Papers

AVS1996 Session FP-ThM: Electroluminescent Displays

Thursday, October 17, 1996 9:00 AM in Room 204B

Thursday Morning

Start	Invited?	Item
9:00 AM		FP-ThM-3 Electroluminescent Materials and Displays R. Mauch (Heinrich-Hertz-Institut Berlin Gmbh, Ag Elektrolumineszenz, Germany) Recently, thin film electroluminescence research is mainly directed towards the realization of full color displays. Therefore, various new types of luminescent materials are under investigations, in particular, such being able to generate blue emission. The most promising candidates belong to alkaline-earth compounds which are doped with rare earth ions acting as luminescence centers. Two different compound types are from main interest. The first is the group of alkaline-earth-sulfides with the most prominent candidate SrS doped with Ce. The second are the alkaline-earth thiogallates doped with Ce. In this group SrGa\sub 2\S\sub 4\:Ce is the phosphor with the best blue color coordinate. Not only new luminescent materials are under investigations but also different display concepts where the respective phosphors are applied to. The classical approach is the structured phosphor concept where R, G and B are generated by dedicated luminescent materials. Since the thiogallates are emitting a pure blue, they are perfectly matched to the requirements of this approach. In contrary, the blue-green emitting SrS:Ce combined with the yellowish-red emitting ZnS:Mn is more adequate for the so-called color by white approach. Here, the emission of a non-structured white emitting luminescent material is subtractively filtered by a color filter matrix. Hence, the color filter determines the pixel color. The recent achievements of basic phosphor research as well as the status of the respective display concepts are summarized. The advantages and drawbacks of the different concepts are discussed. In addition, latest monochrome, multi- and full color developments are introduced.
9:40 AM		FP-ThM-5 Effects of Annealing and Stress on the SrS:Ce Thin Film Electroluminescent Phosphor P. Rack (University of Florida, Gainesville); S. Sun, T. Nguyen (Planar Systems, Inc.); P. Holloway (University of Florida, Gainesville) SrS:Ce thin films have been rf magnetron sputter deposited at 300C and subsequently annealed in a nitrogen ambient at various temperatures. Brightness-Voltage (B-V) curves reveal that the threshold voltage decreases and the brightness increases with increasing annealing temperature. Furthermore, the emission color demonstrates a blue-shift in the CIE color coordinates with increasing annealing temperature. X-ray diffraction analysis of the thin films show an increase in the SrS peak intensities and a decrease in the full-width-at-half-maximum (FWHM) which suggest that the crystal quality is improved with annealing temperature. Thin film stress measurements were also performed with a Tencor FLX-2320 which revealed a change in the stress state when the films were annealed. It is suggested that the brightness improvements and color shifts are due to changes in the thin film stresses which change the Cerium crystal field. The experimental results will be reviewed and the mechanisms responsible for the color shifts will be discussed in the context of general crystal field theory. Finally, the effects of the lattice perturbations on the oscillator strengths will be discussed.
10:00 AM		FP-ThM-6 Electron Spectroscopy Studies of Interface Formation between Metal Electrodes and Luminescent Organic Materials Y. Park, V. Choong, Y. Gao (University of Rochester); B. Hsieh (Xerox Wilson Center for Research & Technology); C. Tang (Eastman Kodak Company); K. M\um u\llen (MPI Mainz, Germany) We have studied the formation of interfaces that occur in organic electroluminescent (EL) devices using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) techniques. These interfaces determine carrier injection barriers that are crucial for device performance. Oligomers of commonly used EL polymer poly(p-phenylene vinylene) (OPV) and the molecular EL material tris-(8-hydroxyquinoline)aluminum (Alq3) were used as the organic materials, while indium-tin oxide (ITO), Ca, and Au were employed as the metallic electrodes. Interfaces formed both by depositing organics on metals and metals on organics were studied.For Ca/OPV, clear sign of interface states was observed in UPS and the XPS peak movement was consistent with energy level bending. The evolution of XPS and UPS spectra for OPV/Ca with increasing organic layer thickness confirmed the level bending and revealed discontinuous vacuum levels across the interface. For OPV/ITO, although the results were similar to those of OPV/Ca, the vacuum levels of the organic layer was determined by several factors such as the work function of metal substrate and the amount of charge transfer.The results for Ca/Alq3 indicated that the chemistry of the interface formation is complicated due to the various chemical states of C and the presence of N and O. However, it was clear that no direct Ca-C bonding occurs at the interface. Also, based on the XPS intensity analysis, we deduced that a Ca oxide layer may have formed at the interface. We also present the results for the interfaces of Alq3/Ca, Alq3/ITO, and Alq3/Au. We discuss the implications of our results in the context of performance of EL devices.
10:20 AM		FP-ThM-7 Effects of Metals on Luminescence of Organic Materials V. Choong, Y. Park, Y. Gao (University of Rochester); B. Hsieh (Xerox Wilson Center for Research & Technology); C. Tang (Eastman Kodak Company); K. M\um u\llen (MPI Mainz, Germany) Organic materials have been demonstrated to have the necessary attributes for display applications. In typical organic light-emitting devices, metallic electrodes are used to inject charged carriers into the organic electroluminescent (EL) medium. We report severe photoluminescence (PL) quenching of organic thin films comprised of the most useful materials, namely tris-(8- hydroxyquinoline) aluminum and 1,4-bis[4-(3,5-di-tert- butylstyryl)styryl]benzene (4PV), upon sub-monolayer deposition of Al, Ag, and Ca in an ultra high vacuum environment. The severity of the luminescence quenching, which depends on the type of metal used, can greatly affect the EL device performance. For example, a sub-monolayer coverage of the various metals on a 300 \Ao\ 4PV thin film can reduce the PL by as much as 50%. Depositing the 4PV layer onto a metal substrate also exhibits PL quenching. An exciton diffusion length of 200 \Ao\ can be estimated from the quenching data. Work supported in part by NSF under grant DMR-9303019.
10:40 AM		FP-ThM-8 Measurement of ZnS Valence Band by XPS P. Rack, P. Holloway, V. Sosa, J. Li, G. Sanders, C. Stanton (University of Florida, Gainesville) The valence band of an n-type ZnS electroluminescent thin film was measured by XPS, in order to analyze optical reflectivity data and correlate them to density of occupied states (DOS). Our measurements correlate well with the calculated DOS of bulk ZnS, showing the s and p structures. Contribution from Zn-3d shows to be below the valence band. Peak splitting near the top of the band was resolved, and the origin will be discussed. Reflectivity was interpreted based on possible interband transitions.These optical results will be discussed and related to the luminescence response of the material.
11:00 AM		FP-ThM-9 Luminescence Characteristics of Tb Doped Yttrium Oxysulphide Films Deposited by Spray Pyrolysis C. Falcony, A. Esparza (CINVESTAV-IPN, Mexico); M. Garcia (UNAM, Mexico) The luminescent characteristics of yttrium oxysulfide films deposited by spray pyrolysis are presented for Tb doped and undoped samples. These characteristics as well as the structural characteristics of the films have been studied as a function of the deposition parameters. It has been found that the crystallinity as well as the luminescent characteristics of the films depend on the substrate temperature during deposition. The films were deposited on a Corning glass slide and the aerosol generated used for the deposition was generated by ultrasonic means. The surface morphology was studied by means of Atomic Force Microscopy and SEM.
11:20 AM		FP-ThM-10 Molecular Beam Epitaxy of Electroluminescent Phosphors B. Wagner (Georgia Tech Research Institute) Among the flat panel display technologies, thin film electroluminescent (TFEL)displays have the simplest structure along with a wide viewing angle, high brightness and a good contrast ratio. However, the development of full color TFEL displays has lagged because of the lack of a suitable blue phosphor material. Molecular beam epitaxy (MBE) has emerged as a flexible deposition technique capable of producing new blue phosphor materials at relatively low temperatures. A review of recent developments in the MBE of phosphor materials and novel device structures will be presented. The discussion will cover topics such as Sr\sub x\Ca\sub 1-x\Ga\sub 2\S\sub 4\:Ce and SrS:Ce phosphors and multilayer thin film structures, as well as a comparison of MBE with other deposition techniques such as atomic layer epitaxy and sputtering.