Field Emitter Arrays Research Articles

Fabrication of a high-density emitter array for electrospray thrusters using field emitter array process

To improve the thrust density of electrospray thrusters using ionic liquids as the propellant, we have fabricated a high-density emitter array utilizing the fabrication technique of a field emitter array (FEA) or gated nano electron sources. The density of the emitter array can reach approximately 4 million cm−2, which is four orders of magnitude higher than that of conventional electrospray thrusters. Performing deep reactive ion etching from the back side of the same structure as FEAs and removing their Ni cones by dilute nitric acid, we have successfully fabricated sub-μm scale capillary emitters with self-aligned extractor electrodes and ionic liquid reservoirs behind them. Current measurements were also conducted with EMI-BF4 and EMI-DCA as the propellant. Although the ion was not extracted with EMI-BF4 owing to its electrochemical reaction with the electrodes, both positive and negative ion currents were detected using EMI-DCA, where the current density exceeded that of conventional electrospray thrusters.

Carbon Nanotube Fiber Field Emission Array Cathodes

Field emission cathodes made from single bulk carbon nanotube (CNT) fibers have demonstrated high emission currents and long lifetimes. This paper investigates the goal of achieving even higher current levels from field emission array (FEA) cathodes comprised of multiple CNT fibers arranged in various array configurations. Arrays were fabricated with 25- $\mu \text{m}$ -diameter CNT fibers in 2-fiber ( $1 \times 2$ ), 4-fiber ( $2 \times 2$ ), and 25-fiber ( $5 \times 5$ ) configurations, and their field emission properties were measured. The $1 \times 2$ and $2 \times 2$ FEA cathodes achieved the maximum current values that scaled to greater than 2 mA per emitter. The $5 \times 5$ FEA cathode achieved a high maximum current value of 22 mA and exhibited stable emission for 10 h at ~9 mA at an applied field strength of 0.11 V/ $\mu \text{m}$ . The Fowler–Nordheim theory was used to calculate the effective field enhancement factor ( $\beta _{\mathrm {eff}}$ ) values for all of the arrays. Electrostatic simulations were performed using COMSOL Multiphysics modeling software to model the field enhancement of a perfectly uniform $5 \times 5$ array. The $\beta _{\mathrm {eff}}$ value for the total array was compared to the individual $\beta $ values at the fiber tips predicted by a surface potential model that predicted the field amplification of all 25 CNT fibers in the array.

Modeling of diamond field emitter arrays for a compact source of high brightness electron beams

Many applications, such as compact accelerators and electron microscopy, demand high brightness electron beams with small source size and ultralow emittance. Diamond emitters manufactured with semiconductor processes can be employed in such compact beam sources. The micrometer-scale pyramid structure of the emitter allows enhancement of the external field compared to that at the substrate, leading to electron emission with small beam size. We investigate the dependence of the field enhancement on the shape of the emitter and the resulting emission characteristics. The beam formation and dynamics are simulated with the LSP [D. Welch, D. Rose, R. Clark, T. Genoni, and T. Hughes, Comput. Phys. Commun. 164, 183 (2004)] particle-in-cell code to obtain the macroscopic observables. To account for the semiconductor charge transport in the bulk material and the tunneling through the surface, a first-principle semiclassical Monte Carlo emission model is developed and applied to the diamond pyramid. Using this Monte Carlo emission model and the result from the geometric field enhancement calculation, we construct a simple model to qualitatively explain the measured emission characteristics. A comparison between our model and experiments indicates that the beam current is mostly emitted at the apex of the emitter.

Fabrication of large-area ZnO nanowire field emitter arrays by thermal oxidation for high-current application

Large-area ZnO nanowire field emitter arrays have important applications in flat-panel displays, light sources, photodetectors and X-ray sources, where it is crucial to realize a high field emission current and current density over a large-area. In this paper, large-area ZnO nanowire arrays with various array spacings were prepared by the thermal oxidation technique from patterned Zn film arrays, and their field emission properties were studied. The results show that the Zn array density is related to the nanowire length and population density. A mechanism is proposed wherein the stress-induced process and mass transport process both play key roles in the growth. Herein, ZnO nanowire arrays in a 4.7 × 4.7 cm2 area exhibited a sufficient quantity of emission sites and diminished field screening effect and a high current of 12.9 mA with a current density of 0.58 mA/cm2. This work provides useful guidance for optimizing ZnO nanowires and enhancing field emission current performance for large-area ZnO nanowire field emission devices.

A Comparative Study of Field Emission From Pristine, Ion‐Treated and Tungsten Nanoparticle‐Decorated p‐Type Silicon Tips

The field electron emission characteristics of individual tips of a silicon field emitter array are analyzed. The array of conical‐shaped tips is fabricated on a p‐type silicon wafer by using reactive ion etching and sharpening oxidation. The tips are decorated with single tungsten nanoparticles at their apexes. Furthermore, the focused ion beam is also used to increase surface conductivity of some of the tips. Comparative measurements of field emission are performed by using the scanning anode probe field emission microscopy technique. All types of tips demonstrated emission activation consisting of a sudden current increase at a certain value of the applied voltage. Compared to the pristine tips, a noticeable reduction of the saturation effect in the current–voltage characteristics and a smaller light sensitivity for the decorated tips is found. For ion‐treated tips, saturation effects and light sensitivity are completely suppressed. Scanning electron microscopy observations reveal the formation of single nanoscale protrusions extending from the metal particles and from the apexes of bare ion‐treated tips after exposure under strong electric fields during the field emission measurements. The influence of protrusions growth on characteristics of silicon field emitter arrays is discussed.

Effects of short-time plasma treatment in the magnetron sputtering equipment on various carbon nanotubes for applied to LCD backlight unit

The field emission and luminescent properties of various (single-walled CNTs, double-walled CNTs, multi-walled CNTs, and carbon nanofibers) screen-printed carbon nanotubes (SP-CNTs) that were straightened using plasma treatment in the magnetron sputtering equipment (MSE) for a short time were investigated. The SP-CNTs investigated CNT field emitter arrays for back light unit (BLU) in liquid crystal display (LCD). The plasma treatment in the MSE enhanced the emission properties of the single-walled CNTs (SWCNTs) and double-walled CNTs (DWCNTs): the emission uniformity and number of emission sites increased. Recurring problems associated with SW and DW SP-CNTs such as bent and/or buried CNTs and degradation of binder–residue-induced emission were ameliorated by inducing protrusion from the binder and permanent straightening of the SW and DW SP-CNTs as a result of treatment. The multi-walled CNTs (MWCNTs) and carbon nanofibers (CNFs) had partial emission sites with poor emission factors. The results of the present study suggest that plasma treatment in the MSE of SW and DW SP-CNTs is an effective method for achieving uniform field emission and reducing the aging.

Stable and environment-hard vacuum nanoelectronic devices for aerospace applications

Extremely stable field-emitter arrays (FEAs) having low work function as well as resisting harsh environments have been prepared by the transfer mold emitter fabrication method to make remarkably resistant and low operation-voltage vacuum nanoelectronic devices in harsh environments such as a strong radical atmosphere. Amorphous carbon and titanium nitride, which have a low work function as well as an environmental hardness, were used as the emitter material. The current emission fluctuations of amorphous carbon FEAs having no resistive layers, were as low as ±1.7%, (the lowest ever reported value), as compared to 5–100% of general FEAs fabricated by the conventional method and containing resistive layers. The amorphous carbon FEAs show the most stable emission characteristics, as compared to transfer mold FEAs coated with titanium nitride or platinum (Pt), because amorphous carbon FEAs have the most uniform tip radii. The amorphous carbon FEAs exhibit stable fluctuations of less than ±5.1% under in situ oxygen radical treatment. Panel and pixel electric thrusters have been proposed that use transfer mold FEAs. These electric thrusters make it easy to produce large and small thrusters. transfer mold amorphous carbon FEAs can be used to create highly efficient and reliable vacuum electronic devices such as electric propulsion engines and field-emission displays.

Novel Nanofabricated Mo Field-Emitter Array for Low-Cost and Large-Area Application

This paper demonstrates a novel nanofabrication technology for producing high-density gated molybdenum (Mo) field-emitter array (FEA) based on modified nanosphere lithography (NSL). The arc-shaped side wall of gate apertures introduced by the conventional NSL process put obstacles in the formation of sharp cone-type emitters. In this paper, a two-method solution was employed to facilitate the realization of gate apertures with sharper cutting side wall which are beneficial for the formation of sharper gated Mo emitters. One method was softening nanosphere to enlarge the contact area between nanosphere and substrate. The other was optimizing gate film deposition conditions to dramatically reduce the diffraction effects. Due to the improvements in the geometrical shape of gated Mo emitters, a low-voltage working high-density gated Mo FEA was demonstrated in this paper. This is the first observation of remarkable electron emission from high-density gated Mo FEA fabricated using such as low-cost, high-throughput, and high-efficiency technique which is expected to substantially overcome the challenges in the traditional fabrication process and promotes the low-cost and large-area application of high-density gated field-emitter devices. In addition, the novel nanofabricating method is expected to offer a simple and scalable route which allows fabrication of many gated field-emission devices.

A Flat Panel Photodetector Formed by a ZnS Photoconductor and ZnO Nanowire Field Emitters Achieving High Responsivity From Ultraviolet to Visible Light for Indirect-Conversion X-Ray Imaging

Flat panel photodetectors have been well studied for their applications in large area detection such as indirect-conversion X-ray imaging. Even though a few of photodetectors have become commercially available, there is still a room for advancement in responsive wavelength range and sensitivity. This paper reports a sensitive broadband flat panel photodetector that integrates a ZnS photoconductor with an array of ZnO nanowire field emitters. The photodetector exhibits a spectral responsivity of 5.93 × 102–3.79 × 103 A/W with an external quantum efficiency of 1.03 × 105–1.34 × 106% from 350 to 700 nm wavelengths. Additionally, a short signal rise time of 55 ms and a fall time of 1.03 s are demonstrated. The flat panel photodetector with broadband detection and a high responsivity shows a great promise for use in indirect-conversion X-ray detector incorporated with a broad range of X-ray scintillators.

Extraction of the current distribution out of saturated integral measurement data of p-type silicon field emitter arrays

At the moment, only complicated techniques are known for the determination of array properties of field emitter arrays such as the number of active tips, the current distribution, or the individual tip radii. In this work, a method for extracting these parameters from integral measurement data is presented. A model describing the characteristics of a single emitter, including the saturation as a function of the applied voltage and the emitter radius, is developed. It is shown that experimental data of field emitter arrays can be represented as the sum of these functions and the characteristic parameters can be fitted to field emission data of an array. Using this method, the values of the radii as well as the parameters of distribution models can be determined directly. Analysis of experimental data from p-type Si emitter arrays shows that only 1–2% of the tips contribute significantly.

The behavior of the Fowler–Nordheim plot for ZnO–Cu virtual emitter arrays

Field emission arrays are attracting the attention of the scientific community due to their unique emission properties and potential technological applications. The present work deals with the Fowler–Nordheim (FN) plot behavior for virtual arrays composed of ZnO and Cu emitters with predefined geometrical properties. Electron emission is assumed to occur in these arrays based on fundamental electron tunneling with preserved emission characteristics in both semiconducting and metallic emitters (i.e., degree of field penetration near the emitter surface, band bending effects and saturation of the conduction band current at high fields). The effect of the emitter’s size distribution and the dependence of conduction band current saturation on emitter’s diameter for ZnO emitters are taken into consideration. Results show that the FN plot characteristics are greatly influenced by the investigated parameters signifying the complicated nature of field emission from these arrays. The present work is believed to be of significance for interpretation of field emission data and can assist further development of field emission arrays for future applications.

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.

Fabrication of ZnO Nanowire Field-Emitter Arrays With Focusing Capability

ZnO nanowire field-emitter arrays (FEAs) show great application potentials in large-area vacuum microelectronics devices. However, the electron divergence is an issue which should be addressed. In this paper, ZnO nanowire FEAs with co-planar focus electrode were designed. A four-mask fabrication process and lift-off process were adopted to achieve the integration of focus electrode. The device demonstrated line-addressing and focusing capability with a spacing of 5 mm between anode and cathode, and the linewidth of field-emission images was reduced from 2.25 to 1.35 mm when the focus voltage changed from 50 to −50 V. Besides, line-shaped emission patterns were observed during focusing process, which was attributed to the emission pattern from the ZnO nanowire and the asymmetrical focusing electrical field.

Field emission investigations of single crystal LaB6 FEA fabricated by femtosecond laser direct writing

The femtosecond laser direct writing method has been used to fabricate the single crystal lanthanum hexaboride (LaB6) field-emission tip arrays (FEAs). The morphologies, structure phase, and field emission of the single crystal LaB6 FEAs are systematically studied. The nanostructures on the surface of tips with the LaB6 phase were formed, resulting in favor of improving field emission, particularly for samples with the nanohill shaped bulges having the size of about 100 nm. The produced single crystal LaB6 FEAs have a uniform structure and a controllable curvature radius of about 0.5–3.0 μm. The FEAs with a curvature radius of about 0.5 μm as field emitters have the best field emission performance, which the field emission turns on and the threshold electric fields are as low as 2.2 and 3.8 V/μm with an emission current of 1.0 A/cm2 at 8.0 V/μm, and the emission current exhibits high stability. These indicate that the processed LaB6 FEAs have a good prospect applied in vacuum microelectronic devices and the simple femtosecond laser direct writing method could lead to an approach for the development of electron sources.

Highly stable field emission from ZnO nanowire field emitters controlled by an amorphous indium–gallium–zinc-oxide thin film transistor

Lowering the driving voltage and improving the stability of nanowire field emitters are essential for them to be applied in devices. In this study the characteristics of zinc oxide (ZnO) nanowire field emitter arrays (FEAs) controlled by an amorphous indium–gallium–zinc-oxide thin film transistor (a-IGZO TFT) were studied. A low driving voltage along with stabilization of the field emission current were achieved. Modulation of field emission currents up to three orders of magnitude was achieved at a gate voltage of 0–32 V for a constant anode voltage. Additionally, a-IGZO TFT control can dramatically reduce the emission current fluctuation (i.e., from 46.11 to 1.79% at an emission current of ∼3.7 µA). Both the a-IGZO TFT and ZnO nanowire FEAs were prepared on glass substrates in our research, demonstrating the feasibility of realizing large area a-IGZO TFT-controlled ZnO nanowire FEAs.

Emission properties of undoped and boron-doped nanocrystalline diamond films coated silicon carbide field emitter arrays

A new process for the fabrication of two-tier silicon carbide field emission array (FEA) of nanoscale tips coated with thin nanocrystalline undoped and highly boron-doped diamond films has been developed to improve the electron emission characteristics of the pure silicon carbide tips. The effects of boron-doped film on field emission properties have been studied in comparison with undoped ones, using a diode configuration. The FEA covered with highly boron-doped film demonstrated the lower turn on electric field and higher emission current due to lower work function, 1.5 times lower than for undoped one. Emission current at various values of the electrode gap from 10 to 500 μm has been studied. It was shown that field enhancement factor of two-tier FEA decreases at small interelectode gap (<100 μm) due to perturbations of electric field and mutual screening of microtips, which leads to significant increase in an applied electric field required for maintaining the preset emission current. Both fabricated FEAs demonstrated high current stability with fluctuations not exceeding 5% at relatively low vacuum (10−5–10−6 Torr).

Electron emission properties of graphene-oxide-semiconductor planar-type electron emission devices

The electron emission properties of planar-type electron emission devices based on a graphene-oxide-semiconductor (GOS) structure before and after vacuum annealing were investigated. The fluctuation of the electron emission current was around 0.07%, which is excellent stability compared to the conventional field emitter array. The GOS devices were operable in very low vacuum of 10 Pa without any deterioration of their electron emission properties. Improvement of the electron emission properties of the GOS devices was achieved by vacuum annealing at 300 °C. The electron emission efficiency of the GOS type electron emission devices reached 2.7% from 0.2% after vacuum annealing. The work function of the graphene electrode was found to decrease 0.26 eV after vacuum annealing by Kelvin force probe microscopy analysis. These results indicated that the improvement of the electron emission efficiency of the GOS devices by vacuum annealing is due to the decrease in the work function of the graphene electrode.

Low turn-on and uniform field emission from structurally engineered carbon nanotube arrays through growth on metal wire mesh substrates

A simple and novel approach to improve the field emission properties from carbon nanotube (CNT) arrays is presented through the direct growth of CNTs onto metal wire mesh substrates. We utilized the curvilinear profiles of the wires to create a cleaved CNT array structure, which was uniformly distributed throughout the entire emitter array area. Investigation of the factors governing the emission properties showed that increased wire diameter, increased CNT array height, and decreased catalyst thickness led to decreased turn-on fields (TOFs) within our experimental range. In addition, we found that the underlying mechanism explaining these empirical results stemmed from the creation of sharp edges which were sufficiently spaced and homogeneously distributed throughout the emitter array. Through this method, a low TOF of ~0.95 V µm−1 for a 10 µA cm−2 emission current density was achieved, which is among the lowest values reported for field emission arrays.

Influence of adsorbates on the performance of a field emitter array in a high voltage triode setup

In the present work, black-silicon field emitter arrays (FEAs) are investigated regarding the influence of residual gas pressure on the characteristics and lifetime in the high voltage triode setup. Current-voltage-characteristics at different pressure levels are recorded and show a decreasing emission current with rising pressure. This decrease can be explained by an increase of the work function and charging of the emitter surface caused by adsorbates. The emission current can be restored to its initial value by heating of the FEA up to 110 °C during active emission. With this regeneration procedure, an extended lifetime from about 20 h to 440 h at a residual gas pressure of 10−5 mbar is achieved.

A double-sided radiating flat-panel X-ray source using ZnO nanowire field emitters

A double-sided radiating flat-panel X-ray source device has been developed. The device has a diode configuration composed of an anode target of a tungsten thin film and field-emitter arrays of ZnO nanowires prepared on glass substrates. The X-ray radiation comes from both transmission and reflection of X-rays induced by electron bombardment. When operated at 24 kV with an anode current of 3.16 mA, radiation dose rates of ∼200 mGy/min and ∼210 mGy/min were recorded positioning at 4 cm away from the front and back sides of the vacuum-sealed flat-panel X-ray source, respectively. Clear X-ray images with high contrast were taken by the radiation from both sides of the device. The reported device has great potential in making compact X-ray radiography and irradiation systems.