Vacuum Nanoelectronic Devices Research Articles

Breakdown of plane-wave-based theories of field emission from a subnanometer-radius tip

We present a combined experimental and theoretical study of field electron emission from an ultrasharp carbon tip with a subnanometer radius formed by a field-emission-induced structural modification of a diamond needle-like microcrystallite. The tip dimensions were measured by high-resolution transmission electron microscopy and used to calculate the potential distribution outside the emitter. Substitution of the calculated field into the Fowler-Nordheim (FN) theory and the more refined theory based on the Simmons equation overestimate the field emission current by six and five orders, respectively. We show that such a strong discrepancy is due to the incorrect use of plane wavefunctions to describe electrons inside a subnanometer emitter. We present a theory based on more realistic non-planar wavefunctions, which takes into account the effect of localization and one-dimensional density of states inside the emitter. These modifications of the FN theory allowed us to obtain a good order-of-magnitude agreement with the experimental values of the field emission current. The presented results are of great practical importance, since field emitters with nanometer radius are intensively studied experimentally and are the basis of state-of-the-art vacuum nanoelectronic devices.

Using static linear response theory to describe field emission field enhancement and a field-induced insulator-conductor transition

Experiments on field electron emission (FE) of single carbon nanotubes (CNTs) indicate that they exhibit a nearly linear Fowler–Nordheim plot, and the field enhancement factor (FEF) near the CNT apex is independent of the applied (macroscopic) field (FM) for small radii field emitters. Recent results, based on density functional theory calculations considering CNTs with small radii, retrieved the constancy of the FEF defined in terms of the corresponding induced electron density. As a consequence, it has been reported that the constancy of the FEF with FM could be connected with the linear response of the CNT. In this paper, we reinforce this connection, considering the problem of a floating (6,6) hybrid single-walled nanotube, whose cylindrical body is an insulating one and composed of alternating boron and nitrogen atoms end-capped with carbon atoms. Our results show that the constancy of the FEF is achieved when a linear dependence between the longitudinal component of the induced system dipole moment (μi,z) and FM is observed. Two regimes of constant polarizabilities have been found at sufficiently low and high FM-values. In the intermediate range 0.3V/nm≲FM≲5 V/nm, a crossover from insulating-to-conducting behavior, exhibiting a nonlinear dependence of μi,z on FM, is found accompanied by an increase of the FEF with FM. This result reveals circumstances that could lead to dependence of the FEF on FM, being timely for interpretation of FE characteristics in the context of vacuum nanoelectronic devices.

Recent Progress on ZnO Nanowires Cold Cathode and Its Applications.

A cold cathode has many applications in high frequency and high power electronic devices, X-ray source, vacuum microelectronic devices and vacuum nanoelectronic devices. After decades of exploration on the cold cathode materials, ZnO nanowire has been regarded as one of the most promising candidates, in particular for large area field emitter arrays (FEAs). Numerous works on the fundamental field emission properties of ZnO nanowire, as well as demonstrations of varieties of large area vacuum microelectronic applications, have been reported. Moreover, techniques such as modifying the geometrical structure, surface decoration and element doping were also proposed for optimizing the field emissions. This paper aims to provide a comprehensive review on recent progress on the ZnO nanowire cold cathode and its applications. We will begin with a brief introduction on the synthesis methods and discuss their advantages/disadvantages for cold cathode applications. After that, the field emission properties, mechanism and optimization will be introduced in detail. Then, the development for applications of large-area ZnO nanowire FEAs will also be covered. Finally, some future perspectives are provided.

Coulomb blockade in field electron emission from carbon nanotubes

We report the observation of Coulomb blockade in electron field emission (FE) from single-wall carbon nanotubes (SWCNTs), which is manifested as pronounced steps in the FE current–voltage curves and oscillatory variations in the energy distribution of emitted electrons. The appearance of the Coulomb blockade is explained by the formation of nanoscale protrusions at the apexes of SWCNTs due to the electric field-assisted surface diffusion of adsorbates and carbon adatoms. The proposed adsorbate-assisted FE mechanism is substantially different from the well-known resonant tunneling associated with discrete electronic states of adsorbed atoms. The simulations based on the Coulomb blockade theory are in excellent agreement with the experimental results. The SWCNT field emitters controlled by the Coulomb blockade effect are expected to be used to develop on-demand coherent single-electron sources for advanced vacuum nanoelectronic devices.

General Vacuum Electronics

The electron devices in which electrons do not collide with other particles or in which the collision probability is very small in the transport process can be theoretically regarded as general vacuum electron devices. General vacuum electron devices include microfabricated vacuum nano-electronic devices, which can work in atmosphere, and some solid-state electron devices with nanoscale channel for electrons whose material characteristics are close to those of vacuum channels. Vacuum nano-electron devices (e.g., nanotriodes) are expected to be the fundamental elements for high-speed, radiation-resistant large-scale vacuum integrated circuits. The solid-state electron devices with spin semiconductor materials, multiferroics or topological crystal insulators are quite different from traditional semiconductor devices and are expected to operate under novel principles. Understanding vacuum electron devices from a microcosmic perspective and understanding solid-state electron devices from a vacuum perspective will promote a union of vacuum electronics and microelectronics, as well as the formation and development of general vacuum electronics.

(Invited) Electron Emission Study of Planar-Type Electron Emission Devices Based on Nanocrystalline Silicon

A planar-type electron emission device having a thin film diode structure such as a metal-oxide-semiconductor (MOS) [1] is a promising candidate as a fine electron source for applications of vacuum nanoelectronic devices, including environment-hard electronic devices, flat panel displays and high-sensitive image sensors. This is because the cathode operates at low extraction voltage and produces uniform and highly directional emission, compared with conventional field emitters with needle structure. Furthermore, the cathode is insensitive to the environment. However, the device is limited in emission current by its low efficiency (i.e., the ratio of the emission current to the total tunneling current) of typically less than 0.1 % due to strong scattering of hot electrons while traveling through the conduction band of both the oxide and the gate metal, after tunneling through the potential barrier in the oxide [2]. The reduction of the work function of the gate electrode by adsorption of alkali metals onto the surface [3] and the use of films containing nano- and micro-crystalline structure taking the place of the oxide layer have been proposed to improve the performance on the MOS cathode [4]. Another strategy is to reduce electron scattering within the topmost metal electrode by decreasing the electrode thickness [5-7]. In order to improve the emission efficiency and understand the emission mechanism we have fabricated the planar-type electron emission devices based on oxidized nanocrystalline Si (nc-Si) prepared by a pulsed laser ablation (PLA) technique and investigated their emission properties. The emission area of the device was designed 500 mm in diameter. The emission current and the efficiency are higher for thinner in the gate electrode, indicating tunneling is sensitive to the thickness of the gate. The electron emission efficiency is predicted to reach ~ 10 % if electron scattering on the inside of the topmost metal electrode is perfectly suppressed. In our presentation, we will discuss the detailed electron emission properties of the nc-Si based planar-type electron emission device.

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.

Review on peculiar issues of field emission in vacuum nanoelectronic devices

Some of the modern aspects of field emission based electron sources have been collated in a short and comprehensive review. The usually overlooked peculiar aspects in this research field have been particularly emphasized in order to increase the interest in further fundamental studies and technological applications. The vast material was roughly split in two main branches which occasionally overlap: the electron emission devices based on chemically homogeneous nanostructured surfaces and the more complex nanocomposite emitting surfaces.

Multiphoton photoemission of gold nanopillars fabricated by carbon nanotube templates

High-aspect ratio metallic nanopillars are promising as electron emitters in frequency ranges of terahertz and higher. However, producing nanopillars with aspect ratio of more than a factor of 10 by lithographic methods is still a challenge. Here, the authors report the fabrication of Au nanopillars using carbon nanotube forests as templates and a study on their near infrared photocurrent characterization. The pillars had the diameter of 100 nm and the aspect ratio up to 28. The authors observed an increase in the photocurrent from Au nanopillars with the increase in the nanopillar height. The observed photocurrent is compatible with the multiphoton photoemission and is uncorrelated to the pillar density. The results indicate that our proposed method is promising to produce electron sources for high frequency vacuum nanoelectronic devices.

Generalized Mechanism of Field Emission from Nanostructured Semiconductor Film Cathodes

Considering the effect of both the buffer layer and substrate, a series of ultrathin multilayered structure cathodes (UTMC) is constructed to simulate the field emission (FE) process of nanostructured semiconductor film cathodes (NSFCs). We find a generalized FE mechanism of the NSFCs, in which there are three distinct FE modes with the change of the applied field. Our results clearly show significant differences of FE between conventional emitters and nanofilm emitters, which the non-Fowler-Nordheim characteristics and the resonant FE will be inevitable for NSFCs. Moreover, the controllable FE can be realized by fine-tuning the quantum structure of NSFCs. The generalized mechanism of NSFCs presented here may be particularly useful for design high-speed and high-frequency vacuum nano-electronic devices.

Book Reviews (8 Reviews)

The following books are reviewed: Fundamentals of Electric Power Engineering, by: I.D. Mayergoyz and P. McAvoy; Vacuum Nanoelectronic Devices, by A. Evtukh, H. Hartnagel, O. Yilmazoglu, H. Mimura, and D. Pavlidis; High Voltage Direct Current Transmission - Converters, Systems and DC Grids, by: D. Jovcic and K. Ahmed; Micromanufacturing Engineering and Technology, 2nd Edition by: Y. Qin, Editor William Andrew; Condensed Matter Optical Spectroscopy by: I. Ionita; Resistive, Capacitive, Inductive, and Magnetic Sensor Technologies by: W.Y. Du; Nanomaterials for Lithium-Ion Batteries by: R. Yazami; Light Sources, 2nd Edition by: S. Kitsinelis.

Highly stable carbon nanotube cathode for electron beam application

The authors fabricated an optimized electron beam (e-beam) with a carbon nanotube (CNT) cathode and triode configuration. CNT emitters grown with a resist-assisted patterning process were used as an electron source. The gate mesh was aligned with the CNT emitter islands for a lower leakage current, resulting in a higher electron emission current and transmission ratio. Additionally, the width between CNT islands and the pitch between CNTs were optimized to enhance the electric field at the tip of the CNT emitters. With the optimized e-beam module, consisting of a CNT cathode and gate mesh, CNT pitch of 30 μm, dot size of 3 μm, line width of 210 μm, and gate mesh width of 75 μm, the emission current showed an increase of 165 times and operated for more than 500 h with DC driving. The optimized e-beam can be a building block for vacuum nanoelectronic devices.

Chemically activated growth of CuO nanostructures for flexible cold cathode emission

A generalized protocol for solution-based seeding and self-assembly of a wide range of cupric oxide (CuO) nanostructures is developed. Parameters of KMnO4 activation are found to play a crucial role in chemical seeding and vis-a-vis in evolution of various morphologies. To demonstrate the devised two-stage seeding and self-assembly protocol, various types of CuO nanostructure films are fabricated on flexible carbon fabric. Such systems are found to retain the native substrate's innate flexibility and the structural integrity of the grown nanostructures showed no deterioration upon rolling up or even twisting to a large degree. Among the various CuO morphologies, the nanoneedles exhibited excellent field-emitting traits such as a lower turn-on field of 0.9 V μm−1 and an appreciably good current density of 1.65 mA cm−2 at an applied macroscopic field of 1.25 V μm−1 as well as high stability under large applied fields. Theoretical estimation of the local electric field distribution supported the experimental outcomes and indicated that the field emission enhancement stems from the emitters' geometry as well as the spatial separation. The devised growth protocol is efficient, economic and high-throughput, thus it lays a promising path for exploiting CuO (and possibly other) nanostructures for flexible field emission electronics/displays as well as in vacuum nano-electronic devices.

High current field emission from individual non-linear resistor ballasted carbon nanotube cluster array

Field emission (FE) electron sources based on carbon nanotubes (CNTs) have the potential to serve as cold cathodes for various vacuum microelectronic and nanoelectronic devices. Emission currents are extremely sensitive to variation in emitter geometry and local surface states, both of which are difficult to synthesize uniformly when fabricating a CNT field emission array (FEA). Such non-uniformities cause unstable emission, limiting the current output. Here, we propose a method for simulating and fabricating a high performance CNT-FEA with emission units that are individually connected to a single crystalline silicon pillar (SP), which acts as an non-linear ballast resistor. Results showed that the driving field for this CNT-FEA was greatly reduced relative to CNT-FEAs on a flat silicon substrate. This improvement was due to the high aspect ratio of the CNT clusters combined with SPs. The FE behavior demonstrated that the emission current was limited by the non-linear resistors (NLRs). Emitted currents density over 1.65A/cm2 at a low extraction field of 5.8V/μm were produced by a 1mm2 emmiting area. The proposed technology may be used to fabricate cathodes capable of reliable, uniform, and high current emission.

Hierarchical cupric oxide nanostructures on copper substrate for cold cathode emission: an experimental venture with theoretical correlation.

In this paper we report a facile route for the synthesis of controlled CuO nanoarchitectures directly grown on a copper substrate by a one-step simple chemical route with varying concentration of non-ionic surfactant PEG-6K. The phase purity and degree of crystallinity of the as-developed nanostructures were systemically investigated by X-ray diffraction, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy (HRTEM). A detailed analysis by field emission scanning electron microscopy confirmed the uniformity of the prepared nanostructures on the substrates. These architectures displayed substantial improvement of field emission properties with respect to other structures of CuO reported so far. A particular nanostructure (needle) among them showed a down shift of the turn-on field to 2.2 V μm(-1) coupled with a good enhancement factor (β) ∼516, which are deemed as sufficient for electron emission based applications such as field emission displays and vacuum nanoelectronic devices. The origin of this efficient field emission from CuO nanoarchitectures, were probed computationally by investigating the local electric field distribution through finite element based simulation method using the ANSYS Maxwell simulation package.

Stable, ruggedized, and nanometer-order size transfer mold field emitter array in harsh oxygen radical environment

Extremely sharp, uniform, stable, rugged, low operating voltage, and nanometer size molybdenum field emitter arrays (Mo FEAs) were developed by transfer mold fabrication. These can be used in stable and reliable vacuum nanoelectronic devices such as those used in electric propulsion engines and in environmentally demanding applications. Nanometer size transfer mold Mo FEAs were fabricated with base lengths of 36–370 nm. The 36 nm base length transfer mold Mo FEAs are the smallest FEA's ever reported. The tip radii were as sharp as 2.8–3.5 nm. The turn-on fields of the transfer mold Mo FEAs with a base length of 36–370 nm were found to be as low as 9.0–23.6 V/μm for the short anode-to-apex distance of less than 10 μm. These are lower than those of conventional FEA's such as carbon nanotube FEAs, Spindt-type FEAs, and Gray-type FEAs with approximate turn-on fields of 50–600 V/μm. They can thus be used in reliable vacuum nanoelectronic devices. The emission fluctuation of the 36 nm base length transfer mold Mo FEAs without a resistive layer were found to be as low as ±1.6% and this value is the lowest ever reported. Conventional FEAs with and without resistive layers have values of 5%–100% and higher than 100%, respectively. The field emission characteristics of the FEAs have been evaluated by in-situ oxygen radical treatment with a flux of 1015 atoms cm−2 s−1. This value is 107–108 times higher than that of typical low earth orbit fluxes. The environmental characteristics of the transfer mold Mo FEAs were compared with those of transfer mold Ni FEAs. The turn-on fields of the transfer mold Mo FEAs changed slightly from 10.8 to 22.5 V/μm at 1 min from the initiation time. It was almost saturated at 29.4 V/μm upon 5 min of in-situ oxygen radical treatment. A rapid increase in the turn-on field from 14.9 to 59.4 V/μm upon in-situ radical treatment for 3 min was observed for the Ni FEAs. The experimental work functions measured by ultraviolet photoelectron spectroscopy and the work functions calculated by the slope of Fowler–Nordheim plots for the transfer mold Mo FEAs changed slightly from 4.3 to 5.2 eV and from 4.3 to 5.3 eV, respectively, after oxygen radical treatment for 5 min. The calculated work functions are similar to the experimental work functions because the transfer mold Mo FEAs have extremely uniform geometric factors and stable emission characteristics. The emission fluctuation rates determined without the radical treatment of the transfer mold Mo FEAs without a resistive layer were found to be as low as ±1.7%. Even with in-situ radical treatment for 30 min the transfer mold Mo FEAs exhibited remarkably stable emission without spike noise and with fluctuations as low as ±4.5%. Therefore, the extremely stable and nanometer size transfer mold Mo FEAs are expected to be useful in the production of vacuum nanoelectronic devices.

Optical and field emission properties of layer-structure GaN nanowires

A layer-structure gallium nitride (GaN) nanowires, grown on Pt-coated n-type Si (111) substrate, have been synthesized using chemical vapor deposition (CVD). The results show: (1) SEM indicates that the geometry structure is layer-structure. HRTEM indicates that GaN nanowire’s preferential growth direction is along [001] direction. (2) The room temperature PL emission spectrum of the layer-structure GaN nanowires has a peak at 375nm, which proves that GaN nanowires have potential application in light-emitting nano-devices. (3) Field-emission measurements show that the layer-structure GaN nanowires film has a low turn-on field of 4.39V/μm (at room temperature), which is sufficient for electron emission devices, field emission displays and vacuum nano-electronic devices. The growth mechanism for GaN nanowires has also been discussed briefly.

Efficient and persistent cold cathode emission from CuPc nanotubes: a joint experimental and simulation investigation.

In the current report, chemically synthesized copper phthalocyanine (CuPc) nanotubes are shown to exhibit unprecedentedly well cold cathode emission characteristics with turn-on field (3.2 V μ m(-1)) and stable emission during long intervals (200 min). Simulation of electric field distribution via finite element method around an isolated nanotube emitter in a manner parallel to the experimental setup (inter-electrode distance = 180 μm) exhibits good corroboration of theoretical premises with experimental findings. Obtained results strongly indicate CuPc nanotubes to be potential candidate as cold cathode emitter for electron emission based applications such as field emission displays and vacuum nano-electronic devices.

Highly oriented cupric oxide nanoknife arrays on flexible carbon fabric as high performing cold cathode emitter

Sharp knife edged copper oxide (CuO) nano architectures were directly grown on the surface of flexible carbon fabric by a facile chemical route with the assistance of nonionic surfactant PEG-6000. The carbon fabric substrate preserves its high flexibility even after the growth of the entire nanostructure. Moreover, it can be rolled-up and twisted to a large degree without affecting the electrical characteristics. The phase purity and degree of crystallinity of the developed nanostructures were systemically supported by X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy and high-resolution transmission electron microscopy. A proposed growth mechanism has been offered by thoroughly analyzing the field emission scanning electron microscopic images of the nanostructures grown at different concentrations of PEG. Among the as grown nanostructures, copper oxide nanoknives have exhibited remarkable field emission properties and high stability with a lower turn-on field of 0.9 V μm−1 (10 μA cm−2) at room temperature, which is sufficient for electron emission based devices like field emission displays and vacuum nano-electronic devices.

Paper No 16.3: Low‐Operation Voltage, High Aspect Ratio, Position‐Controlled Transfer Mold Carbon Nanotube Field Emitter Arrays

AbstractLow operation voltage, high aspect ratio, and position‐controlled Transfer Mold carbon nanotube (CNT) FEAs have been developed by the combination of the Transfer Mold FEAs and CNTs to realize highly efficient, reliable, and high performance displays. The Transfer Mold FEAs have the highest uniform and the most reliable field emission characteristics compared with the conventional FEAs such as CNT FEAs having the nonuniform and unstable emission characteristics, because the Transfer Mold FEAs can give the uniform emitter morphology and can decide the uniform emission sites. Moreover, if the nonuniform emission happens in a pixel, it is difficult to make uniform electron beam in a pixel even by using focusing electrode. However, the emission points of Transfer Mold FEAs are uniformly distributed in a pixel. Transfer Mold CNT FEAs can improve the emission characteristics by combining the Transfer Mold FEAs and CNTs, because Transfer Mold CNT FEAs have the higher aspect ratio than those of Transfer Mold FEAs. The turn‐on fields of Transfer Mold CNT FEAs and Transfer Mold Ni FEAs are 12.5 V/μm and 14.9 V/μm, respectively; these represent one of the lowest turn‐on voltages obtained to date. The turn‐on voltages are reduced by about 16% due to the high aspect ratio of Transfer Mold CNT FEAs. Both values of the turn‐on voltages, 12.5 and 14.9 V/µm, are lower than those of other conventional FEAs such as CNTs FEAs of 50−100 V/µm at the short distance less than 10 μm. Therefore, Transfer Mold CNT FEAs can operate at low voltages and are applicable for fabricating uniform and reliable FEDs and other vacuum nanoelectronic devices.