Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7April 27, 2009 1O’Dwyer, C. et al. Bottom-up growth of fully transparent contact layers of indium tin oxide nanowires for light emitting devices. Nature Nanotech. 4, 239 – 244 (2009).Jaeseok (“Jae”) JeonDept. of Electrical Engineering & Computer SciencesUniversity of California at BerkeleyBerkeley, California 94720-1774Monday, April 27, 2009April 27, 2009 2Indium Tin Oxide (ITO)•Indium Tin Oxide (ITO); versatile material used in various applications» Strong ultraviolet absorption higher efciency solar cells⇒» High visible and near-infrared transmission transparent contact electrodes for light-⇒emitting devices (LEDs)» High far-infrared refectivity anti-refection coatings⇒» Strong microwave atenuation radiation protection⇒•In the visible and near-infrared wavelength regime;» Trade-off between optical transparency and electrical conductivity for ITO and even other transparent, conductive oxides» Optically transparent, but highly resistive contact electrodes for LEDs•To overcome the trade-off, ITO nanowires botom-up grown using custom-developed Molecular Beam Epitaxy (MBE)April 27, 2009 3Big-picture view of ITO nanowire growth•MBE is one of the widely used methods of depositing epitaxial films•ITO nanowire growth;Indium (In) and Tin (Sn) as precursors in O2In and Sn heated in separate effusion cellsIn and Sn slowly sublimated (evaporated)gas-phase In and Sn condensed on substrateIn and Sn react with each other, forming ITO nanowiresTypical MBE systemApril 27, 2009 4Atomistic view of ITO nanowire growth I•Entire process in a single step» Seeding → nucleation → growth → progressive branch seeding and growth » No heterogeneous catalysts neededSEM image of oxidized In-Sn droplet seed crystals, MBE deposition conditions: Tsub = 400 °C, TIn = 835 °C, TSn = 1000 °C, 0.1 nm/sec, In:Sn = 10:90, scale bar = 100nm Seeds crystal nucleated, and non-tapered ITO nanowires precipitatively grown and homogeneously branched, scale bar =50 nmCross-sectional SEM image of ITO nanowire with seed crystal FIB milledApril 27, 2009 5Atomistic view of ITO nanowire growth IICross-sectional SEM images of 8 to 50 nm long ITO nanowires, MBE deposition conditions: Tsub = 400 °C, TIn = 835 °C, TSn = 1000 °C, 0.1 nm/sec, In:Sn = 10:90, scale bar = 200 nmMorphological variations High-quality, defect-free single-crystal nanowires formed at high deposition ratesPorous nanowires created at lower deposition ratesSEM images of 100 to 25nm thick ITO nanowiresSEM images of 100 to 300 nm thick ITO nanowires showing increased surface roughness and internal porosityApril 27, 2009 6Atomistic view of ITO nanowire growth IIISEM images of ITO nanowires showing the overall long-range growth uniformity, cross-sectional diameters of ITO nanowires = 10 – 15 nm, scale bar on the first image = 1 umApril 27, 2009 7ITO nanowires: optical and electrical characteristicsTransmission measurements of ITO nanowires, MBE deposition conditions: Tsub = 575 °C (red curve), Tsub = 450 °C (green curve) Effect of ITO nanowire morphology on optical transmission; transparency in the visible wavelength regime improved to ~ 90 % with increasing growth rate up to 0.2 nm/secAs-deposited nanowires showing a linear I-V characteristic no need ⇒for forming gas anneal or metal interlayer to offset high contact
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