Electrical and optical studies on carbon nanotube arrays

Single-walled carbon nanotubes (SWNTs) have been studied as a prominent class of high performance electronic materials for next generation electronics. Their geometry dependent electronic structure, ballistic transport and low power dissipation due to quasi one dimensional transport, and their capability of carrying high current densities are some of the main reasons for the optimistic expectations on SWNTs. However, device applications of individual SWNTs have been hindered by uncontrolled variations in characteristics and lack of scalable methods to integrate SWNTs into electronic devices. One relatively new direction in SWNT electronics, which avoids these issues, is using arrays of SWNTs, where the ensemble average may provide uniformity from device to device, and this new breed of electronic material can be integrated into electronic devices in a scalable fashion. This dissertation describes (1) methods for characterization of SWNT arrays, (2) how the electrical transport in these two-dimensional arrays depend on length scales and spatial anisotropy, (3) the interaction of aligned SWNTs with the underlying substrate, and (4) methods for scalable integration of SWNT arrays into electronic devices. The electrical characterization of SWNT arrays have been realized by polymer electrolyte-gated SWNT thin film transistors (TFTs). Polymer electrolyte-gating addresses many technical difficulties inherent to electrical characterization by gating through oxide-dielectrics. Having shown polymer electrolyte-gating can be successfully applied on SWNT arrays, we have studied the length scaling dependence of electrical transport in SWNT arrays. Ultrathin films formed by sub-monolayer surface coverage of SWNT arrays are very interesting systems in terms of the physics of two-dimensional electronic transport. We have observed that they behave qualitatively different than the classical conducting films, which obey the Ohm’s law. The resistance of an ultrathin film of SWNT arrays is indeed non-linear with the length of the film, across which the transport occurs. More interestingly, a transition between conducting and insulating states is observed at a critical surface coverage, which is called percolation limit. The surface coverage of conducting SWNTs can be manipulated by turning on and off the semiconductors in the SWNT array, leading to the operation principle of SWNT TFTs. The percolation limit depends also on the length and the spatial orientation of SWNTs. We have also observed that the percolation limit increases abruptly for aligned arrays of SWNTs, which are grown on single crystal quartz substrates. In this dissertation, we also compare our experimental results with a two-dimensional stick network model, which gives a good qualitative picture of the electrical transport in SWNT arrays in terms of surface coverage, length scaling, and spatial orientation, and briefly discuss the validity of this model. However, the electronic properties of SWNT arrays are not only determined by geometrical arguments. The contact resistances at the nanotube-nanotube and nanotube-electrode (bulk metal) interfaces, and interactions with the local chemical groups and the underlying substrates are among other issues related to the electronic transport in SWNT arrays. Different aspects of these factors have been studied in detail by many groups. In fact, I have also included a brief discussion about electron injection onto semiconducting SWNTs by polymer dopants. On the other hand, we have compared the substrate-SWNT interactions for isotropic (in two dimensions) arrays of SWNTs grown on Si/SiO2 substrates and horizontally (on substrate) aligned arrays of SWNTs grown on single crystal quartz substrates. The anisotropic interactions associated with the quartz lattice between quartz and SWNTs that allow near perfect horizontal alignment on substrate along a particular crystallographic direction is examined by Raman spectroscopy, and shown to lead to uniaxial compressive strain in as-grown SWNTs on single crystal quartz. This is the first experimental demonstration of the hard-to-achieve uniaxial compression of SWNTs. Temperature dependence of Raman G-band spectra along the length of individual nanotubes reveals that the compressive strain is non-uniform and can be larger than 1% locally at room temperature. Effects of device fabrication steps on the non-uniform strain are also examined and implications on electrical performance are discussed. Based on our findings, there are discussions about device performances and designs included in this dissertation. The channel length dependences of device mobilities and on/off ratios are included for SWNT TFTs. Time response of polymer-electrolyte gated SWNT TFTs has been measured to be ~300 Hz, and a proof-of-concept logic inverter has been fabricated by using polymer electrolyte gated SWNT TFTs for macroelectronic applications. Finally, I dedicated a chapter on scalable device designs based on aligned arrays of SWNTs, including a design for SWNT memory devices.

Seed and vegetative propagation methods for the rare Florida native species Chionanthus pygmaeus (pygmy fringetree)

Chionanthus pygmaeus Small (pygmy fringetree) (Oleaceae) is an endemic and rare Florida species, which has an attractive, small habit giving it great potential for use in managed landscapes. Members of the genus Chionanthus are difficult to propagate via cuttings and possess complex seed dormancies that are not well understood. Conservation of pygmy fringetree and its potential for commercial propagation for use in managed landscapes is contingent on a better understanding of its complex seed dormancy and enhancement of its propagation. I conducted two experiments to assess sexual and asexual propagation methods for pygmy fringetree. The first experiment was conducted to determine what factors are involved in overcoming seed dormancy. Various scarification treatments, which mimicked conditions seeds are exposed to in the wild, were investigated to determine their effects on germination of 20-year-old seeds originally collected from the species’ native range. Treatments included endocarp removal, sulfuric acid, boiling-water, and smoke-water treatments. Prior to treatment initiation, seed viability was estimated to be 12%. Treated seeds went through two cold- and two warm-stratification periods of 4°C and 25°C, respectively, in a dark growth chamber. After 180 days, none of the treatments induced early germination. Seeds were then tested for viability, which was 11%. Seed dormancy of the species is apparently complex, allowing some of the seeds to retain some degree of viability, but without dormancy requirements satisfied. The second experiment was conducted to assess if pygmy fringetree could be successfully propagated via hardwood or root cuttings if the appropriate combination of environmental conditions and hormones were applied. Hardwood and root cuttings were treated with either 1000 ppm IBA talc, 8000 ppm IBA talc, or inert talc. All cuttings were placed on a mist bench in a greenhouse for 9 weeks. Hardwood cuttings were supplemented with bottom heat at 24 °C. No treatments were successful in inducing adventitious root formation. I conclude that pygmy fringetree seeds possess complex dormancy that was not able to be overcome by the treatments utilized. However, this result is confounded by the age of the seeds used in the experiment. I also conclude that vegetative propagation of pygmy fringetree is highly dependent on the time of year cuttings are harvested. Further research of both seed and asexual propagation methods need to be explored before pygmy fringetree can be propagated on a commercial scale.

Material characterization and process development for indium-arsenide / indium-gallium-antimonide channel, high electron mobility transistors for low power, high speed applications

Efforts to push the performance of transistors for millimeter-wave and microwave applications have borne fruit through device size scaling and the use of novel material systems. III-V semiconductors and their alloys hold a distinct advantage over silicon because they have much higher electron mobility which is a prerequisite for high frequency operation. InGaAs/InP pseudomorphic heterojunction bipolar transistors (HBTs) have demonstrated fT of 765 GHz at room temperature and InP based high electron mobility transistors (HEMTs) have demonstrated fMax of 1.2 THz. The 6.1 A lattice family of InAs, GaSb, AlSb covers a wide variety of band gaps and is an attractive future material system for high speed device development. Extremely high electron mobilities ~ 30,000 cm^2 V^-1s^-1 have been achieved in modulation doped InAs-AlSb structures. The work described in this thesis involves material characterization and process development for HEMT fabrication on this material system.