Cathodoluminescence Emission Intensity Research Articles

STEM‐CL investigations on the influence of stacking faults on the optical emission of cubic GaN epilayers and cubic GaN/AlN multi‐quantum wells

AbstractWe report the influence of {111} stacking faults on the cathodoluminescence (CL) emission characteristics of cubic GaN (c‐GaN) films and cubic GaN/AlN multi‐quantum wells. Transmission electron microscopy (TEM) measurements indicate that stacking faults (SFs) on the {111} planes are the predominant crystallographic defects in epitaxial films, which were grown on 3C‐SiC/Si (001) substrates by plasma‐assisted molecular beam epitaxy. The correlation of the SFs and the luminescence output is evidenced with a CL setup integrated in a scanning TEM (STEM). By comparing the STEM images and the simultaneously measured CL signals it is demonstrated that SFs in these films lead to a reduced CL emission intensity. Furthermore, the CL emission intensity is shown to increase with increasing film thickness and decreasing SF density. This correlation can be connected to the reduction of the full width at half maximum of X‐ray diffraction rocking curves with increasing film thickness of c‐GaN films. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Strong red emission in Eu3+/Bi3+ ions codoped CaWO4 phosphors for white light-emitting diode and field-emission display applications

The CaWO4:xEu3+ and CaWO4:0.08Eu3+/yBi3+ phosphors were synthesized via a high-temperature solid-state reaction method. The crystal structure and luminescence properties were investigated by X-ray diffraction patterns, photoluminescence excitation and emission spectra. The results indicate that both CaWO4:xEu3+ and CaWO4:0.08Eu3+/yBi3+ phosphors can be excited by near-ultraviolet and blue lights, and a strong red emission located at 616nm (5D0→7F2) corresponding to the characteristic emission of Eu3+ ions was observed in all the samples. Furthermore, the cathodoluminescence (CL) spectra as a function of filament current and accelerating voltage were also measured. Compared to the CaWO4:0.08Eu3+ phosphor, the CaWO4:0.08Eu3+/0.02Bi3+ phosphor had a stronger CL emission intensity with good Commission International de I’Eclairage chromaticity coordinate. The CaWO4:0.08Eu3+/0.02Bi3+ phosphor shows great potential for applications in white light-emitting diodes and field-emission displays as an excellent red-emitting phosphor.

Photoluminescence and cathodoluminescence properties of Li<sup>+</sup> doped Gd<sub>1.88</sub>Eu<sub>0.12</sub>O<sub>3</sub>

Li+-doped cubic Gd1.88Eu0.12O3 phosphors were synthesized at 1200 degrees C in air by co-precipitation (CP) and solid state reaction (SS) methods. X-ray diffraction analysis revealed that, regardless of synthetic method, the average crystallite size for Li+-free Gd1.88Eu0.12O3 was approximately 400 nm, then increased with the amount of Li+ and at 20 mol%, reached about 2.3 mu m. Under excitation of the charge transfer band of Eu3+ at 245 nm, Gd1.88Eu0.12O3 exhibited a dominant photoluminescence (PL) red emission peak at 611 nm assigned to the electric dipole transition D-5(0) -> F-7(2) of Eu3+. The red emission peak intensity increased consistently with the amount of Li+-doping, while CP method was found to be effective in improving the red emission intensity at lower amounts of L-i+-doping from 0 to 8 mol%. Cathodoluminescence (CL) property was studied by mounting the synthesized phosphor on a vacuum fluorescent display (VFD) operated at an anode voltage of 50V. The Gd1.88Eu0.12O3 exhibited a similar spectrum with an intense red emission peak at 611 nm, and the highest luminance intensity for the CL red emission was achieved for 8 mol% Li+-doped Gd1.88Eu0.12O3 synthesized by SS method. The evaluation results of PL and CL properties suggested that, besides the crystallite size of Gd1.88Eu0.12O3, dispersion property of Eu3+ ions in the host Gd2O3 was an important factor for improving luminescent properties of Gd1.88Eu0.12O3. Moreover, under the present VFD operating condition at the low excitation voltage, it was thought to be essential for improving CL emission intensity to maintain a sufficient surface area of Gd1.88Eu0.12O3. (C)2015 The Ceramic Society of Japan. All rights reserved.

Spatially resolved optical emission of cubic GaN/AlN multi-quantum well structures

ABSTRACTIn this contribution we report on the optical properties of cubic AlN/GaN asymmetric multi quantum wells (MQW) structures on 3C-SiC/Si (001) substrates grown by radio-frequency plasma-assisted molecular beam epitaxy (MBE). Scanning transmission electron microscopy (STEM) and spatially resolved cathodoluminescence (CL) at room temperature and at low temperature are used to characterize the optical properties of the cubic AlN/GaN MQW structures. An increasing CL emission intensity with increasing film thickness due to the improved crystal quality was observed. This correlation can be directly connected to the reduction of the linewidth of x-ray rocking curves with increasing film thickness of the c-GaN films. Defects like stacking faults (SFs) on the {111} planes, which also can be considered as hexagonal inclusions in the cubic crystal matrix, lead to a decrease of the CL emission intensity. With low temperature CL line scans also monolayer fluctuations of the QWs have been detected and the observed transition energies agree well with solutions calculated using a one-dimensional (1D) Schrödinger-Poisson simulator.

Composition-dependent electronic properties of indium–zinc–oxide elongated microstructures

Microrods and hierarchical structures of indium–zinc–oxide (IZO) with different compositions were grown by thermal treatments of mixtures of InN and ZnO powders. In long rods, an increase in Zn content along the growth axis is revealed by energy dispersive spectroscopy. The structures obtained range from Zn-doped indium oxide with a few atomic per cent of Zn, to IZO compounds of the type ZnkIn2Ok+3. X-ray photoelectron spectroscopy measurements with spatial resolution show that IZO microstructures degenerate at room temperature, with carrier concentration of the order of 1020cm−3. Electron accumulation has been found for undoped (100) and (111) surfaces, whereas depletion of carriers at the surface is observed in IZO samples. The Fermi level position correlates with the Zn concentration at the surface which, taking into account the surface dependence of the ionization potentials, work functions and band gaps, could lead to tunable material properties for device applications. Cathodoluminescence emission intensity is enhanced by the presence of Zn, which induces spectral changes and broadening of the emission band compared with undoped material. The results are discussed in terms of the charge neutrality level and the band structure of the material.

Effects of low energy e‐beam irradiation on cathodoluminescence from GaN

AbstractWe present cathodoluminescence (CL) studies on low energy e‐beam irradiated (LEEBI) metal‐organic vapor phase epitaxy (MOVPE) grown GaN films. High intensity LEEBI has been reported to reduce the band‐edge photoluminescence intensity of MOVPE grown GaN films. Here we observe similar reduction of band‐edge CL intensity with increasing LEEBI dose. The irradiation damage is found to be concentrated in the LEEBI energy dissipation depth by CL depth profiling. We have previously attributed the LEEBI induced reduction of band‐edge intensity to the activation of in‐grown Ga‐vacancies. Here we observe no increase in the relative intensity of defect related yellow or blue CL emission peaks in the LEEBI treated samples. This indicates that blue or yellow emission in undoped GaN is not related to in‐grown Ga‐vacancies.

FIB-SEM cathodoluminescence tomography: practical and theoretical considerations

Focused ion beam-scanning electron microscope (FIB-SEM) tomography is a powerful application in obtaining three-dimensional (3D) information. The FIB creates a cross section and subsequently removes thin slices. The SEM takes images using secondary or backscattered electrons, or maps every slice using X-rays and/or electron backscatter diffraction patterns. The objective of this study is to assess the possibilities of combining FIB-SEM tomography with cathodoluminescence (CL) imaging. The intensity of CL emission is related to variations in defect or impurity concentrations. A potential problem with FIB-SEM CL tomography is that ion milling may change the defect state of the material and the CL emission. In addition the conventional tilted sample geometry used in FIB-SEM tomography is not compatible with conventional CL detectors. Here we examine the influence of the FIB on CL emission in natural diamond and the feasibility of FIB-SEM CL tomography. A systematic investigation establishes that the ion beam influences CL emission of diamond, with a dependency on both the ion beam and electron beam acceleration voltage. CL emission in natural diamond is enhanced particularly at low ion beam and electron beam voltages. This enhancement of the CL emission can be partly explained by an increase in surface defects induced by ion milling. CL emission enhancement could be used to improve the CL image quality. To conduct FIB-SEM CL tomography, a recently developed novel specimen geometry is adopted to enable sequential ion milling and CL imaging on an untilted sample. We show that CL imaging can be manually combined with FIB-SEM tomography with a modified protocol for 3D microstructure reconstruction. In principle, automated FIB-SEM CL tomography should be feasible, provided that dedicated CL detectors are developed that allow subsequent milling and CL imaging without manual intervention, as the current CL detector needs to be manually retracted before a slice can be milled. Due to the required high electron beam acceleration voltage for CL emission, the resolution for FIB-SEM CL tomography is currently limited to several hundreds of nm in XY and up to 650 nm in Z for diamonds. Opaque materials are likely to have an improved Z resolution, as CL emission generated deeper in the material is not able to escape from it.

Sol-Gel Synthesis and Characterization of SiO2 @ NaGd ( WO4 ) 2 : Eu3 + Core-Shell-Structured Spherical Phosphor Particles

Monodisperse, core-shell-structured particles were prepared by the sol-gel method. The samples were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, photoluminescence (PL), and low-voltage cathodoluminescence (CL) as well as time-resolved PL spectra and lifetimes. PL and CL study revealed that the core-shell-structured particles show strong red emission dominated by the transition of at with a lifetime of . The PL and CL emission intensity can be tuned by the coating number of phosphor layers on and by accelerating voltage and the filament current, respectively.

Luminescence properties of sol–gel derived spherical SiO 2@Gd 2(WO 4) 3:Eu 3+ particles with core–shell structure

Monodisperse, core–shell structured SiO 2@Gd 2(WO 4) 3:Eu 3+ particles were prepared by the sol–gel method. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy, transmission electron microscopy, photoluminescence (PL) and low-voltage cathodoluminescence (CL). PL and CL study revealed that the core–shell structured SiO 2@Gd 2(WO 4) 3:Eu 3+ particles show strong red emission dominated by the 5D 0– 7F 2 transition of Eu 3+ at 615 nm with a lifetime of 0.89 ms. The PL and CL emission intensity can be tuned by the coating number of Gd 2(WO 4) 3:Eu 3+ phosphor layers on SiO 2 particles, the size of the SiO 2 core particles, and by accelerating voltage and the filament current, respectively.

Scanning tunneling and cathodoluminescence spectroscopy of indium nitride

Indium nitride epilayers grown by metalorganic vapor-phase epitaxy have been studied by cathodoluminescence (CL) spectroscopy, scanning tunneling microscopy and scanning tunneling spectroscopy (STS). A broad CL emission peak centered at 0.8 eV was observed at 80 K. This peak was attributed to an excitonic radiative recombination mechanism as its emission intensity exhibited a super-linear dependence on beam current with a power-law exponent of m=2. A large spatial variation in the CL emission intensity was ascribed to the presence of threading dislocations, which act as non-radiative recombination centers. A surface band gap of ∼1.4 eV was estimated from STS I– V curves.

Growth and cathodoluminescent characteristics of blue emitting cerium-doped zinc aluminate layers synthesized by spray pyrolysis technique

Cerium doped ZnAl2O4 cathodoluminescent films have been deposited by ultrasonic spray pyrolysis deposition technique. X-ray diffraction measurements on these layers have shown that their crystalline structure depends on the substrate temperature. Cerium ions have been introduced into zinc aluminate films for produce violet-blue cathodoluminescence emissions. The behavior of cathodoluminescence emission intensity has been studied, as a function of the variations on doping concentrations, in the start spraying solution, substrate temperatures and the electron accelerating voltage. Concentration quenching of the cathodoluminescence occurs at about 0.8 percent of activator (Ce) concentrations inside the synthesized films. The surface morphology characteristics of these films, as a function of the substrate temperature, and the chemical composition, as measured by energy dispersive X-ray spectroscopy, are also exhibited. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface-state origin for the blueshifted emission in anodically etched porous silicon carbide

Anodic etching of SiC yields a highly monodisperse distribution of nanometer dimension porous structures which extend to a significant depth. Cathodoluminescence (CL) studies of the porous layers yield luminescence peaks in the UV region, above the band gap energy of bulk SiC. Higher etching current densities produce porous silicon carbide (PSiC) with peak CL emission wavelengths deeper in the ultraviolet. Photoluminescence (PL) is also blueshifted in anodically etched PSiC, although not to the extent of the CL emission, suggesting that different emissive states are accessed in CL and PL. Raman investigations of the polar A1 LO mode, which couples strongly to the macroscopic electric field accompanying the LO phonon, were conducted in an attempt to discern whether quantum confinement effects could effectively explain the blueshifted emission. The principal feature of the Raman spectra was a significant low-frequency shoulder on the A1 LO mode, the magnitude of which correlates with the magnitude of the blueshift in PL and the intensity of the blueshifted CL emission. The shoulder was fit quantitatively with a model incorporating the effects of extraordinary LO modes and longitudinal and transverse Fröhlich modes. The Fröhlich mode widths derived from the fit are too wide to be due solely to Fröhlich modes and likely indicate the combined effects of diffuse scattering, broadening of spectral lines, and violation of the symmetry selection rules. The preponderance of the evidence, especially the inability to fit the low-frequency shoulder in the Raman spectra with a phonon confinement model, support an interpretation in which defect structures or surface states are responsible for the UV emission.

Indentation-induced damage in GaN epilayers

The mechanical deformation of wurtzite GaN epilayers grown on sapphire substrates is studied by spherical indentation, cross-sectional transmission electron microscopy (XTEM), and scanning cathodoluminescence (CL) monochromatic imaging. CL imaging of indents which exhibit plastic deformation (based on indentation data) shows an observable “footprint” of deformation-produced defects that result in a strong reduction in the intensity of CL emission. Multiple discontinuities are observed during loading when the maximum load is above the elastic-plastic threshold, and such a behavior can be correlated with multiple slip bands revealed by XTEM. No evidence of pressure-induced phase transformations is found from within the mechanically damaged regions using selected-area diffraction patterns. The main deformation mechanism appears to be the nucleation of slip on the basal planes, with dislocations being nucleated on additional planes on further loading. XTEM reveals no cracking or delamination in any of the samples studied for loads of up to 250 mN.

Simultaneous TEM and Cathodoluminescence Imaging of Non Uniformity in In0.1Ga0.9N Quantum Wells

ABSTRACTMonochromatic cathodoluminescence (CL) imaging of metal-organic vapour phase epitaxy (MOVPE) grown In0.1Ga0.9N single quantum wells (QW) has been performed in a scanning transmission electron microscope (STEM). Spatially resolved fluctuations in the CL emission wavelength and intensity of the QW luminescence were recorded. The presence of regions with luminescent features asymmetrically distributed either side of the QW peak emission was inferred. These fluctuations may be attributed to, by for example, variations of ±0.01 in the In fraction of the In0.1Ga0.9N alloy, to changes of up to 0.6nm in the QW thickness. However these factors do not explain the gross fluctuations in QW emission intensity observed in TEM-CL on the scale of ∼1μm.

High intensity luminescence from pulsed laser annealed europium implanted sapphire

Sapphire samples (Al2O3) were implanted with 400-keV ions at a dose of 1×1016 ions cm−2. A comparison was made between furnace annealing and pulsed laser annealing of the implanted samples. Furnace annealing to 1200 °C, followed by excimer laser anneals, resulted in an increase of the cathodoluminescence emission intensity of the implanted europium by a factor of ∼20. This enhanced intensity is ∼50 times that of the signal prior to any form annealing treatment. It is proposed that the laser anneals dissociate Eu related clusters. The Eu 622-nm lifetime reached 1.53 ms compared with an original postimplant value of 0.14 ms.

Luminescence properties of tetragonal Tl 2Ba 2Cu 1O 6+δ superconducting ceramics

Luminescence properties of tetragonal Tl 2Ba 2Cu 1O 6+δ have been investigated by cathodoluminescence (CL) in the scanning electron microscope (SEM). The intensity of CL emission is inhomogeneous in the sample and its distribution has been found to be independent of impurity phases. Present results, together with previous characterization of the samples by different techniques, indicate that CL emission is related to an inhomogeneous oxygen distribution.

A comparison between the furnace annealing and ruby laser annealing of Yb ion implants in CdTe

Abstract The cathodoluminescence (CL) spectra due to the Ybi3+ and YbCd3+ centres obtained from Yb+ ion implants (50 keV energy and 3 × 1015 ions cm-2) in cleaved [110] faces of CdTe have been compared after furnace annealing (FA) and after laser annealing with a Q-switched ruby laser (PLA) using energy densities between 0.2 and 0.8 J cm-2. Comparable emission intensities between FA and PLA were obtained. Co-implants of Te were needed to observe the YbCd3+ emission in both FA and PLA. The laser energy density of PLA for maximum CL emission intensity from the Ybi3+ centre was approximately 0.5 J cm-2.

Cathodoluminescence of laser-annealed erbium-implanted zinc selenide

After zinc selenide crystals implanted with erbium ions had been annealed with a continuous wave (cw) krypton ion laser it was found that the cathodoluminescence emission intensity of the implanted erbium ions had been considerably enhanced. In order to gain some understanding of the physical processes involved in such an enhancement a comparison was made between thermal annealing and laser annealing of the implanted crystals. Clear differences were found in the cathodoluminescence spectra of the thermally annealed and laser-annealed samples. The increase in cathodoluminescence emission intensity produced by laser annealing was attributed to the production of selenium vacancies in the vicinity of the implanted erbium ions and leads us to suggest the possibility of site selective annealing by this technique.