April 27, O’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”) Jeon Dept. of Electrical Engineering & Computer Sciences University of California at Berkeley Berkeley, California Monday, April 27, 2009

April 27, Indium Tin Oxide (ITO) Indium Tin Oxide (ITO); versatile material used in various applications »Strong ultraviolet absorption ⇒ higher efficiency solar cells »High visible and near-infrared transmission ⇒ transparent contact electrodes for light- emitting devices (LEDs) »High far-infrared reflectivity ⇒ anti-reflection coatings »Strong microwave attenuation ⇒ 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 bottom-up grown using custom- developed Molecular Beam Epitaxy (MBE)

April 27, Big-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 O 2 In and Sn heated in separate effusion cells In and Sn slowly sublimated (evaporated) gas-phase In and Sn condensed on substrate In and Sn react with each other, forming ITO nanowires Typical MBE system

April 27, Atomistic view of ITO nanowire growth I Entire process in a single step »Seeding → nucleation → growth → progressive branch seeding and growth »No heterogeneous catalysts needed SEM image of oxidized In-Sn droplet seed crystals, MBE deposition conditions: T sub = 400 °C, T In = 835 °C, T Sn = 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 nm Cross-sectional SEM image of ITO nanowire with seed crystal FIB milled

April 27, Atomistic view of ITO nanowire growth II Cross-sectional SEM images of 8 to 50 nm long ITO nanowires, MBE deposition conditions: T sub = 400 °C, T In = 835 °C, T Sn = 1000 °C, 0.1 nm/sec, In:Sn = 10:90, scale bar = 200 nm Morphological variationsHigh-quality, defect-free single- crystal nanowires formed at high deposition rates Porous nanowires created at lower deposition rates SEM images of 100 to 25nm thick ITO nanowires SEM images of 100 to 300 nm thick ITO nanowires showing increased surface roughness and internal porosity

April 27, Atomistic view of ITO nanowire growth III SEM 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 um

April 27, ITO nanowires: optical and electrical characteristics Transmission measurements of ITO nanowires, MBE deposition conditions: T sub = 575 °C (red curve), T sub = 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/sec As-deposited nanowires showing a linear I-V characteristic ⇒ no need for forming gas anneal or metal interlayer to offset high contact resistance