Field Emission Threshold Research Articles

Origin of low threshold field emission from nitrogen-incorporated nanocrystalline diamond films

Highly conductive, nitrogen-incorporated nanocrystalline diamond films with quasimetallic character emit electrons at low turn-on fields (∼3 V μm−1). These films exhibit stronger delocalization of carriers, indicative of smaller energy separation between the defect bands in the band gap. We show that the emission level derived from the measured emission characteristic and electron affinity shifts upward (up to a few eV) with increasing the film conductivity, thereby decreasing the effective potential barrier height for the emission. This is attributed to higher probabilities of electron injection into upper defect levels during the transport process, originating from internal band bending and increasing band continuity.

Field emission properties of Si tip arrays coated with N-doped SrTiO3 thin films at different substrate temperature

The effect of substrate temperature (TS) on the behavior of field emission, microstructure, optical band gap, and the surface energy of N-doped SrTiO3 thin films coated on silicon tip arrays has been examined in detail. Results indicate that the TS dominates the chemical states of nitrogen added to the sputtered SrTiO3 films and hence the observations. At the critical temperature of 600 °C, nitrogen atoms incorporate into the oxide film with sp-hybridization features. The generation of the nonbonding lone pair states narrows the optical band gap and the lone pair induced antibonding dipoles lower the threshold field for electron emission substantially. At lowered TS, molecular adsorption of nitrogen dominates. Contact angle measurements further evidence for the presence of antibonding dipole states at the surfaces which is responsible for the adsorbate-induced surface stress.

Carbon nanostructures as thermal field emitters for waste heat recovery

Traditional vacuum-based thermal energy conversion (VTEC) devices with “flat” metal emitters have been compromised by high working temperature mainly due to high work function and electron space charge effect. However, a VTEC system is inherently efficient due to unimpeded flow of electrons through vacuum and reduction of heat conduction by the vacuum medium. Consequently, VTEC devices are attractive in terms of space, weight and energy efficiency for waste heat recovery in a TEC system. Carbon nanostructured emitters including multiwalled carbon nanotubes (CNTs) and “ridge” nanodiamond thin films have shown relatively low threshold fields for electron field emission mainly due to their nanoscaled, high aspect ratio, emitting surfaces. In this study, the field-enhancing features of carbon nanostructures were investigated as thermal-field emitters for waste heat recovery. The turn-on field of CNTs was found to decrease from ~ 1.9 V/μm at room temperature to ~ 0.9 V/μm at 400 °C. A high emission current of ~ 26 μA was achieved at relatively low field of ~ 1.5 V/μm and low temperature of 400 °C. The maximum efficiency of the VTEC device is estimated to be ~ 15% at 300 °C with a current density of 54 mA/cm 2.

Effect of film structure on field emission properties of nitrogen doped hydrogenated amorphous carbon films

In this study, we investigated the effect of bonding structure of nitrogen doped a-C:H ( a-C:N:H) film on the electrical and field emission properties. The bonding structure in nitrogen doped a-C:H films were controlled via growth using an acetonitrile methane mixture. Nitrogen atoms in the a-C:N:H films were incorporated in termination structure such as C≡N and N–H. Current–voltage measurements for a-C:N:H films showed that the electrical conductivity was high for the lightly nitrogen doped films. The lightly doped a-C:N:H film exhibited lower threshold field emission voltage than heavy nitrogen doped films. The behavior observed in this research suggests that nitrogen atoms do not act as electron donors. The results can be interpreted as that incorporated nitrogen atoms do not acts as electron donor because they are involved in the C≡N and N–H termination structures. Characterizations of the doping system, not only doping species, is necessary to design the electronic structures of a film to obtain high efficiency field emission.

A new approach to inspect the internal pressure of vacuum switchgear

A new approach is developed to inspect the internal pressure within the vacuum switchgear by no means of magnet-exciting coil, micro-discharge threshold voltage U d and field emission threshold voltage U e, all of which are available in laboratory. Experimental results show that internal pressure is a function of the ratio of the micro-discharge threshold voltage to the field emission threshold voltage i.e., U d/U e. By this method, the internal pressure in the range of 100–10−4 Pa within vacuum chambers can be inspected without magnet-exciting coil which is required in conventional magnetron discharge method.

Laser‐induced self organization of silicon–germanium hillocks for field emission displays

AbstractWe report here on the formation of a self‐organized hillock structure in polycrystalline silicon–germanium (SiGe) thin films with a Ge content of 30 to 70%, due to the irradiation of the films with a sequence of laser pulses. By varying the crystallization parameters it is possible to control the periodicity length of the structure from 0.4 to 2 µm. The self organization phenomenon can be switched on and off by using substrates with different thermal conductivities. In addition, we show that the self‐organized hillock structure exhibits a low field emission threshold of 20 V/µm and therefore has an immense potential for field emission displays. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

An empirical study of dynamic properties of an individual carbon nanotube electron source system

In this paper we present an empirical study of some dynamic properties of an individual carbon nanotube (CNT) field emission electron source system. We propose a circuit model that represents the CNT cathode to anode diode as a capacitor in parallel with a voltage-controlled variable resistor. The transient response of the CNT electron source system to the falling edge of a voltage step input was evaluated. For input voltages below the threshold voltage for field emission, the nanotube loop is effectively open and the circuit response is consistent with a discharging capacitor. On the other hand, for input voltages above field emission threshold, the nanotube loop conducts and now the capacitor discharges to a certain extent through the nanotube loop as well. Field emission current versus voltage data also shows that the resistance across the CNT cathode to anode diode varies as a function of applied voltage. Below turn-on voltage, the diode behaves as an open circuit (4 TΩ at the ammeter noise floor). Above turn-on voltage, resistance falls exponentially, as expected from the Fowler–Nordheim equation for cold field emission current. Experimental current–voltage data is presented for a simple emitter array consisting of two CNTs with equal lengths. Despite the similarity in their lengths the turn-on voltages of the nanotubes varied significantly, viz. 26 V versus 109 V. This large difference in the turn-on voltages can be attributed to tip imperfections. For advanced array applications such as high-throughput parallel e-beam lithography, in which precise dose control is necessary, the diode circuit model will be useful for controlling individually addressed nanotubes to account for dissimilar field emission properties. The model may also be applied to optimize the design of a SEM incorporating a single CNT electron source.

Enhancement of electron field emission property with silver incorporation into diamondlike carbon matrix

Effects of silver doping on the electron field emission properties of diamondlike carbon films deposited on silicon substrates by the rf reactive sputtering technique were studied in detail. It was found that the threshold field and effective emission barrier were reduced by Ag doping and the emission current strongly depends on the Ag doping percentage. The threshold field was found to decrease from 6.8to2.6V∕μm with a variation of Ag at. % from 0 to 12.5. The field enhancement factor was calculated and we have explained the emission mechanism.

Stable field emission performance of SiC-nanowire-based cathodes

In this paper we report the fabrication and testing of diode-type low-voltage fieldemission display (FED) devices with SiC-nanowire-based cathodes. The SiC-nanowireFEDs (flat vacuum lamps) were characterized by low emission threshold fields(∼2 V µm−1), high current density and stable long-term performance. The analysis of field emissiondata evidenced that the Schottky effect would have a considerable influence on the fieldemission from nanowire-based samples, leading to the true values of the field enhancementfactor being significantly lower than those derived from Fowler–Nordheim plots.

Surface modification by vacuum annealing for field emission from heavily phosphorus-doped homoepitaxial (1 1 1) diamond

The relationship between field emission properties and C 1s core level shifts of heavily phosphorus-doped homoepitaxial (1 1 1) diamond is investigated as a function of annealing temperature in order to optimize surface carbon bonding configurations for device applications. A low field emission threshold voltage is observed from surfaces annealed at 800 °C for hydrogen-plasma treated surface, while a low field emission threshold voltage of wet-chemical oxidized surface is observed after annealing at 900 °C. The C 1s core level by X-ray photoelectron spectroscopy (XPS) showed a shoulder peak at 1 eV below the main peak over 800 and 900 °C annealing temperature for hydrogen-plasma treated and wet-chemical oxidized surfaces, respectively. When the shoulder peak intensity is less than 10% of the main peak intensity, lower threshold voltages are observed. This is due to the carbon-reconstruction which gives rise to a small positive electron affinity. By increasing annealing temperature, the shoulder peak ratios also increase, which indicates that a surface graphitization takes place. This leads to higher threshold voltages.

Field emission from as grown and nitrogen incorporated tetrahedral amorphous carbon/silicon heterojunctions grown using a pulsed filtered cathodic vacuum arc technique

This article reports the field emission measurements on as grown tetrahedral amorphous carbon (ta-C) and nitrogen incorporated tetrahedral amorphous carbon (ta-C: N) films grown using a pulsed filtered cathodic vacuum arc technique. The effect of varying thickness on field emission in the as grown ta-C films and the effect of varying nitrogen content in ta-C: N films have also been studied. The values of threshold field of emission (Eturnon) increase with decrease of thickness in the as grown ta-C films. Nitrogen incorporation up to 5.2at.% in ta-C films decreases the value of Eturnon from 9.9to5.1V∕μm and thereafter it starts increasing again. To understand the mechanism of electron emission, a realistic energy band diagram of ta-C:N∕n++Si heterojunction has been proposed from the experimentally measured valence and conduction band offsets, using in situ x-ray photoelectron spectroscopy and optical spectroscopy data already published in DRM 9 (2000) 1148. The data are explained using the Fowler and Nordheim theory. The field emission results obtained reveal that there exists a barrier to emission and the main barrier is at the front surface and this is related to the conduction band offset of the ta-C:N∕n++Si heterojunction.

Morphology and temperature-dependent electron field emission from vertically aligned carbon nanofibers

Effects of temperature and the aspect ratio on the electron field emission properties of vertically aligned carbon nanofibers in thin-film form were studied in detail. Vertically aligned carbon nanofibers have been synthesized on silicon substrates via a direct current plasma enhanced chemical vapor deposition technique. Surface morphologies of the films were studied by an atomic force microscope. It was found that the length of the nanofibers increased and the diameter decreased as the thickness of the Ni catalyst film decreased. The threshold field for the electron field emission was found to be in the range from 4.3 to 5.4 V/μm for carbon nanofibers having different aspect ratios. The threshold field for carbon nanofibers having diameter ∼ 200 nm and aspect ratio ∼7.5 was found to decrease from 4.8 to 2.1 V/μm when the temperature was raised from 27 to 350 °C. This dependence was due to the change in work function of the nanofibers with temperature. The field enhancement factor, the current density and the effective work function were calculated and used to explain the emission mechanism.

Field emission characteristics of a lanthanum monosulfide cold cathode array fabricated using microelectromechanical systems technology

Using microelectromechanical systems technology, an array of cold cathodes was fabricated by pulsed laser deposition of chemically and thermally stable lanthanum monosulfide (LaS) thin film anode and cathode contacts. The latter were defined via etching and processing of two different pieces of (100) Si wafers separated via a highly resistive sputter deposited aluminum nitride (AlN) layer, whose thickness was used to control the anode to cathode spacing. The top and bottom Si wafers were aligned and glued together using high temperature, vacuum compatible epoxy. Field emission characteristics were recorded in a vacuum chamber with a base pressure less than 10−7Torr. An average electric field threshold for Fowler-Nordheim field emission in the range of 100V∕μm was measured. The largest emission current measured was about 5×10−7A, above which thermal runaway occurred, leading to a failure of the cathode. The failure mechanism is analyzed in terms of a patchwork field emission model from the LaS thin film reported recently. Suggestions for improvements in the cathode design are discussed.

Improving the electron emission properties of ion-beam-synthesized Ag–SiO[sub 2] nanocomposites by pulsed laser annealing

The effects of pulsed laser annealing on the structural and electron field emission (FE) properties of Ag–SiO2 nanocomposites are reported. The nanocomposites were synthesized by Ag+ implantation into thermally oxidized Si substrates with a dose of 7×1016Ag+∕cm2. The annealing was performed using a 248nm KrF excimer laser. Multiple pulses, with duration of 25ns, at an energy density of 250mJ∕cm2 were accumulated onto the same area of the sample. Surface protrusion structures of the as-implanted samples, causing geometrical field enhancement, are eliminated after laser annealing, and a reduction in concentration and coalescence of the Ag nanoclusters is observed from atomic force microscopy and transmission electron microscopy measurements. The threshold field for emission for the as-implanted sample is altered from 28 to 104, 16, and 35V∕μm after annealing with 5, 10, and 20 laser pulses, respectively. The FE properties of laser annealed samples are discussed in terms of two distinct field enhancement effects using their structural properties.

Effect of nitrogen addition on the band gap, core level shift, surface energy, and the threshold field of electron emission of the SrTiO3 thin films

The effect of nitrogen (N) doping on the behavior of field emission, surface energy and the band structure of strontium titanate (SrTiO3) thin films coated on silicon tip arrays has been examined in detail. Measurements using x-ray photoelectron spectroscopy, ellipsometry, water contact angle and field emission testing revealed that the optimal 50%-nitrogen partial pressure (PN) could improve substantially the threshold field of electron emission of the SrTiO3 films accompanied with narrowed band gap, lowered surface energy and work function and a negative energy shift of the N 1s level from 404 to 396 eV. Results evidence consistently the presence of the nonbonding lone pairs and the lone pair induced antibonding dipoles upon tetrahedron formation which is responsible for the observations. At PN below and above the optimal value physisorption and hydrogen bond likes formation like to occur.

High specific surface area porous SiC ceramics coated with reticulated amorphous SiC nanowires

High specific surface area porous SiC ceramics, coated completely with reticulated amorphous SiC nanowires, have been fabricated with commercially available phenolic resin and silicon powders using a novel method. The results indicate that the specific surface area and porosity of the as-synthesized materials can be up to 112 m 2 g −1 and 81%, respectively. The coated amorphous SiC nanowires have uniform structure and smooth surfaces. The electron emission turn-on field and threshold field are about 2.9 and 6.7 V μm −1, respectively. This kind of materials may simultaneously possess the unique properties of porous materials, SiC, and nanowires. And they may have promising applications in a wide variety of areas.

Single- and Few-Walled Carbon Nanotubes Grown at Temperatures as Low as 450 °C: Electrical and Field Emission Characterization

Single-wall (SW-) and few-walled (FW-) carbon nanotubes (CNTs) were synthesized on aluminum/ cobalt coated silicon at temperatures as low as 450 degrees C by plasma enhanced chemical vapor deposition technique (PECVD). The SWCNTs and FWCNTs grow vertically oriented and well separated from each other. The cold field emission studies of as-grown SWCNTs and FWCNTs showed low turn-on field emission threshold voltages, strongly dependent of the nanotubes morphology. Current-voltage curves of individual CNTs, measured by conductive atomic force microscopy (CAFM), showed an electrical resistance of about 90 Komega, that is attributed mainly to the resistance of the contact between the CNTs and the conductive CAFM tip (Au and Pt).

Ultralow threshold field emission from ZnO nanorod arrays grown on ZnO film at lowtemperature

ZnO nanorod arrays have been synthesized on silicon substrate covered withZnO film by thermal evaporation of zinc particles at a low temperature of550 °C. Their field emission has been investigated: the turn-on electric field (at the current density of1 µA cm−2) isabout 3.8 V µm−1, and the threshold electric field (at the current density of1 mA cm−2) is6.3 V µm−1 at the workingdistance of 100 µm. In comparison, the turn-on and threshold electric fields ofthe not well-aligned ZnO nanorod arrays and ZnO film are 9.8,15.8 V µm−1 and13.7, 26.0 V µm−1 at 100 µm, respectively. These behaviors indicate that such an ultralow threshold field emission isattributed to the aligned structure, the good electric contact with the conducting substratewhere they grow, and weaker field-screening effect. Our results demonstrate thatwell-aligned nanorod arrays with excellent field-emission performance grown at such a lowtemperature can provide the possibility of application in glass-sealed flat panel displays.

Field emission from multiwall carbon nanotubes on paper substrates

The authors report extremely low electron field emission thresholds from acid oxidized multiwall carbon nanotubes deposited on paper substrates by dip coating in an aqueous nanotube ink. Using paper substrates of differing surface roughness, field emission threshold fields ranging from 0.8to11.6V∕μm were observed, varying in an approximate inverse linear log relationship with the surface roughness of the underlying paper substrate. This study shows how field emission from supported nanotube films can be tailored via the morphology of the scaffold substrate, and how these composite electrodes can be straightforwardly fabricated on cheap, flexible substrates.

Electron field emission from boron doped microcrystalline diamond

Field emission properties of hot filament chemical vapor deposited boron doped polycrystalline diamond have been studied. Doping level ( N B) of different samples has been varied by the B/C concentration in the gas feed during the growth process and doping saturation has been observed for high B/C ratios. Threshold field ( E th) for electron emission as function of B/C concentration has been measured, and the influences of grain boundaries, doping level and surface morphology on field emission properties have been investigated. Carrier transport through conductive grains and local emission properties of surface sites have been figured out to be two independent limiting effects in respect of field emission. Emitter current densities of 500 nA cm −2 were obtained using electric fields less than 8 V/μm.