Enhanced Emission Efficiency Research Articles

Enhanced emission efficiency due to an excited subband resonance in a GaAs-based quantum-well system

The emission energies and their intensity distributions of excitonic radiative recombination in a non-uniform GaAs-based quantum well (QW) system are investigated by spatially resolved cathodoluminescence (CL) spectroscopy. By measuring the spectrally discriminated CL intensities originating from the different QWs as a function of excitation position, we are able to map out how the compositional distribution of the alloy barriers affects the CL intensities of the respective QW exciton bands. Surprisingly intense high-energy emission is observed due to the presence of an unintentionally introduced interface layer formed in the barrier heterostructure, when it is in resonance with the excited subband transition of a neighboring QW.

InAs/GaAs quantum dots grown on different GaAs substrates with graded In x Ga1 − x As strain-reducing layer

Self-assembled InAs quantum dots with graded composition strain-reducing layer (SRL) grown on exact substrates were studied. It is shown that a graded InxGa1 − xAs SRL leads to growth quality improvement, emission efficiency enhancement, and wavelength blueshift. Samples grown on 2° misoriented substrates with different In contents in graded InxGa1 − xAs SRL were also investigated, and emission efficiency enhancement and wavelength blueshift were found when graded SRL was introduced and when the change rate of In content in graded InxGa1 − xAs SRL was enlarged.

Preparation and photoelectric properties of mesoporous ZnO films

Mesoporous ZnO films doped with Ti4+ (M-ZnO) have been prepared by doping process and sol–gel method. The films have mesoporous structures and consist of nano-crystalline phase, as evidenced from small angle X-ray diffraction and high resolution transmission electron microscopy. The wide angle X-ray diffraction of M-ZnO films confirms that M-ZnO has hexagonal wurtzite structure and ternary ZnTiO3 phases. Ultraviolet–visible transmittance spectra, absorbance spectra and energy gaps of the films were measured. The Eg of M-ZnO is 3.25 eV. Photoluminescence intensity of M-ZnO centered at 380 nm increases obviously with the excitation power, which is due to the doping process and enhanced emission efficiency. M-ZnO thin films display a positive photovoltaic effect compared to mesoporous TiO2 (M-TiO2) films.

Controlled synthesis and improved luminescent properties of (Gd 1−x,Eu x) 3GaO 6 phosphors fabricated via spray pyrolysis

Eu 3+ activated trigadolinium gallate, Gd 3GaO 6:Eu, was successfully synthesized using various aqueous solutions via ultrasonic spray pyrolysis. The corresponding phosphors illuminated an efficient red emission under Hg-discharge condition with enhanced color purity, which is related to prominent peaks centered at 615 and 629 nm induced by the electric dipole transition of 5D 0 → 7F 2 of Eu 3+. By using a proper Ga source for aqueous solution, especially gallium sulfate, uniformly distributed Gd 3GaO 6:Eu 3+ phosphor with controlled morphology and enhanced emission efficiency was obtained.

Enhanced photoluminescence caused by localized excitons observed in MgZnO alloy

Temperature-dependent photoluminescence of MgZnO alloy film has been studied, and it is found that the emission intensity increases significantly at a certain temperature range and then decreases when increasing temperature further. The anomalous increase is resulted from the localized excitons in MgZnO alloy, as revealed by the enhanced second-order longitudinal optical phonon in the Raman spectrum of the MgZnO film. A schematic model was suggested to depict the carrier transportation process in the MgZnO film considering the existence of localized exciton states. The results reported in this paper indicate that localized excitons in MgZnO alloy can result in greatly enhanced emission efficiency, which is eagerly wanted for the application of ZnO-based materials in high-efficiency light-emitting devices.

Blue emitting fluorophores of phenyleneethynylenes substituted by diphenylethenyl terminal groups for organic light-emitting diodes

Phenyleneethynylene motifs substituted by diphenylethenyl groups at both ends were prepared successfully by use of double elimination protocol of β-substituted sulfones for introducing phenyleneethynylene arrays followed by Wittig–Horner reaction for introducing diphenylethenyl moiety. The hybrid blue fluorophores exhibited strong emission even in the solid-state films ( Φ F ≧ 0.60) while, in CHCl 3 solution, incorporation of substituents on the central phenylene unit significantly enhanced emission efficiency up to Φ F = 0.57. The OLED devices with use of these blue fluorophores as an emitting material provided maximum external quantum efficiency of η ext = 2.4%.

Enhancing the emission efficiency in photonic crystal waveguide with coupled waveguide arrays

We propose a system of enhanced emission efficiency by employing coupled waveguide arrays to a single-mode photonic crystal waveguide. The FDTD simulation results show that, when the coupled waveguide arrays are placed properly at the exit plane of the single-mode photonic crystal waveguide, the output light can be split into many light beams, and the interference of these light beams can produce a focusing emission beam, which greatly enhances the emission efficiency. In addition, when the row number of the waveguide arrays is larger than a certain value (2N=8), the state of the emission is nearly steady and the most compact structure can be obtained. This type of photonic crystal has potential applications in near-field optics or integrated optics.

Enhanced efficiency and reduced spectral shift of green light-emitting-diode epitaxial structure with prestrained growth

The enhanced emission efficiency and reduced spectral shifts of a green InGaN/GaN quantum-well (QW) light-emitting-diode epitaxial structure by using the prestrained growth technique when compared with a control sample of the same emission spectrum with conventional growth are demonstrated. By adding an ∼7%-indium InGaN/GaN QW to the structure before the growth of designated emitting high-indium QWs, the growth temperature of the emitting QWs can be raised by 30 °C while keeping about the same emission wavelength around 544 nm in photoluminescence (PL) and 525 nm in electroluminescence (EL). The internal quantum efficiency, room-temperature PL intensity, and EL intensity at the injection current of 20 mA are increased by ∼167%, ∼140%, and ∼182%, respectively. Also, the spectral blueshift range in increasing injection current in the range of 50 mA is decreased by 46%. Based on the pump-power dependent PL measurement, it is found that the quantum-confined Stark effect (QCSE) becomes weaker in the prestrained growth sample. Also, from the calibration of the Arrhenius plots, the carrier localization effect is observed to become weaker under prestrained growth. Therefore, the enhanced emission efficiency is mainly attributed to the decreased defect density and the reduced QCSE in the prestrained sample.

Enhanced emission efficiency of GaN∕InGaN multiple quantum well light-emitting diode with an embedded photonic crystal

A photonic crystal (PC) structure of periodic SiO2 pillar cubic array is embedded in n-GaN layer of InGaN∕GaN multiple quantum well (MQW) blue (480nm) light-emitting diode (LED). The diameter, period, and depth of SiO2 pillar are 124±6, 230±10, and 130±10nm, respectively. The increments of 70% for external quantum efficiency, 17% for internal quantum efficiency, and 45% for light extraction efficiency from photoluminescence measurement, and 33% for optical output power at 20mA are observed for LEDs with an embedded PC layer. This improvement can be attributed to the increased extraction efficiency by PC effect as well as increased internal quantum efficiency due to the decrease of dislocation density in n-GaN layer because of an epitaxial lateral over-growth process.

Integrated ZnO nanotips on GaN light emitting diodes for enhanced emission efficiency

Enhancement of light extraction from an integrated ZnO nanotips/GaN light emitting diode (LED) is demonstrated. The device is composed of a GaN LED with a Ga-doped ZnO (GZO) transparent conductive layer and ZnO nanotips grown on GZO for light extraction. The light output power of a ZnO nanotips/GZO/GaN LED exhibits 1.7 times enhancement, in comparison with a conventional Ni∕Au p-metal LED. The higher emission efficiency is attributed to the enhanced light transmission and scattering in the ZnO∕GaN multilayer.

Single-mode room-temperature emission with a silicon rod lattice

The authors experimentally evidence an increase of light emission efficiency at room temperature in a silicon-on-insulator photonic crystal. The photonic crystal is made of a triangular lattice of silicon rods and operates as a single-mode light extractor. It exhibits a luminescence intensity two orders of magnitude higher than silicon-on-insulator substrate. In light of photoluminescence experiments, emission diagram measurements, and finite difference time domain calculations, they identify the different optical properties of the photonic crystal and they demonstrate the existence of at least a fivefold emission efficiency enhancement per surface unit.

Metamorphic quantum dot nanostructures for long wavelength operation with enhanced emission efficiency

In this work, we study the effect of additional InAlAs or GaAs barriers to enhance the confinement of carriers into quantum dots (QDs) in metamorphic QD nanostructures for long-wavelength light-emission. Such barriers are used in order to compensate for the reduction in band discontinuities that occurs when using InGaAs confining layers (CL) to red-shift the emission towards the spectral windows of optoelectronic interest; as a side effect, such a reduction causes a decrease of carrier confinement and then an inefficient emission at room temperature.By photoluminescence characterization of metamorphic QD structures grown by molecular beam epitaxy, we study the effect of barriers with different compositions, thicknesses and positions relative to QDs on the QD emission and the activation energy for confined carriers' thermal escape; our experimental data clearly show that the insertion of additional barriers results in larger emission efficiencies at room temperature and in blue-shifts of emission wavelengths.To counterbalance the blue-shift resulting from the use of these additional barriers, we introduce a red-shift by using the QD strain engineering approach, based on the reduction of QD strain, deriving from the decrease of the QD-CL mismatch; the combined use of QD strain engineering and enhanced barriers to confine carriers into QDs allows us to obtain room temperature emission up to 1.46 μm. These findings indicate that QD strain engineering with the use of additional barriers is a viable approach to achieve efficient room temperature emission from QD structures in the long wavelength spectral windows for telecom application.

Enhanced emission efficiency of InGaN films with Si doping

AbstractHigh emission efficiency of InGaN films were obtained with Si doping, and the influence of Si doping on structural and optical properties of InGaN epilayers with different Si concentration were investigated in detail by means of high‐resolution X‐ray diffraction (HRXRD), scanning electron microscope (SEM), Cathodoluminescence (CL) and photoluminescence(PL). V‐defects were observed on the surface of InGaN films, and further investigation by CL showed that V‐defects act as nonradiative center, which lower the emission efficiency of InGaN. Meanwhile, it was found that Si doping could improve the crystal quality of InGaN and suppress the formation of V‐defects in InGaN, and hence enhance the emission efficiency of InGaN. One possible mechanism of influence of Si doping on the formation of V‐defects in InGaN was also proposed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced emission efficiency in organic light-emitting diodes using deoxyribonucleic acid complex as an electron blocking layer

Enhanced electroluminescent efficiency using a deoxyribonucleic acid (DNA) complex as an electron blocking (EB) material has been demonstrated in both green- and blue-emitting organic light-emitting diodes (OLEDs). The resulting so-called BioLEDs showed a maximum luminous efficiency of 8.2 and 0.8cd∕A, respectively. The DNA-based BioLEDs were as much as 10× more efficient and 30× brighter than their OLED counterparts.

Effect of strain relaxation and exciton localization on performance of 350-nm AlInGaN quaternary light-emitting diodes

The optical and structural properties of AlInGaN quaternary single and multiple quantum-well structures have been investigated by means of photoluminescence and x-ray diffraction. This comparative study of single quantum-well (SQW) and multiple quantum-well (MQW) structures was carried out in terms of the exciton localization effect and the strain relaxation. A detailed analysis indicated that 13% strain relaxation occurs in the MQW compared to the SQW, which is assumed to be fully strained. Furthermore, the AlInGaN SQW structure showed a stronger localization effect than the MQW. Both these effects result in enhanced emission efficiency for the SQW structure, indicating that it is better suited as the active region for ultraviolet light-emitting diodes (UV-LEDs). Finally, the UV-LEDs with an emission wavelength of about 350nm based on such SQW and MQW active regions were grown. The output power of the SQW UV-LEDs is around 2.3 times higher than that of MQW UV-LEDs.

Enhanced Emission Efficiency and Excited State Lifetime Due to Restricted Intramolecular Motion in Silole Aggregates

The aggregation-induced emission (AIE) properties of 1,1,2,3,4,5-hexaphenylsilole (HPS) and poly{11-[(1,2,3,4,5-pentaphenylsilolyl)oxy]-1-phenyl-1-undecyne} (PS9PA) were studied by time-resolved fluorescence technique. The enhanced fluorescence and long fluorescent lifetime were obtained for the sample in an aggregate state as compared to the sample in solution. The time-decay of fluorescence of HPS and PS9PA in high viscosity solvents and low-temperature glasses has also been measured in detail to further investigate the possible mechanism for AIE. Enhanced light emission and long fluorescence lifetime were detected for both HPS and PS9PA in the solution-thickening and -cooling experiments. These results provided direct evidence that the enhanced photoluminescence (PL) efficiency is due to restricted intramolecular motion, which ascribes AIE to the deactivation of nonradiative decay caused by restricted torsional motions of the molecules in the solid state or aggregate form.

Fullerene-functionalized gold nanoparticles: electrochemical and spectroscopic properties.

Fullerene (C(60))-tethered gold nanoparticles were synthesized by the coupling of the fullerene molecules with peripheral amine moieties on the particle surface. The particle composition was determined by thermogravimetric analysis and FT-IR spectroscopy. The resulting particles exhibited unique optical and electrochemical properties. UV-visible measurements showed that the C(60) characteristic absorption remained practically invariant whereas the fluorescence demonstrated rather drastic enhancement of emission efficiency as compared to the behaviors of C(60) monomers. Tethering of C(60) on the particle surface has virtually no effect on the particle molecular capacitance when C(60) is in neutral state, whereas when C60 is electroreduced, the particle effective capacitance increases drastically, reflected in the quantized capacitance charging measurements. The strong affinity of C(60) to amine moieties was also exploited to assemble multilayers of C(60) and gold particle nanocomposite structures. Quartz crystal microbalance measurements showed quite efficient adsorption of C(60) and particles up to two repeated cycles. However, the voltammetric responses of the surface-confined C(60) and gold particle composite structures were found to be complicated by the inaccessibility of electrolyte counterions due to the compact nature of the surface assemblies.

III-Nitride Quantum Devices—Microphotonics

Photonic devices based on III-nitrides offer benefits such as UV/blue emission, large band offsets of InN/GaN/AlN heterostructures allowing novel quantum well (QW) device design, and inherently high-emission efficiencies. Furthermore, due to their mechanical hardness and larger band gaps (when compared with conventional semiconductor devices), III-nitride-based devices may operate at much higher temperatures and voltages/power levels for any dimensional configuration and in harsher environments than other semiconductor devices and are expected to provide much lower temperature sensitivities. These are crucial advantages for many applications.Over the last decade, the physics of microsize photonic devices has been investigated. New physical phenomena and properties are expected to dominate as the device size scales down. The microsize light emitters offer benefits over edge emitters such as the ability to create arrays of individually controllable pixels on a single chip, enhanced quantum efficiency, and greatly reduced lasing threshold. Rapid progress in the area of III-nitride microphotonics has been made. The growth and fabrication of micron and submicron size photonic structures based on III-nitride wide bandgap semiconductors has been achieved, and the technology has made it possible to integrate arrays of optical elements to form active photonic-integrated devices.One example is an interconnected µ-LED with enhanced emission efficiency over the conventional LEDs for the same device area. Another example is a µ-LED array with independently addressed pixels or III-nitride microdisplay. III-nitride microdisplay may offer performance that is superior to microdisplays fabricated from liquid crystals and organic LEDs. The third example presented is III-nitride UV Focal Plane Arrays (UV-FPA) of detectors. So far, the operation of AlGaN UV-FPA with size up to 256×256 pixels with 30×30 μm2 unit cells has been demonstrated. Together with the nature of their two-dimensional array, these active micro-photonic devices show promise in many important applications, such as optical communications, signal and image processing, optical interconnects, computing, enhanced energy conversion and storage, chemical, biohazard substances, and disease detection, missile and shellfire, atmospheric ozone-level, and flame sensing.III-nitride microlens arrays have been fabricated successfully for blue and UV wavelength applications on GaN and AlN. The successful fabrication of microlens arrays based on III-nitride materials opens the possibility for monolithically integrating nitride-based micro-size photonic devices, as well as coupling light into, out of, and between arrays of III-nitride emitters and detectors, especially for short wavelengths covering the green-blue to deep UV (200 nm) region.Nanofabrication and characterization of photonic crystals with diameter/periodicity as small as 100/180 nm on InGaN/GaN MQW has been achieved. An unprecedented maximum enhancement factor of 20 was obtained under optical pumping.Single-mode ridged optical waveguide devices using GaN/AlGaN heterostructures have been designed, fabricated, and characterized for operation in 1550 nm wavelength window. The feasibility of developing novel photonic integrated circuits based on III-nitride wide bandgap semiconductors for fiber-optical communications has been investigated.

Spin-dependent delayed luminescence from nongeminate pairs of polarons inπ-conjugated polymers

We measured cw photoinduced absorption (PA) and spin-$\frac{1}{2}$ PA- and photoluminescence detected magnetic resonance (PADMR and PLDMR, respectively) in films of two representative $\ensuremath{\pi}$-conjugated polymers; poly(phenylene-vinylene) and polythiophene. From the dependences of the polaron PA and PLDMR on the laser excitation intensity we conclude that the PLDMR is due to spin-dependent delayed luminescence from nongeminate pairs of polarons. Our findings imply that spin-dependent exciton formation in polymer light-emitting diodes leads to enhanced emission efficiency.

Epitaxial core-shell and core-multishell nanowire heterostructures.

Semiconductor heterostructures with modulated composition and/or doping enable passivation of interfaces and the generation of devices with diverse functions. In this regard, the control of interfaces in nanoscale building blocks with high surface area will be increasingly important in the assembly of electronic and photonic devices. Core-shell heterostructures formed by the growth of crystalline overlayers on nanocrystals offer enhanced emission efficiency, important for various applications. Axial heterostructures have also been formed by a one-dimensional modulation of nanowire composition and doping. However, modulation of the radial composition and doping in nanowire structures has received much less attention than planar and nanocrystal systems. Here we synthesize silicon and germanium core-shell and multishell nanowire heterostructures using a chemical vapour deposition method applicable to a variety of nanoscale materials. Our investigations of the growth of boron-doped silicon shells on intrinsic silicon and silicon-silicon oxide core-shell nanowires indicate that homoepitaxy can be achieved at relatively low temperatures on clean silicon. We also demonstrate the possibility of heteroepitaxial growth of crystalline germanium-silicon and silicon-germanium core-shell structures, in which band-offsets drive hole injection into either germanium core or shell regions. Our synthesis of core-multishell structures, including a high-performance coaxially gated field-effect transistor, indicates the general potential of radial heterostructure growth for the development of nanowire-based devices.