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Title: Tunneling and Transport in Nanowires

Abstract

The goal of this program was to study new physical phenomena that might be relevant to the performance of conductive devices and circuits of the smallest realizable feature sizes possible using physical rather than biological techniques. Although the initial scientific work supported involved the use of scanning tunneling microscopy and spectroscopy to ascertain the statistics of the energy level distribution of randomly sized and randomly shaped quantum dots, or nano-crystals, the main focus was on the investigation of selected properties, including superconductivity, of conducting and superconducting nanowires prepared using electron-beam-lithography. We discovered a magnetic-field-restoration of superconductivity in out-of-equilibrium nanowires driven resistive by current. This phenomenon was explained by the existence of a state in which dissipation coexisted with nonvanishing superconducting order. We also produced ultra-small superconducting loops to study a predicted anomalous fluxoid quantization, but instead, found a magnetic-field-dependent, high-resistance state, rather than superconductivity. Finally, we developed a simple and controllable nanowire in an induced charged layer near the surface of a masked single-crystal insulator, SrTiO 3. The layer was induced using an electric double layer transistor employing an ionic liquid (IL). The transport properties of the induced nanowire resembled those of collective electronic transport through an array of quantummore » dots.« less

Authors:
 [1]
  1. Univ. of Minnesota, Minneapolis, MN (United States)
Publication Date:
Research Org.:
Univ. of Minnesota, Minneapolis, MN (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1295659
Report Number(s):
1
TRN: US1700254
DOE Contract Number:
FG02-02ER46004
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
77 NANOSCIENCE AND NANOTECHNOLOGY; 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; SUPERCONDUCTIVITY; NANOWIRES; ELECTRON BEAMS; QUANTUM DOTS; TUNNEL EFFECT; ELECTRIC CONDUCTIVITY; SCANNING TUNNELING MICROSCOPY; MAGNETIC FIELDS; RANDOMNESS; STATISTICS; ENERGY LEVELS; MAGNETIC FLUX; QUANTIZATION; SPECTROSCOPY; STRONTIUM TITANATES; electrical transport; electron-beam-lithography

Citation Formats

Goldman, Allen M. Tunneling and Transport in Nanowires. United States: N. p., 2016. Web. doi:10.2172/1295659.
Goldman, Allen M. Tunneling and Transport in Nanowires. United States. doi:10.2172/1295659.
Goldman, Allen M. 2016. "Tunneling and Transport in Nanowires". United States. doi:10.2172/1295659. https://www.osti.gov/servlets/purl/1295659.
@article{osti_1295659,
title = {Tunneling and Transport in Nanowires},
author = {Goldman, Allen M.},
abstractNote = {The goal of this program was to study new physical phenomena that might be relevant to the performance of conductive devices and circuits of the smallest realizable feature sizes possible using physical rather than biological techniques. Although the initial scientific work supported involved the use of scanning tunneling microscopy and spectroscopy to ascertain the statistics of the energy level distribution of randomly sized and randomly shaped quantum dots, or nano-crystals, the main focus was on the investigation of selected properties, including superconductivity, of conducting and superconducting nanowires prepared using electron-beam-lithography. We discovered a magnetic-field-restoration of superconductivity in out-of-equilibrium nanowires driven resistive by current. This phenomenon was explained by the existence of a state in which dissipation coexisted with nonvanishing superconducting order. We also produced ultra-small superconducting loops to study a predicted anomalous fluxoid quantization, but instead, found a magnetic-field-dependent, high-resistance state, rather than superconductivity. Finally, we developed a simple and controllable nanowire in an induced charged layer near the surface of a masked single-crystal insulator, SrTiO3. The layer was induced using an electric double layer transistor employing an ionic liquid (IL). The transport properties of the induced nanowire resembled those of collective electronic transport through an array of quantum dots.},
doi = {10.2172/1295659},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2016,
month = 8
}

Technical Report:

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  • We present the results of a three year LDRD project that focused on understanding the impact of defects on the electrical, optical and thermal properties of GaN-based nanowires (NWs). We describe the development and application of a host of experimental techniques to quantify and understand the physics of defects and thermal transport in GaN NWs. We also present the development of analytical models and computational studies of thermal conductivity in GaN NWs. Finally, we present an atomistic model for GaN NW electrical breakdown supported with experimental evidence. GaN-based nanowires are attractive for applications requiring compact, high-current density devices such asmore » ultraviolet laser arrays. Understanding GaN nanowire failure at high-current density is crucial to developing nanowire (NW) devices. Nanowire device failure is likely more complex than thin film due to the prominence of surface effects and enhanced interaction among point defects. Understanding the impact of surfaces and point defects on nanowire thermal and electrical transport is the first step toward rational control and mitigation of device failure mechanisms. However, investigating defects in GaN NWs is extremely challenging because conventional defect spectroscopy techniques are unsuitable for wide-bandgap nanostructures. To understand NW breakdown, the influence of pre-existing and emergent defects during high current stress on NW properties will be investigated. Acute sensitivity of NW thermal conductivity to point-defect density is expected due to the lack of threading dislocation (TD) gettering sites, and enhanced phonon-surface scattering further inhibits thermal transport. Excess defect creation during Joule heating could further degrade thermal conductivity, producing a viscous cycle culminating in catastrophic breakdown. To investigate these issues, a unique combination of electron microscopy, scanning luminescence and photoconductivity implemented at the nanoscale will be used in concert with sophisticated molecular-dynamics calculations of surface and defect-mediated NW thermal transport. This proposal seeks to elucidate long standing material science questions for GaN while addressing issues critical to realizing reliable GaN NW devices.« less
  • The formulation of carrier transport through heterojunctions by tunneling and thermionic emission is derived from first principles. The treatment of tunneling is discussed at three levels of approximation: numerical solution of the one-band envelope equation for an arbitrarily specified potential profile; the WKB approximation for an arbitrary potential; and, an analytic formulation assuming constant internal field. The effects of spatially varying carrier chemical potentials over tunneling distances are included. Illustrative computational results are presented. The described approach is used in exploratory physics models of irradiated heterojunction bipolar transistors within Sandia's QASPR program.
  • A cold stage was built for in-situ UPS measurements. Preliminary results show a Fermi edge that disappears as the sample is warmed up in a vacuum. This surface degradation is reversible by annealing at 400C in pure oxygen. The films made by co-evaporation have improved. Films on bare silicon have zero resistance by 73K. The best films grow on SrTiO{sub 3} (110) with zero resistance by 84K and critical current densities greater than l million A/cm{sup 2} below 67K. A unique system of loadlocks was implemented that allows transfer of a film from the synthesis chamber to the low temperaturemore » STM for analysis. This has yielded, for the first time, good images of the YBCO surface. Spectroscopic measurements indicate thus far that most of the surface is normal, with a few spots showing superconducting behavior and a large superconducting gap.« less
  • We have measured reproducible tunneling spectra on YBa{sub 2}Cu{sub 3}0{sub 7} thin films with a cryogenic scanning tunneling microscope. We find that the I-V curves are generally of three types. The most common type, featured in a large majority of the data, shows a region of high conductance at zero bias. The amplitude of this region is inversely proportional to the tunneling resistance between the tip and sample. It is possible that this can be explained in terms of Josephson effects with the films, although an alternative is given based on electronic self-energy corrections. Data showing capacitive charging steps aremore » analyzed in terms of two ultrasmall tunnel junctions in series.« less
  • Thin films of high-temperature superconductors were made by sputtering and by co-evaporation. The former method produced thin films of YBaCuO which were completely superconducting at 82 K with 6 K transition widths. A new method was developed at the University of Texas which produced films of YBaCuO on strontium titanate that were superconducting at 84 K. This method minimizes the process temperature and produces films that are superconducting without the need for annealing. The films were also grown on silicon and sapphire substrates with zero resistance of 68 K or better. A low-temperature scanning tunneling microscope was used to studymore » the superconducting properties of these and other samples. High quality spectroscopic data was obtained which yields a value of approximately 11 for the ratio of the superconducting gap to the transition temperature.« less