ICMCTF2005 Session C6: Optical Thin Films for Active Devices

Tuesday, May 3, 2005 1:30 PM in Room Royal Palm 4-6

Tuesday Afternoon

Start	Invited?	Item
1:30 PM	Invited	C6-1 Mechanics of Optical Coatings in Extreme Thermal Environments J.J. Talghader (University of Minnesota) The mechanical requirements for optical coatings under high power illumination can be quite severe relative to those of their "normal" counterparts, particularly if the structure is thin or lightweight such as in micromachined or space-based systems. Perhaps the most serious problem for these coatings is deformation due to thermal expansion mismatch. Solving this problem requires both new materials and changes in optical design. When large temperature gradients exist across optical coatings, optical design must include the mechanical structure of the coating. This can be handled in most systems by analyzing the strains, forces and moments in a free multilayer stack and then empirically modifying the solution for any fixed boundary conditions on the coatings. Recent work in our group demonstrates that optical coating stacks can be designed to minimize thermal deformation. Shape deviations of less than δ/60 can be achieved even with micromirrors only 2μm thick. Among the most important aspects of optomechanical design is having optical coatings with a variety of mechanical properties so that high and low expansion materials can be traded off of one another. Among the most exciting new materials with potential for optical coatings are negative thermal expansion thin films, such as zirconium tungstate. Optical coatings made from these materials will contract with higher temperatures. This behavior is extremely beneficial for adaptive optics where thin film deformable mirrors will heat up to much higher temperatures than their support structures. Compressive stress due to positive thermal expansion in such mirrors will cause buckling, which inhibits them from functioning. Results on the deposition of amorphous negative thermal expansion thin films of zirconium tungstate will be presented. Thermal expansion coefficients of less than -10ppm/K have been achieved. The mechanical, thermal, and optical properties of negative expansion films will also be discussed. Early work shows that density plays a key role in determining the coefficient of thermal expansion, and the right deposition conditions can even induce "normal" materials, such as tungsten oxide, to take on negative thermal expansion in thin film form.
2:10 PM		C6-3 White OLED Using β-Diketones Rare Earth Bi-Nuclear Complex as Emitting Layer M. Cremona, W.G. Quirino, C. Legnani (Pontificia Universidade Catolica do Rio de Janeiro, Brazil); P.P. Lima, S.A. Junior, O.L. Malta (Universidade Federal de Pernambuco, Brazil) In the last decades there was a huge development of the research activities to obtain integrated light sources based on organic materials. Due to their applications in many different areas and manufacturing simplicity, organic electroluminescent diodes (OLEDs) represent a promising research in the development of new optoelectronic and photonic devices. The OLED emission wavelength can be tailored by choosing different combinations of organic materials. Moreover, by using trivalent rare earth (RE³⁺) complexes as emitting layers, electroluminescence (EL) can be obtained throughout the visible spectral region [1]. Today's handheld electronics like PDAs, cell phones, pagers and others, commonly integrate color Liquid Crystal Display (LCD) with white LEDs as dominant backlight solution. A promising substitute could be represented by the new OLED technology which can offer more saturated colors and the possibility to employ flexible substrates. In this work, we report the fabrication and the characterization of a white triple-layer OLED using a β-diketonate binuclear complex [Eu(btfa)₃ phenterpy Tb(acac)₃] as emitting layer for backlight purposes. The devices were assembled using a heterojunction between three organic molecular materials: the the N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)benzidine (NPB) as hole-transporting layer, the β-diketonate binuclear complex as emitting layer and tris(8-hydroxyquinoline aluminum)(Alq3) as electron transporting layer. All the organic layers were sequentially deposited under high vacuum environment by thermal evaporation onto ITO substrates and the final structure was ITO/NPB/ [Eu(btfa)₃phenterpy Tb(acac)₃]/Alq3/Al. Continuous EL emission was obtained varying the applied bias voltage from 10 to 22 V showing a wide emission band from 450 to 750 nm. [1] J. Kido, and Y. Okamoto, Chem. Rev. 102, 2537 (2002) This work is supported by RENAMI, CNPq, and FAPERJ.
2:30 PM		C6-4 Structures and Electrochromic Properties of Ta_0.3W_0.7O_x Thin Films Deposited by Pulsed Laser Ablation D. Yang, L. Xue (National Research Council Canada) Electrochromic materials based on mixed metal oxides are of growing importance since improved durability, coloration efficiency and chemical stability, as well as a desirable neutral color could be accomplished in those multicomponent films. In this work, we have used the pulsed laser deposition technique to deposit thin films of Ta_0.3W_0.7O_x on ITO-coated glass substrates in reactive O₂ gas environment at a substrate temperature range of 20 to 700°C. X-ray diffraction results showed that Ta_0.3W_0.7O_x films crystallized mainly to the cubic phase at substrate temperatures near 700°C, while films with amorphous structure were obtained at lower substrate temperatures. The lattice constants of the polycrystalline films are similar to those of stochiometric Ta_0.3W_0.7O_2.85 bulk materials. Optical transmittance of Ta_0.3W_0.7O_x films decreases as the O₂ pressures during deposition decreasing from 40 mTorr to 1 mTorr particularly at wavelength > 500 nm. Electrochromic properties of the Ta_0.3W_0.7O_x films were evaluated in 0.1 M H₃PO₄ electrolyte and the results were compared with those of WO₃ and Ta_0.1W_0.9O_x films deposited also by pulsed laser ablation. The results have demonstrated that addition of one metal oxide (e.g., Ta₂O₅) into another (e.g., WO₃) is an effective way to alter the electrochromic properties of the individual constituents.
2:50 PM		C6-5 Combinatorial Deposition of EL Phosphor Thin Films by r.f. Magnetron Sputtering using a Subdivided Powder Target T. Minami, Y. Mochizuki, T. Miyata (Kanazawa Institute of Technology, Japan) This paper describes a new technique incorporating combinatorial deposition to develop thin-film phosphors by r.f. magnetron sputtering. By sputtering with a powder target that is subdivided into two parts, phosphor thin films with a chemical composition and/or impurity content that varied across the substrate surface were successfully prepared. The results show that this technique can be effectively used to optimize the chemical composition and/or impurity content in various phosphor thin films for the purpose of obtaining higher luminous efficiency. Thin-film electroluminescent (TFEL) devices that incorporate newly developed rare earth- or manganese-activated phosphors consisting of host materials such as multicomponent oxynitrides (nitride combined with an oxide) were fabricated. As an example, the EL characteristics are described for TFEL devices fabricated using Eu-activated gallium oxynitride-based phosphors: host material, multicomponent oxynitrides composed of GaN and an oxide phosphor such as Ga₂O₃ or ZnO. The thin films were prepared by rf magnetron sputtering using a powder target subdivided into two parts with each part containing a different mixture of nitride phosphor powder and oxide phosphor powder. The thin-film emitting layers, thickness of approximately 1μm, were deposited at 350°C with an Eu content of 1 at.% and postannealed at a temperature up to 1100°C in various atmospheres for 30-60 min. The results showed that the obtained luminance in devices with an as-deposited ((ZnO)_1-X-(GaN)_X):Eu thin-film emitting layer exhibited a tendency to decrease as the GaN content was decreased from 100 to 50 mol.%, whereas it exhibited a tendency to increase with this same decrease in GaN content in devices fabricated using postannealed phosphor thin films. In addition, there was an improvement in the rate of increase in luminance relative to the applied voltage for TFEL devices postannealed at 950°C; there was also a tendency for greater improvement as the GaN content was increased.
3:10 PM	Invited	C6-6 Material Aspects for Transparent and Conductive ZnO Films being Importnat for a-Si:H/µc-Si:H Thin Film Solar and Display Applications B. Szyszka, V. Sittinger, A. Pflug, F. Ruske (Fraunhofer Institute for Surface Engineering and Thin Films, Germany) ZnO is an extraordinary material for various applications in optoelectronics and sensor technology. It is the hexagonal crystal structure which gives rise to the growth of crystalline films at low temperature as well as to the piezoelectricity of ZnO films and its excitonic characteristics being important for optoelectronics. Many device applications such as surface acoustic wave devices or a-Si:H / µc-Si:H solar cells having ZnO as a transparent front contacts make use of that crystallinity of ZnO films. For the formation of fine patterned electrodes for display applications, on the other hand, the crystalline structure of ZnO films is a problem due to the inhomogeneity of wet chemical etching of the crystalline films and thus, new modifications of ZnO films are necessary allowing for processing characteristics similar to a-ITO. Starting on our experience on ZnO:Al films deposition for a-Si:H / µc-Si:H thin film solar cells by reactive AC magnetron sputtering we report on the development of ZnO based TCO films for display applications where thin films in the order of 100 nm in film thickness as well as reliable etching characteristics for fine pattering are crucial. We report on the deposition of doped ZnO TCO films by reactive in-line magnetron sputtering and on the films properties obtained by optical spectroscopy and conductivity measurements. The micro structure and morphology of the films has been studied by X-ray diffraction and X-ray reflectometry as well as by atomic force and scanning electron microscopy.
3:50 PM		C6-9 The Effects of Oxygen Partial Pressure on Local Structural Properties for Ga-Doped ZnO Thin Films M. Osada (National Institute for Materials Science, Japan); T. Yamamoto, S. Kishimoto (Kochi University of Technology, Japan) Transparent and conductive Ga-doped ZnO (GZO) films have been deposited at a glass substrate temperature of 200°C using a reactive plasma deposition method. A detailed understanding of lattice defects on electrical properties is still lacking. In this work we report on Raman studies of GZO thin films in order to study the influences of Ga doping and O₂-partial pressure on lattice and electrical properties. A series of GZO thin films were deposited with varied O₂-partial pressure (0.2 - 310^-4 Torr). The GZO thin films with Ga₂O₃ wt% exhibit low resistivity (of 2.8 10^-4 Ω cm) and high carrier concentration of 8.210²⁰ cm^-3, Hall mobility of 27 cm² (Vs)^-1 and high average transmittance above 90% in the visible range. Under reducing conditions, carrier concentration decreases very slowly with increasing oxygen partial pressure. Excess O₂ gas flow rates above 15 sccm, O₂ partial pressure above 2.4610^-4 Torr, substantially decrease both carrier concentration and Hall mobility, and consequently increase resistivity of the GZO films. Raman spectra revealed vibrational modes at 470, 570 and 640 cm^-1 in addition to the host phonons of ZnO. The peak at 570 cm^-1 is corresponding to the A₁ mode, which is caused by the defects of oxygen vacancies, Zn-interstitial(Zn_i) or these complexes. Other two modes are interpreted as local vibrational modes associated with Ga_Zn or Zn vacancies, indicating a possible formation of defect complexes, such as Ga_Zn-V_Zn and Ga_Zn-O_i. With increasing O₂-partial pressure, the intensity of the 570-cm^-1 mode correlates linearly with the reduced carrier concentration in this system. These facts suggest that the increased O₂-partial pressure is attributable to the formation of the compensating-defects trapping carriers, such as Ga_Zn-O_i and Ga_Zn-V_Zn.