Fujia Zhang

Synthesis and characterization of tris-(8-hydroxyquinoline)aluminum

Abstract A comprehensive understanding of the characteristics of the organic electroluminescent (EL) material tris-(8-hydroxyquinoline)aluminum (Alq 3 ) is meaningful for organic light-emitting devices (OLEDs). Thin films of Alq 3 are among the most efficient EL molecular films known to date. Alq 3 sample was synthesized and purified. The purity of Alq 3 is up to 98.5%. The electronic structure and interaction in Alq 3 molecule have been studied. The results of a joint experimental investigation based on a combination of X-ray diffraction (XRD) spectra, hydrogen nuclear magnetic resonance (HNMR) spectra, infrared absorption (IRA) spectra, mass spectra (MS), X-ray emission (XRE) spectra and fluorescence spectra of Alq 3 have been obtained. The analysis on HNMR spectra of Alq 3 demonstrated that the coordinate interaction between Al 3+ and O ion is more intensive than that between Al 3+ and N ion. The AlO bond is covalent bond in Alq 3 molecule. Alq 3 has two types of geometrical isomers, namely mer isomer and fac isomer. IRA spectra of Alq 3 indicate a very intensive chelation of aromatic ring. From the MS spectra the ratio of charge to mass of Alq 3 was obtained. Fluorescence data confirmed the existence of a fluorescent species formed between Al ion and 8-hydroxyquinolinol.

Surface and interface analysis for copper phthalocyanine (CuPc) and indium-tin-oxide (ITO) using X-ray photoelectron spectroscopy (XPS)

Indium-tin-oxide (ITO) coated glass has been widely used as a hole injection electrode for organic light-emitting devices (OLEDs), but the work function of ITO film usually mismatches the highest occupied molecular orbital (HOMO) of the hole transport materials. Copper phthalocyanine (CuPc) has been used as a hole injection buffer to enhance the hole injection from ITO to the hole transport layer. A thin CuPc layer was thermally evaporated onto the ITO-coated glass substrate, and the surface and interface electron states of the CuPc/ITO close contact were measured and analyzed by X-ray photoelectron spectroscopy (XPS) technology. Results show that, in CuPc molecule, copper atom has a valence of +2 and interacts with nitrogen atoms through coordinate bonds. There are two kinds of carbon atoms: eight carbon atoms bonding with two nitrogen atoms and other 24 carbon atoms with an aromatic hydrocarbon character. The nitrogen atoms are also in two kinds of chemical environment: four nitrogen atoms only bond with two carbon atoms forming C-N=C bonds, and other four nitrogen atoms not only bond with carbon atoms but also bond with copper atom through coordinate bonds. Argon ion beam sputtering was used to study the interface characteristics of the CuPc/ITO contact. As sputtering time increases, the peaks of C 1s and N 1s spectra gradually become weaker, the peaks of Cu 2p, O 1s, In 3d and Sn 3d spectra get stronger. The core-levels of C 1s, N 1s, O 1s, In 3d and Sn 3d spectra all have chemical shifts towards higher or lower binding energy, but their behavior are different.

Surface and interface analysis of tris-(8-hydroxyquinoline) aluminum and indium-tin-oxide using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS)

Abstract An understanding of the surface and interface state of the organic material and the underlying anode material is meaningful for organic light-emitting devices (OLEDs). The morphology, surface and interface electron states of the tris-(8-hydroxyquinoline) aluminum (Alq 3 ) and the underlying indium–tin-oxide (ITO) have been investigated using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) technology. AFM observations indicated that the Alq 3 particles show a pin-like shape and are parallel to each other. XPS analysis of the Alq 3 /ITO surface suggests that the Al atoms have a valence of +3. The C atoms mainly bond to C, N and O atoms. The N and O atoms also interact with Al atoms through coordination bonds, in additional to bonding to C atoms. The O atoms basically originate from quinolate rings and the absorbed O 2 and water. Argon ion beam sputtering and XPS were used to analyze the interface between Alq 3 and ITO films. Results demonstrate that, as sputtering time increases, the core levels of Al 2p, C 1s, N 1s, O 1s and In 3d spectra all have chemical shifts to higher or lower binding energy, but the alteration of the peak shape, position and area of Al 2p, C 1s, N 1s, O 1s and In 3d spectra are different.

Organic light-emitting devices based on aromatic polyimide doped by electrophosphorescent material fac tris(2-phenylpyridine) iridium

We report on the performance of a fabricated organic light-emitting device employing the electrophosphorescent material, fac tris(2-phenylpyridine) iridium (Ir(ppy)3), doped into the new polyimide type of polymer, aromatic polyimide host. The luminance and the external quantum efficiency of the host emitter can be improved to ~50000 cd m−2 and ~7%, respectively, by doping the host with Ir(ppy)3. The photoluminescence (PL) and electroluminescence (EL) have been investigated. The EL spectra shift with the changes of the concentration of Ir(ppy)3. Bright green emission was observed from the EL devices. The current–voltage (I–V) characteristics suggest that, when the voltage is higher than the turn-on voltage, the current of the doped cell is higher than that of the undoped cell at the same voltage, indicating that the doping has an effect on the I–V characteristics.

Pb/Zn/Pd ohmic contacts to p-GaP

Low resistance ohmic contacts have been fabricated and analyzed to Zn-doped p-GaP using an annealed Pd/Zn/Pd metalization. It has been found that the Pd/Zn/Pd multilayer structure shows good ohmic characteristics after heat-treatment. The results show that the alloying temperature and alloying time are very important factors for the quantity of the ohmic contact layer. When the surface hole concentration of the p-GaP is 2 x 10(17) cm(-3), a specific contact resistance of 6 x 10(-5) Omega cm(-2) has been obtained at the optimum alloying temperature of 550 degrees C and the optimum alloying time of 3 min

AES and SIMS analyses of a low-resistance ohmic contact layer of GaP

Abstract On the basis of a study of the metallurgical properties and the interface characteristics of the ohmic contact layer formed by p-GaP with Pd/Zn/Pd metal films [F. Zhang, Z. Song and J. Peng, Appl. Surf. Sci. 62 (1992) 83], we further analyze the profile distribution and atomic binding state of the elements in the contact layer by AES and SIMS.

Metallurgical properties and characteristics of the Pd/Zn/Pd to p-type GaP ohmic contact interface layers

The metallurgical properties and characteristics of a Pd-based ohmic contact layer to p-type GaP have been studied by SEM XRD, EDAX and AES profiling measurements. The results show that (1) the alloying temperature is a very important factor for the quality of the ohmic contact layer; when the hole concentration of the p-type GaP is 2 × 1017 cm−3 a specific contact resistance of 6 × 10−5 ω cm−2 has been obtained at an alloying temperature of 550°C; (2) a layer with a M−p+−p structure is formed at the interface, the main metallurgical composition being ZnP2, Zn3P2 and (Ga4Pd7Zn3)14R. The correlation between the quality of the ohmic contact and the microstructure of the contact layer has also been discussed.

X-ray double crystal diffraction and Raman spectra measurements of AlP/GaP short-period superlattices

The growth quality and structure parameters of (AlP)n/(GaP)n short-period superlattices (n = 4,6) grown by MOVPE using TMGa, TMAl and PH3 were measured by x-ray double crystal diffraction. Based on this, Raman spectra of the samples were measured and analysed and they were studied by comparing with the Raman spectra of a GaP single crystal. For the samples, the double-phonon modes of the first-order Raman scattering all exist. Moreover, the second-order Raman scattering peaks are observed to exist.

X-ray photoelectron spectroscopy analyses of the surface and interface of Pd-based p-GaP ohmic contact layer

Abstract The surface and interface of the low resistance ohmic contact layers of Pd-based Pd/Zn/Pd multilayer to the p-type GaP have been analysed using X-ray photoelectron spectroscopy. The atom states and the electron chemical shift at the surface and interface of this contact system are given. The metallurgical properties of the contact layers are also discussed.

The investigation of growth characteristics of single quantum wells grown on (311) GaAs substrates

Abstract High quality AlxGa1−xAs/GaAs single quantum wells have been grown successfully on (311)A and (311)B Si-doped GaAs substrates by molecular beam epitaxy (MBE) at 600 °C and 650 °C. Photoluminescence (PL) spectra of these samples are measured at 1.8 K. The calculations of the PL peak energies agree with those of experiments, suggesting that the conduction band offset at the interface between AlxGa1−xAs and GaAs is 65% and that the electron effective mass in both AlxGa1−xAs and GaAs in growth temperature on (311)A and (311)B substrates is discussed. We find that the surface of GaAs in (311)B can capture impurities more easily than that in (311)A.