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Conformal Charge Barrier Layers To Minimize Current In Organic Electro-Optic Devices

Organic electro-optic (EO) molecules have several advantages over incumbent inorganic EO materials, and are slated to be used in modulators for much faster next-generation fiber-op¬tic telecommunications networks if their performance can be further improved. Here we investigate increasing the efficiency of EO molecules and utilizing materials that are more suitable for modulator devices. Organic EO molecules in this application must first be aligned. Alignment is typically done by a process called electric field poling, where a thin film of organic EO material is placed between two electrodes and a strong electric field is applied, causing the molecules to reorient. We use chromophore molecules at high number density to produce higher EO coefficients (r33), which is the figure of merit for comparing EO molecules. However, because of their high degree of conjugation (delocalized electron sharing), these molecules can have significant leakage current during poling, reducing the electric field and thus the degree of molecular alignment. Although solvent-cast charge barrier layers have been successful in reducing current in parallel plate test devices, they cannot be applied as conformal (consistent in depth) coatings in the microscopic trenches in real world silicon modulators. We hypothesized that four new types of charge barrier layers could meet this requirement: phenylphosphonic acid self-assembled monolayer (SAM), 4-cyanobenzoic acid SAM, aminopropylphosphonic acid SAM, and layer-by-layer (LbL) films of poly(sodium styrene sulfonate)/poly(diallyldimethylammonium) chloride. We discuss fabrication methods as well as effects of barrier layers on r33, on conductance during poling, and on EO thickness and roughness.

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Conformal Charge Barrier Layers To Minimize Current In Organic Electro-Optic Devices

  1. 1. Conformal Charge Barriers For Organic Electro-Optics Data /Results Figure 2. Electrical trace of typical test Figure 3. Optical trace of typical test • Real-time monitoring and data collection for temperature (green), current (blue) and voltage (black) during poling of sample devices. • EO material must be heated to glass transition temperature (Tg) to allow molecules to move into alignment under poling voltage. • Optical data is also monitored and collected. • Im is proportional to r33, the figure of merit for comparing EO materials. Introduction • Organic electro-optic (EO) molecules can dramatically increase the speed of fiber-optic communications while reducing the size and power requirements of the EO modulator. • Molecules must first be aligned using a strong electric field. Leakage current through EO material reduces alignment and thus performance. • Solvent-cast charge barrier layers reduce current in parallel plate test devices, but cannot be applied as conformal (consistent in depth) coatings in the microscopic trenches in silicon fiber-optic modulators. • We hypothesized that a self-assembled monolayer or LbL film could work as an effective charge barrier layer and be applied as a conformal coating. Scott P. Merry1,2, Delwin L. Elder2, and Larry R. Dalton2 1Nanotechnology Department, North Seattle College, Seattle, WA 98103 2Department of Chemistry, University of Washington, Seattle, WA 98195 This material is based upon work supported by the National Science Foundation (Grant No. DMR-1303080). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. The authors acknowledge partial financial support from the Air Force Office of Scientific Research (FA9550-10-1-0558, FA9550-15-1-0319). Conclusions • EO devices were successfully fabricated with LbL or SAM barrier layers. • Peak conductance measurements (Figure 4a), show devices with barrier layers had no decrease in leakage current compared to devices without a barrier layer. • The 4.5 bilayer charged polymers barrier layer devices exhibited leakage current similar to those with no barrier layer, while SAMs showed increased current (Figure 4a). • Charge barrier layers had little effect on r33, the figure of merit for comparing electro-optic activity (Figure 4b). • Barrier layers did not negatively effect EO thickness. Phenyl-phosphonic acid barrier layers were associated with increased film thickness (Figure 4c). • Little change in surface roughness was observed between different charge barrier layers (Figure 4d). References • Dalton, L. R.; Sullivan, P. A.; Bale, D. H. Chem. Rev. 2010, 110, 25. • Jin, W.; Johnston, P. V.; Elder, D. L.; Tillack, A. F.; Olbricht, B. C.; Song, J.; Reid, P. J.; Xu, R., Robinson, B. H.; Dalton, L. R. App. Phy. Lett., 2014, 104, 243304. • An, M.; Hong, J.-D. Thin Solid Films 2006, 500, 74. • Havare, A. K.; Can, M.; Demic, S.; Okur, S.; Kus, M.; Aydin, H.; Yagmurcukardes, N.; Tari, S. Synth. Met. 2011, 161, 2397. • Bardecker, J. A.; Ma, H.; Kim, T.; Huang, F.; Liu, M. S.; Cheng, Y.-J.; Ting, G.; Jen, A. K.-Y. Adv. Funct. Mater. 2008, 18, 3964. Acknowledgments A special thanks to Prof. Bruce H. Robinson and Dr. Andreas F. Tillack of the University of Washington, Department of Chemistry, for their knowledge of chemical physics. Nathan Sylvain, Huajun Xu, Kerry Garrett, and Peter Johnston of the Dalton Lab provided background on chromophores and their chemistry. Thanks also to Alissa Agnello and Dr. Peter Kazarinoff for their work on the Nanotechnology program at North Seattle College. Figure 1. 50/50 blend of two EO molecules used in our tests Figures 4. a, b, c, d. Characterization by barrier layer type. Methods V Device Structure Electric Field Poling 50 V/µm Gold electrode ITO electrode V Barrier Layer Self-Assembled Monolayers (SAM) Heat 140 C (Promotes covalent attachment) ITO 1. Sonicate 2. Plasma clean Dip in 1 mM sol’n ITO OH OHOH OH OH ITO OH OH OH OH ITO Bilayers of oppositely charged polymers ITO ITO ITO ++ + + + - - - - - ++++ ++ ++ ++ ++ + + + ++ + + + ++ + + + - - - - - - - - - - - - - - - ITOITO 1. Sonicate 2. Plasma clean 3. NH4OH/H2O2 clean PSS/ PDADMAC 4-Cyanobenzoic acid Phenyl- phosphonic acid Aminopropyl- phosphonic acid Materials used for barrier layers Spin coat from 20 mM polymer sol’n + ++++ Temperature Current Voltage 0 1 2 3 4 5 6 7 8 9 None 4.5 Bilayer Cyanobenzoic acid Phenyl-PA Aminopropyl-PA PeakConductance[uS] Peak Conductance by Barrier Type Average a. 0 5 10 15 20 25 30 35 None 4.5 Bilayer Cyanobenzoic acid Phenyl-PA Aminopropyl-PA r33(pm/V) r33 by Barrier Type Average b. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 None 4.5 Bilayer Cyanobenzoic acid Phenyl-PA Aminopropyl-PA EOLayerFilmThickness(um) EO Layer Film Thickness by Barrier Type Average c. 0 1 2 3 4 5 6 7 8 9 None 4.5 Bilayer Cyanobenzoic acid Phenyl-PA Aminopropyl-PA SurfaceRoughness(Rq,nm) Surface Roughness by Barrier Type Average d.