Yongho Choi

Experimental characterization of single-walled carbon nanotube film-Si Schottky contacts using metal-semiconductor-metal structures

We demonstrate that single-walled carbon nanotube (CNT) films make a Schottky contact on silicon by experimentally characterizing metal-semiconductor-metal (MSM) structures. We find that at temperatures above 240K, thermionic emission is the dominant transport mechanism across CNT film-Si contacts, and at lower temperatures tunneling begins to dominate. At high bias voltages, the CNT film MSM devices exhibit a higher photocurrent-to-dark current ratio relative to that of metal control devices. Our results not only provide insight into the fundamental electronic properties of the CNT film-Si junction but also opens up the possibility of integrating CNT films as Schottky electrodes in conventional Si-based devices.

Resistivity scaling in single-walled carbon nanotube films patterned to submicron dimensions

The authors describe efficient patterning of transparent, conductive single-walled carbon nanotube thin films by photolithography and e-beam lithography followed by reactive ion etching, and study the transport characteristics of the films patterned down to 200nm lateral dimensions. The resistivity of the films is independent of device length, while increasing over three orders of magnitude compared to the bulk films, as their width and thickness shrink. This behavior is explained by a geometrical argument. Such “top-down” patterning of nanotube films should permit their integration into submicron device structures; however, the strong resistivity scaling will have to be taken into account.

Nanolithographic patterning of transparent, conductive single-walled carbon nanotube films by inductively coupled plasma reactive ion etching

The authors report successful patterning of transparent, conductive single-walled carbon nanotube films down to 100nm lateral dimensions by photolithography or e-beam lithography and subsequent O2 plasma etching using an inductively coupled plasma reactive ion etching (ICP-RIE) system. They systematically study the effect of ICP-RIE etch parameters, such as substrate bias power, chamber pressure, and substrate cooling, on the nanotube film etch rate and etch selectivity. They also characterize the effect of the linewidth etched on the nanotube film etch rate for widths ranging from 50μm down to 100nm. Furthermore, by fabricating standard four point probe structures using the patterning capability developed, the authors investigate the effect of different resist processes on the resistivity of patterned single-walled carbon nanotube films and the effect of ICP reactive ion etching on the resistivity of partially etched nanotube films. In addition, they demonstrate that using an ICP-RIE system provides sign...

Metal-semiconductor-metal photodetectors based on single-walled carbon nanotube film-GaAs Schottky contacts

We demonstrate the Schottky behavior of single-walled carbon nanotube (CNT) film contacts on GaAs by fabricating and characterizing metal-semiconductor-metal (MSM) photodetectors with CNT film electrodes. We extract the Schottky barrier height of CNT film contacts on GaAs by measuring the dark I-V characteristics as a function of temperature. The results show that at temperatures above ∼260 K, thermionic emission of electrons with a barrier height of ∼0.54 eV is the dominant transport mechanism in CNT film–GaAs junctions, whereas at lower temperatures, tunneling begins to dominate suggested by the weak dependence of current on temperature. Assuming an ideal MS diode, this barrier height corresponds to a CNT film workfunction of ∼4.6 eV, which is in excellent agreement with the previously reported values. Furthermore, we characterize the effect of device geometry on the dark current and find that dark currents of the MSM devices scale rationally with device geometry, such as the device active area, finger ...

GaN nanowire and Ga2O3 nanowire and nanoribbon growth from ion implanted iron catalyst

The authors experimentally demonstrate a simple and efficient approach for nucleating the catalytic chemical vapor deposition (CVD) growth of GaN nanowires, Ga2O3 nanowires, and Ga2O3 nanoribbons by using ion implantation of Fe+ into thermally grown SiO2 layers and subsequent annealing to form the catalyst nanoparticles. This work shows that ion implantation can be used as a versatile method to create catalyst nanoparticles for wide band gap nanowire/nanoribbon growth. They also demonstrate that ion implanted catalyst nanoparticles prepared under identical conditions can be used to grow different types of nanowires/nanoribbons containing Ga by simply changing the gas types and flow rates during CVD growth. Furthermore, they systematically characterize the structural properties of the as-grown nanomaterials, and find that the distance between the Ga source and the substrate, growth temperature, growth time, and flow rates are all critical parameters for nanowire growth. They explain the growth of single-crystal wurtzite GaN and monoclinic β-Ga2O3 nanowires by the vapor-liquid-solid (VLS) growth model, whereas the growth of monoclinic β-Ga2O3 nanoribbons by a combination of the VLS and vapor-solid (VS) mechanisms. This work opens up the possibility of controlling the origin of wide band gap nanowires/nanoribbons at the nanometer scale using the technique of catalyst ion implantation through a lithographically defined mask, of integrating nanowires/nanoribbons into nonplanar three-dimensional device structures, and of growing different Ga-based wide band gap semiconductor nanostructures on the same substrate by simply changing the gas types and flow rates.The authors experimentally demonstrate a simple and efficient approach for nucleating the catalytic chemical vapor deposition (CVD) growth of GaN nanowires, Ga2O3 nanowires, and Ga2O3 nanoribbons by using ion implantation of Fe+ into thermally grown SiO2 layers and subsequent annealing to form the catalyst nanoparticles. This work shows that ion implantation can be used as a versatile method to create catalyst nanoparticles for wide band gap nanowire/nanoribbon growth. They also demonstrate that ion implanted catalyst nanoparticles prepared under identical conditions can be used to grow different types of nanowires/nanoribbons containing Ga by simply changing the gas types and flow rates during CVD growth. Furthermore, they systematically characterize the structural properties of the as-grown nanomaterials, and find that the distance between the Ga source and the substrate, growth temperature, growth time, and flow rates are all critical parameters for nanowire growth. They explain the growth of single-cr...

Single-walled carbon nanotube growth from ion implanted Fe catalyst

The authors present experimental evidence that single-walled carbon nanotubes can be grown by chemical vapor deposition from ion implanted iron catalyst. They systematically characterize the effect of ion implantation dose and energy on the catalyst nanoparticles and nanotubes formed at 900°C. They also fabricate a micromachined silicon grid for direct transmission electron microscopy characterization of the as-grown nanotubes. This work opens up the possibility of controlling the origin of single-walled nanotubes at the nanometer scale and of integrating them into nonplanar three-dimensional device structures with precise dose control.

Patterned growth of silicon oxide nanowires from iron ion implanted SiO2 substrates

We demonstrate experimentally a simple and efficient approach for silicon oxide nanowire growth, by implanting Fe(+) ions into thermally grown SiO(2) layers on Si wafers and subsequently annealing in argon and hydrogen to nucleate the nanowires. We study the effect of implantation dose and energy, growth temperature, H(2) gas flow, and growth time on the silicon oxide nanowire growth. We find that sufficiently high implant dose, high growth temperature, and the presence of H(2) gas flow are crucial parameters for silicon oxide nanowire growth. We also demonstrate the patterned growth of silicon oxide nanowires in localized areas by lithographic patterning and etching of the implanted SiO(2) substrates before growth. We propose a simple physical model to explain the growth results. This works opens up the possibility of growing silicon oxide nanowires directly from solid substrates, controlling the location of nanowires at the submicron scale, and integrating them into nonplanar three-dimensional nanoscale device structures.

Field-emission properties of individual GaN nanowires grown by chemical vapor deposition

Single crystalline GaN nanowires were synthesized using chemical vapor deposition. Devices containing individual GaN nanowires were fabricated using contact printing. The local turn-on electric field at the tip of the GaN nanowires was compared to that of other nanomaterials. The quality of contact between GaN nanowires and metal electrodes was found to affect the field-emission behavior significantly. It was also observed that the field-emission behavior of individual GaN nanowires follows the conventional Fowler-Nordheim model in the range of applied electric fields.

High-aspect-ratio, 3-D micromachining of carbon-nanotube forests by micro-electro-discharge machining in air

This paper reports micro-electro-discharge machining of vertically aligned carbon nanotube forests for the formation of high-aspect-ratio, three-dimensional microstructures in the material. The developed forest machining method is a dry process performed in air, generating high-frequency pulses of electrical discharge to locally machine the nanotubes in order to create target shapes in a forest. With this approach, forest microstructures can be fabricated to have varying shapes along their height, unachievable with conventional pre-patterned chemical vapor deposition growth techniques. The use of the pulses with a minimized discharge energy defined with 35 V and 10 pF in the discharge generation circuit leads to an aspect ratio of 20 with the smallest feature of 5 µm in forests without disordering the vertical orientation of the nanotubes. Micromachining of multilayer geometries as well as arrayed needle-like microstructures with angled surfaces is demonstrated.

Percolation transport in single-walled carbon nanotube films: experiment and simulation

We present the scaling of percolation resistivity in nanotube films as a function of nanotube and device parameters both experimentally and using simulations. We first characterize the resistivity of these films down to 200 nm lateral dimensions by fabricating standard four-point-probe structures. We find that the film resistivity starts to increase at device widths below 20 microns, and exhibits an inverse power law dependence on width below a critical width of 2 microns. We then use quasi-3D Monte Carlo simulations to model and fit these experimental results. In addition to fitting the experimental data, we also study the effect of four parameters, namely nanotube density, length, alignment, and measurement direction on resistivity and its scaling with device width. We explain these simulation results by simple physical and geometrical arguments. Nanoscale study of percolation transport mechanisms in nanotube films is essential for understanding and characterizing their performance in nanosensing device applications.