Physics
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Item Nanoelectronic Materials(2010) Moore, Tracy; Williams, Ellen D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis explores fabrication methods and characterization of novel materials used in field effect transistors, including metallic nanowires, carbon nanotubes, and graphene. Networks of conductive nanotubes are promising candidates for thin film electrode alternatives due to their desirable transparency, flexibility, and potential for large-scale processing. Silver nanowire and carbon nanotube networks are evaluated for their use as thin film electrode alternatives. Growth of silver nanowires in porous alumina membranes, dispersion onto a variety of substrates, and patterning is described. Metallic carbon nanotubes are suspended in aqueous solutions, airbrushed onto substrates, and patterned. The conductivity and transparency of both networks is evaluated against industry standards. Graphene is a two dimensional gapless semimetal that demonstrates outstanding room temperature mobilities, optical transparency, mechanical strength, and sustains large current densities, all desirable properties for semiconductors used in field effect transistors. Graphene's low on/off ratio and low throughput fabrication techniques have yet to be overcome before it becomes commercially viable. Silicon oxide substrates are common dielectrics in field effect transistors and instrumental in locating mechanically exfoliated graphene. The morphology of two different silicon oxides have been studied statistically with atomic force microscopy and scaling analysis. Tailoring the physical properties of these substrates may provide a control of graphene's electrical properties. A silicon oxide substrate may also be chemically altered to control the properties of graphene. I have modified silicon oxide with self-assembled monolayers with various terminal groups to control the field near the graphene. I characterize the monolayers with atomic force microscopy, x-ray photospectroscopy, and contact angles. I characterize graphene on these substrates using Raman microscopy and transport measurements. Finally, I examine low frequency noise in graphene field effect transistors on conventional silicon oxide substrates. As devices become smaller, the signal to noise ratio of these devices becomes important. Low frequency noise occurs on long time scales and must be controlled for device stability. I measure novel behavior of low frequency noise in multiple graphene devices. The noise may be described electron-hole puddles in the graphene that are caused by trapped charges near the surface of silicon oxide.Item semiconducting carbon nanotube transistors: electron and spin transport properties(2006-04-25) Chen, Yung-Fu; Fuhrer, Michael S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Single-walled carbon nanotubes (SWNTs) have attracted great interest both scientifically and technologically due to their long mean free paths and high carrier velocities at room temperature, and possibly very long spin-scattering lengths. This thesis will describe experiments to probe the charge-and spin-transport properties of long, clean individual SWNTs prepared by chemical vapor deposition and contacted by metal electrodes. A SWNT field-effect transistor (SWNT-FET) has been shown to be sensitive to single electrons in charge traps. A single charge trap near a SWNT-FET is explored here using both electronic and scanned-probe techniques, and a simple model is developed to determine the capacitances of the trap to the SWNT and gate electrode. SWNTs are contacted with ferromagnetic electrodes in order to explore the transport of spin-polarized current through the SWNT. In some cases spin-dependent transport was observed, verifying long spin scattering lengths in SWNT. However, in many cases no spin-dependent effects were observed; these results will be discussed in the context of the present state of results in the literature. Semiconducting SWNTs (s-SWNTs) with Schottky-barrier contacts are measured at high bias. Nearly symmetric ambipolar transport is observed, with electron and hole currents significantly exceeding 25 µA, the reported current limit in m-SWNTs. Four simple models for the field-dependent velocity (ballistic, current saturation, velocity saturation, and constant mobility) are studied in the unipolar regime; the high-bias behavior is best explained by a velocity saturation model with a saturation velocity of 2 x 10^7 cm/s. A simple Boltzmann equation model for charge transport in s-SWNTs is developed with two adjustable parameters, the elastic and inelastic scattering lengths. The model predicts velocity saturation rather than current saturation in s-SWNTs, in agreement with experiment. Contact effects in s-SWNT-FET are explored by electrically heating the devices. These experiments resolve the origin of nanotube p-type behavior in air by showing that the observed p-type behavior upon air exposure cannot be explained by change in contact work function, but is instead due to doping of the nanotube. Modest doping of the SWNT narrows the Schottky Barriers and provides a high-conductance Ohmic tunnel contact from electrode to SWNT.Item Carbon Nanotube Devices: Growth, Imaging, and Electronic Properties(2006-01-11) Brintlinger, Todd Harold; Fuhrer, Michael S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation focuses on growth, fabrication, and electronic characterization of carbon nanotube (CNT) devices. A technique for imaging CNTs on insulating substrates with the scanning electron microscope (SEM) will be described. This technique relies on differential charging of the CNT relative to the surrounding insulator. In addition, it is not only quicker than using scanning probe microscopy (SPM), but is also useful for identifying conducting pathways within an assortment of CNTs and metallic contacts. CNT field effect transitors (FETs) fabricated on strontium titanate gate dielectric show transconductances normalized by channel width of 8900 S/m, greatly exceeding that in Si FETs. Intriguingly, the transconductance cannot be explained within the conventional FET or Schottky-barrier models. To explain this, it is proposed that there is Schottky-barrier lowering due to high electric fields at the nanotube/contact interface. Exploring novel CNT-FET lithography, I demonstrate focused electron beam induced deposition (FEBID) of pure gold for CNT device electrodes. In examination of the CNT/electrode interface, equivalence between FEBID leads and leads deposited using conventional electron beam lithography is found with the majority device resistance in the CNT. Lastly, CNTs are suspended across wide trenches (>100 microns). These trenches are formed without lithography or etching and have metallic leads on either side of the trench for electrical transport measurements. Using a mechanical probe as a mobile gate, electrical transport can be performed on these suspended CNT devices, which show minimal hysteresis consistent with the absence of charge trapping.