AVS1997 Session TF+FP-MoA: Flat Panel Emissive Displays and Oxides

Monday, October 20, 1997 2:00 PM in Room B3/4

Monday Afternoon

Start	Invited?	Item
2:00 PM	Invited	TF+FP-MoA-1 Chromaticity and Electroluminescent Efficiency of Atomic Layer Epitaxy SrS:Ce Thin Films W.L. Warren, C.H. Seager (Sandia National Laboratories); E. Soininen (Planar International Ltd., Finland); P. Benalloul, C. Barthou (Universite Pierre et Marie Curie, France); S.-S. Sun (Planar Systems, Inc.) We have studied various factors that control the luminous properties of ALE SrS:Ce electroluminescent (EL) devices by using a combination of analytical tools. EL and photoluminescent (PL) light is green shifted and the radiative decay time shorter in ALE SrS:Ce films, compared to powders and thin films prepared with some other techniques. We have found that Ce ³⁺ lies in two locations in SrS:Ce: in a nearly cubic site and in another site of lower symmetry, assigned to Ce ³⁺-V _Sr. This study suggests that the green shift may be collectively induced by symmetry lowering at Ce ³⁺ sites and by enhanced Ce ³⁺-Ce ³⁺ interactions in Ce doping planes. Moderate heat treatments of as-deposited SrS:Ce devices were found to dramatically improve the EL efficiency and increase the tranferred charge density through the phosphor film. This was found to be due to removal of defects leading to nonradiative Ce ³⁺ decay and to reduction of Ce ⁴⁺ to radiative Ce ³⁺. The heat treatments also induce macroscopic stress in the films and atomic level stress around the Ce ³⁺ sites. However, the stresses were not found to directly correlate with the EL efficiency. The ALE SrS:Ce films are of high crystalline quality as deposited at 300-400C. The growth temperature of 400C is favored since this results in higher Ce ³⁺ excitation efficiencies. We will discuss electronic traps in SrS:Ce films and suggest that some traps can be beneficial in providing necessary charge for the Ce ³⁺ excitation process and others can be deleterious in that they suppress radiative Ce ³⁺ decay events. It is envisaged that through a suitable selection of the trap density, or trap type, one might enhance the former and suppress the latter effect. To determine the effects of Ce doping, undoped SrS films were also deposited and fully characterized. Finally, we compare the properties of films prepared by ALE to those deposited via rf sputtering. The part of this work performed at SNL was supported by DARPA and the US Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the US Department of Energy under contract DE-AC04-94AL85000.”
2:40 PM		TF+FP-MoA-3 XPS Analysis and Semiempirical SCF/CI Modeling of Ce⁺³ Core Level Electrons in Ca_{x√sub 1-x}Ga₂S₄ P.D. Rack, P.H. Holloway, T.A. O'Brien, M.C. Zerner (University of Florida, Gainesville) The addition of oxygen in the Ca_{x√sub 1-x}Ga₂S₄:Ce phosphor was found to nearly double the luminance and blue shift the emission spectrum of the Ce⁺³ 4f-5d transition. XPS analysis was used to investigate the local chemistry of the Ce⁺³ luminescent center in the undoped and oxygen doped Ca_{x√sub 1-x}Ga₂S₄ phosphor. The shake up structure in the Ce⁺³ 3d_3/2 and 3d_5/2 XPS spectra is due to a ligand-to-metal charge transfer process, and it was found that the relative intensity of the charge transfer peaks increased when oxygen was introduced into the Ca_{x√sub 1-x}Ga₂S₄:Ce. Because the charge transfer probability is controlled by the local ionicity of the Ce bonds, the increase in the charge transfer peak intensity suggested the formation of Ce-O bonds. Furthermore, the binding energy of the charge transfer peaks and the Ce MNN Auger peak shifted, which confirmed the idea that oxygen was bonding to the Ce⁺³ luminescent center. To understand the effects that oxygen has on the Ce⁺³ 4f-5d emission spectrum, we used a self-consistent-field configuration interaction (SCF/CI) model to calculate the positions of the 4f and 5d energy levels. By replacing the appropriate number of sulfur ligands with oxygen the SCF/CI model predicted a 650cm^-1 blue shift in the 4f-5d transition energy which was in excellent agreement with the observed 730cm^-1 blue shift.
3:00 PM		TF+FP-MoA-4 Deposition and Luminescence Characterizations of ZnGa₂O₄ Thin Films Prepared by rf magnetron sputtering Y.J. Kim, Y.H. Jeong, S.G. Kang, K.K. Lee (Kyonggi University, Korea); Y.K. Park, J.I. Han (Korea Electronic Technology Institute) ZnGa₂O₄ and Mn doped ZnGa₂O₄ thin films were deposited on ITO coated glass substrates and Si wafers by rf magnetron sputtering using Ar/O₂ mixture gases. The effects of deposition parameters and heat treatments on the growing structure, growth rate, Ga/Zn ratio, grain size, and luminescence properties were investigated. There were discrepancies in the structure and the grain size between ZnGa₂O₄ thin films deposited on Si and ITO/glass substrates. Also, the growing orientational changes such as the transformations from <311> direction to <400> were observed by varying the substrate temperature. In case of Mn doped ZnGa₂O₄ thin films, the transformation from <311> to <400> direction was occurred at lower substrate temperature than undoped ZnGa₂O₄ thin films because of the surface energy differences. By heat treatment, the sharper PL spectrum could be obtained because it diminished the strain energy and the defects in as-deposited thin films. Also, PL spectrums of thin films deposited on Si wafers were compared to those on ITO/glass substrates.
3:20 PM	Invited	TF+FP-MoA-5 FED Display Manufacturing Challenges K.B. Hilgers (Motorola Inc.) The potential of Field Emission Devices (FED) using Spindt tip deposition techniques for Flat Panel Displays has been shown to be viable. Typically, these "prototype" displays have been less than five inches in diagonal and fabricated on relatively small glass substrates. When fabricating these FED displays on large glass panels there are new manufacturing challenges that must be examined. These challenges not only deal with the technological challenges to fabricate the displays, but also with the associated costs of large volume production. Manufacturabilty indices can be derived from device yield, initial tool cost, tool cost of ownership, and tool throughput. This presentation will take a look at these issues along with an overview of the FED display technology at Motorola.
4:00 PM		TF+FP-MoA-7 Properties of Fluorine-Doped Tin Oxide Films Produced by Low-Pressure Chemical Vapor Deposition using Tetramethyltin. X. Li, T.A. Gessert, H.R. Moutinho, P. Sheldon (National Renewable Energy Laboratory) Conductive tin oxide films on glass substrates are used extensively for transparent electrodes in electrochromic devices, flat-panel displays, and thin-film photovoltaic solar cells. The typical-film deposition process is atmospheric-pressure, chemical-vapor deposition (APCVD) using tintetrachloride (TTC) and oxygen source. Although the TTC APCVD process produces films of sufficient quality for many device applications, other researchers have demonstrated that higher-quality films result when TTC is replaced with tetramethyltin (TMT). However, due to the greater toxicity of TMT compared to TTC, relatively few detailed studies have been performed on the properties of TMT-produced films. In this study, fluorine-doped tin oxide films have been produced using low-pressure, chemical-vapor deposition (LPCVD) of TMT, oxygen, and bromotrifluoromethane. We have found that the TMT chemistry, combined with the LPCVD process, produces film properties that are easily controlled and reproduced. Further, the films demonstrate higher transparency, higher conductivity, and reduced surface roughness as compared to commercially available films produced using TTC. For F-doped films, Hall measurements indicate an electron mobility of >40 cm/volt-s and an electron concentration of 4.0e20 per cubic cm. Films that are 1 µm thick have been produced that have an average transmission of >80% in the visible spectral range and a sheet resistance of <5 Ohms per square. Undoped film resistivities of ~1 Ohm-cm have also been achieved at deposition temperatures of ~500 centigrade. By carefully controlling flow rate, chamber pressure, and injector design, thickness uniformities of ±5% have been achieved over a deposition area of 10 cm x 30 cm. Finally, we have found that through careful design of the reactor containment region and the effluent treatment system, we can deposit films with no detectable TMT in the effluent.
4:20 PM		TF+FP-MoA-8 Preparation of Transparent Conducting ZnO-In₂O₃-SnO₂ System Multicomponent Oxide Films by R.F. Magnetron Sputtering S. Takata, T. Minami, K. Shimokawa, I. Fukuda (Kanazawa Institute of Technology, Japan) Recently, we reported that ternary compounds such as ZnSnO₃, Zn₂In₂O₅ and In₄Sn₃O₁₂ are very promising as new materials for transparent conducting oxide films. In this paper, we demonstrate the preparation of transparent conducting multicomponent oxide thin films consisting of combinations of these ternary ZnO-In₂O₃-SnO₂ system compounds. Thin films of multicomponent oxides such as Zn₂In₂O₅-In₄Sn₃O₁₂ and ZnSnO₃-In₄Sn₃O₁₂ were prepared by conventional r.f. planar magnetron sputtering using slightly compressed powder targets. The sputter deposition was carried out in an Ar and O₂ mixed gas atmosphere. Highly transparent and conductive Zn₂In₂O₅-In₄Sn₃O₁₂ films were prepared on substrates at room temperature (RT) and 350°C using targets with In₄Sn₃O₁₂ contents of 0 to 100wt%. The Zn₂In₂O₅-In₄Sn₃O₁₂ films exhibited resistivities of 3x10^-4 to 7x10^-4 ohm-cm as well as average transmittances above 80% in the visible range. The etching rate in acid solution decreased as the In₄Sn₃O₁₂ content was increased. ZnSnO₃-In₄Sn₃O₁₂ films prepared on substrates at RT and 350°C using targets with In₄Sn₃O₁₂ contents of 0 to 100wt% exhibited resistivities from 10^-3 to 10^-4 ohm-cm with an average transmittance above 80% in the visible range. The ZnSnO₃-In₄Sn₃O₁₂ films were chemically stable, regardless of the In₄Sn₃O₁₂ content. The optical properties such as refractive index and band-gap energy in these multicomponent oxide films changed monotonically as the In₄Sn₃O₁₂ content was increased.
4:40 PM		TF+FP-MoA-9 High Dielectric Constant BaSrTiO₃ Thin Films for EL Displays G.S. Stauf (Advanced Technology Materials, Inc.); S. Moehnke, S. Robinson, R. Tuenge (Planar Systems, Inc.) We report on development of high dielectric constant BaSrTiO₃ (BST) thin films for use as insulators in electroluminescent displays. These films were made via metalorganic chemical vapor deposition (MOCVD), a technique capable of producing complex oxide films of high purity, crystalline quality, and uniformity, over large areas. A novel liquid delivery system was used for injection of precursors into the MOCVD reactor. Electrical measurements, including dielectric constant, leakage current and breakdown strength will be reported for these BST insulators. We will also discuss results on the integration of these insulators at Planar Systems, Inc. into complete active matrix electroluminescent (AMEL) test structures on Si wafers, including measurements of threshold voltage, brightness, and efficiency. Comparisons with standard aluminum oxide based dielectrics shows up to 25% reduction in threshold voltages with BST dielectrics, with comparable emission brightness and efficiency. This may be due to the higher dielectric constants attainable with this material, up to 200, leading to higher levels of charge storage. Some discussion of interaction with various types of phosphors and electrodes will also be presented.
5:00 PM		TF+FP-MoA-10 The Effect of the Base Layer on the Conductivity and Structure of dc Magnetron Sputtered Thin Silver Films. M. Arbab (PPG Industries, Inc., Glass Technology Center) Many low emissivity - solar control optical coatings on glass consist of a repeating dielectric-silver-dielectric thin film period. We have investigated the effect of structure of the dielectric base layer on the conductivity, structure, and thermal stability of contiguous silver films up to approximately 500 Å thick. All individual layers were deposited by dc-magnetron cathode sputtering. Specifically, a detailed comparison of thin films of amorphous zinc stannate with crystalline zinc oxide showed that the latter significantly improves the conductivity and emissivity of 50-100 Å, and thicker, silver layers. X-ray diffraction analysis showed that zinc oxide with a preferred basal plane orientation, results in the preferred growth of the Ag(111) planes parallel to the float glass substrate. Transmission electron microscopy results support the above observation. The TEM results also indicate that the silver-dielectric interface is of similar quality in both cases, thus eliminating the role of the interface as a major cause of the above difference. Our data on the structure and electrical properties of heat treated samples suggest that the preferential growth of the silver layer results in its higher thermal stability.foot1 ¹Pending US patent.