S.Y. Liu

White light emission induced by confinement in organic multiheterostructures

A technique for inducing white-light emission from organic multiheterostructures is proposed. The configuration of organic multiheterostructure white-light emitting diodes is ITO/TPD(50 nm)/BePP2(5 nm)/TPD(4 nm)/BePP2:rubrene(5 nm)/TPD(4 nm)/Alq3(10 nm)/Al. Triphenyldiamine derivative (TPD) is used as a hole-transporting layer and the potential barrier layers. Blue fluorescent phenylpyridine beryllium (BePP2), orange fluorescent rubrene, and green fluorescent aluminum complex (Alq3) are used as three primary colors. BePP2 and BePP2 doped with rubrene act as the potential wells sandwiched between TPD barrier layers, in which excitons are confined. Alq3 is used as an electron-transporting green-color emitter. The white-light emission spectrum covers a wide range of the visible region, and the Commission Internationale de l’Eclairage coordinates of the emitted light are (0.32, 0.38) at 9 V. The maximum brightness and luminous efficiency of this device are 4000 cd/m2 (at 17 V) and 0.4 lm/W, respectively.

Synthesis of ultrafine gallium nitride powder by the direct current arc plasma method

Ultrafine gallium nitride (GaN) powder has been synthesized by the dc arc plasma method through the reaction of metal gallium (Ga) with the mixture gas of nitrogen ( N2) and ammonia ( NH3). The analyses of the produced powder by x‐ray diffraction, transmission electron microscopy, and selected‐area electron diffraction showed that the GaN particles in wurtzite structure consisted of nanometer‐sized polycrystals and monocrystals. The conversion of Ga to GaN was determined by the mixture ratio of NH3 and N2 in the mixture gas. The morphology of the GaN particles was mainly hexagonal with the size about 20–200 nm. When heated in air or nitrogen atmosphere, the thermostablity of the GaN powder was different.

Efficient red electroluminescence from organic devices using dye-doped rare earth complexes

Using rare earth complexes with electron-transporting properties as host materials, and 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as a dopant, bilayer doped electroluminescent (EL) devices with efficient red light emission were fabricated. When a europium complex was adopted, the EL spectrum consisted of emissions from DCJTB and Eu3+ ions. At optimal dopant concentration, an EL efficiency of 5.7 cd/A at 0.04 mA/cm2 was observed. Although the EL efficiency decreased with an increase in current density, it remained higher than 2.0 cd/A with brightness of 347.0 cd/m2 at 5.7 V bias. DCJTB and Eu3+ ions collected the energy of singlet and triplet excitons, respectively, and then gave rise to pure red color emission, suggesting a promising way by which to utilize both singlet and triplet excited states in EL devices.

Vibrational analysis of oxygen-plasma treated indium tin oxide

Abstract High-resolution electron-energy-loss spectroscopy (HREELS) showed that the dominant feature in the HREELS spectrum of the ‘as-received’ ITO was the loss peaks associated with the CH x group. Oxygen-plasma treatment of ITO led to the disappearance of the CH x loss peaks, and a concomitant increase in the intensity of the phonon peaks at 71 and 134 meV. The result indicates the removal of the CH x group and additional oxidation of the ITO surface by the oxygen-plasma treatment. Both processes are proposed to be responsible for the increase in work function of ITO by the oxygen-plasma treatment.

Role of ytterbium and ytterbium/cesium fluoride on the chemistry of poly(9,9-dioctylfluorene-co-benzothiadiazole) as investigated by photoemission spectroscopy

Since ytterbium (Yb) possesses a low work function of 2.6 eV and Yb fluoride generally has a high negative free energy of formation, it is conceivable to use Yb, either directly or parasitically, with a metal fluoride, as a cathode in organic light-emitting diodes (OLEDs). In this work, the electronic structure and chemistry at the interface of Yb/poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) were investigated by ultraviolet and x-ray photoemission spectroscopy (UPS and XPS, respectively). We found that the deposition of Yb on top of F8BT foremost formed organometallic complexes with the sulfur but interacted partly with the nitrogen of F8BT, and eventually formed a Yb–C complex at higher Yb coverages. In the UPS spectra, Yb deposition increased the relative peak intensities corresponding to the σ-bonds originated from the aliphatic side chains, implying that some of the π-conjugated carbons in the polymer backbone may be destroyed. These results agree well with the disappearance of the π-to-π* tra...

Formation and characterization of (553)B In0.15Ga0.85As/GaAs quantum wire structure

Strained In0.15Ga0.85As/GaAs quantum wires (QWRs) are naturally formed on a (553)B-oriented GaAs substrate by molecular beam epitaxy. The QWR structure with a corrugated upper interface and a flat lower interface is directly confirmed by atomic force microscopy. The average lateral period and step height of (553)B QWRs are revealed to be about 25 and 1.6 nm, respectively. Accordingly the density of (553)B QWRs is as high as 4.0×105 cm−1, which is several times larger than that of QWR arrays fabricated by the finest lithography. Moreover, the (553)B QWRs are intensively studied by photoluminescence (PL). In comparison with the reference (100) In0.15Ga0.85As/GaAs quantum wells, the PL peaks of the (553)B QWRs show slight redshifts and strong polarization dependence at 12 K, indicating a two-dimensional carrier confinement effect due to the corrugated structure. The narrow full width at half maximum of the PL peak at λ=861 nm is as small as 9.2 meV, which indicates the high uniformity and good optical qualit...

Band structures and characteristics of InGaAs/InGaAsP strain-compensated quantum well lasers

Analysis is performed for valence band structures and some characteristics of InGaAs/InGaAsP strain-compensated quantum well lasers lattice-matched to InP substrate. The computed results show that band offsets are functions of strain compensation instead of constants; strain compensation changes the band structures and the density of states, and hence affects the optical gain and the threshold current density. Under the condition of zero net strain, the values of the well width, cavity length and relative threshold carrier density and threshold current density are determined for realization of 1.55 μm wavelength emission.

Self-organized (553)BIn0.15Ga0.85As/GaAs quantum-wire field-effect transistors

Extremely uniform and high-density In0.15Ga0.85As/GaAs quantum wires (QWRs) were naturally formed on a (553)B-oriented GaAs substrate by molecular-beam epitaxy. The density of the QWRs is as high as 4.0×105 cm−1. The strong photoluminescence peak at λ=868 nm from the (553)B QWRs shows a large polarization anisotropy [p=(I∥−I⊥)/(I∥+I⊥)=0.22] and a very small full width at half maximum of 9.2 meV at 12 K. Based on the modulation-doped (553)B QWR structure, self-organized QWR field-effect transistors were fabricated (the channel along the QWRs’ direction). The devices demonstrate very good saturation characteristics and pinch-off behavior at room temperature. A maximum transconductance (gm) of 135 mS/mm is measured for 2 μm gate-length devices.