Fabrication of self-defined gated field emission devices on silicon substrates using PECVD-grown carbon nano-tubes

Fabrication of novel self-defined gated field-emission devices on silicon substrates using vertically grown carbon nano-tubes (CNT) is reported. Carbon nano-tubes are grown in a PECVD reactor from the Ni catalyst islands at a pressure of 1.6 Torr with a mixture of C₂H₂ and H₂ gases with 5 and 30 sccm flows, respectively. The growth occurs at temperatures ranging between 550 and 650 °C and CNT’s are electrically isolated by a TiO₂ film. Silver is used as the metal gate and complete fabrication of transistors requires removing the insulating layer from top of the tip followed by one step of plasma ashing. With a voltage applied between gate and the cathode electrode, the emission current from cathode to anode shows a significant drop, indicating proper control of gate on the anode current.     

Quantum-mechanical simulations of field emission from carbon nanotubes

We present simulations of field emission from 2-nm long open (5,5), closed (5,5) and open (10,0) carbon nanotubes. Besides usual effects associated with the field-emission process, the total-energy distributions of the field-emitted electrons present peaks that are shifted by the electric field. Their sharpness and the evolution of their amplitude when changing the electric field depend on the semiconducting or metallic character of the nanotube.     

Synthesis of patterned carbon nanotube arrays for field emission using a two layer Sn/Ni catalyst in an ethanol flame

Patterned carbon nanotube (CNT) arrays on Si substrate have been fabricated by using a two layer Sn/Ni catalyst in a diffusion ethanol flame. Vertically well-aligned CNT arrays were achieved on a Si substrate without any catalyst pretreatment. The Sn underlayer activated the substrates for CNT growth with Ni as catalyst, and provided a good contact between CNTs and the substrate, which is useful for field emission. Since the adhesion of Sn/Ni nanoparticles to the substrate is very strong, the growth of the CNTs follows a base-growth mode. The thickness of the Sn underlayer largely determines the diameter and diameter distribution of the as-grown CNTs. The morphologies and field electron emission properties of CNT arrays grown on Si substrates with different thicknesses of Sn and growth times have been investigated. The variation of emission current density was less than 5% during a 4 h test under a field of 1.77 V/µm.     

In-situ studies of electron field emission of single carbon nanotubes inside the TEM

Electron field emission characteristics of individual multi-walled carbon nanotubes (MWCNTs) were investigated in situ inside the transmission electron microscope (TEM). For a single MWCNT it was found that while field-emission can hardly occur from the side of the nanotube, a curved nanotube may result in finite side emission and the best emission geometry is the top emission geometry. Current-voltage (I-V) measurements made at different vacuum conditions and voltage sweeps emphasize the importance of the adsorbates on the electron field emission of MWCNTs. For a contaminated MWCNT, although the field emission current was reduced, the stability of its emission was improved. A current of up to several tens of μA was observed for a single MWCNT, but it was found that long time emission usually results in drastic structure damage that may lead to sudden emission failure.     

Improved field emission stability from single-walled carbon nanotubes chemically attached to silicon

ABSTRACT: Here we demonstrate the simple fabrication of a single walled carbon nanotube (SWCNT) field emission electrode which shows excellent field emission characteristics and remarkable field emission stability without requiring post treatment. Chemically functionalized SWCNTs were chemically attached to a silicon substrate. The chemical attachment led to vertical-alignment of SWCNTs on the surface. Field emission sweeps and Fowler-Nordheim plots showed that the Si-SWCNT electrodes field emit with a low turn-on electric field of 1.5 V mu m-1 and high electric field enhancement factor of 3965. The Si-SWCNT electrodes were shown to maintain a current density of > 740 mu A cm -1 for 15 hr with negligible change in applied voltage. The results indicate that adhesion strength between SWCNTs and substrate is a much greater factor in field emission stability than previously reported.     

Thermionic emission of amorphous diamond and field emission of carbon nanotubes

Thermal-field emission characteristics from nano-tips of amorphous diamond and carbon nanotubes at various temperatures are reported in this study. Amorphous diamond emitted more than 13 times more electrons at a temperature of 300 °C than at room temperature. In contrast, CNTs exhibited no increase of emitted current upon heating to 300 °C. The thermally agitated emission of amorphous diamond is attributed to the presence of defect bands. The formation of these defect bands raises the Fermi level into the upper part of the band gap, and thus reduces the energy barrier that the electrons must tunnel through. From defect bands within the band gap, the conduction band electrons were significantly increased due to electron tunnels from defect bands. The enhanced thermal-field emission originating from defect bands was observed in this study. This thermally agitated behavior of field emission for amorphous diamond was highly reproducible as observed in this research.     

Density control of carbon nanotubes and filaments films by wet etching of catalyst particles and effects on field emission properties

Carbon nanotubes and filaments (CNT&F) films with controlled density were grown by low pressure thermal chemical vapour deposition from acetylene on nickel nanoparticles. Density control was achieved by wet etching of the catalyst particles before carbon growth. Field emission measurements were carried out on several films with different CNT&F densities obtained with this method. Despite strong morphological changes, only slight differences in the field emission characteristics between the highest and the lowest density films were detected, suggesting that almost none of the CNT&F suppressed by the etching step took part to the field emission. However, a maximum field amplification factor was reached for the medium density film. Taking into account the field amplification factor distribution, a model is proposed to link the particles diameter distribution, the CNT&F film morphology and the field emission properties.     

Array geometry, size and spacing effects on field emission characteristics of aligned carbon nanotubes

Microwave plasma-enhanced chemical vapor deposition (MPECVD) has been shown capable of producing vertically aligned mutli-walled CNTs as a result of self-bias of the microwave plasma. These CNTs are relevant to field emission applications. However, it is also known that closely packed or mat-like CNTs are not effective field emitters due to field screening effects among neighboring tubes. In this study, an approach whereby “micro-” patterning of CNT arrays, adjusting their geometry, size and array spacing by conventional photolithography, rather than “nano-” patterning a single CNT by electron-beam lithography, is employed to fabricate efficient emitters with enhanced field emission characteristics. MPECVD with catalysts are used on Si substrate to fabricate micropatterned vertically aligned CNT arrays with various geometries, sizes and spacing. The field emission results show that a circular array with 20 μm spacing has the lowest turn-on field of 2 V/μm at 1 μA/cm² and achieves the highest current density of 100 μA/cm² at 3 V/μm. Investigation on the array spacing effect shows that 10 × 10 μm CNT square array with an array spacing of 20 μm displays the lowest turn-on field of 9 V/μm and achieved a very high current density of 100 mA/cm² at 20 V/μm. Furthermore, the results suggest that the array spacing of the 10 × 10 μm CNT square array can be reduced to at least 20 μm without affecting the field enhancement factor of the emitter. The results clearly indicate further optimization of spacing in the arrays of CNT emitters could result in lower turn-on field and higher current density.     

Interaction between carbon nanotubes and substrate and its implication on field emission mechanism

The interaction between carbon nanotubes (CNTs) and substrate plays an important role in the process of field emission. A double-barrier model is adopted to analyze the difference of field emission for various CNT films. Result shows that the width of interface barrier determines the emission performance. For CNTs on titanium, the best emission performance is attributed to the removal of interlayer barrier by the formation of conductive titanium carbide. These facts might shed new light on the field emission mechanism for CNTs.     

Carbon nanotubes field emission integrated triode amplifier array

Because the high frequency operation of a field emission triode amplifier is dictated by the cutoff frequency and not the electron transit time, a high ratio of transconductance, g_m to the overlapping interelectrode capacitance, C_g is the desired outcome. Consequently, to achieve high frequency performance of the CNT amplifier array in this study, C_g was reduced by performing a dual-mask photolithography process to minimize the overlapping gate area, and, the insulating layer's thickness was increased. Moreover, wedge-shaped CNT emitter arrays are employed to increase emission sites, resulting in return higher g_m. Both dc and ac performance of the amplifier were characterized. The triode amplifier array exhibited a high current of  0.32 mA (74 mA/cm²), g_m of  63 μS and voltage gain of  18 dB. Frequency response of the triode amplifier up to 20 kHz was also investigated. A theoretical cutoff frequency of > 70 MHz could be achieved with proper shielding of the test setup.     

Fabrication of efficient field emitters with thin multiwalled carbon nanotubes using spray method

Thin multiwalled carbon nanotube (t-MWCNTs)-based field emitters are made by use of a spray method. The number of tube walls is between 2 and 6, with the corresponding outer diameters between 3 and 6 nm. They were dispersed in dichloroethane and sprayed onto metal-deposited indium tin oxide glass. After heat treatment, they were found to be tightly adhered to metal electrode. Excellent field emission characteristics were exhibited, with a large field enhancement factor and low turn-on voltage, comparable to those of singlewalled CNTs. However, the t-MWCNTs demonstrated a significantly lower degradation rate than SWCNTs in the emission current. This high emission stability was attributed to their stable edge structures, similar to conventional large-diameter MWCNTs. Therefore, t-MWCNTs could be utilized as an alternative material for field emitters.     

Influence of electric field and emission current on the configuration of nanotubes in carbon nanotube layers

The influence of applied electric field (E_av) and emission current (I_FE) on the configuration of conical layers carbon nanotubes (CLNTs) grown by CVD on the edge of Ni foil has been investigated. TEM profile imaging revealed a high concentration of nanotubes near the foil edge surface, whereas on the nanotube layers’ outer surfaces single, non-oriented nanotubes with open ends free of catalytic particles, were observed. After sufficient electric field application many nanotubes became oriented towards the anode, but one or two of them were found to be always a few microns more extended. In situ SEM investigation showed that below E_av = 3.2–3.9 V/μm, emission was achieved at the expense of originally existing free nanotube ends. Configuration changes began at larger electric fields. On the observed foil edge length (14.6–17.8 μm, with an edge thickness of 200 μm) one or two nanotubes extended towards the anode and probably became the main emitters. Upon further increasing the field to E_av = 5.7–8 V/μm and at an emission current I_FE = 2 × 10⁻⁵ A these tubes disappeared (or essentially shortened). At E_av = 8 V/μm and higher and at an exposure time up to 40 min, several tens of extended nanotubes appeared, with one or two extended well beyond the others. This nanotube configuration pattern is connected with electrostatic screening between the nanotubes. Our interpretation of the data suggests that in the investigated range of E_av and I_FE, a limited number of nanotubes are emitting and these nanotubes are constantly changing as E_av, I_FE and exposure time increase.     

Enhancement of the field emission properties of low-temperature-growth multi-wall carbon nanotubes by KrF excimer laser irradiation post-treatment

Multi-wall carbon nanotube (CNT) films were fabricated by microwave plasma chemical vapor deposition at low temperatures ( 500 °C). The films when properly post-treated by laser irradiation exhibited a factor of 2–3 enhancement in the emission current, while the turn-on field (E_on) was reduced from 4.89–5.22 to 2.88–3.15 V/μm. The introduction of excessive oxygen during laser irradiation, however, degrades the performance of field emission properties drastically. Raman spectroscopy measurements revealed the intimate correlation between the parameter I_D/I_G (intensity ratio between the two representative Raman peaks seen in carbon nanotubes) and the field emission performance. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses showed that the irradiation-induced modification of the tube morphology and crystallinity might be responsible for the observations.     

Carbon nanotubes field emission devices grown by thermal CVD with palladium as catalysts

The effects of palladium (Pd) catalyst film thickness and ammonia (NH₃) in thermal chemical vapor deposition (CVD) growth of carbon nanotubes (CNTs) are systematically compared per the resulting morphologies, Raman spectra and field emission characteristics. The CNT field emitters were tested under identical experimental configurations. Field emission characteristics were described with Fowler-Nordheim field emission theory. Experimental results demonstrate that thermally grown CVD CNTs configured as diode field emitters exhibit low turn-on fields and high emission current density. The work is extended to include the study of gated field emitters or field emission triode, important to achieving high-resolution, full gray-scale imaging for field emission, flat-panel displays. The gated device was fabricated utilizing single-mask, self-aligned gate electrode with conventional integrated-circuit (IC) fabrication process. The CNT-triode showed gate-controlled modulation of emission current where higher gate voltage gives rise to higher anode currents. The triode fabrication process using silicon-on-insulator (SOI) wafers is discussed.     

Patterned growth of carbon nanotubes over vertically aligned silicon nanowire bundles for achieving uniform field emission

A fabrication strategy is proposed to enable precise coverage of as-grown carbon nanotube (CNT) mats atop vertically aligned silicon nanowire (VA-SiNW) bundles in order to realize a uniform bundle array of CNT-SiNW heterojunctions over a large sample area. No obvious electrical degradation of as-fabricated SiNWs is observed according to the measured current-voltage characteristic of a two-terminal single-nanowire device. Bundle arrangement of CNT-SiNW heterojunctions is optimized to relax the electrostatic screening effect and to maximize the field enhancement factor. As a result, superior field emission performance and relatively stable emission current over 12 h is obtained. A bright and uniform fluorescent radiation is observed from CNT-SiNW-based field emitters regardless of its bundle periodicity, verifying the existence of high-density and efficient field emitters on the proposed CNT-SiNW bundle arrays.     

A novel fabrication approach for carbon nanotube lateral field emission devices

A novel fabrication approach for growing carbon nanotubes (CNTs) laterally and selectively on the tip region of lateral micro-fingers with built-in metallic anode utilizing a single-mask microfabrication process is presented. The selective growth of the CNTs was achieved with a two-step microwave plasma-enhanced chemical vapor deposition process involving a pre-growth hydrogen plasma treatment. Without plasma pretreatment, CNTs were found to grow along edges of the sandwiched tri-metal layer including the anode. Interestingly, with plasma pretreatment, CNTs grew selectively near the sharp tip region. Moreover, specific CNTs could be selectively synthesized on the tip region by optimizing the plasma pretreatment and the growth time. In essence, a lateral field emission device having CNT emitters with integrated metallic anode can be fabricated in just a single-mask microfabrication process. This approach can enhance the feasibility of integrating CNTs into vacuum integrated circuits.     

Effects of CF4 plasma on the field emission properties of aligned multi-wall carbon nanotube films

We present a simple method to functionalize the surface and to modify the structures of aligned multi-wall carbon nanotube (CNT) arrays grown on silicon substrates using CF₄ plasma produced by reactive ion etching (RIE). Field emission (FE) measurements showed that after 2 min of plasma treatment, the emission currents were enhanced compared with as-grown CNTs; however, extended treatment over 2 min was found to degrade the FE properties of the film. Scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy have been employed to investigate the mechanism behind the modified FE properties of the CNT film. The FE enhancement after 2 min of etching could be attributed to favorable surface morphologies, open-ended structures and a large number of defects in the aligned CNT films. On the other hand, deposition of an amorphous layer comprising carbon and fluorine during extended CF₄ plasma treatment may hamper the field emission of CNT films.     

Pulsed laser deposition-assisted patterning of aligned carbon nanotubes modified by focused laser beam for efficient field emission

Patterned carbon nanotube (CNT) arrays have been synthesized on patterned substrates created via pulsed laser deposition (PLD) of the precursor catalyst films with a mask. Arrays of CNTs in square and hexagonal patterns with tube lengths of 8 μm and 16 μm were created on silicon or quartz substrates, respectively. Using the method of laser cutting, as-grown CNT patterns were pruned by focused He-Ne laser beam. It is found that after pruning, CNTs tend to cluster together and form welded junctions. The comparison of field emission properties of CNTs before and after pruning shows that laser modification of CNT morphologies effectively enhanced the emission currents.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.     

Electrophoretic deposition of carbon nanotubes

Electrophoretic deposition (EPD) has been gaining increasing interest as an economical and versatile processing technique for the production of novel coatings or films of carbon nanotubes (CNTs) on conductive substrates. The purpose of the paper is to present an up-to-date comprehensive overview of current research progress in the field of EPD of CNTs. The paper specifically reviews the preparation and characterisation of stable CNT suspensions, and the mechanism of the EPD process; it includes discussion of pure CNT coatings and CNT/nanoparticle composite films. A complete discussion of the EPD parameters is presented, including electrode materials, deposition time, electrode separation, deposition voltage and resultant electric field. The paper highlights potential applications of the resulting CNT and CNT/composite structures, in areas such as field emission devices, fuel cells, and supercapacitors.