Wei-Kai Hong

Controlling Steps During Early Stages of the Aligned Growth of Carbon Nanotubes Using Microwave Plasma Enhanced Chemical Vapor Deposition

Vertically aligned carbon nanotubes (CNTs) with controllable length and diameter fabricated by microwave plasma enhanced chemical vapor deposition (MPECVD) are of continuing interest for various applications. This paper describes the role of process gas composition as well as the pre-coating catalytic layer characteristics. It is observed that nucleation of CNTs was significantly enhanced by adding nitrogen in the MPECVD process, which also promoted formation of bamboo-like structures. The very first key step toward growth of aligned CNTs was the formation of high-density fine carbon onion encapsulated metal (COEM) particles under a hydrogen plasma. Direct microscopic investigation of their structural evolution during the very early stages revealed that elongation, necking, and splitting of the COEM particles occurred accompanying the growth of CNTs, such that one of the split portions rode on the top of the growing tube while the remaining one resided on the root. Our results suggest that CNTs grow via the “tip-growth” as well as “root-growth” mechanisms.

Low turn-on voltage field emission triodes with selective growth of carbon nanotubes

A low turn-on voltage, field emission triode array has been fabricated using the selective deposition of carbon nanotubes (CNTs) in a microwave plasma chemical vapor deposition (MPCVD) system. The field emission triodes exhibited a low turn-on voltage of 13 V and a large emission current of 23 /spl mu/A with the gate voltage at 60 V. Short-term stress reveals a 10% current fluctuation within 1800 sec. The excellent electric properties suggest that the array shows potential for application in field emission displays and vacuum microelectronics.

Fabrication and Characterization of Carbon Nanotube Triodes

To achieve high emission current density, carbon nanotube triodes have been proposed using recessed oxide as the insulator between the gate and the carbon nanotubes to replace the conventional spacer. An anode current of 10 µA was achieved with a gate voltage of 98 V for a nanotube sample prepared with a growth time of 12 min. Carbon nanotubes synthesized with various growth times were analyzed using a scanning electron microscope and Raman spectroscopy. By increasing the growth time, longer carbon nanotubes were obtained. Effects of the length of the carbon nanotubes on the field emission of the triodes are discussed. Enhanced luminance was obtained as the anode voltage increased from 600 V to 1000 V. Such carbon nanotube triodes are promising for utilization in future field-emission displays.

Field-Emission Properties of Aligned Carbon Nanotubes

Dense, well-separated, and aligned carbon nanotubes have been prepared via bias-enhanced microwave plasma chemical vapor deposition. The turn-on fields defined at the emission current density of 10 µA/cm2 are about 3.35 V/µm, 2.54 V/µm, and 3.54 V/µm, for the immersion times in PdCl2 of 1 min, 20 min, and 40 min, respectively. The corresponding emission current densities are about 0.97 mA/cm2, 4.5 mA/cm2, and 0.44 mA/cm2 at the electric field of 5 V/µm. The higher emission current obtained from the aligned carbon nanotubes for the immersion time of 20 min is ascribed to the denser and sharper nanotubes formed in this condition.

Improved Field-Emission Properties of Carbon Nanotube Field-Emission Arrays by Controlled Density Growth of Carbon Nanotubes

The density distribution of CNTs is one of the crucial parameters determing the field-emission property of CNTs. To effectively control the density of CNTs, an inactive thin-film layer was deposited on a catalyst. The results showed that improved field emission property could be obtained with a thin SiO layer on the catalyst layer as the precursor. For 3.5 nm Fe and 3.5 nm SiO on 3.5 nm Fe as a catalyst, the turn-on field could be decreased from 3.7 V/µm. to 2.2 V/µm and the field-emission current density increased from 2.6×10-8 A/cm2 to 2.4×10-4 A/cm2 when the applied field was 4 V/µm

Improvement of field emission characteristics of carbon nanotubes by excimer laser treatment

The density of carbon nanotubes (CNTs) deposited by microwave plasma chemical vapor deposition (MPCVD) is extremely high, which results in screening effect in the electric field. To achieve excellent field emission characteristics, an excimer laser treatment (ELT) was introduced to reduce the density of CNTs. Scanning electron microscopy (SEM) micrographs showed reduced densities of the CNTs, and the measurement of electrical characteristics results revealed the improved field emission properties under suitable ELT conditions. The turn-on field decreased from 3 V/µm to 2.1 V/µm, and the emission current density increased from 5.73 mA/cm2 to 87.13 mA/cm2 at the applied field of 5 V/µm.

Fabrication and Characterization of Various Carbon-Clad Silicon Microtips with Ultra-Small Tip Radii.

Various types of ultra sharp Si microtips and multitips with carbon-clading films were fabricated by microwave plasma chemical vapor deposition (MPCVD). The radii of these Si tips prepared by bias assisted carburization (BAC) can be reduced below 300 A under a low deposition temperature (<550°C). Field emission characterization was performed in a high vacuum environment. With an applied anode voltage of 1100 V, emission currents of 169 µA, 198 µA, and 385 µA can be achieved from an array of 50×50 BAC-clad Si monotips, Si multitips via high bias, and Si multitips via the Ar presputtering technique, respectively. Both the auger electron spectroscopy (AES) and X-ray photo-electron spectroscopy (XPS) studies of the C 1 s peak suggest that the BAC-cladding is more likely to be a carbon-rich SiC layer or a SiC layer mixed with a small amount of diamond nuclei. This BAC-carbon can be used as an effective nucleation layer for further diamond nuclei. Due to the low field emission, low temperature, and large area growth capability, the sharp BAC-clad Si multitip field emitter arrays are attractive for flat panel display applications.