Thin Film Field Emission Research Articles

Enhanced field emission characteristics of WS2 nano-films by diamond film and Mo film

WS2 is a type of transition metal chalcogenide materials (TMDCs) that exhibits exceptional photoelectric properties. Due to its atomically sharp edges, WS2 has the capability to enhance the electric field around its edges, thus presenting significant potential in field emission applications. In this study, the electron beam vapor deposition (EBVD) system and microwave plasma chemical vapor deposition (MPCVD) system were used to prepare a series of single-layer WS2 films，single-layer diamond films and diamond/WS2 nano-composite films on N-type highly doped Si substrate, and Mo/WS2 nano-composite films were prepared on Al2O3 ceramic substrate. Based on the characterization and analysis of the microstructure and chemical composition of each thin film layer in the above film devices, the field emission (FE) characteristics of each thin film device are compared and studied. The results demonstrate that the maximum FE current density of diamond/WS2 nano-composite film device is 912 μA/cm2, which is 2.7 times of the maximum FE current density of Mo/WS2 nano-composite film device 330 μA/cm2, and 3.4 times of the maximum FE current density of the single-layer diamond thin film device 267 μA/cm2. The single layer WS2 film did not exhibit measurable FE performance. The working principles of thin film field emission devices with various structures are introduced. It is considered that the diamond layer plays the role of electron accelerating layer, which not only effectively improves the FE characteristics of diamond/WS2 nanocomposite film, but also improves the repeatability and long-term stability of diamond/WS2 nanocomposite film FE devices.

Enhanced Field Emission Properties of ZnO:Al/SrTiO3 Perovskite Composite Films by ZnO:Al Film

ZnO:Al(Al‐doped zinc oxide)/SrTiO3 composite thin film field emission (FE) devices are fabricated on heavily doped n‐type silicon substrates using a vacuum electron beam evaporation vapor deposition technique and hydrogen plasma treated technique. The morphology and thickness of SrTiO3 film and ZnO:Al film are controlled by adjusting and optimizing the growth conditions, deposition time, and post‐treatment conditions. FE experimental results show that the addition of ZnO:Al films can significantly improve the FE properties of ZnO:Al/SrTiO3 composite film. The maximum current density of the FE sample increases from 230 μA cm−2 of the monolayer SrTiO3 film sample, which is treated by hydrogen plasma (H–SrTiO3) to 951 μA cm−2 of the ZnO:Al/H–SrTiO3 composite film sample, which increases by more than 4 times. Meanwhile, at the applied electric field intensity of 3.6 V μm−1, the FE current density of ZnO:Al/H–SrTiO3 composite film is 48 times that of monolayer H–SrTiO3 film. Each field emission sample device has good working stability and experimental repeatability.

Perovskite-type SrTiO3 thin film preparation and field emission properties

By using vacuum electron beam vapor deposition (EBVD) technology, perovskite-type SrTiO3 was deposited on indium tin oxide (ITO) conductive glass. The SrTiO3 orientation and its film thickness were well-controlled by appropriately modifying the growth conditions and deposition time. The elemental composition, microstructure, surface morphology and field emission character of SrTiO3 film were characterized by energy dispersive spectrum (EDS), X-ray diffraction (XRD) spectrum, field emission scanning electron microscope (FE-SEM) and field emission (FE) detecting system. The FE test results showed that the SrTiO3 film showed good field emission characteristics. The maximum field emission current density reach 88.96 μA/cm2 at 10.45V/μm applied electric field. And the experimental results show that the SrTiO3 film has certain potential in FE field.

Enhanced electron emission from coated metal targets: Effect of surface thickness on performance

In this work, we establish an analytical formalism to address the temperature dependent electron emission from a metallic target with thin coating, operating at a finite temperature. Taking into account three dimensional parabolic energy dispersion for the target (base) material and suitable thickness dependent energy dispersion for the coating layer, Fermi Dirac statistics of electron energy distribution and Fowler’s mechanism of the electron emission, we discuss the dependence of the emission flux on the physical properties such as the Fermi level, work function, thickness of the coating material, and operating temperature. Our systematic estimation of how the thickness of coating affects the emission current demonstrates superior emission characteristics for thin coating layer at high temperature (above 1000 K), whereas in low temperature regime, a better response is expected from thicker coating layer. This underlying fundamental behavior appears to be essentially identical for all configurations when work function of the coating layer is lower than that of the bulk target work function. The analysis and predictions could be useful in designing new coated materials with suitable thickness for applications in the field of thin film devices and field emitters.

Preparations and field emission properties of multilayer AlGaN nanofilm

We report on the electron field emission (FE) from multi-layer AlGaN nanofilm grown by pulsed laser deposition, and the investigation of the multi-layer quantum structure effect on the field emission performance. The results show that the as-grown film has a good crystallinity, and the thickness values of GaN, AlN, and GaN film are 25 nm, 50 nm, and 25 nm, respectively. The FE measurement indicates that compared with single layer, the multilayer filmhas a low turn-on field and large threshold current. The turn-on filed is found to be 0.93 V/m, and the electric current density reaches to 30 mA/cm2 at 5.5 V/m. The improvement of the FE performance is attributed to resonant tunneling in the quantum well structure, and the accumulated electrons lower the effective surface barrier. The outstanding performance of multi-layer filed emission film should provide a feasible technical solution for large current and high power density thin film field emission device.

Modelling of field emitter array characteristics in diode system

In terms of a mathematical model, this paper studies features of the current characteristics of a thin-film matrix field emission array cathode with vertical blade-shaped emitters. Microscopical distribution of electric field strength was calculated and its dependence on geometric parameters of the structures was defined.

Properties of a field emission lighting plane employing highly crystalline single-walled carbon nanotubes fabricated by simple processes

We developed a new cathode for a visible ray flat plane-emission device formed from a mixture of highly crystalline single-walled carbon nanotubes (SWNTs) dispersed into an organic In2O3–SnO2 precursor solution and a non-ionic surfactant. A thin film field emission cathode having highly homogeneously dispersed SWNTs was able to be fabricated with low driving voltage, high electron emission homogeneity in the plane, and high brightness efficiency by employing a simple scratching process for the film.The turn-on field of a diode using the optimized cathode was 1.2V/μm, with the brightness homogeneity in that plane being within 5% against the averaged brightness in the lighting plane. Favorable brightness homogeneity from the lighting device to control the content of SWNTs in the coated film was achieved by employing highly crystalline SWNTs. Furthermore, brightness efficiency and emission life-time, which are important factors when comparing luminance devices, totaled more than 70lm/W and over 5000h until reaching the half time against the initial field emission current density.This flat plane-emission device has the potential to provide a new approach to lighting in everyday life as it contributes to energy saving through its low power consumption.

(Invited) Flat-Panel X-ray Image Sensor Using Thin Film Transistors and Field Emitter Arrays

Active matrix flat panel imagers (AMFPI) have been commercialized for a wide range of x-ray imaging applications. They have demonstrated superior imaging performance except in fluoroscopy, where the electronic noise degrades the image quality, making it inferior to x-ray imaging intensifier (XRII). In this paper we will present two FPI approaches under investigation to overcome the electronic noise limitation. They use an amorphous selenium layer with programmable avalanche gain to detect light generated from an x-ray scintillator upon x-ray absorption. Two charge readout methods are being investigated: a thin-film transistor (TFT) array; and a field emitter array (FEA). The amorphous selenium (a-Se) avalanche photoconductor is called HARP (high-gain avalanche rushing photoconductor). The avalanche gain of HARP depends on both a-Se thickness and applied electric field ESe. At ESe of > 80 V/μm, the avalanche gain can enhance the signal at low dose (e.g. fluoroscopy) and make the detector x-ray quantum noise limited down to a single x-ray photon. At high exposure (e.g. radiography), the avalanche gain can be turned off by decreasing ESe to < 70 V/μm, thus ensuring a wide dynamic range without burdening the readout electronics. The potential x-ray imaging performance of both FPI approaches, especially the aspect of programmable gain to ensure wide dynamic range and x-ray quantum noise limited performance at the lowest exposure in fluoroscopy, have been investigated.

Effect of few-walled carbon nanotube crystallinity on electron field emission property

We discuss the influence of few-walled carbon nanotubes (FWCNTs) treated with nitric acid and/or sulfuric acid on field emission characteristics. FWCNTs/tetraethyl orthosilicate (TEOS) thin film field emitters were fabricated by a spray method using FWCNTs/TEOS sol one-component solution onto indium tin oxide (ITO) glass. After thermal curing, they were found tightly adhered to the ITO glass, and after an activation process by a taping method, numerous FWCNTs were aligned preferentially in the vertical direction. Pristine FWCNT/TEOS-based field emitters revealed higher current density, lower turn-on field, and a higher field enhancement factor than the oxidized FWCNTs-based field emitters. However, the unstable dispersion of pristine FWCNT in TEOS/N,N-dimethylformamide solution was not applicable to the field emitter fabrication using a spray method. Although the field emitter of nitric acid-treated FWCNT showed slightly lower field emission characteristics, this could be improved by the introduction of metal nanoparticles or resistive layer coating. Thus, we can conclude that our spray method using nitric acid-treated FWCNT could be useful for fabricating a field emitter and offers several advantages compared to previously reported techniques such as chemical vapor deposition and screen printing.

Integration of thin film transistors and vertical thin film field emitter arrays using ion-induced bending

A vertical thin film field emitter array (VTF-FEA) was integrated with a thin film transistor (TFT) through a simple fabrication process that is compatible with the existing TFT manufacturing process. Specifically, the authors integrated a poly-Si TFT and a previously reported VTF-FEA. The VTF-FEA was fabricated using the ion-induced bending (IIB) technique. The IIB is a technique that can bend a cantilever by ion-beam irradiation and can be used with a wide range of materials. The emission from the TFT-VTF-FEA was controlled from subnanoampere to microampere levels using the built-in TFT. This work demonstrated that VTF-FEAs can be integrated with poly-Si TFTs and other electronic devices, such as metal-oxide semiconductor field effect transistors.

Evaluation of a simplified simulation approach for thin film type gated field emitters

A simplified simulation approach aiming at reducing computational complexity has been proposed for gated field emitters. In this approach, a thin-film cold cathode consisting of quasi-one-dimensional nanoemitters is modeled as a flat metallic thin-film. The surface of the thin-film is divided into equal-size grids. Fowler–Nordheim parameters fitted with experimental current-voltage data are used to calculate emission current from these individual grids. The statistics of the total emission current and its distribution between the gates and the anode are analyzed by summing the currents from all of the grids. The validity of the proposed approach is evaluated by comparing the simulation results with experimental data from gated carbon nanotube (CNT) thin-film field emitters. CNTs at the edge of the CNT film stand out as superior emitters, and proper consideration of such emitters plays a critical role in determining the validity of the proposed approach.

SnO2 nanoribbons: excellent field-emitters

Nanoribbon structures have the feature of sharp edges, providing the marked geometrical field enhancement for field emitters even in a randomly arranged thin film structure. In this work, the field emission of SnO2 nanoribbons with thin walls (10–30 nm) prepared on a large scale via a rapid oxidation reaction is investigated. It is found that macroscopically, thin films made of SnO2 nanoribbons have an extremely low electron threshold field as low as 1.23 V μm−1. The SnO2 nanoribbon field emitter also exhibits excellent emission stability, with a degradation lower than 2.7% and a huge field enhancement factor as high as 2680. Furthermore, the emission current decrease slightly, while the stability remains excellent even for a poor vacuum of 10−4 Pa. These field emission properties of the present SnO2 nanoribbons surpass any other reported SnO2 nanostructures. This work opens the avenues for SnO2 nanoribbon structures as promising thin film field emitters.

Development of a CdTe x-ray imaging device driven by a vertical thin film field emission array

The authors describe a novel CdTe x-ray imaging device that is driven by an electron beam from a field-emitter array. This CdTe x-ray imaging device comprises a CdTe Schottky diode with a vertical thin-film field-emitter array (VTF-FEA). The VTF-FEA was fabricated by an ion-induced bending process and an etch-back technique. The output signal from the detector was obtained through the recombination of x-ray-generated holes and electrons emitted from the VTF-FEA. The output signal clearly depended on the intensity of the x-ray beam. The shape of a Cu plate was successfully detected by the VTF-FEA-driven CdTe diode.

Improved Field Emission Characteristics with Different Electrode Materials by Forming Process in a Quasi-Planar Thin Film Field Emission Diode

The essay was reported the different electrode materials in a quasi-planar thin film field emission diode for different forming process. The forming process causes an increased surface roughness of emitters or reduced the work function of the emitter and results in a higher field enhancement factor, which shows better field emission characteristics. The turn on voltage could from 15 V to 8 V and 6.5V with Co and Pd electrode at the emission current of 100 nA for samples.

Improved Field Emission Characteristics with Different Electrode Materials by Forming Process in a Quasi-planar Thin Film Field Emission Diode

not Available.

Formation of TiO 2 nano-network on titanium surface increases the human cell growth

Objectives This study was to improve human cell growth on titanium (Ti) used for dental implants through formation of a nano-network surface oxide layer created by an electrochemical anodization treatment. Methods An electrochemical anodization treatment was used to produce a network oxide layer on Ti surface. Surface characterization of the network layer was carried out using thin film X-ray diffractometer and field emission scanning electron microscopy. Human bone marrow mesenchymal stem cells (hMSCs) were made to express green fluorescent protein (GFP) by retroviral transduction. The GFP signal was measured in situ to assess in vitro and in vivo cell growth on Ti surfaces. In vivo experiments on Ti-supported cell growth were carried out on the back skin of nude mice. Alizarin red staining and immunofluorescent staining were used to observe cell differentiation. Results A multilayer TiO 2 nano-network was produced rapidly on Ti surface using a simple electrochemical anodization treatment. The TiO 2 nano-network layer on the anodized Ti surfaces significantly improved in vitro and in vivo hMSC growth, as assessed by measurement of GFP fluorescence, relative to hMSC growth on untreated Ti surface. The TiO 2 nano-network layer on the anodized Ti surfaces can also induce the differentiation of hMSCs after 28-day in vivo test. Significance The formation of TiO 2 nano-network on the Ti surfaces can increase the hMSC growth in vitro and in vivo.

Field emission from graphene based composite thin films

Field emission from graphene is challenging because the existing deposition methods lead to sheets that lay flat on the substrate surface, which limits the field enhancement. Here we describe a simple and general solution based method for the deposition of field emitting graphene/polymer composite thin films. The graphene sheets are oriented at some angles with respect to the substrate surface leading to field emission at low threshold fields (∼4Vμm−1). Our method provides a route for the deposition of graphene based thin film field emitter on different substrates, opening up avenues for a variety of applications.

A Quasi-Planar Thin Film Field Emission Diode

A novel quasi-planar thin-film field emitter is fabricated by thin-film deposition and wet etching processes. The spacing between the emitters and collectors could be well controlled on the basis of the thicknesses of Cr thin films, which create submicron gaps. A forming process increases emitter surface roughness and results in a higher field enhancement factor, which shows better field emission characteristics. The turn-on voltage (at which the current level is 100 nA) of the device with a Cr thin film thickness of 300 nm is as low as 12 V, and the current fluctuation in 1 hour test at a driving voltage of 20 V represents a variation from -86 to +114%.

Room-temperature deposition of diamond-like carbon field emitter on flexible substrates

Room-temperature fabrication of diamond-like carbon electron field emitters on flexible polyimide substrate is reported. These thin film field emitters are made using an Ar gas cluster ion beam assisted C60 vapor deposition method. The bond structure of the as-deposited diamond-like carbon film was studied using Raman spectroscopy. The field emission characteristics of the deposited films were also measured. Electron current densities over 15mA/cm2 have been recorded under an electrical field of about 65V/μm. These diamond-like carbon field emitters are easy and inexpensive to fabricate. The results are promising for flexible field-emission fabrication without the need of complex patterning and tip shaping as compared to the Spindt-type field emitters.

Modeling of emitted current distribution and electron trajectories in the thin-film field-emission triode

Conformal mapping was used to find a closed-form solution to Laplace’s equation in the thin-film cathode field-emission triode, a structure commonly used in field-emission display applications. The equation was used to find the electric field on the surface of the cathode and this, together with the Fowler–Nordheim equation and published data on a carbon nanotube film, was used to find the emitted current density across the surface of the cathode. The equation of motion of the electron was integrated using a Runge–Kutta algorithm to find the electron trajectories. A range of anode voltages was investigated, and it was found that the beam divergence that occurred at low anode voltages could be turned into beam convergence by an increase in the anode voltage sufficient such that the vertical field in the device due to the anode was greater than that due to the gate. Furthermore, the modeling showed that when the anode voltage was this high, emission occurred preferentially from the center of the cathode, rather than from the edges, which occurred at lower anode voltages. This modeling provides a method of controlling beam divergence in field-emission-display pixels, without having to resort to additional focusing electrodes, hence, reducing fabrication complexity and system cost.