Gated Field Emitter Arrays Research Articles

Modeling the Temporal Response of Gated ZnO Nanowire Field Emitter by Considering the Charging and Self-Heating Effect for Improving the Response Speed

ZnO nanowire is a promising candidate for large-area gated field emitter arrays. How to improve its temporal response is one of the key problems to be solved for applications. In this work, a device model for a gated ZnO nanowire field emitter with consideration of charging and self-heating effect has been established to investigate its temporal response. It is found that while the charging effect is responsible for the delay at the beginning of the pulse, the self-heating effect which induces delay due to the thermal conduction process can shorten the charging time because of its lowering of nanowire resistance. The response time can be minimized when these two effects are balanced at an optimal field which is below the critical field for thermal runaway. We further investigate the optimal response time of a nanowire with the same resistance but a different length, radius, and electrical properties. The results imply that a lower heat capacity and higher critical temperature for thermal runaway are in favor of a shorter response time, which must be taken into account in the reduction in nanowire resistance for improving response speed. All the above should be useful for the device design of a fast-response gated ZnO nanowire field emitter array.

Effects of gases on the field emission performance of silicon gated field emitter array

Effects of gases on field emission performance were measured using silicon-gated field emitter arrays. Gas was injected into a vacuum chamber with a 1000 × 1000 tip array, which was driven by a DC gate and collector voltages. The collector voltage was fixed at 200 V while the gate voltage was swept to 40 V. For the gas exposure study, N2, He, and Ar were used. The sets of partial pressures, 5 × 10−6, 5 × 10−5, and 5 × 10−4 Torr, were used for the experiment. It was observed that N2 had the least effect and Ar had the worst effect on emission current performance. The degradation of collector current at 5 × 10−4 Torr pressure for Ar was ≈65% where for the N2, at the same level of pressure, the degradation was ≈41%. However, further experiments with high purity Ar gas showed that it was the water vapor present in the gas itself that was the primary cause of reduction in emission current and not the gas itself. The results expressed in reduction in emission current versus Langmuir exposure versus the current clearly showed the effect of water vapor. After the vacuum was recovered, the work function again restored partially to its original value. After ultraviolet light cleaning, the emission current was restored completely to the original state.

Demonstration of a silicon gated field emitter array based low frequency Colpitts oscillator at 400 °C

Silicon gated field emitter arrays have been used as a vacuum transistor to demonstrate a 152 kHz Colpitts oscillator. The transfer and output characteristics of the 1000 × 1000 silicon arrays were measured using a collector placed ≈ 1 mm away with a gate voltage up to 40 V and a collector voltage up to 200 V. The data were used to establish an LTspice transistor model based on a field emission tip model and a collector current model that fit the characteristics. Then, the LTspice model was used to design a low frequency Colpitts oscillator. Furthermore, experiments were carried out to successfully demonstrate the oscillation. Oscillation frequency was 152 kHz with a peak to peak voltage of 25 V for a tip to ground series resistance value of 10 kΩ at 50 V on the gate and 210 V on the collector. Further, the oscillator was also tested at 50, 100, 200, 300, and 400 °C. It was observed that frequency shifts for each temperature which is due to the change in the overall capacitance of the test setup. This type of device could be used as a temperature sensor in harsh environments.

Quantitative Analysis on the Field Strength in the Addressable Gated ZnO Nanowire Field Emitter Arrays: Model and Experiment

Addressable gated field emitter arrays (FEAs) have important applications in vacuum microelectronic devices. To fabricate high-performance device, a comprehensive understanding on the field emission characteristics of gated FEAs is necessary, which requires a quantitative analysis method. In this work, a general model based on Fowler-Nordheim (FN) theory has been established to fulfill this blank. It is found that nonlinear FN plot with positive and negative slopes could occur in gated FEAs, which its turning point is related to the proportion of anode and gate field. This provides a way for obtaining the field strengths applied by the anode and gate structures. Besides, the transconductance of the gated FEAs increases exponentially with the total surface field, which indicates that both the anode and gate field need to be increased for achieving a high transconductance device. As a demonstration, the addressable gated ZnO nanowire FEAs using multimicrosize pattern with radius as small as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.5 ~\mu \text{m}$ </tex-math></inline-formula> have been designed and fabricated. The relationship between the transconductance and anode voltage can be well fitted by using the model, which deduces the proportion of anode and gate field in the device to be about 3:1.

Ultraviolet light stimulated water desorption effect on emission performance of gated field emitter array

The performance of silicon gated field emission arrays (GFEAs) was characterized before and after ultraviolet (UV) light exposure. Emission and gate leakage currents were measured on 1000 × 1000 tip arrays by sweeping the gate voltage to 40 V DC with a fixed DC collector voltage of 100 V DC. UV light exposure was used to desorb water molecules from the GFEA surfaces. It was found that, before UV exposure, the gate current was 6 mA at 40 V, whereas after 70 min of UV exposure, the gate current decreased to 0.46 mA, indicating a more than ten times reduction in leakage current between the gate and the emitter. Similarly, the observed collector current was 94 μA at 40 V before exposure, and after UV exposure, the collector current increased to 1.33 mA, indicating an improvement of more than 14 times. During the experiments with UV light, residual gas analyzer measurements showed that the partial pressure for water increased by greater than ten times after 60 min of exposure and then decreased by 1 order of magnitude after 100 min of exposure. The emission and leakage current changes remained even after turning off the UV lamps for several tens of minutes; however, upon the exposure to the atmosphere for a few days, those changes reversed. The enhancement could again be observed after additional UV exposure indicating that the adsorbates (mainly water along with others) on the surface affected the leakage between gate and emitter and field emission. Based on analysis of the IV characteristics before and after UV exposure, the work function of the emitter surfaces increases while the portion of the array tips that emits expands resulting in a decrease in the calculated array tip sharpness as duller tips now emit.

Coplanar-gate ZnO nanowire field emitter arrays with enhanced gate-control performance using a ring-shaped cathode

Nanowire field emitters have great potential for use as large-area gated field emitter arrays (FEAs). However, the micrometer-scale cathode patterns in gated FEA devices will reduce regulation of the gate voltage and limit the field emission currents of these devices as a result of field-screening effect among the neighboring nanowires. In this article, a ring-shaped ZnO nanowire pad is proposed to overcome this problem. Diode measurements show that the prepared ring-shaped ZnO nanowire pad arrays shows uniform emission with a turn-on field of 5.9 V/µm and a field emission current density of 4.6 mA/cm2 under an applied field of 9 V/µm. The ZnO nanowire pad arrays were integrated into coplanar-gate FEAs and enhanced gate-controlled device characteristics were obtained. The gate-controlled capability was studied via microscopic in-situ measurements of the field emission from the ZnO nanowires in the coplanar-gate FEAs. Based on the results of both simulations and experiments, we attributed the enhanced gate-controlled device capabilities to more efficient emission of electrons from the ZnO nanowires as a result of the increase edge area by designing ring-shaped ZnO nanowire pad. The results are important to the realization of large-area gate-controlled FEAs based on nanowire emitters for use in vacuum electronic devices.

Optimizing the Field Emission Properties of ZnO Nanowire Arrays by Precisely Tuning the Population Density and Application in Large-Area Gated Field Emitter Arrays.

Zinc oxide (ZnO) nanowires are prepared for application in large area gated field emitter arrays (FEAs). By oxidizing Al-coated Zn films, the population density of the ZnO nanowires was tuned precisely by varying the thickness of the Al film. The nanowire density decreased linearly as the thickness of the Al film increased. Optimal field emission properties with a turn-on field of 6.21 V μm-1 and current fluctuations less than 1% are obtained. This can be explained by the minimized screening effect and good electrical conductivity of the back-contact layer. The mechanism responsible for the linear variation in the nanowire density is investigated in detail. Addressable FEAs using the optimal ZnO nanowire cathodes were fabricated and applied in a display device. Good gate-controlled characteristics and the display of video images are realized. The results indicate that ZnO nanowires could be applied in large area FEAs.

微細加工で作るフィールドエミッタアレイ～新しい応用に向けて～

Fabrications of gated field emitter array (FEA), such as Si-FEA and Spindt-type FEA, is overviewed from the conventional method to the latest one. The invention of an etch-back method makes it possible to form the gate electrode on the emitter tip, irrespective of the emitter shape, and also possible to form the multi-stacked gate electrode for beam focusing. A hafnium carbide coating can enhance the electron emission and emission lifetime, significantly. The historical Spindt-type FEA fabrication is also progressing. Using a double-layered photoresist, instead of aluminum parting layer, makes it possible to apply the multi-gate formation even on the Spindt-type FEA. These FEAs are applicable for many kinds of attractive devices, such as ultra-high sensitive image sensor, radiation tolerant electronics, and stationary X-ray source for medical applications.

Integration of ZnO nanowires in gated field emitter arrays for large-area vacuum microelectronics applications

Addressable field emitter arrays (FEAs) have important applications in vacuum electronic devices. However, it is important to integrate nanowire emitters into a gated structure without influencing the device structure and maintain the excellent field emission properties of nanowire emitters in the FEAs after the fabrication process. In this study, gate-structure ZnO nanowire FEAs were fabricated by a microfabrication process. The structure combines a planar gate and an under-gate, which is compatible with the preparation of ZnO nanowire emitters. The effect of electrode materials on the field emission properties of ZnO nanowires was studied using a diode structure, and it was found that ZnO nanowire pads on indium-tin-oxide (ITO) electrode showed better field emission performance compared with chromium (Cr) electrode. In addition, effective emission current modulation by the gate voltage was achieved and the addressing capability was demonstrated by integrating the ZnO nanowire FEAs in a vacuum-encapsulated field emission display. The reported technique could be a promising route to achieve large area addressable FEAs.

Experimental study of ultra-sharp silicon nano-tips

This paper presents two approaches on fabrication gated field emitter array from uniform single crystalline nano-tips. Investigation into electrical characterization of these silicon field emitters under an ultra high vacuum system is reported. Extensive experiment results were analyzed, in particular for the field emitter arrays with PECVD silicon dioxide as the insulating layer. Emission current fluctuation was significantly reduced after long time seasoning treatment. A low turn on voltage of 29 V was obtained. Emission current of 43.3 μA at a gate voltage of 92 V was available for a 10×10 field emitter array.

Optimization of Electron Beam Focusing for Gated Carbon Nanotube Field Emitter Arrays

We fabricated gated field emitter arrays with a novel focusing structure of electron beams, where the focusing electrode concentrically surrounded each gate hole. Carbon nanotube emitters were screen printed inside an amorphous-Si concave well far below the gate. It was theoretically and experimentally verified that the concave well structure effectively focused the emitted electron beams to their designated phosphor pixels by modulating focusing gate voltages. For the vacuum packaged field emission displays with the pixel specification fitting high-definition televisions, color reproducibility of approximately 71% was achieved at the brightness of 400 cd/m/sup 2/.

Self-aligned fabrication of single crystal diamond gated field emitter array

This paper describes a self-aligned fabrication process for diamond gated field emitter array (FEA). Utilizing the non-conformal coverage sputtering conditions of silicon oxide, an interesting “sphere on cone” structure is formed on diamond nano tip array, which is the key point of gate hole opening process. This structure causes shadowing at certain regions of side-wall during Ti / Au gate metal deposition. Removal of “sphere” by wet etching leads to the successful fabrication of a single crystalline diamond gated FEA. Scanning electron microscope observations reveal the fabrication of a uniform emitter array with tip radius of curvature (20 nm) and gate hole (1.4 μm). We also confirmed that no noticeable physical damage exists on tip. In field emission characteristics of the fabricated single crystal diamond gated FEA, gate voltage control of field emission current is realized.

Effect of Al and catalyst thicknesses on the growth of carbon nanotubes and application to gated field emitter arrays

Multi-walled carbon nanotubes (CNTs) were synthesized on Fe–Ni–Co alloy-coated glass substrates by thermal chemical vapor deposition at 550 °C. Thickness of the Al and catalyst markedly affected the growth rates, diameters, and population density of CNTs. The catalyst thickness had a greater effect on diameter and growth rate than that of Al. The population density of CNTs was governed mainly by the Al thickness. The Al and catalyst thickness were optimized in terms of field emission characteristics to grow the CNTs in the gated field emitter arrays. The smaller diameters and lower density of CNTs resulted in the best emission characteristics.

Carbon nanotube field emitter arrays having an electron beam focusing structure

An electron beam focusing structure was incorporated into the gated field emitter arrays where the emitters were screen-printed carbon nanotubes. The focusing structure was comprised of 8-μm-thick bulky SiOx focus gate insulator and Cr focus gate, and exhibited negligible leakage between the gate and the focus gate. In current–voltage measurements, it is found that the anode current strongly depends on both the focus gate and the anode bias voltages. Electron beams were focused well at the anode with a slight overfocusing effect, which is due to the wide electron beam divergence from carbon nanotubes. A new focusing structure based on the simulation is proposed to overcome the overfocusing.

Growth of Vertically Aligned Carbon Nanotubes inside Dome-structured Amorphous Silicon Holes by Plasma-enhanced Chemical Vapor Deposition

Vertically aligned carbon nanotubes (CNTs) were synthesized inside an array of dome-structured amorphous silicon (a-Si) holes on glass substrates. An a-Si layer swelled up as amorphous carbon (a-C) was grown to penetrate beneath the a-Si layer through patterned holes during thermal chemical vapor deposition (CVD), leading to an array of a-Si domes filled with a-C. Following the etching of a-C inside the domes, vertically aligned CNTs were selectively grown inside an array of hollow dome-structured holes using alternating-current plasma-enhanced chemical vapor deposition. The potential of applying this structure to gated field emitter arrays was discussed.

Fabrication and characterization of gated field emitter arrays with self-aligned carbon nanotubes grown by chemical vapor deposition

Field emitter arrays with multiwall carbon nanotubes (CNTs) grown inside their gated holes were fabricated on glass substrates. The Fe–Ni–Co alloy catalyst dots on which the CNTs would be grown were deposited into the gated holes by a self-aligned method to maintain a constant distance between CNT emitters and gate electrodes. The CNTs were synthesized by thermal chemical vapor deposition using a gas mixture of CO and H2 at 500 °C. The CNT lengths were controlled by changing ratios of CO to H2. Field emission currents and images were monitored as a function of gate and anode voltages. It was shown that the CNT emitters grown just up to the gate electrode height operated best in a triode mode.

Gated field emitter arrays

The successful fabrication of field emission (FE) devices is directly related to process simplicity and device performance, which depends to a large extent on the tip material and emitter geometry. On the other hand, these characteristics are the most important issues influencing industrial applicability of the FE devices. In most typical solutions single emitter cells consist of a sharp tip located in the hollow, with the tip apex surrounded by a gate. The fabrication process described here is simple and allows self-aligned gate electrode formation. The field emission of the emitter tips is supported by a 50-nm diamond-like-carbon (DLC) film formed by chemical vapor deposition.

Emission current influence of gated structure and diamond emitter morphologies in triode-type field emission arrays

Field emission display is evolving as a promising technique for the future generation of flat panel displays. Characteristics affecting the power of field emitter arrays include the shape and work function of emission materials, distance between tip and gate, and the environmental vacuum condition. In this work, we present a novel scheme that involves a new fabrication process of gate structure metal–insulator–semiconductor (MIS) diode using IC technology. Deposition of the diamond film in this MIS diode forms a column-like diamond with gated field emission arrays (FEAs). This process is completed using a bias-assisted microwave plasma chemical vapor deposition system. Our results indicate that the threshold voltage of column-like diamond FEAs is about 10 V, and the field emission current density is about 139 mA cm −2 (at V gc =20 V). The diamond field emitter array devices with the new gate structure and tip morphology significantly influence the electron field emission characteristics.

Process development of gated field emitter arrays with dry etched amorphous silicon microtips on glass substrates

A low temperature fabrication process for gated field emitter arrays is developed with etched amorphous silicon microtips on glass substrates. Reactive ion etching using a SF6/O2 plasma is employed to sharpen microtips without any thermal oxidation or wet etching technique. Field emitter arrays with a triode structure are successively fabricated by utilizing plasma enhanced chemical vapor deposition and electron beam evaporation for a SiO2 gate oxide and a niobium gate, respectively.

Gated Si field emitter array prepared by using anodization

High aspect-ratio gated field emitter arrays have been fabricated using silicon tip prepared by the anodization of a silicon wafer with periodic n/p junctions. The tips were formed by the preferential growth of porous silicon in p-type material and were transferred to a separate silicon substrate by direct bonding. Gate electrodes were then formed by depositing a WSi2 film over the tips, and apertures were subsequently created by etching with an argon milling process. The resulting gated tip arrays demonstrated field emission, although higher than usual gate voltages were required.