catalysts

Direct integration of nanostructures into macroscopic substrates is very important for their practical applications. In this work, we report a simple method that can be introduced for the Sn‐catalyzed growth of alumina nanowires on ceramic substrates such as porous disk, monolith, and foam. Our study focuses on the role of the Sn catalysts in the formation mechanisms governing nanowire growth. Using the proposed approach, hair‐ or grass‐like tufts of 20 nm diameter nanowires grow on the surface of the ~3 μm diameter Sn particles, in a tip growth mechanism. The nanowires of α‐phased polycrystalline structure grow and are packed via a complex process involving batch‐by‐batch, branching, and amalgamation growth. The detailed observations reveal that the Sn catalyst is key to tailoring the growth patterns of the nanowires. In addition, cathodoluminescence studies highlight the potential optical applications of the alumina nanowires.

Cathodoluminescence study of optical properties along the growth direction of ZnO films on GaN substrate

Abstractc‐plane ZnO films were eptiaxially grown on c‐GaN/sapphire substrate by MOCVD. Cathodoluminescence (CL) line scan was performed on the cross‐sectional ZnO/GaN films. These CL spectra shapes show a variation along the growth direction with a peak position shift, which can be divided into three stages. The study of the three stages reflects the growth mode alteration from the interface between ZnO and GaN to the surface. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)