Papers

ICMCTF2002 Session C1-1: Recent Advances in Optical Coating Technology

Tuesday, April 23, 2002 8:30 AM in Room Sunset

Tuesday Morning

Start	Invited?	Item
8:30 AM		C1-1-1 A New Method for Vacuum Deposition of Polymer Films J.D. Affinito (Senior Director, Moltech Corporation) Vacuum web coating of acrylate films, utilizing the Polymer Multi-Layer (PML) process for flash evaporation of monomer fluids, is well known. Widespread use of PML has not occurred, in part due to the difficulty of obtaining a license, and in part due to certain process limitations and stability issues. PML deposition requires a relatively long time to reach equilibrium once liquid monomer flow, to the evaporator, is initiated, and a much longer time is required for gaseous monomer output to subside once the liquid monomer flow is terminated. This lack of instant-on/instant-off capability will be seen to be problematic for in-line batch coaters. Control of the traditional PML process becomes increasingly difficult as the web speed decreases too, making it difficult to match in-line deposition rates and line speeds with slower processes such as sputtering. With traditional PML evaporators the deposition rate tends to decrease steadily with time as monomer polymerizes inside the evaporator and interferes with heat transfer between the walls and the freshly injected liquid monomer. A new Vacuum Polymer Technique (VPT), utilizing new source designs, has been developed allowing vacuum deposition of acrylate films with the same properties (ultra smooth, pinhole free) as PML films. The VPT process should permit sub-micron, or multiple-micron, thick film deposition at web speeds in excess of 100 m/min as in the PML process. Unlike the PML process, the VPT process should also permit controlled deposition of very thin films at very low web speeds (10's of Angstroms thick at a fraction of a m/min). The VPT process should also be more applicable to in-line batch coating processes than the PML process as it can be turned on, and off, for practical purposes, instantly. The VPT process will be described, a complete mathematical description and model for the process will be developed, and data relevant to barrier films and optical coatings will be presented.
9:10 AM		C1-1-3 Investigation Of Sputtered HfF₄ Films and Application to Interference Filters For Thermophotovoltaics P.M. Martin, L.C. Olsen, J.W. Johnston (Battelle Pacific Northwest Laboratory); D.M. DePoy (Knolls Atomic Power Laboratory) The optical properties of sputtered deposited hafnium fluoride films and their use in fabricating a front surface, interference filter for thermovoltaic energy conversion (TPV) were investigated. In particular, HfF₄ films are being considered for the low index component in a multilayer interference filter that will selectively transmit photons in the wavelength range of 1.0 um to 2.5 μm,, and strongly reflect all other wavelengths. Processes were developed for sputter depositing HfF₄ films with an index of refraction between 1.5 to 1.6 and an extinction coefficient below 0.001 in the 1.0 to 2.5 μm wavelength range. All HfF₄ coatings were deposited by RF reactive magnetron sputtering using a 3" (7.5 cm) Mighty Mac (US Corp.) magnetron cathode with pressed powder targets. Chamber pressure during depositions ranged from 2 to 10 mTorr, and RF power ranged from 50 W (0.91 W/cm²) to 150 W (2.73 W/cm²). Deposition rates depended primarily on target power and ranged from .3 to 1 Å/s. All depositions were performed with Ar. Resputtering effects played an important role in sputter deposition of hafnium fluoride. Film thickness and composition varied strongly with substrate distance from the target axis. Resputtering of growing films by energetic argon atoms resulted in strongly absorptive, fluorine-deficient films on the substrate platform near the target axis. However, films became increasingly more stochiometric and clear as the substrate was moved away from the target axis. The optical properties of the films were related to deposition conditions and placement in the substrate holder. Front surface interference filters for TPV cells were fabricated utilizing HfF₄ and an Si:H alloy for the low-index and high-index components, respectively. Transmission, reflection and absorption properties of the filters will be presented.
9:30 AM		C1-1-4 High-Luminance Thin-Film Electroluminescent Devices Using Y₂O₃:Mn Phosphor T. Minami, Y. Kobayashi, T. Shirai, T. Miyata, M. Yamazaki (Kanazawa Institute of Technology, Japan) It is well known that Y₂O₃:Eu oxide phosphor, consisting of cubic Y₂O₃ in a crystal structure, exhibits a highly efficient red emission in photoluminescence (PL) and cathodoluminescence (CL). However, except for Ga₂O₃ phosphors, high-luminance oxide phosphor thin-film electroluminescent (TFEL) devices using a binary compound had not been realized until recently. In this paper, we demonstrate that high-luminance TFEL devices could be fabricated using Mn-activated Y₂O₃ phosphor. The EL characteristics were investigated using a single-insulating-layer TFEL device structure; Y₂O₃:Mn thin films were deposited by r.f. magnetron sputtering onto BaTiO₃ ceramic sheets. The EL characteristics of TFEL devices using Y₂O₃:Mn thin films were strongly dependent on the crystal structure as well as the postannealing temperature and the content of doped Mn. Although as-deposited Y₂O₃:Mn thin films were crystallized, the crystallinity was considerably improved by postannealing at temperatures above about 800°C. PL and EL emissions were only observed from Y₂O₃:Mn thin films postannealed above about 800°C. However, it should be noted that PL and EL emissions were not observed when using cubic Y₂O₃:Mn thin films, the same crystal structure as conventional Y₂O₃:Eu phosphor. On the contrary, monoclinic Y₂O₃:Mn thin films exhibited high-luminance yellow emissions in both PL and EL. TFEL devices using monoclinic Y₂O₃:Mn thin films postannealed around 1000°C produced high-luminance yellow emissions; 5110 and 210 cd/m² for yellow emission with a sinusoidal wave voltage at 1 kHz and 60 Hz, respectively.
9:50 AM		C1-1-5 Photoluminescence from Highly Oriented Mg_xZn_1-xO Films Grown by Chemical Spray Pyrolysis T. Terasako, S. Shirakata (Ehime University, Japan); T. Kariya (Kochi University, Japan) Polycrystalline thin films of ternary oxide alloys Mg_xZn_1-xO (0≤x<0.2) have been grown by chemical spray pyrolysis for the first time. Photoluminescence (PL) properties have been studied systematically in terms of substrate temperature and alloy composition x. Mg_xZn_1-xO films were grown on glass, Si(100), and a- and c-plane sapphire substrates. Substrate temperature was changed in the range of 200-500^oC. A solution of zinc acetate dihydrate (Zn(CH₃COO)₂2H₂O) and magnesium acetate tetrahydrate (Mg(CH₃COO) ₂4H₂O) in ethanol was used as a precursor. The Mg content in the film determined by EPMA increases linearly with increasing that in the spray solution. This result shows that the alloy composition in the film can be well controlled by the Mg content in the solution. X-ray diffraction patterns of the Mg_xZn_1-xO films grown at 400-450^oC indicate that the films are highly oriented to c-axis. For ZnO/Si(100), PL measurements at 8K have been carried out. The films grown at 400 and 450^oC exhibited PL peaks related to excitons bound to neutral impurities. For the film grown at 500^oC, the near-band-edge (NBE) PL peak appears at higher energy than the A exciton energy. This is considered to be due to the Burstein-Moss effect. The Mg_xZn_1-xO (0≤x<0.2) films exhibited dominant unstructured NBE emission with long low energy tail in UV region at 77K. As the alloy composition x increases from 0.0 to 0.13, the PL peak of the NBE emission shifts continuously from 3.30 to 3.57eV accompanied with broadening. Observation of the unstructured NBE emissions with long low energy tail implies that there is considerable compositional inhomogeneity in alloy films. .
10:30 AM		C1-1-7 Photoelectric and Luminescent Properties of Pentacene Films Deposited on n-Si by Thermal Evaporation S.S. Kim, S.P. Park, S. Im (Yonsei University, South Korea) We report on the fabrication of a new light-emitting and light-detecting photodiode using an organic thin film for an active layer. Pentacene polycrystal films were deposited on n-Si substrate at various temperatures ranging from room temperature to 120°C. The energy bandgap of pentacene was characterized by ellipsometry and photoluminescence (PL), determined to be 1.82 eV. The most intense PL was obtained from pentacene films deposited at RT and 40°C, which means those films contain less amount of deep defects than the films deposited at higher temperatures. Finally we prepared Au/p-pentacene/n-Si diodes by sequentially evaporating pentacene and Au to shadow masks attached on n-Si. Under a forward bias of 12 V the diode begins to show electroluminescence while under a reverse bias it does show large photocurrents. The photoelectric properties are related to the quality of the organic thin films and will be discussed in detail.
10:50 AM		C1-1-8 Investigation of Amorphous Tungsten Oxide Thin Film from Far Infrared to Ultraviolet Spectrum H.N. Cui, V. Teixeira, A. Monteiro (Universidade do Minho, Portugal); E Bertran (Universitat de Barcelona, Portugal); D.M. Wang, J Wang (Chinese Academy of Sciences, China) Tungsten Oxide (WO₃) is one of the most studied electrochromic (EC) materials. It can be used as EC layer in EC devices. Electrochromism can be described as reversible change optical properties, by an applied external voltage. Here amorphous tungsten oxide thin films were deposited by reactive DC magnetron sputtering onto glass substrates for EC application. Growth temperature, oxygen partial pressure, and total gas pressure have been varied to obtain thin films with different densities. The thin films were analyzed by spectroscopes in the coverage of far infrared (reflecting absorption method), near infrared (NIR, both absorption and reflecting methods), visible and ultraviolet (UV-vis) spectra, because of the using glass substrate. X-ray diffraction, X-ray photoelectron spectroscopy and atomic force microscopy were also used in order to characterize the film structure. We consider thin film porosity, and therefore analyzed the experimental data by an effective medium approach. We obtain information on the tungsten oxide optical properties, a percentage of void fraction, surface roughness and film thickness. An IR spectrum was constituted of many broad bands in the 1000-3600 cm^-1 region. These bands are assigned to O-H modes of adsorbed water and moisture incorporation, respectively. The corresponding tungsten oxide vibrations are characterized by infrared region of 1000-200 cm^-1, which corresponds to (O-W-O) stretching, bending and lattice modes. Results from UV-Vis-NIR-IR analysis for film thickness, surface roughness, structure and volume fraction of moisture incorporation are consistent between models for XRD and XPS data analysis, as well as with AFM results.
11:10 AM		C1-1-9 Growth and Characterization of New OLED with Samarium Complex as Emission Layer M. Cremona (Pontifícia Universidade Católica do Rio de Janeiro, Brazil); R. Reyes (Universidad Nacional de Ingenieria, Brazil); E. Hering (Universidade Catolica do Rio de Janeiro, Brazil); C.F.B. da Silva, H.F. de Brito (Instituto de Quimica, Brazil); C.A. Achete (COPPE/UFRJ, Brazil) Organic light emitting diodes (OLEDs) represent today an active and promising area for the development of new optoelectronic devices. In this work the results of the growth and of the electrical and optical characterization of a new emitting OLEDs produced in our laboratory are presented. The device is assembled using a heterojunction between three organic molecular materials. The junction is formed by a 40 nm thickness film of 1-(3-methylphenyl)-1,2,3,4 tetrahydroquinoline-6-carboxyaldehyde-1,1´-diphenylhydrazone (MTCD), working as a hole transporting molecular layer (HTL), a 40 nm thick layer of samarium complex, tris(α -thenoyltrifluoroacetonato)bis(triphenylphosphine oxide) samarium (Sm(TTA)₃(TPPO)₂) working as light-emitting layer and finally by another 45 nm thick layer of tris 8-hydroxyquinoline aluminum (Alq₃), which is the electron transport layer. The device is grown in high vacuum environment (6x10^-6 Torr) on glass substrates coated with an hole injecting indium tin oxide (ITO) transparent layer. Finally, an electron injecting Al electrode (150 nm) is deposited onto the system. Polarizing the ITO as anode and the Al as cathode it was possible to observe the electroluminescence (EL) of the device at room temperature and to built the EL curve as a function of the applied bias voltage. Due to the Sm, the EL is peaked around 600 nm and the OLED orange light emission is almost linear with the current density. The I-V curves show an exponential dependence of I as a function of the applied voltage, as it is expected for a semiconductor diode. Measurements of optical absorption and photoluminescence (PL) as a function of different excitation wavelengths were performed in the OLED device and in the single layers of HTL, emission layer samarium complex and ETL. An investigation about the device degradation as a function of the environmental conditions and optical and electrical excitation is also reported.}