Showing papers on "Cathodoluminescence published in 2004"
TL;DR: It is shown by direct thermally detected optical absorption measurements that the true band gap of InN is markedly wider than the currently accepted 0.7 eV.
Abstract: The deviations on the band-gap of InN which were linked to the precipitation of indium in the metallic phase that leads to additional optical losses associated with Mie resonances were discussed. The two sets of InP epilayers were examined by both plasma-assisted molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) on sapphire. The Mie resonance for InN layers were important because of the activation in the In aggregation phenomena in the alloys. The results show that bright infrared emission arises in a close vicinity of In inclusions and was likely associated with surface states at the metal/InN interfaces.
197 citations
TL;DR: In this article, high-density ZnO nanowires doped with 4 atom % sulfur (S) and pure ZnOs were grown vertically aligned on a silicon substrate, and X-ray diffraction data suggest that the incorporation of S would expand the lattice constants.
Abstract: High-density ZnO nanowires doped with 4 atom % sulfur (S) and pure ZnO nanowires were grown vertically aligned on a silicon substrate. They were synthesized via chemical vapor deposition of a Zn or Zn/S powder mixture at 500 °C. The S-doped ZnO nanowires usually form bundles. The average diameter of the S-doped ZnO nanowires and ZnO nanowires is 20 and 50 nm, respectively. They consist of single-crystalline wurtzite ZnO crystals with a uniform growth direction of [001]. Elemental mapping reveals that the S doping takes place mainly at the surface of the nanowires with a thickness of a few nanometers. X-ray diffraction data suggest that the incorporation of S would expand the lattice constants of ZnO. The photoluminescence and cathodoluminescence of S-doped ZnO nanowires exhibit a significantly enhanced green emission band that comes from the S-doped surface region of the nanowires.
194 citations
TL;DR: In this article, the s-shape temperature dependence is systematically analyzed as a function of aluminum content and quantitatively correlated with a model of alloy disorder, where the shift in the luminescence peak position with respect to the usual temperature dependence of the band gap was quantified by −σE2/kBT, where σE is the standard deviation of the potential fluctuations.
Abstract: AlxGa1−xN layers with 0.05⩽x⩽0.25 were studied using spectrally and time resolved cathodoluminescence (CL). Continuous wave spectra were taken at temperatures ranging from 5 to 300 K. The near-band-edge peak emission energy exhibits an s-shaped temperature dependence characteristic of disordered systems. This effect is quantitatively explained within a model of potential fluctuations caused by alloy disorder. An s-shape temperature dependence has been observed in other alloy systems including InGaN, however, no systematic study exists for AlGaN. In this work, the s-shape temperature dependence is systematically analyzed as a function of aluminum content and quantitatively correlated with a model of alloy disorder. The shift in the luminescence peak position with respect to the usual temperature dependence of the band gap has been quantified by −σE2/kBT, where σE is the standard deviation of the potential fluctuations. Its dependence on aluminum concentration, x, was found to systematically increase from 7...
130 citations
TL;DR: In this paper, the effects of preparation conditions on the cathodoluminescence properties of thin film phosphors were studied, and the luminance and efficiency of these phosphors at a current density of 57 μA/cm2 were found to be linearly dependent on the excitation voltage.
125 citations
TL;DR: In this article, single-crystal ZnO nanowires were fabricated at a low temperature of 500°C without using any metal catalysts via the simple thermal oxidation of metallic Zn precursors.
Abstract: We fabricated single-crystal ZnO nanowires at a low temperature of 500 °C without using any metal catalysts via the simple thermal oxidation of metallic Zn precursors. Scanning electron microscopy and high-resolution transmission electron microscopy (HRTEM) studies showed that the ZnO nanowires with orientation-selectivity grew laterally along the sides of the hexagonal-shaped ZnO matrix transformed from the metallic Zn precursors. It was found that the nanowires grew along the 〈1120〉 direction and were single crystals by HRTEM study. Typically, the lengths of the ZnO nanowires ranged from 0.5 to 3 μm and their mean diameter was 20±5 nm. Photoluminescence and cathodoluminescence measurements at room temperature showed a strong ultraviolet emission peak with negligibly weak green emissions, confirming that the individual ZnO nanowire was of excellent optical quality. Based on these results, we conclude that the simple thermal oxidation of metal Zn precursors is a useful, feasible approach for fabricating ...
106 citations
TL;DR: In this paper, the authors studied the evolution of defects as a function of depth in oxygen-deficient and -surplus ZnO thin films using cathodoluminescence, photoluminecence, Rutherford backscattering spectroscopy/ion channeling and Auger electron spectrograph.
95 citations
TL;DR: In this article, a series of peaks from the cubic phase of BN single crystal were assigned as the phonon-assisted free-exciton luminescence bands from which the indirect exciton gap energy was estimated to be 6.17 eV.
Abstract: Boron nitride (BN) single crystals grown by employing a temperature gradient method under high-pressure and high-temperature conditions using a carefully purified Ba-B-N solvent system showed exciton-related cathodoluminescence peaks in the ultraviolet region around 200 nm. A series of peaks from the cubic phase of BN single crystal was assigned as the phonon-assisted free-exciton luminescence bands from which the indirect exciton gap energy was estimated to be 6.17 eV. The exciton binding energy was estimated to be 80 meV based on the temperature dependence of the exciton luminescence intensity. A dominant luminescence peak from hexagonal-phase BN single crystal was observed at 215 nm, which is 1000 times larger than the indirect exciton luminescence of type IIa diamond. This intense luminescence suggests that hexagonal BN is a direct band gap material.
92 citations
TL;DR: In this paper, the cathodoluminescence spectra of In 2 O 3 -Sn thin films taken at room temperature, present two emission peaks, the first one at 410 nm corresponding to indirect band gap emission and the second one at 650 nm is related to oxygen deficiencies acting as defects.
84 citations
TL;DR: In this article, the authors investigate hexagonal micropillars grown by high pressure pulsed laser deposition on sapphire with spatially and spectrally resolved cathodoluminescence.
Abstract: Using ZnO a variety of micro- and nanostructures can be fabricated. We investigate hexagonal micropillars grown by high pressure pulsed laser deposition on sapphire with spatially and spectrally resolved cathodoluminescence. We demonstrate that the impurity and defect incorporation is strongly inhomogeneous, at least under the present growth conditions, dramatically influencing the optical properties.
83 citations
TL;DR: In this article, the authors used shadow mask technique of InGaN layers grown by metalorganic chemical vapor deposition on Si(111) substrates to analyze the spectra ofCathodoluminescence spectra.
Abstract: Cathodoluminescence spectra employing a shadow mask technique of InGaN layers grown by metalorganic chemical vapor deposition on Si(111) substrates are reported. Sharp lines originating from InGaN quantum dots are observed. Temperature dependent measurements reveal thermally induced carrier redistribution between the quantum dots. Spectral diffusion is observed and was used as a tool to correlate up to three lines that originate from the same quantum dot. Variation of excitation density leads to identification of exciton and biexciton. Binding and anti-binding complexes are discovered.
82 citations
TL;DR: In this article, a ZnO/Zn0.6Mg0.4O double-heterojunction structure was grown without intentional doping and the formation of a deep potential well for electrons was confirmed using cathodoluminescence and transmittance spectra.
Abstract: This paper describes a strong piezoelectric carrier confinement at a ZnO/Zn0.6Mg0.4O heterointerface grown on an a-plane sapphire substrate by molecular beam epitaxy. A ZnO/Zn0.6Mg0.4O double-heterojunction structure was grown without intentional doping and the formation of a deep potential well for electrons was confirmed using cathodoluminescence and transmittance spectra. Photoluminescence spectra at 4.5 K consisted of an intense near-band-edge emission at 3.359 eV and a broad and weak peak on the low-energy side. The results of Hall effect measurement revealed that the conduction is n-type with a high carrier concentration of ?1.2?1013 cm-2. The mobilities are ?170 cm2/V?s at 300 K and ?400 cm2/V?s at 77 K without deterioration at lower temperatures; these values are much higher than those of a thick single-layer ZnO film grown on an a-plane sapphire substrate. We attribute these optical and electrical properties to the formation of two-dimensional electron gas at the ZnO/Zn0.6Mg0.4O heterointerface by the piezoelectric polarization in a strained ZnO well.
TL;DR: In this paper, the binding energy for the free-exciton-A in AlN was found to be nearly twice that in GaN and the electron effective mass was deduced by using the thermal quenching behavior of these emission lines.
Abstract: Cathodoluminescence experiments were performed on a high-quality AlN epitaxial film grown by organometallic vapor phase epitaxy on a large single crystal AlN substrate. The low-temperature near-bandedge spectra clearly show six very narrow lines. The thermal quenching behavior of these emission lines provides insight on how to assign them to free and bound exciton recombination processes. The binding energy for the free-exciton-A in AlN was found to be nearly twice that in GaN. The observation of the free-exciton-A first excited state permitted us to estimate its reduced effective mass and, by using recent reported values for the hole effective mass in Mg-doped AlN, the electron effective mass in AlN has been deduced.
TL;DR: In this article, Europium was implanted into GaN through a 10nm thick epitaxially grown AlN layer that protects the GaN surface during the implantation and also serves as a capping layer during the subsequent furnace annealing.
Abstract: Europium was implanted into GaN through a 10nm thick epitaxially grown AlN layer that protects the GaN surface during the implantation and also serves as a capping layer during the subsequent furnace annealing. Employing this AlN layer prevents the formation of an amorphous surface layer during the implantation. Furthermore, no dissociation of the crystal was observed by Rutherford backscattering and channeling measurements for annealing temperatures up to 1300°C. Remarkably, the intensity of the Eu related luminescence, as measured by cathodoluminescence at room temperature, increases by one order of magnitude within the studied annealing range between 1100 and 1300°C.
TL;DR: In this article, the authors used thermal sintering in argon and air atmospheres at temperatures ranging from 1100 to 1500 degreesC from powders of anatase and rutile phases.
Abstract: Polycrystalline samples of titanium oxide were prepared by thermal sintering, in argon and air atmospheres at temperatures ranging from 1100 to 1500 degreesC, from powders of anatase and rutile phases. The samples sintered in argon were further treated in oxygen at the temperature of 800 degreesC for intervals of time up to 8 h. The luminescence emission of the initial powders was situated in the green region, with the peak at 580 nm. The sintering treatment in argon leads to a decrease of the luminescence intensity that appears as a broad band peaked at 550 nm in the case of anatase and as a band peaked at 450 nm in the case of the rutile phase. The sintering treatment in air causes the quenching of the emission in the visible region. In both cases, a sharp and intense emission appears in the infrared region at 800 nm for rutile and 820 nm for the anatase phase. The blue emission is sensitive to the treatment in oxygen atmosphere which causes mainly an intensity increase of the band at 450 nm. The treatment in oxygen results in the reconstruction of the polycrystalline sample surface as hexagonal shape protrusions and large terraces.
TL;DR: In this paper, the average grain sizes of Tb 3+ :Y 3 Al 5 O 12 powders have been determined by X-ray powder diffraction measurements and the concentration dependence of the luminescence spectra and lifetimes have been investigated as a function of the YAG grains sizes.
TL;DR: In this paper, 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 determine whether quantum confinement effects could effectively explain the blueshifted emission.
Abstract: 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 bl...
TL;DR: Measurements of Boron nitride whiskers with a special structure show the specially structured BN emits strong UV and visible luminescences, which is a promising material for deep-blue and UV applications.
Abstract: Boron nitride whiskers with a special structure have been synthesized by a thermal reaction process. The as-prepared BN whiskers have a length of tens of micrometers and a mean diameter of 500 nm. High-resolution TEM analysis shows that the as-prepared BN whiskers can be described as a nanofiber-interweaved network. Infrared and electron energy loss spectra reveal that the BN whiskers are composed of both sigma-sp2 and sigma-sp3 chemical bonds. The UV-vis absorption spectrum displays the energy band gap of the BN whiskers and multiple fine absorption peaks of the phonon-electron coupling. Both photoluminescence (PL) and cathodoluminescence (CL) measurements show the specially structured BN emits strong UV and visible luminescences, which is a promising material for deep-blue and UV applications.
TL;DR: InxGa1−xN quantum dots have been fabricated by the selective growth of GaN micropyramid arrays topped with InGaN∕GaN quantum wells.
Abstract: InxGa1−xN quantum dots have been fabricated by the selective growth of GaN micropyramid arrays topped with InGaN∕GaN quantum wells. The spatially, spectrally, and time-resolved emission properties of these structures were measured using cathodoluminescence hyperspectral imaging and low-temperature microphotoluminescence spectroscopy. The presence of InGaN quantum dots was confirmed directly by the observation of sharp peaks in the emission spectrum at the pyramid apices. These luminescence peaks exhibit decay lifetimes of approximately 0.5ns, with linewidths down to 650μeV (limited by the spectrometer resolution).
TL;DR: In this article, thin films of CaB2O4:Eu were deposited on glass substrates by spray pyrolysis and the abnormal reduction of Eu3+ to Eu2+ in as-grown thin films was found at the growth temperatures from 300to500°C.
Abstract: Thin films of CaB2O4:Eu were deposited on glass substrates by spray pyrolysis. The abnormal reduction of Eu3+ to Eu2+ in as-grown thin films was found at the growth temperatures from 300to500°C. Blue cathodoluminescence was observed by annealing films in air, correlating with further abnormal reduction of Eu ions and crystallization of compounds. These phenomena were explained by an electron transfer mechanism. The chromaticity coordinates were x=0.193 and y=0.089 with a dominant wavelength of 455nm. Our results indicate calcium borate doped with Eu is a blue phosphor at temperatures suitable for the use of glass substrates in flat panel applications.
TL;DR: In this paper, the main emission bands appear centered at about 2.58, 2.25, and 1.94 eV depending on the sintering treatment, and the evolution of the luminescence bands with mechanical milling is explained by formation and recovery of defects during milling.
Abstract: Cathodoluminescence (CL) in the scanning electron microscope (SEM) has been used to investigate the luminescence mechanisms in tin oxide. Sintered material prepared from high purity powder has been found to show a strong dependence of the CL emission on the thermal treatments applied during sample preparation. SEM images show the presence of nano and microcrystalline grains. The correlation of the grain size and morphology with the optical emission is analyzed by CL microscopy and spectroscopy. The main emission bands appear centered at about 2.58, 2.25, and 1.94 eV depending on the sintering treatment. CL images reveal that the 2.25 and the 2.58 eV bands are associated at specific crystal faces. The evolution of the luminescence bands with mechanical milling shows a complex evolution of the 1.94 and 2.58 eV emissions which is explained by formation and recovery of defects during milling.
TL;DR: In this article, the optical properties of blue InGaN-based light-emitting diodes aged at high current levels have been studied by electroluminescence and cathodoluminecence.
Abstract: The optical properties of blue InGaN-based light-emitting diodes aged at high current levels have been studied by electroluminescence and cathodoluminescence. The onset of a broad optical band peaked at about 3.1 eV in devices aged without a heat sink (junction temperature higher than 300 °C) has been correlated to an electrothermal threshold effect. The band is attributed to the dissociation of Mg–H complexes inside the p-type layers and to the consequent formation of Mg-related metastable complexes acting as acceptors. Subsequent electron-beam irradiation determines the almost complete quenching of the band.
TL;DR: A high-density array of vertically aligned ZnGa(2)O(4) nanowires has been synthesized on Si substrates via CVD of ZnO-Ga at 1000 degrees C consisting of a single-crystalline cubic spinel structure grown in a [111] direction and exhibiting strong photolumineinescence and cathodoluminescence in the blue wavelength region.
TL;DR: The self-assembled growth of ordered ZnO nanowires on GaN/Si layers has been observed at the low temperature of 500°C through Zn evaporation and oxidation as discussed by the authors.
Abstract: The self-assembled growth of ordered ZnO nanowires on GaN/Si layers has been observed at the low temperature of 500 °C through Zn evaporation and oxidation. The nanowires have a nearly uniform diameter of 40 nm and length of nm. No metal catalyst was used. Interestingly, the nanowires grow on a wetting film of an interconnected vortex-like structure. An empirical model is proposed to explain the growth process. Spatially resolved cathodoluminescence (CL) measurements show a sharp and intense emission (I8 line) from the nanowires, while weak and redshifted luminescence from the wetting layer. The CL indicates the existence of tensile stress in the wetting layer while the nanowires are fully relaxed.
TL;DR: In this article, metal-organic vapor-phase epitaxy growth of 3J solar cells where the epitaxial Ga 0.44 In 0.56 P/Ga 0.92 In 0.08 As middle subcells are grown lattice-mismatched on a Ge substrate.
TL;DR: In this paper, the authors report on the study of ZnS and X-doped X-ZnS thin films, prepared by spray pyrolysis technique using chloride precursors, which consist of mixed hexagonal (α) and cubic (β) phases with a predominance of cubic phase.
Abstract: Here we report on the study of ZnS and X-doped ZnS (with 4 at% of X = Al, Sn) thin films, prepared by spray pyrolysis technique using chloride precursors. Cathodoluminescence imaging and spectroscopy, x-ray diffraction, x-ray energy dispersive spectrometry and spectrophotometry have been used for their characterization. Deposited at their optimal substrate temperature (Ts = 773 K), these films are polycrystalline and consist of mixed hexagonal (α) and cubic (β) phases with a predominance of the cubic phase. Their growth is preferentially oriented along the (111)β direction and their optical bandgap always remains close to 3.56 eV regardless of the sample considered. The cathodoluminescence spectra of ZnS and Al–ZnS films are similar and are characterized by a blue emission peak at 407 nm (3.05 eV) and a broad blue–green one located at 524 nm (2.36 eV) due to the presence of chlorine. The insertion of Sn2+ ions in the ZnS material leads to the formation of the SnCl2 compound and to the disappearance of the blue–green emission associated with Cl ionized donors.
TL;DR: In this article, a uniform growth of (1-101)GaN was performed on coalesced stripes of GaN which had been prepared by selective metalorganic vapor phase epitaxy on a 7° off-axis (001)Si substrate via an AlN intermediate layer.
Abstract: Uniform growth of (1-101)GaN was performed on coalesced stripes of GaN which had been prepared by selective metalorganic vapor phase epitaxy on a 7° off-axis (001)Si substrate via an AlN intermediate layer. The cathodoluminescence spectra at 4 K exhibited a donor bound excitonic emission at 358 nm followed by defect-related emission peaks at 363, 371, and 376 nm. The 363 and 376 emission bands are observed upon the coalescence region. The Hall measurements exhibited p-type conduction at 80–300 K (the hole carrier density 6.3×1012 cm−2 and hole mobility 278 cm2/V s at 100 K). The activation energy of the acceptor was estimated to be 60 meV. The possible origin of the p-type conduction is discussed in relation to the unintentionally doped carbon.
TL;DR: In this paper, a single-phase, insulating AlCrN is produced whose band gap shows a small (0.1 − 0.2eV) decrease from the value for undoped AlN (6.2 eV), a decrease in aplane lattice constant and the introduction of two absorption bands at 3 and 5eV into the band gap.
Abstract: Cr concentrations of ∼2at.% were incorporated into AlN during growth by molecular beam epitaxy. Under optimized conditions, single-phase, insulating AlCrN is produced whose band gap shows a small (0.1–0.2eV) decrease from the value for undoped AlN (6.2eV), a decrease in a-plane lattice constant and the introduction of two absorption bands at 3 and 5eV into the band gap. This material shows ferromagnetism with a Curie temperature above 300K as judged from the difference in field-cooled and zero-field-cooled magnetization. For nonoptimized growth, second phases of Cr2N and AlxCry are produced in the AlN and the material is conducting (∼1000Ωcm) with activation energy for conduction of 0.19eV and apparent band gap of 5.8eV.
TL;DR: In this paper, a cubic-structure yttrium aluminum garnet (YAG) phase was formed by the reaction of crystalline Y 2 O 3 and the aluminum component, and the prepared particles had spherical morphology.
Abstract: Europium-doped yttrium aluminum garnet (YAG) phosphor particles were prepared from mixed nitrate solutions by the FEAG (filter expansion aerosol generator) process. The crystallinity, morphology, and luminescence of the YAG:Eu particles were investigated. The prepared particles had an amorphous phase, which turned into phase-pure YAG particles after annealing above 1000°C. A cubic-structure YAG phase was formed by the reaction of crystalline Y 2 O 3 and the aluminum component. The prepared particles had spherical morphology. The mean size of the YAG:Eu particles increased from 0.45 to 1.0 μm when the overall solution concentrations were increased from 0.02 to 1.2 mol/L. The optimum doping concentration of europium for the maximum brightness of phosphor particles was 1.3 at.%. The cathodoluminescence (CL) intensity was strongly affected by the annealing temperatures. The maximum CL value of the particles was 55 cd/m 2 .
TL;DR: In this paper, a single ZnO nanowires of about 100 nm diameter was grown by a thermal vapor-liquid-solid (VL-solid) process and the low temperature spectra of the ZnOs showed three main peaks at 3.366, 3.362 and 3.358 eV.
Abstract: Cathodoluminescence (CL) investigations of selected single ZnO nanowires grown by a thermal vapor-liquid-solid process demonstrate their very good structural quality. The low temperature CL of the ZnO nanowires of about 100 nm diameter shows in the spectral range of the donor bound exciton three main peaks at 3.366, 3.362, and 3.358 eV. The full width at half maximum of the bound exciton peak is 1.1 meV for ZnO nanowires at 9 K. In comparison, the peak widths of epitaxial ZnO PLD thin films and a ZnO bulk single crystal (Eagle Picher) are 1.2 meV and 0.7 meV, respectively. In contradiction to ZnO nanowires grown by MOCVD at 750°C, the CL spectra of the thermally (915°C) grown ZnO nanowires show no additional peaks from donor or acceptor transitions or phonon replica.
TL;DR: In this article, the characteristics of V-defects in quaternary AlInGaN epilayers and their correlation with fluctuations of the In distribution are investigated, and the geometric size of the defect is found to depend on the In composition of the alloy.
Abstract: The characteristics of V-defects in quaternary AlInGaN epilayers and their correlation with fluctuations of the In distribution are investigated. The geometric size of the V-defects is found to depend on the In composition of the alloy. The V-defects are nucleated within the AlInGaN layer and associated with threading dislocations. Line scan cathodoluminescence (CL) shows a redshift of the emission peak and an increase of the half width of the CL spectra as the electron beam approaches the apex of the V-defect. The total redshift decreases with decreasing In mole fraction in the alloy samples. Although the strain reduction may partially contribute to the CL redshift, indium segregation is suggested to be responsible for the V-defect formation and has a main influence on the respective optical properties. (C) 2004 American Institute Of Physics.