Vacuum Nanoelectronics Research Articles

真空ナノエレクトロニクスに期待する

真空ナノエレクトロニクスに期待する

A general computational method for electron emission and thermal effects in field emitting nanotips

Electron emission from nanometric size emitters becomes of increasing interest due to its involvement to sharp electron sources, vacuum breakdown phenomena and various other vacuum nanoelectronics applications. The most commonly used theoretical tools for the calculation of electron emission are still nowadays the Fowler-Nordheim and the Richardson-Laue-Dushman equations although it has been shown since the 1990s that they are inadequate for nanometrically sharp emitters or in the intermediate thermal-field regime. In this paper we develop a computational method for the calculation of emission currents and Nottingham heat, which automatically distinguishes among different emission regimes, and implements the appropriate calculation method for each. Our method covers all electron emission regimes (thermal, field and intermediate), aiming to maximize the calculation accuracy while minimizing the computational time. As an example, we implemented it in atomistic simulations of the thermal evolution of Cu nanotips under strong electric fields and found that the predicted behavior of such nanotips by the developed technique differs significantly from estimations obtained based on the Fowler-Nordheim equation. Finally, we show that our tool can be also successfully applied in the analysis of experimental I-V data.

Chemical Vapor Deposited Few-Layer Graphene as an Electron Field Emitter.

Chemical vapor deposition (CVD) growth of graphene on polycrystalline copper (Cu) foil in a low pressure conditions has been presented, aiming to achieve the highest quality with large-scale fabrications, which requires comprehensive understanding and effective controlling of the growth process. Herein, few-layer graphene (FLG) films with large-domain sizes were grown on Cu metal catalyst substrates using a vertical mass-flow hot-filament CVD reactor, with the intention of large scale production, by optimizing the CVD system and three of the process parameters: (i) gas flow compositions, (ii) substrate annealing time and (iii) graphene deposition time. The detailed scanning electron microscope and Raman spectroscopy analysis indicate that all the above mentioned process parameters affect growth of FLG film on Cu substrate. The presence of two intense peaks, G and 2D-band at 1583.6 and 2702.6 cm⁻¹ for synthesized sample at optimized conditions (H₂/CH₄ ratio of 50:1 at graphene deposition time of 10 minutes and substrate annealed time for 20 minutes) revealed the formation of FLG films with large domain size. These graphene films on Cu have shown the room temperature field electron emission characteristics, hence appears to be prospective candidate for vacuum nanoelectronics.

Enhanced electron emission of directly transferred few-layer graphene decorated with gold nanoparticles

We demonstrate the possibility for integrating field emitters with two-dimensional (2D) graphene for directly transferred vacuum nanoelectronics.

Electron emission from conduction band of heavily phosphorus doped diamond negative electron affinity surface

Hydrogen (H)-terminated surfaces of diamond have attracted significant attention due to their negative electron affinity (NEA), suggesting high-efficiency electron emitters. Combined with n-type doping technique using phosphorus (P) as donors, the unique NEA surface makes diamond a promising candidate for vacuum cold-cathode applications. However, high-electric fields are needed for the electron emission from the n-type doped diamond with NEA.Here we have clarified the electron emission mechanism of field emission from P-doped diamond having NEA utilizing combined ultraviolet photoelectron spectroscopy/field emission spectroscopy (UPS/FES). An UP spectrum has confirmed the NEA of H-terminated (1 1 1) surface of P-doped diamond. Despite the NEA, electron emission occurs only when electric field at the surface exceeds 4.2 × 106 V cm−1. Further analysis by UPS/FES has revealed that the emitted energy level is shifted, indicating that the electron emission mechanism of n-type diamond having NEA surface does not follow a standard field emission theory, but is dominated by potential barrier formed within the diamond due to upward band bending. The reduction of internal barrier is the key to achieve high-efficiency electron emitters using P-doped diamond with NEA, of which application ranges from high-resolution electron spectroscopy to novel vacuum nanoelectronics devices.

Dovetail Tip: A New Approach for Low-Threshold Vacuum Nanoelectronics

An innovative approach for the fabrication of low-threshold, nanoscale field-emitter cathodes and arrays is proposed. By shaping a titanium cathode as dovetail geometry and using a thin vertical wall of high-permittivity dielectric, Si3N4, to isolate the tip from the gate, the electric field intensity near the tip edges is strongly increased, and a turn-ON voltage in the range of a few volts is predicted numerically and observed experimentally. The resulting cathode structure is compact, self-aligned to the vacuum cavity and is fabricated through the standard silicon fabrication processes. Besides, unlike all existing approaches, vacuum is achieved at the wafer level and not at the package level, thereby reducing fabrication costs. Measurements on a first prototyped array of $5\times 4$ diode-connected parallel devices with a silicon area of $5~\mu \text {m}\times 4~\mu \text{m}$ , confirm Fowler–Nordheim emission with a 2 V turn-ON voltage, while displaying a total emission current of 25 $\mu \text{A}$ at a gate-cathode bias of 20 V.

Modelling of the Influence of a Pointed Field Emission Cathode Design from the Silicon Carbide with Graphene Film on the Electric Field Strength

Graphene film on silicon carbide is considered to be promising material for high-frequency vacuum nanoelectronics. However, the possibility of graphene application in this area is still poorly understood. We have carried out the simulation of the electric field distribution in interelectrode gap of the anode-cathode system pointed field emission cathode based on silicon carbide with graphene film on its surface subject to the rounding-off radius of the top, interelectrode gap, height and cathode forming half-angle of the cone opening by the finite element method. The influence of constructional parameters on the electric field strength in the test structure was analyzed. It is shown that the values of rounding-off radius of the cone point and interelectrode distance has the biggest influence on the electric field in the investigated structure. Changing of the height and cathode forming half-angle of the cone opening does not lead to a significant increase or decrease of the electric field value.

Studying fringe field effect of a field emitter array

Field emitter arrays on heavy As-doped Si wafer are studied in vacuum nanoelectronics diode configuration. Different shapes of emitters are considered: cone-shaped point-emitters and cylinder-shaped sharp-edge-emitters are compared. Micro scale field enhancement factor on the edge of cylindrical emitter was calculated via home-developed Matlab application and the results are presented. Two types of anode geometry are proposed: plane anode and spherical anode. Experimental and modelling results of surface electric field distribution are presented. The spherical shape of anode allows higher voltage (and higher field emission current) without destructive arcs risk.

Field emission characteristics from graphene on hexagonal boron nitride

An attempt has been made to utilize uniquely high electron mobility of graphene on hexagonal boron nitride (h-BN) to electron emitter. The field emission property of graphene/h-BN/Si structure has shown enhanced threshold voltage and emission current, both of which are key to develop novel vacuum nanoelectronics devices. The field emission property was discussed along with the electronic structure of graphene investigated by Fowler-Nordheim plot and ultraviolet photoelectron spectroscopy. The result suggested that transferring graphene on h-BN modified its work function, which changed field emission mechanism. Our report opens up a possibility of graphene-based vacuum nanoelectronics devices with tuned work function.

Enhanced Field Emission of WS2 Nanotubes

Results on electron field emission from free standing tungsten disulfide (WS2) nanotubes (NTs) are presented. Experiments show that the NTs protruding on top of microstructures are efficient cold emitters with turn-on fields as low as 1 V/μm and field enhancement of few thousands. Furthermore, the emission current shows remarkable stability over more than eighteen hours of continuous operation. Such performance and long-term stability of the WS2 cathodes is comparable to that reported for optimized carbon nanotube (CNTs) based emitters. Besides this, it is found that the WS2 cathodes prepared are less sensitive than CNTs in chemical reactive ambients. The high field enhancement and superior reliability achieved indicates a potential for vacuum nanoelectronics and flat panel display applications.

Intensive electron emission in a strong electric field in vacuum nanoelectronics and highpower electronics

Intensive electron emission in a strong electric field in vacuum nanoelectronics and highpower electronics

Miniature ion-sorption vacuum pump with CNT field-emission electron source

Generation and maintenance of the high vacuum in the MEMS-type (micro-electro-mechanical system) microsystems and vacuum nanoelectronics devices remain a major problem today. The phenomena of gas desorption from the surface of a microcavity and outgassing of materials limit the vacuum to the level of about 10−1 Pa. In this paper, a new MEMS-type micropump for generating a high vacuum in a microcavity is presented. The main component of the ion-sorption micropump is a carbon nanotube (CNT) field-emission electron source. Test structures of the electron source with electrophoretically deposited CNT have been fabricated and measured. A satisfactory value of the emission current and a low turn-on voltage have been obtained. The elaborated electron source has been applied to the micropump structure; it has enabled us to achieve a vacuum level below 10−3 Pa.

Historical development and future trends of vacuum electronics

Vacuum electronics (VE) have dominated development and industrial growth in their application areas from the end of the 19th century to the end of 20th century. VE have contributed to basic concepts of physics and have enabled important basic inventions. Despite this bright past, in the meantime also a complete or partial replacement by new technologies such as solid-state electronics (SSE) occurred in several applications areas, triggered by the demand for new features and leading to new applications. Based on a review of the historical development of vacuum electronics from the basic inventions to the modern state of the art, the aim of this paper is to identify future trends and prospects of this field. The appearance of generic technology cycles, as in the case of radio-receiving tubes and cathode-ray display tubes, is discussed. Microwave tubes did experience only a partial replacement by solid-state devices and defended the high-power, high-frequency domain. The reason for their superiority in this domain is discussed. The development of the base technologies for VE, namely vacuum technology and electron source technology, is outlined, enabling further improvements. Besides the high-power, high-frequency domain of microwave tubes, VE technology applications with positive future prospects are addressed, e.g., space applications (long-lived microwave tubes, ion thrusters); thermionic energy converters; e-beam lithography; x-ray tubes; vacuum-based high-resolution characterization, and high-brightness beams for free electron lasers or particle accelerators. The continuous growth and increase in performance of solid-state electronics is shortly reviewed, SSE taking the lead with respect to total sales in the 1980s. Now, despite inherent advantages, solid-state electronics also seem to approach technical limitations. These include increasing energy consumption in conjunction with reduced long-term reliability when further scaling down. It is envisioned that vacuum nanoelectronics can help to overcome these limitations when scaling down feature sizes of integrated circuits below 22 nm.

Vacuum nanoelectronics: Back to the future?—Gate insulated nanoscale vacuum channel transistor

A gate-insulated vacuum channel transistor was fabricated using standard silicon semiconductor processing. Advantages of the vacuum tube and transistor are combined here by nanofabrication. A photoresist ashing technique enabled the nanogap separation of the emitter and the collector, thus allowing operation at less than 10 V. A cut-off frequency fT of 0.46 THz has been obtained. The nanoscale vacuum tubes can provide high frequency/power output while satisfying the metrics of lightness, cost, lifetime, and stability at harsh conditions, and the operation voltage can be decreased comparable to the modern semiconductor devices.

Fabrication of graphene nanogap with crystallographically matching edges and its electron emission properties

We demonstrate the fabrication of graphene nanogap with crystallographically matching edges on SiO2/Si substrates by divulsion. The current-voltage measurement is then performed in a high-vacuum chamber for a graphene nanogap with few hundred nanometers separation. The parallel edges help to build uniform electrical field and allow us to perform electron emission study on individual graphene. It was found that current-voltage (I-V) characteristics are governed by the space-charge-limited flow of current at low biases while the Fowler–Nordheim model fits the I-V curves in high voltage regime. We also examined electrostatic gating effect of the vacuum electronic device. Graphene nanogap with atomically parallel edges may open up opportunities for both fundamental and applied research of vacuum nanoelectronics.

A Study on Field Emission Characteristics of Planar Graphene Layers Obtained from a Highly Oriented Pyrolyzed Graphite Block

This paper describes an experimental study on field emission characteristics of individual graphene layers for vacuum nanoelectronics. Graphene layers were prepared by mechanical exfoliation from a highly oriented pyrolyzed graphite block and placed on an insulating substrate, with the resulting field emission behavior investigated using a nanomanipulator operating inside a scanning electron microscope. A pair of tungsten tips controlled by the nanomanipulator enabled electric connection with the graphene layers without postfabrication. The maximum emitted current from the graphene layers was 170 nA and the turn-on voltage was 12.1 V.

Recent progress in vacuum nanoelectronics

Recent progress in vacuum nanoelectronics

New evolution of vacuum nanoelectronics

New evolution of vacuum nanoelectronics

Electron emission mechanism of hydrogenated natural type IIb diamond (111)

Clarification of the electron emission mechanism of diamond has been one challenging topic in the field of vacuum nanoelectronics. Electric field of less than 5 V/µm is enough to extract electrons from diamond, which is orders of magnitude lower than the value required for electron emission from metal emitter in general. There have been number of studies on the clarification of electron emission mechanism, however, unified model has not been proposed. The difficulty largely lies in determining the origin of emitted electrons. In this study, we succeeded in clarifying the electron emission mechanism of hydrogenated natural type IIb diamond (111) surface by using combined x-ray photoemission spectroscopy/ultraviolet photoemission spectroscopy/field emission spectroscopy (XPS/UPS/FES) system. Obtained spectra clearly defined the origin of emitted electrons at the valence band maximum (VBM). As applied voltage was increased, the number of emitted electrons increased, however, the origin remained at VBM independent of the applied voltage.

Electron emission mechanism of diamond characterized using combined x-ray photoelectron spectroscopy/ultraviolet photoelectron spectroscopy/field emission spectroscopy system

Clarification on electron emission mechanism of diamond is one essential approach to realize the clear vision of vacuum nanoelectronics. Electric field of less than 5V∕μm is enough to extract electrons from diamond, whereas field of one to two orders of magnitude higher is needed to extract electrons from conventional metal emitter tips. Diamond has various advantages as an electron emitter in addition to the low-threshold voltage, such as negative electron affinity and high thermal conductivity. The difficulty in clarification of electron emission mechanism is the factor preventing diamond from being used in a practical way. In this study, combined spectroscopy of x-ray photoelectron spectroscopy/ultraviolet photoelectron spectroscopy/field emission spectroscopy was performed to characterize the electron emission mechanism of diamond. The results indicated the first successful observation of applied voltage dependence on the origin of field-emitted electrons.