Effects of the morphologies and structures of light-emitting layers on the performance of organic electroluminescent devices

Organic electroluminescent devices with a structure of ITO/ploy (9-vinylcarbazole)/tris (8-hydroxyquinoline) aluminum (Alq3)/Mg:Ag are fabricated at different substrate temperatures (77, 298, and 438 K) during Alq3 deposition. It is found that the surface morphologies of Alq3 thin films greatly affect the I-V characteristics of the devices by the contact area between metal cathode and light-emitting layer. There is an increase in the luminous efficiency of the devices in the order 77 K < 298 K < 438 K. We attribute this trend to different structures of Alq3 thin films. (C) 2001 American Institute of Physics.

Synthesis and high efficiency green light-emitting diode of a soluble polymer

A soluble polymer emitting green color with high efficiency was synthesized. Bright green electroluminescence devices, both single layer and multilayer, were fabricated. The luminous efficiency was improved dramatically. Carrier injection from the electrodes to the emissive layer and concomitant green electroluminescence from the emissive layer were observed. A luminance of 920 cd/m(2) and luminous efficiency of 5.35 1m/W were achieved at a drive voltage of 15 V for the multilayer device. (C) 1997 Elsevier Science S.A.

A collinear light-emitting diode-induced fluorescence detector for capillary electrophoresis

A novel fluorescence detector based on collinear scheme using a brightness light-emitting diode emitting at 470 nm as excitation source is described. The detector is assembled by all-solid-state optical-electronic components and Coupled with capillary electrophoresis using on-column detection mode. Fluorescein isothiocyanate (FITC) and FITC-labeled amino acids and small molecule peptide as test analyte were used to evaluate the detector. The concentration limit of detection for FITC-labeled phenylalanine was 10 nM at a signal-to-noise ratio (S/N) of 3. The system exhibited good linear responses in the range of 1 x 10(-7) to 2 x 10(-5) M (R-2 = 0.999). (c) 2004 Elsevier B.V. All rights reserved.

Large Scale Fabrication Of Periodical Bowl-Like Micropatterns Of Single Crystal Zno

Nanostructured ZnO materials are of great significance for their potential applications in photoelectronic devices, light-emitting displays, catalysis and gas sensors. In this paper, we report a new method to produce large area periodical bowl-like micropatterns of single crystal ZnO through aqueous-phase epitaxial growth on a ZnO single crystal substrate. A self-assembled monolayer of polystyrene microspheres was used as a template to confine the epitaxial growth of single crystal ZnO from the substrate, while the growth morphology was well controlled by citrate anions. Moreover, it was found that the self-assembled monolayer of colloidal spheres plays an important role in reduction of the defect density in the epitaxial ZnO layer. Though the mechanism is still open for further investigation, the present result indicates a new route to suppress the dislocations in the fabrication of single crystal ZnO film. A predicable application of this new method is for the fabrication of two-dimensional photonic crystal structures on light emitting diode surfaces.

Multicolor luminescence in oxygen-deficient Tb3+;doped calcium aluminogermanate glasses

In this paper, we report on the multicolor luminescence in oxygen-deficient Tb3+-doped calcium aluminogermanate glasses. A simple method was proposed to control oxygen-deficient defects in glasses by adding metal Al instead of the corresponding oxide (Al2O3), resulting in efficient blue and red emissions from Tb3+-undoped glasses with 300 and 380 nm excitation wavelengths, respectively. Moreover, in Tb3+-doped oxygen-deficient glasses, bright three-color (sky-blue, green or yellow, and red) luminescence was observed with 300, 380, and 395 nm excitation wavelengths, respectively. These glasses are useful for the fabrication of white light-emitting diode (LED) lighting.

An experimental study about the influence of well thickness on the electroluminescence of InGaN/GaN multiple quantum wells

The influence of well thickness on the electroluminescence (EL) of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition is investigated. It is found that the peak wavelength of EL increases with the increase of well thickness when the latter is located in the range of 3.0-5.1 nm. The redshift is mainly attributed to the quantum confined Stark effect (QCSE). As a contrast, it is found that the EL intensity of InGaN/GaN MQWs increases with the increase of well thickness in spite of QCSE. The result of X-ray diffraction demonstrates that the interface become smoother with the increase of well thickness and suggests that the reduced interface roughness can be an important factor leading to the increase of EL intensity of InGaN/GaN MQWs. (C) 2009 Elsevier B.V. All rights reserved.

Mechanism on Effect of Surface Plasmons Coupling with InGaN/GaN Quantum Wells: Enhancement and Suppression of Photoluminescence Intensity

The photoluminescence (PL) intensity enhancement and suppression mechanism on surface plasmons (SPs) coupling with InGaN/GaN quantum wells (QWs) have been systematically studied. The SP-QW coupling behaviors in the areas of GaN cap layer coated with silver thin film were compared at different temperatures and excitation powers. It is found that the internal quantum efficiency (IQE) of the light emitting diodes (LEDs) varies with temperature and excitation power, which in turn results in anomalous emission enhancement and suppression tendency related to SP-QW coupling. The observation is explained by the balance between the extraction efficiency of SPs and the IQE of LEDs

Effects of the morphology of ZnO/Ag interface on the surface-plasmon-enhanced emission of ZnO films

The effects of the surface morphology of Ag on the surface-plasmon-enhanced emission of ZnO films have been studied for a ZnO/Ag/Si system by photoluminescence spectroscopy and atomic force microscopy. The results indicate that the enhancement of ZnO ultraviolet emission is dependent on the deposition conditions of the Ag interlayers. By examining the dependence of the enhancement ratio of surface-plasmon-mediated emission on the characteristic parameters of Ag surface morphology, we found that the surface plasmon coupling to light is determined by both the Ag particle size and density.

Sixfold symmetry of excitonic transition energies in c-plane for wurtzite GaN

The optical properties of the strained wurtzite GaN are investigated theoretically within the nearest neighbor tight-binding method. The piezoelectric effect is also taken into account. The empirical rule has been used in the strained band-structure calculation. The results show that the excitonic transition energies are anisotropic in the c-plane in a high electronic concentration system and have a 60 degrees periodicity, which is in agreement with experiment. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3001937]

Enhanced electroluminescence intensity of InGaN/GaN multi-quantum-wells based on Mg-doped GaN annealed in O-2

InGaN/GaN multi-quantum-well blue (461 +/- 4 nm) light emitting diodes with higher electroluminescence intensity are obtained by postgrowth thermal annealing at 720 C in O-2-ambient. Based on our first-principle total-energy calculations, we conclude that besides dissociating the Mg-H complex by forming H2O, annealing in O-2 has another positive effect on the activation of acceptor Mg in GaN. Mg can be further activated by the formation of an impurity band above the valence band maximum of host GaN from the passivated Mg-Ga-O-N complex. Our calculated ionization energy for acceptor Mg in the passivated system is about 30 meV shallower than that in pure GaN, in good agreement with previous experimental measurement. Our model can explain that the enhanced electroluminescence intensity of InGaN/GaN MQWs based on Mg-doped p-type GaN is due to a decrease in the ionization energy of Mg acceptor with the presence of oxygen. (C) 2008 American Institute of Physics.

Fabrication and characterization of GaN-based LEDs grown on nanopatterned sapphire substrates

Sapphire substrates were nanopatterned by dry (inductively coupled plasma, ICP) etching to improve the performance of GaN-based light-emitting diodes (LEDs). GaN-based LEDs on nanopatterned sapphire substrates (NPSS) were fabricated by metal organic chemical vapor deposition (MOCVD). The characteristics of LEDs fabricated on NPSS prepared by dry etching were studied. The light output power and wall-plug efficiency of the LEDs fabricated on NPSS were greater than those of the conventional LEDs fabricated on common planar sapphire substrates when the injection currents were the same. The LEDs on NPSS and common planar sapphire substrates have similar I-V characteristics.

Fabrication of nano-patterned sapphire substrates and their application to the improvement of the performance of GaN-based LEDs

Nano-patterned sapphire substrates (NPSSs) were fabricated by a chemical wet etching technology using nano-sized SiO2 as masks. The NPSS was applied to improve the performance of GaN-based light emitting diodes (LEDs). GaN-based LEDs on NPSSs were grown by metal organic chemical vapour deposition. The characteristics of LEDs grown on NPSSs and conventional planar sapphire substrates were studied. The light output powers of the LEDs fabricated on NPSSs were considerably enhanced compared with that of the conventional LEDs grown on planar sapphire substrates.

Energy band alignment of SiO2/ZnO interface determined by x-ray photoelectron spectroscopy

Thin SiO2 interlayer is the key to improving the electroluminescence characteristics of light emitting diodes based on ZnO heterojunctions, but little is known of the band offsets of SiO2/ZnO. In this letter, energy band alignment of SiO2/ZnO interface was determined by x-ray photoelectron spectroscopy. The valence band offset Delta E-V of SiO2/ZnO interface is determined to be 0.93 +/- 0.15 eV. According to the relationship between the conduction band offset Delta E-C and the valence band offset Delta E-V Delta E-C=E-g(SiO2)-E-g(ZnO)-Delta E-V, and taking the room-temperature band-gaps of 9.0 and 3.37 eV for SiO2 and ZnO, respectively, a type-I band-energy alignment of SiO2/ZnO interface with a conduction band offset of 4.70 +/- 0.15 eV is found. The accurate determination of energy band alignment of SiO2/ZnO is helpful for designing of SiO2/ZnO hybrid devices and is also important for understanding their carrier transport properties. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3204028]

Nonpolar growth and characterization of InN overlayers on vertically oriented GaN nanorods

Nanostructured hexagonal InN overlayers were heteroepitaxially deposited on vertically oriented c-axis GaN nanorods by metal-organic chemical vapor deposition. InN overlayers grown in radial directions are featured by a nonpolar heteroepitaxial growth mode on GaN nanorods, showing a great difference from the conventional InN growth on (0001) c-plane GaN template. The surface of InN overlayers is mainly composed of several specific facets with lower crystallographic indices. The orientation relationship between InN and GaN lattices is found to be [0001](InN) parallel to [0001](GaN) and [1100](InN)parallel to[1100](GaN). A strong photoluminescence of InN nanostructures is observed. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3177347]

The junction temperature and forward voltage relationship of GaN-based laser diode

In GaAs-based light-emitting diode (LED) or laser diode (LD), the forward voltage (V) will decrease linearly with the increasing junction temperature (T). This can be used as a convenient method to measure the junction temperature. In GaN-based LED, the relationship is linear too. But in GaN-based LD, the acceptor M (g) in p-GaN material can not ionize completely at-room temperature, and the carrier density will change with temperature. But we find finally that, this change won't lead to a nonlinear relationship of V-T. Our experiments show that it is Linear too.