SnO 2 Nanowires Used to Fabricate Fully Transparent Thin-Film Transistor Devices
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be too long for practical use, and those based on birefringence, while small, are too lossy to be practical. To address this, the researchers decided to pursue a hybrid device, which combines a central birefringent section with mode-matching taperedwaveguide input/output couplers to reduce optical loss. The researchers fabricated samples of the devices in an InGaAsP heterostructure consisting of an InP top cladding layer, an InGaAsP waveguide layer, and an InP buffer layer. A silica layer was deposited to form a hard mask. The waveguide was formed by writing the waveguide pattern into a 200-nm thick layer of polymethylmethacylate (PMMA) using electron-beam lithography, transferring the pattern into a hard-mask layer of silica with reactive ion etching using CHF3 chemistry, and finally deep etching into the InP layers using chemically assisted ion beam etching. The final devices were all less than 50 µm long. The group then tested the devices by launching polarized light at a wavelength of 1.55 µm into them and measuring the rotation of the plane of polarization as a function of the width of the central birefringent section. The rotation angle increased with decreasing width, and ranged from nearly zero for a 1500-nmwide central section to over 250 degrees for a 500-nm-wide central section. The overall insertion losses were only about 1 dB, dominated by propagation loss rather than back reflections, which can degrade photonic circuit performance. The devices were also found to operate over a 100 nm wavelength range, demonstrating that they could be used in a broadband photonic circuit configuration. Given these results, hybrid polarization-control devices of this nature may form a key component of future integrated microphotonic systems. COLIN MCCORMICK
SnO2 Nanowires Used to Fabricate Fully Transparent Thin-Film Transistor Devices The extraordinary properties of semiconductor nanowires have led many researchers to pursue novel device applications. For example, conventional transparent thin-film transistors (TFTs) that use single-crystal channel materials typically require growth and annealing at high temperature and expensive singlecrystalline substrates. In contrast, highperformance, nanowire-based TFTs have been fabricated on glass and plastic substrates. A key advantage to this approach is its capacity to separate the nanowire growth step from the device fabrication, which makes moot the compatibility between the device substrate with nano688
wire growth. High temperatures can therefore be used to obtain nanowires composed of single crystals, which are then transferred to the device substrate, configured into thin-film form, and processed into TFTs using conventional techniques. Heretofore, opaque semiconducting nanowires, (e.g., silicon), were used with electrodes made from normal metals such as Au or Ni but new frontiers such as “invisible electronics” require optical transparency. Recently, W. Lu and co-researchers from the University of Michigan fabricated fully transparent nanowire-based TFT devices that
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