Influence of growth parameters on the properties of InGaN/GaN multiple quantum well grown by metalorganic chemical vapor deposition

The effects of growth parameters such as barrier growth time, growth pressure and indium flow rate on the properties of InGaN/GaN multiple quantum wells (MQWs) were investigated by using photoluminescence (PL), high resolution X-ray diffraction (HRXRD), and atomic force microscope (AFM). The InGaN/GaN MQW structures were grown on c-plane sapphire substrate by using metalorganic chemical vapor deposition. With increasing barrier growth time, the PL peak energy is blue-shifted by 18 nm. For InGaN/GaN MQW structures grown at different growth pressures, the PL intensity is maximized in the 300 Torr – grown structure, which could be attributed to the improved structural quality confirmed by HRXRD and AFM results. Also, the optical properties of InGaN/GaN MQW are strongly affected by the indium flow rate.     

Thermo,Iono and photoluminescence properties of 100 MeV Si7+ ions bombarded CaSiO3:Eu3+ nanophosphor

This paper reports on the thermo (TL), iono (IL) and photoluminescence (PL) properties of nanocrystalline CaSiO₃:Eu³⁺ (1–5 mol %) bombarded with 100 MeV Si⁷⁺ ions for the first time. The effect of different dopant concentrations and influence of ion fluence has been discussed. The characteristic emission peaks ⁵D₀→⁷F_J (J=0, 1, 2, 3, 4) of Eu³⁺ ions was recorded in both PL (1×10¹¹–1×10¹³ ions cm^?2) and IL (4.16×10¹²–6.77×10¹² ions cm^?2) spectra. It is observed that PL intensity increases with ion fluence, whereas in IL the peaks intensity increases up to fluence 5.20×10¹² ions cm^?2, then it decreases. A well resolved TL glow peak at ～304 °C was recorded in all the ion bombarded samples at a warming rate of 5 °C s^?1. The TL intensity is found to be maximum at 5 mol% Eu³⁺ concentration. Further, TL intensity increases sub linearly with shifting of glow peak towards lower temperature with ion fluence.     

Defect related luminescence in Hg0.2Cd0.8Te nano- and micro-crystals grown by the solvothermal method

The photoluminescence (PL) properties of nano- and micro-crystalline Hg_1?xCd_xTe (x≈0.8) grown by the solvothermal method have been studied over the temperature range 10–300 K. The emission spectra of the samples excited with 514.5 nm Ar⁺ laser consist of five prominent bands around 0.56, 0.60, 0.69, 0.78 and 0.92 eV. The entire PL band in this NIR region is attributed to the luminescence from defect centers. The features like temperature independent peak energy and quite sensitive PL intensity, which has a maximum around 50 K is illustrated by the configuration coordinate model. After 50 K, the luminescence shows a thermal quenching behavior that is usually exhibited by amorphous semiconductors, indicating that the defects are related to the compositional disorder.     

Photoluminescence properties of Er-doped β-FeSi2 grown by ion implantation

Photoluminescence (PL) properties of Er-doped β-FeSi₂ (β-FeSi₂:Er) and Er-doped Si (Si:Er) grown by ion implantation were investigated. In PL measurements at 4.2 K, the β-FeSi₂:Er showed the 1.54 μm PL due to the intra-4f shell transition of ⁴I_13/2→⁴I_15/2 in Er³⁺ ions without a defect-related PL observed in Si:Er. In the dependence of the PL intensity on excitation photon flux density, the obtained optical excitation cross-section σ in β-FeSi₂:Er (σ=7×10⁻¹⁷ cm²) is smaller than that in Si:Er (σ=1×10^-15 cm²). In the time-resolved PL and the temperature dependence of the PL intensity, the 1.54 μm PL in β-FeSi₂:Er showed a longer lifetime and larger activation energies for non-radiative recombination (NR) processes than Si:Er. These results revealed that NR centers induced by ion implantation damage were suppressed in β-FeSi₂:Er, but the energy back transfer from Er³⁺ to β-FeSi₂ was larger than Si:Er.     

Modifications in physical,optical and electrical properties of tin oxide by swift heavy Au8+ ion bombardment

Tin oxide thin films were deposited by a novel technique called as modified-SILAR. The preparative parameters were optimized to obtain good quality thin films. As-deposited films were annealed in O₂ atmosphere for 1 h at 500 °C. The annealed films were irradiated using Au⁸⁺ ions with energy of 100 MeV at different fluencies of 1 × 10¹¹, 1 × 10¹², 5 × 10¹² and 1 × 10¹³ ions/cm² using tandem pelletron accelerator. The irradiation-induced modifications in tin oxide thin films were studied using XRD, AFM, optical band gap, photoluminescence and I–V measurements. XRD studies showed formation of tin oxide with tetragonal structure. AFM revealed uniform deposition of the material with increase in grain size after irradiation. Decrease in band gap from 3.51 eV to 2.82 eV was seen with increases in fluency. A decrease in PL intensity, and an additional peak was observed after irradiation. I–V measurements showed a decrease in resistance with fluency.     

Optical properties,luminescence quenching mechanism and radiation hardness of Eu-doped GaN red powder phosphor

We report on the luminescence quenching mechanism of Eu-doped GaN powder phosphor produced with a low-cost, high yield rapid-ammonothermal method. We have studied as-synthesized and acid rinsed Eu-doped GaN powders with the Eu concentration of ～0.5 at.%. The Eu-doped GaN photoluminescence (PL) was investigated with 325 nm excitation wavelength at hydrostatic pressures up to 7.7 GPa in temperature range between 12 K and 300 K. The room temperature integrated Eu³⁺ ion PL intensity from acid rinsed material is a few times stronger than from the as-synthesized material. The temperature dependent PL studies revealed that the thermal quenching of the dominant Eu³⁺ ion transition (⁵D₀ → ⁷F₂) at 622 nm is stronger in the chemically modified phosphor indicating more efficient coupling between the Eu³⁺ ion and passivated GaN powder grains. Furthermore, it was found that thermal quenching of Eu³⁺ ion emission intensity can be completely suppressed in studied materials by applied pressure. This is due to stronger localization of bound exciton on Eu³⁺ ion trap induced by hydrostatic pressure. Furthermore, the effect of 2 MeV oxygen irradiation on the PL properties has been investigated for highly efficient Eu-doped GaN phosphor embedded in KBr–GaN:Eu³⁺ composite. Fairly good radiation damage resistance was obtained for 1.7 × 10¹² to 5 × 10¹³ cm^?2 oxygen fluence. Preliminary data indicate that Eu-doped GaN powder phosphor can be considered for devices in a radiation environment.     

Evaluation of correlation between chemical dosimetry and subharmonic spectrum analysis to examine the acoustic cavitation

Currently several therapeutic applications of ultrasound in cancer treatment are under progress which uses cavitation phenomena to deliver their effects. There are several methods to evaluate cavitation activity such as chemical dosimetry and measurement of subharmonic signals. In this study, the cavitation activity induced by the ultrasound irradiation on exposure parameters has been measured by terephthalic acid chemical dosimetry and subharmonic analysis. Experiments were performed in the near 1 MHz fields in the progressive wave mode and effect of duty cycles changes with 2 W/cm² intensity (I_SATA) and acoustic intensity changes in continuous mode on both fluorescence intensity and subharmonic intensity were measured. The dependence between fluorescence intensity of terephthalic acid chemical dosimetry and subharmonic intensity analysis were analyzed by Pearson correlation (p-value < 0.05). It has been shown that the subharmonic intensity and the fluorescence intensity for continuous mode is higher than for pulsing mode (p-value < 0.05). Also results show that there is a significant difference between the subharmonic intensity and the fluorescence intensity with sonication intensity (p-value < 0.05). A significant correlation between the fluorescence intensity and subharmonic intensity at different duty cycles (R = 0.997, p-value < 0.05) and different intensities (R = 0.985, p-value < 0.05) were shown. The subharmonic intensity (μW/cm²) significantly correlated with the fluorescence intensity (count) (R = 0.901; p < 0.05) and the fluorescence intensity due to chemical dosimetry could be estimated with subharmonic intensity due to subharmonic spectrum analysis. It is concluded that there is dependence between terephthalic acid chemical dosimetry and subharmonic spectrum analysis to examine the acoustic cavitation activity.     

Luminescence enhancement by the reduction–oxidation synthesis in monoclinic RE2O3 (RE=Eu,Gd) phosphors containing Eu3+ activator

A novel synthesis was developed for enhanced luminescence in sesquioxide phosphors containing Eu³⁺ activator. It consisted of two annealing steps: reduction under vacuum with gaseous H₂ at 10 Torr and 1300 °C and re-oxidation at 300–1500 °C in air. The integrated luminescence intensity of the monoclinic Eu₂O₃ phosphor was enhanced ca. 21 times by this method compared with conventional processing. The photoluminescence (PL) intensity was maximized at re-oxidation temperatures of 500–1100 °C. The PL characteristics of monoclinic Eu₂O₃ and Gd₂O₃:0.06Eu samples were compared with a commercial cubic Y₂O₃:Eu phosphor. The evolution of physical characteristics during the two-step annealing was studied by Raman spectroscopy, XPS, XRD, PL decay analysis, and SEM. PL decay lifetime increased proportionally to the PL intensity over the range 0.5–100 μs. Additional vibrational modes appeared at 490, 497, and 512 cm^?1 after the two-step annealing. The increase in PL intensity was ascribed to the formation of excess oxygen vacancies and their redistribution during annealing. Resonance crossovers between the charge transfer state and the emitting ⁵D_J states are discussed in relation to reported luminescence saturation mechanisms for oxysulfides Ln₂O₂S:Eu³⁺ (Ln=Y, La).     

Growth-morphology-luminescence correlation in ZnO-containing nanostructures synthesized in different media

Zinc hydroxide particles were prepared by a two-step process employing zinc nitrate hexahydrate, urea, ethylene glycol, water and p-toluene-sulfonic acid monohydrate (p-TSA). We used different concentrations of the reactants as well as different volume ratios of the solvents. ZnO particles were obtained by thermal treatment of the reaction products at two different temperatures: 350 °C and 500 °C. The samples were characterized by scanning field emission electron microscopy (SEM), X-ray diffraction (XRD) spectroscopy, BET analysis, thermogravimetry (TG) analysis and photoluminescence (PL) spectroscopy. It was found that after the thermal treatment particles become smaller, with the p-TSA concentration strongly affecting the morphology of the particles. Luminescence properties of the samples probed by PL at 8 K and room temperature exhibited a remarkable correlation with specimens′ nanomorphology. Luminescent features at ～2.0–2.2 eV, ～2.4–2.5 eV, ～2.65 eV, ～2.9 eV, ～3.0–3.1 eV and ～3.3 eV were observed in most specimens, although their relative intensity and temperature dependence were specific to an individual group of samples vis-à-vis their growth history and morphology.     

Luminescence and defect studies of YAlO3:Dy3+, Sm3+ single crystals exposed to 100 MeV Si7+ ion beam

Ionoluminescence (IL) and photoluminescence (PL) spectra for different rare earth ions (Sm³⁺ and Dy³⁺) activated YAlO₃ single crystals have been induced with 100 MeV Si⁷⁺ ions with fluence of 7.81×10¹² ions cm^?2. Prominent IL and PL emission peaks in the range 550–725 nm in Sm³⁺ and 482–574 nm in Dy³⁺ were recorded. Variation of IL intensity in Dy³⁺ doped YAlO₃ single crystals was studied in the fluence range 7.81×10¹²–11.71×10¹² ions cm^?2. IL intensity is found to be high in lower ion fluences and it decreases with increase in ion fluence due to thermal quenching as a result of an increase in the sample temperature caused by ion beam irradiation. Thermoluminescence (TL) spectra were recorded for fluence of 5.2×10¹² ions cm^?2 on pure and doped crystals at a warming rate of 5 °C s^?1 at room temperature. Pure crystals show two glow peaks at 232 (Tg₁) and 328 °C (Tg₂). However, in Sm³⁺ doped crystals three glow peaks at 278 (Tg₁), 332 (Tg₂) and 384 °C (Tg₃) and two glow peaks at 278 (Tg₁) and 331 °C (Tg₂) in Dy³⁺ was recorded. The kinetic parameters (E, b s) were estimated using glow peak shape method. The decay of IL intensity was explained by excitation spike model.     

Thermoluminescence,photoluminescence and EPR studies on Mn2+ activated yttrium aluminate (YAlO3) perovskite

Thermoluminescence (TL) measurements were carried out on undoped and Mn²⁺ doped (0.1 mol%) yttrium aluminate (YAlO₃) nanopowders using gamma irradiation in the dose range 1–5 kGy. These phosphors have been prepared at furnace temperatures as low as 400 °C by using the combustion route. Powder X-ray diffraction confirms the orthorhombic phase. SEM micrographs show that the powders are spherical in shape, porous with fused state and the size of the particles appeared to be in the range 50–150 nm. Electron Paramagnetic Resonance (EPR) studies reveal that Mn ions occupy the yttrium site and the valency of manganese remains as Mn²⁺. The photoluminescence spectrum shows a typical orange-to-red emission at 595 nm and suggests that Mn²⁺ ions are in strong crystalline environment. It is observed that TL intensity increases with gamma dose in both undoped and Mn doped samples. Four shouldered TL peaks at 126, 240, 288 and 350 °C along with relatively resolved glow peak at 180 °C were observed in undoped sample. However, the Mn doped samples show a shouldered peak at 115 °C along with two well defined peaks at ～215 and 275 °C. It is observed that TL glow peaks were shifted in Mn doped samples. The kinetic parameters namely activation energy (E), order of kinetics (b), frequency factor (s) of undoped, and Mn doped samples were determined at different gamma doses using the Chens glow peak shape method and the results are discussed in detail.     

Improved photoluminescence of BaMgAl10O17 blue phosphor prepared by spray pyrolysis

BaMgAl₁₀O₁₇:Eu²⁺ (BAM) blue phosphor particles with improved photoluminescence (PL) intensity under vacuum ultraviolet (VUV) excitation were prepared by a spray pyrolysis process. In order to improve the PL intensity, Er³⁺ and Nd³⁺ ions were used as co-doping elements. The VUV characteristics of BAM:Eu²⁺, M⁺ (M=Er, Nd) were monitored with varying the Er³⁺ and Nd³⁺ content in order to find the optimal co-doping concentration when they were prepared by spray pyrolysis. It was found that doping Er³⁺ or Nd³⁺ enhances the PL intensity of BAM:Eu²⁺ blue phosphor particles. In particular, the M³⁺ doping effect on the PL intensity was pronounced when the prepared BAM:Eu²⁺, M³⁺ particles were excited by 172 nm VUV. The maximum intensity was obtained when the M³⁺ content was 1.0 at% with respect to Ba element. The PL intensity of BAM:Eu²⁺, M⁺ (M=Er³⁺, Nd³⁺) particles was also further improved by producing them in a spherical shape, which was successfully achieved by controlling the spray solution. The optimized BAM:Eu²⁺, M⁺ particles had about 10% higher PL intensity than that of the commercial particles, which are made by a conventional solid-state reaction.     

Highly efficient flexible organic light-emitting devices utilizing F4-TCNQ/m-MTDATA multiple quantum well structures

Flexible organic light-emitting devices (FOLEDs) based on multiple quantum well (MQW) structures, which consist of alternate layers of 2,3,5,6-Tetrafluoro-7,7,8,8,-tetracyano-quinodimethane (F4-TCNQ) and 4,4′,4″-tris-(3-methylphenylphe-nylamino)tripheny-lamine (m-MTDATA) have been fabricated. The Alq₃-based device with double quantum well (DQW) structure exhibits the remarkable electroluminescent (EL) performances for the brightness of 23,500 cd/m² at 14 V and the maximum current efficiency of 7.0 cd/A at 300.3 mA/cm², respectively, which are greatly improved by 114% and 56% compared with the brightness of 10,958 cd/m² at 14 V and the maximum current efficiency of 4.5 cd/A at 174.0 mA/cm² for the conventional device without MQW structures. These results demonstrate that the EL performances of FOLEDs could be greatly improved by utilizing the novel MQW structures, and the reason for this improvement has also been explained by the effect of interfacial dipole and interfacial doping between F4-TCNQ and m-MTDATA in this article.     

Optical and electrical properties of Ge-implanted SiO2 layers on n-Si and p-Si

Ge ions of 100 keV were implanted into a 120 nm-thick SiO₂ layer on n-Si at room temperature while those of 80 keV were into the same SiO₂ layer on p-Si. Samples were, subsequently, annealed at 500°C for 2 h to effectively induce radiative defects in the SiO₂. Maximum intensities of sharp violet photoluminescence (PL) from the SiO₂/n-Si and the SiO₂/p-Si samples were observed when the samples have been implanted with doses of 1×10¹⁶ and 5×10¹⁵ cm⁻², respectively. According to current–voltage (I–V) characteristics, the defect-related samples exhibit large leakage currents with electroluminescence (EL) at only reverse bias region regardless of the type of substrate. Nanocrystal-related samples obtained by an annealing at 1100°C for 4 h show the leakage at both the reverse and the forward region.     

Structure and site selective luminescence of sol–gel derived Eu:Sr2SiO4

Strontium silicate (Sr₂SiO₄) samples doped with varying Eu³⁺content were prepared via sol–gel route and characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), dynamic light scattering (DLS) and photoluminescence (PL) techniques. The synthesis temperature could be brought down to 600 °C for formation of a singe phase sample. The concentration of the dopant ion and the temperature of annealing were optimized for maximum PL intensity. The critical energy-transfer distance for the Eu³⁺ ions was evaluated based on which the quenching mechanism was verified to be a multipole–multipole interaction. Based on the time-resolved emission data (TRES), it was inferred that, two different types of Eu³⁺ ions were present in the matrix. The first type was a long lived species (τ=4.7 ms) present at 10-coordinated ‘Sr’sites and the other was a short lived species (τ=1.2 ms) present at the 9-coordinated ‘Sr’sites which gets selectively excited at 296 nm. Judd–Ofelt (J–O) intensity parameters for both the species were evaluated. The trend observed for the two species in the J–O parameters, Ω₂ and Ω₄ were different confirming their existence in two different environments. The color coordinates of the system were evaluated and plotted on a CIE index diagram. Commercial utility of the phosphor was investigated by comparing it with commercial red phosphor.     

Effect of phosphorus irradiation on the structural,electrical, and optical characteristics of ZnO thin films

Phosphorus irradiation at a low energy (50 keV) and at a dosage of 8×10¹⁴ ions/cm² was carried out on 〈002〉 ZnO films grown by using a pulsed laser deposition technique (Sample A). Subsequent rapid thermal annealing at 650 °C and 750 °C was performed to remove defects resulting from the irradiation (samples B and C, respectively). Atomic force microscopy was used to determine the root mean square roughness, which was 10.07, 8.66, and 9.31 nm for samples A, B, and C, respectively. Low-temperature photoluminescence measurements revealed increased deep-level defect peaks following irradiation; however, the subsequent annealing minimized the defects. Although the dominant donor-bound exciton peak verifies the n-type conductivity of the films, the free–electron–to–acceptor and donor-to-acceptor pair peaks in the irradiated samples confirm an increase in acceptor concentration.     

Synthesis and photoluminescent behavior of Eu3+-doped alkaline-earth tungstates

Eu³⁺-doped alkaline-earth tungstates MWO₄ (M=Ca²⁺, Sr²⁺, Ba²⁺) were prepared by a polymeric precursor method based on the Pechini process. The polymeric precursors were calcined at 700 °C for 2 h in order to obtain well-crystallized powders and then characterized by X-ray diffraction (XRD), thermogravimetric analysis (TG), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy and photoluminescence spectroscopy (PL). All prepared samples showed a pure crystalline phase with scheelite-type structure confirmed by XRD. It was noted that the charge-transfer band shifted from 260 to 283 nm when calcium is replaced by strontium. However, this band was not observed for Eu³⁺-doped barium tungstate. Upon excitation at 260 nm, the emission spectra are dominated by the red ⁵D₀→⁷F₂ transition at 618 nm. By analyzing of the emission lines, it was inferred that Eu³⁺ ions occupy low symmetry sites in the host lattice. It was also found that Eu³⁺-doped SrWO₄ displays better chromaticity coordinates and greater luminescence intensity than the other samples.     

Synthesis,characterization and studies of radiative properties on Eu3+-doped ZnAl2O4

Eu³⁺-doped ZnAl₂O₄ phosphors were successfully synthesized in air atmosphere at 900 °C. The phosphors were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermally stimulated luminescence (TSL) and photoluminescence (PL) techniques. The average particle size of the system as determined from SEM was found to be 100–150 nm (for samples annealed at 900 °C). PL spectra of the doped phosphors showed emission peaks corresponding to Eu³⁺ ions. Lifetime studies revealed Eu³⁺ ions to be in two different sites. The asymmetric ratio (I₆₁₆/I₅₉₂) was observed to be about 3.75. This suggested that Eu³⁺ ion entered the host mainly substituting Al³⁺ site distorting the local environment and is partly located on surface of the phosphors. A prominent glow peak at 430 K was observed in the TSL of γ-irradiated Eu³⁺-doped ZnAl₂O₄ phosphors. Trap parameters for this peak have been determined and the probable mechanism for the glow peak is proposed. CIE chromaticity coordinates for the system were evaluated. It was observed that, the system could be employed as a potential red emitting phosphor. Commercial utility of the phosphor was investigated by comparing it with commercial red phosphor. The PL intensity of the as prepared phosphors was 63% of that of the commercial phosphor. Apart from this, various radiative properties such as the Judd–Ofelt intensity parameters, spontaneous emission probabilities, luminescence branching ratios, radiative lifetimes and quantum efficiency were evaluated for the system.     

Luminescent vacuum ultraviolet spectroscopy of Cr3+ ions in nanostructured aluminum oxide

Impurity Cr³⁺ centers in submicron and nanostructured Al₂O₃ crystals of different phase compositions at temperatures of 300 and 7.5 K were studied by a luminescent vacuum ultraviolet (VUV) spectroscopy method. Photoluminescence (PL) spectra and the energies of ²E, ⁴T₂, and ⁴T₁ excited states of Cr³⁺ ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al₂O₃ nanoscale crystals is formed by intracenter transitions (2.5–5.5 eV region), by charge transfer band (6.9 eV) and by effective formation of impurity-bound excitons (9.0 eV region). Such impurity-bound excitons correspond to O₂p→Al₃s electron transition in surroundings of an impurity Cr³⁺ center. The efficiency of impurity-bound excitons formation decreases with the increase of the grain size above 100 nm. The size dependence is noticeably shown in PL excitation spectra in VUV region. Excitons bound to impurity centers do not appear in nanostructured δ+θ-Al₂O₃ crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al₂O₃ at an excitation energy above 19 eV (more than 2E_g).     

Time-resolved spectroscopy in LiCaAlF6 doped with Cr3+ as a function of pressure and temperature: Excited-state crossover and phase-transition effects

This work investigates phase transition (PT) and excited-state-crossover (ESCO) effects on the photoluminescence (PL) properties of LiCaAlF₆: Cr³⁺. The structural requirements for changing the Cr³⁺ PL behavior from a broad-band emission at 1.59 eV (781 nm) at ambient conditions, to ruby-like narrow-line emission at 1.87 eV (663 nm) are analyzed in the 0–35 GPa range. We report a PL study on LiCaAlF₆: Cr³⁺ by means of time-resolved emission as a function of pressure and temperature. In particular we focus on the PL variations occurring around the pressure-induced trigonal-to-monoclinic first-order PT in LiCaAlF₆ at 7 GPa.