Cathode Luminescence Spectra Research Articles

Multi-color emission based on InGaN/GaN micro-truncated pyramid arrays

3D micro-nano devices are expected to become the mainstay of multi-color solid-state lighting in the future because of their broad-band characteristic and the advantage of integrating the monolithic light-emitting diode on a single chip. In this work, InGaN/GaN micro-truncated pyramid arrays with six equivalent (101̄1) semi-polar facets and one (0001) polar facet were successfully prepared by the metal-organic chemical vapor deposition technology. The average diameter of the obtained uniform micro-truncated pyramids was 6.8 µm with a height of 2.4 µm. According to the results of micro-photoluminescence performed, the InGaN/GaN micro-truncated pyramid arrays can achieve multi-color emission from blue to red. The luminescent positions corresponding to different wavelengths were detected by the cathode luminescence spectrum. The multi-color emission was related to the quantum hybrid structures apart from the discrepancy of In composition in different positions. The developed microstructure can create multi-color emission by combining distinct luminescence modes, which can aid in the design of future optoelectronic devices.

Heavy Ge Doping in GaN via Pulsed Sputtering to Tailor the Optical Bandgap Energy

The epitaxial growth of heavily Ge‐doped GaN films using pulsed sputtering deposition (PSD) on AlN (0001)/sapphire substrates is presented and the correlations among their structural, electrical, and optical properties are investigated. High‐quality Ge‐doped PSD‐GaN films are grown with electron concentrations of 0.33–4.4 × 1020 cm−3. Notably, cathode luminescence spectra reveal the absence of defect‐induced emissions associated with Ga vacancies (2.3–2.4 eV), consistent with the high electron mobilities of the films. As the electron concentration increases, the optical bandgap also increases. This correlation can be explained by combining the theoretical formulas for the Burstein–Moss shift and the bandgap renormalization effects. Remarkably, the optical bandgap reaches 3.84 eV at 4.4 × 1020 cm−3 carrier concentration, marking the highest value reported for the optical bandgap of GaN. These results indicate the potential of heavy Ge doping in GaN via PSD for applications in highly transparent conductive layers and tunneling junctions within GaN‐based optoelectronic devices.

The Mg2NiH4 film on nickel substrate: synthesis, properties and kinetics of formation

We studied the synthesis reaction of Mg2NiH4 hydride films when a nickel plate and magnesium hydride powder interacted in the presence of hydrogen at 450 °C. The hydrogen pressure (>6 MPa) was higher than the synthesis equilibrium pressure for both Mg2NiH4 and MgH2 hydrides.Film examination with scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and thermal desorption spectroscopy methods revealed that a dense polycrystalline film of Mg2NiH4 hydride grows on a thin (0.9 μm) underlayer of intermetallic alloy MgNi2. The synthesis time variation made it possible to create Mg2NiH4 films with a width of 0.2 to 4 μm. Based on analysis of the growth kinetics, the crystal structure of the material, and energies of chemical bonds between atoms, we show that the interface between MgNi2 and Mg2NiH4 is the most probable location of the growth reaction.We have obtained cathode luminescence spectra of the Mg2NiH4 hydride film. Mg2NiH4 is found to be an indirect gap semiconductor with a gap value of ∼1.6 eV. However, luminescence with quanta energy higher than the band gap is registered. We associate this fact with Van Hove singularities in the Mg2NiH4 density of states.

Structure and Luminescence Properties of InGaN/AlGaN Multiquantum‐Well Microrod Array Structures

The InGaN/AlGaN multiquantum-well microrod-array structure is grown on c-plane GaN template with SiO2 microholes patterned as a mask. The surface morphology exhibits the accurately ordered microrod-array structure. Temperature-dependent photoluminescence (PL) measurement indicates that the PL peaks redshift as the temperature increases. Room-temperature PL and cathode luminescence spectra show that the different positions on InGaN/AlGaN microrod arrays have different luminous peaks. The successful fabrication of InGaN/AlGaN microrod arrays is of great significance for the realization of multicolor emission and the design of new optoelectronic devices.

Performance enhancement of InGaN/GaN MQWs grown on SiC substrate with sputtered AlN nucleation layer

InGaN/GaN multiple quantum wells (MQWs) based light-emitting diode (LED) structures grown on silicon carbide (SiC) substrates with and without sputtered AlN nucleation layer (NL) were investigated. High resolution X-ray diffraction rocking curve results indicated an improvement of GaN crystalline quality by using sputtered AlN NL. The surface morphology improvement of the LED structures was characterized by atomic force microscope and the root-mean-square roughness decreased from 14.1 nm to 2.1 nm by using sputtered AlN NL. The obvious enhancement of luminescence property was identified by photoluminescence and cathode luminescence spectra. These results pave the way for potential applications in SiC based microwave power devices and light emitting devices.

Simultaneously Breaking the Double Schottky Barrier and Phonon Transport in SrTiO3-Based Thermoelectric Ceramics via Two-Step Reduction.

The low powder factor (PF) of polycrystalline oxide perovskites induced by the resistive grain boundaries or known as double Schottky barrier (DSB) greatly restricts their thermoelectric performance in application. Here, a general protocol including (i) powder and (ii) bulk reduction in H2/Ar forming gas is demonstrated to break the DSB in La and Nb codoped SrTiO3. While the powder reduction guarantees a high carrier concentration by fully stimulating the donor doping effect, the bulk reduction effectively lowers the DSB by influencing the point defects at grain boundaries, which is proved by the combination of cathode luminescence spectra and energy-dispersive X-ray spectroscopy in transmission electron microscopy. The Hall mobility can approach 10 cm2 V-1 s-1 after two-step reduction, which is similar to the level of single crystals. However, the Seebeck coefficient is not compromised, giving rise to high PF values up to 1.70 mW m-1 K-1 under proper reduction strength. Meanwhile, the reduction process also promotes mild precipitation of Nb nanoparticles, thus effectively lowering the lattice thermal conductivity by scattering phonons. As a result, a remarkable figure of merit reaching 0.4 at 700 K is obtained, which validates the two-step reduction as a reliable strategy toward "electron crystal-phonon glass" behavior in SrTiO3-based perovskites.

Effects of indium surfactant and MgN intermediate layers on surface morphology and crystalline quality of nonpolar a-plane AlGaN epi-layers

High quality non-polar a-plane AlGaN epi-layers with dual MgN interlayers were successfully grown on semi-polar r-plane sapphire substrates with the indium-surfactant-assisted metal organic chemical vapor disposition (MOCVD) technology, and characterized with atomic force microscopy, cathode luminescence (CL), and high-resolution X-ray diffraction rocking curve. It was found that both surface morphology and crystalline quality of the non-polar AlGaN films were strongly dependent on the mass flow of indium surfactant in the MOCVD growth process. In fact, the great suppression of the deep energy level impurity-related transitions in the CL spectra indicates a significant enhancement in crystalline quality for the non-polar AlGaN films. Moreover, with the optimization of the indium surfactant mass flow, a root mean square value as small as 10.9 nm was achieved, demonstrating a remarkable improvement in surface morphology for the a-plane AlGaN epi-layer.

Structural and magnetic–luminescent properties of carbon-doped aluminum oxide

Three-phase (corundum + δ phase + amorphous phase) amorphous–nanocrystalline powders of pure and carbon-doped Al2O3 (x C = 1.07–6.6 wt %) have been produced by pulsed electron beam evaporation in vacuum. The corundum hexagonal nanocrystals in the Al2O3–C nanopowder (x C = 1.07 wt %) had sizes about 5 nm. The carbon solubility boundary in the Al2O3 lattice was lower than 1.07 wt % C. The dependence of the form of the cathode-luminescence spectra and the phase compositions of the prepared Al2O3 and Al2O3–C nanopowders has been found. The absence of R lines of Cr3+ ions in photoluminescence spectra of doped nonopowders has been detected. All the nanopowders of the pure and C-doped Al2O3 were ferromagnets at room temperature with the maximum magnetization of ~0.12 emu/g at x C = 6.6 wt %.

On the synthesis, phase optimisation and luminescence of some rare earth pyrosilicates

This work concerns the synthesis, effect of fluxes on the formation of single phase materials, and spectroscopic characterization of rare earth pyrosilicates according to Ln2Si2O7 (with Ln = La, Lu, Y). It turned out that by the support of LiF as a flux it is possible to obtain a single phase of the polymorph Lu2Si2O7, La2Si2O7, and Y1,9La0,1Si2O7 compositions. Tauc-plots of the VUV reflection spectra of these compounds were used to derive the optical band gaps, which are in the range between 6.2 and 7.1eV. The single phase samples show strong photoluminescence in the UV and visible range upon excitation by VUV radiation or by electron bombardment. PL spectra have been measured by 160nm excitation and cathode luminescence (CL) spectra have been recorded at electron beam energies between 2 and 20keV, respectively. All measurements reveal that the pyrosilicates show strong defect luminescence, whereby the observed broad emission bands are assigned to different defect types.

A mechanism of ultraviolet activation of the α-AlH3 decomposition

We investigated the mechanism of activation of α-AlH3 powder decomposition by irradiation with ultraviolet (UV) light using barometry, photoluminescence (PL), cathode luminescence (CL) and X-ray diffraction (XRD) methods. Exposing the samples under a mercury lamp through the filters we have shown that only the line, corresponding to the energy of 4.88 eV, leads to activation, whereas the spectral lines of lower energy are ineffective. XRD shows that after continued UV activation the aluminum clusters with metallic properties are formed on the surface of the AlH3 powder particles at room temperature. The analysis of results of performed calculations of the absorption spectrum of the perfect crystal α-AlH3 using DFT method showed that hydrogen vacancies are involved in activation by UV light. Studying of dynamics of CL and PL spectra at different UV expositions permitted to suggest the following mechanism of activation. Absorption of a photon implies that vacancy captures an electron from a neighboring hydrogen atom. Hydrogen atom then can leave its regular position in the lattice and form a new vacancy next to the first. Clustering of vacancies occurs. Continued UV activation is accompanied by merging of vacancies, which deprives the aluminum atoms of hydrogen bonds connecting them. Thus the aluminum clusters with metallic properties are formed.

High pressure synthesis of BN and BCN crystals and their functionalization

Hexagonal BN (hBN) and cubic BN (cBN) are known as the representative crystal structures of BN. The former is chemically and thermally stable, and has been widely used as an electrical insulator and heat-resistant materials. The latter, which is a high-density phase, is an ultra-hard material second only to diamond. Recently, some progresses in the synthesis of high purity BN crystals were achieved by using Ba-BN as a solvent material at high pressure crystal growth [1]. Band-edge natures (cBN Eg=6.2eV and hBN Eg=6eV) were characterized by their optical properties. The key issue to obtain high purity crystals is to reduce oxygen and carbon contamination in the growth circumstances. It should be emphasized that hBN exhibits attractive potential for deep ultraviolet (DUV) light emitter [2 ] and also superior properties as substrate of graphene devices [3]. In this study, the effect of carbon impurity in BN is investigated. Three types of experimental approaches were carried out; (1) synthesis of high purity hBN single crystals and its characterization with respect to residual carbon, (2) high temperature solid state diffusion of carbon into hBN and its characterization, and (3) high temperature annealing of turbostratic B-C-N(t-BCN) compound under high pressure. t-BCN flakes obtained by chemical vapour deposition process was annealed near 3000C and 2GPa so as to become well crystallized.At annealing near 3000C at 2GPa with graphite, carbon incorporation of 1E21/cm3 in hBN was achieved with exhibiting totally different Cathode Luminescence spectra feature with high purity hBN crystals. Since major carbon contribution may affect the crystal structure of the 2-D layers stacking in hBN system, phase stability of BCN ternary phase will be introduced by the experimental results of high temperature annealing. Furthermore, effect of carbon impurity upon the synthesis of wurtzite BN from highly crystalline hBN via martensitic phase transformation will also be introduced.

Size-induced effect on cathode luminescence spectra of CsI(Na) and CsI(Tl) crystals

We investigated the cathode luminescence characteristics of CsI(Na) and CsI(Tl) crystals by the spectrum and structure properties at room temperature. We fabricated three different sizes of CsI(Na) and CsI(Tl) crystals and measured their luminescence spectra under cathode rays. We found that CsI(Na) cathode luminescence peaks appear at 420 and 305 nm, and CsI(Tl) cathode luminescence peaks are 540 and 410 nm, the grain size affects CsI(Na) luminescence significantly, and the Na-related 420 nm luminescence intensified relatively when the average grain size reaches ∼20 μm, which becomes weak when the grain size is down to nano-scale. But the cathode luminescence spectra of CsI(Tl) crystals with different size have no obvious changes. Our explanations for these phenomena are that the different impurities in the same host material CsI lead to different luminescence mechanisms. These cathode luminescence characteristics indicate the suitability of CsI(Na) and CsI(Tl) crystals to match photomultiplier tube for large area crystal detector development.

Structural and Optical Characterization of Eu<SUP>3+</SUP> Doped <I>β</I>-Ga<SUB>2</SUB>O<SUB>3</SUB> Nanoparticles Using a Liquid-Phase Precursor Method

Eu3+ doped beta-Ga2O3 and non-doped beta-Ga2O3 nanoparticles were synthesized at 800 degrees C using a liquid-phase precursor (LPP) method, with different annealing times and Eu3+ ion concentrations. Eu3+ doped beta-Ga2O3 nanoparticles showed broad XRD peaks, revealing a second phase compared with the non-doped beta-Ga2O3 nanoparticles. The cathode luminescence (CL) spectra of beta-Ga2O3 and Eu3+ doped beta-Ga2O3 nanoparticles showed a broad band emission (300-500 nm) of imperfection and two component emissions. The luminescence quenching properties of Eu3+ dopant ion concentration appeared gradually beyond 5 mol% in our investigation.

Effect of iron doping on the properties of nanopowders and coatings on the basis of Al2O3 produced by pulsed electron beam evaporation

Multiphase nanopowders (NPs) and amorphous/amorphous-nanocrystalline coatings (A-NC) have been prepared by the evaporation of ceramic targets of Al2O3-Fe2O3 (0.1, 3, 5 Fe2O3 mass %) by a pulsed electron beam in vacuum. The specific surface area of NP Al2O3-Fe2O3 reached 277 m2/g. The α and γ phases Al2O3 and other nonidentified phases have been found in the composition of NP Al2O3-Fe2O3. All coatings contained an insignificant fraction of the crystalline γ phase. No secondary phases on the basis of iron have been revealed. According to transmission electron microscopy, the fine fraction of NP Al2O3-Fe2O3 consists of amorphous nanoparticles of an irregular and quasispherical shape no more than 10 nm in size which form agglomerates reaching 1.5 μm. A large fraction of NPs consists of crystal spherical nanoparticles with preferential sizes of about 10–20 nm. All NP Al2O3-Fe2O3 showed ferromagnetic behavior at room temperature. The maximum magnetic response has been established in NPs with a minimum iron content (1.1 mass %). The pulsed cathode luminescence spectra of coatings and NP Al2O3-Fe2O3 have been presented by a wide band in the wavelength range of 300–900 nm regardless of their phase composition. Phase transformations into NP AL2O3-1.1% Fe and coatings from undoped Al2O3 heated to 1400°C occur according to the following scheme: amorphous phase → γ → δ → θ → α, regardless of their initial phase composition. The threshold of thermal stability of the Γ phase in NPs and the coating of undoped Al2O3 does not exceed 830°C. For the first time, the increased thermo and optically stimulated luminescent response comparable with the response of the leading TLD-500K thermoluminescent dosimeter has been reached in A-NC coatings of undoped Al2O3.

Synthesis of boron-doped diamonds by a static high-pressure process

High-quality boron-doped diamonds were synthesized from four boron-doped carbon sources with different boron concentration using a static high-pressure process with metal solvent. Three diamond pellets obtained from the carbon sources with boron concentrations less than the solubility limit showed a homogeneous appearance and obvious X-ray diffraction patterns characteristic of diamond. The electrically-conducting diamond with 0.2 mass% of boron showed the boron-bound exciton peak characteristic of boron doping in the cathode luminescence spectra, indicating its ability as a semiconductor. [TANSO 2013 (No. 257) 95–102]

Optical band-gap of TiO2 nanopowders doped with Al2O3

ABSTRACTOptical band-gap and cathode luminescence (CL) properties of anatase TiO2 nanopowders mixed with γ- Al2O3 powders by planetary ball mill were evaluated as a function of a powder mass ratio (x=Al2O3/TiO2) of 0 to 0.5 and their correlation with XRD spectra was also investigated. The optical band-gap of TiO2 increased from 3.36 eV to 3.41eV with increasing milling time (tm) up to 600 min, which was in good agreement with the blue shifts observed in the CL spectra with increasing tm and it was interpreted as a quantum size effect. In addition, the optical band-gap of TiO2 powders mixed with Al2O3 with tm=60min greatly increased from 3.36 eV to 3.48 eV with increasing x up to x=0.5. On the other hand, the optical band-gap of all the powders was decreased by annealing at temperatures above 600°C, which was evidenced by the XRD spectra to be due to the growth of grain size.

Growth and evaluation of self-standing CVD diamond single crystals on off-axis (001) surface of HP/HT type IIa substrates

As part of the development of diamond radiation detectors for energy spectrometry, quality improvement of CVD diamond single crystals was conducted mainly by measures of single diamond substrates. To suppress stress resulting from unconformity of lattice spacing between a substrate and a grown layer, which sometimes causes breakage, HP/HT type IIa diamond single-crystal substrates were adopted, and it resulted successfully. In addition an off-axis (001) surface was fabricated on the substrate to reduce abnormal growth. A lift-off method was adopted to reuse the HP/HT type IIa substrate with an off-axis (001) surface. Judging from peaks caused by free exciton recombination in cathode luminescence spectra, growth conditions more strongly affected the diamond crystal quality than substrates of any kind. Energy resolution of 0.6% for alpha particles was obtained by one CVD diamond single crystal. Charge collection efficiency of approximately 100% and 95% for holes and electrons was achieved. Step bunching caused by residual gas was observed on the grown CVD diamond surface. From the perspective of charge carriers' transportation measurement, the reduction of residual gas and optimization of growth conditions are indispensable.

Photo- and radioluminescence of lithium hafnate Li2HfO3

Photoluminescence, excitation and pulse cathode luminescence spectra, and decay kinetics of lithium hafnate Li2HfO3 were investigated at temperatures 300 and 7.5K. Time-resolved luminescence spectra were excited with synchrotron radiation in UV–VUV range. Strong emission with the peak position at 4.42eV and with the decay time in μs range was revealed at T=7.5K. The emission was attributed to radiative decay of self-trapped excitons (STE). The features observed in the excitation spectra at 6.6eV were likely to take place because of the exciton formation, with inter-band transitions dominating at 6.7eV. Crystal structure defects underpin the luminescence spectra formation at room temperature in the frame of vacancy model.

Impurity incorporation in semipolar (1-1 0 1) GaN grown on an Si substrate

Carbon doping was investigated in (1-1 0 1) GaN, where the topmost surface is terminated by nitrogen. Intentional doping was performed supplying C2H2 as the source precursor during the MOVPE growth under N2 ambient. The sample showing p-type conduction exhibited a strong but broad peak at 361 nm in a low-temperature cathode luminescence spectrum. FTIR analyses were performed on these samples and a new absorption peak at 945 cm−1 was found along with an AlN–A1(LO) phonon mode at 888 cm−1. SIMS analyses showed unintentionally highly doped Al in the sample. The 945 cm−1 peak was assigned to a local vibration mode related to an Al–C bond.

Growth of Gallium Oxide Nanowires by Pulsed Laser Deposition

We report on the synthesis of gallium oxide nanowires by pulsed laser deposition using a gold catalyst. In the vapor-liquid-solid process, gold thickness was the crucial parameter for deciding the morphology of nanowires. In the case of 1 nm thick gold, homogeneous nanowire growth was confirmed at temperatures of 700°C to 850°C. Transmission electron microscopy and selected area electron diffraction measurements showed that the nanowires were polycrystalline. In the cathode luminescence spectra, UV, blue, green and red emission peaks were observed, as reported in previous studies. As growth temperature was increased, the relative intensities of blue, green, and red emissions decreased. Thermal annealing treatments were effective in decreasing the blue, green and red emission peaks, suggesting that these emission peaks were associated with oxygen vacancies.