Low Extraction Fields Research Articles

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.

Velocity-map imaging at low extraction fields

We present a velocity-map imaging (VMI) setup for photoelectron imaging that utilizes low electric extraction fields. This avoids any complications that could arise from electrostatic interactions between the extraction field and the molecular properties that are probed and has a minimal effect on the trajectory of ions in ion beam experiments. By using an attractive potential supplied to the detector, and keeping the electrodes at ground (zero) potential, we show that fringe fields between the VMI arrangement and the vacuum chamber can be eliminated, which is important in experiments on ions.

Ion–ion coincidence experiments with low extraction fields

The fragmentation of a polyatomic molecular dication has been investigated by means of a Monte Carlo simulation which can take into account realistic experimental conditions. The simulation applied to the three body process CH 4 2+ → CH 2 + + H + + H shows that the low angular acceptance, consequence of the low extraction field, does not prevent the distinction between the initial charge separation and the synchronous concerted decay mechanisms, which lead to the same final state. The comparison of the results with recent experiments [1] confirms that an initial charge separation with a following loss of a hydrogen atom describes better the experimental observations.

TwinGrid: A wafer post-processed multistage Micro Patterned Gaseous Detector

This paper presents a new multistage Micro Patterned Gaseous Detector (MPGD) made by wafer post-processing. The device consists of a double metal grid supported by SU-8 structures on top of a Timepix chip. The detector has been operated with He/iC 4H 10 and Ar/iC 4H 10 gas mixtures. Cosmic rays as well as 55Fe decay events have been recorded using the Timepix chip in 2D and 3D readout modes. The detector can be operated like a Micromegas (high field close to the chip) or like a GEM (amplification stage followed by a low extraction field). This approach for manufacturing GEM-like radiation imaging detectors allows for precise dimensional control and considerable geometrical design freedom.

Sulfur doped nanocrystalline diamond films as field enhancement based thermionic emitters and their role in energy conversion

Sulfur doped nanocrystalline diamond films, like other nanostructured carbon films, exhibit electron emission characterized by a spatial non-uniformity of the field enhancement factor. While field emission effects are observed at room temperature, an increase in emitter temperature is accompanied by an amplified emission current with a simultaneous drop in the threshold field. At low extraction fields a fit of the emission current to the Richardson equation indicates a material work function of ∼2.5 eV. The Schottky formula describes thermionic emission at a moderate field and is utilized to determine the work function at an electric field of 0.8 V/μm with a value of ∼1.7 eV and a concurrently reduced Richardson constant. This significant difference in the work function of 2.5 and 1.7 eV for 0.5 and 0.8V/μm, respectively can be attributed to field enhancement effects.

Application of vitreous and graphitic large-area carbon surfaces as field-emission cathodes

Numerous carbon bulk or thin-film materials have been used as field-emission cathodes. Most of these can be made into large-area and high-current field-emission cathodes without the use of complex IC fabrication techniques. Some of these exhibit low-extraction field, low work-function, high ruggedness, chemical stability, uniform emission, and low-cost manufacturability. A comparison of all of these materials is presented. Two viable cathode materials, reticulated vitreous carbon (RVC) and graphite paste are examined here and compared.

Amorphous selenium photodetector driven by diamond cold cathode

Amorphous selenium is one of the most promising candidates as photoconducting material for future imaging devices. It can exhibit ultra-high photosensitivity by using avalanche multiplication inside the solid. However, the requirement of high vacuum by the electron emitters prevents amorphous selenium from being a realistic imaging device in general. Diamond, with its outstanding properties of negative electron affinity and high chemical stability, can be an ultimate emitter with low extraction field and low vacuum operation, especially for imaging devices. This report presents the first successful operation of an amorphous selenium photodetector with low operational vacuum and extraction field using a diamond cold cathode.

Photodissociation of HBr molecules and clusters: Anisotropy parameters, branching ratios, and kinetic energy distributions

The ultraviolet photolysis of HBr molecules and (HBr)n clusters with average size around n̄=9 is studied at three different wavelengths of 243, 205, and 193 nm. Applying polarized laser light, the kinetic energy distribution of the hydrogen photofragment is measured with a time-of-flight mass spectrometer with low extraction fields. In the case of HBr monomers and at 243.1 nm, an almost pure perpendicular character (β=−0.96±0.05) of the transitions is observed leading to the spin–orbit state Br(2P3/2). The dissociation channel associated with the excited state Br*(2P1/2) is populated by a parallel transition (β*=1.96±0.05) with a branching ratio of R=0.20±0.03. At the wavelength of 193 nm, about the same value of R=0.18±0.03 is found, but both channels show a mainly perpendicular character with β=−0.90±0.10 for Br and β*=0.00±0.10 for Br*. The results for 205 nm are in between these two cases. For the clusters at 243 nm, essentially three different groups appear which can be classified according to their kinetic energy: (i) A fast one with a very similar behavior as the monomers, (ii) a faster one which is caused by vibrationally and rotationally excited HBr molecules within the cluster, and (iii) a slower one with a shoulder close to the fast peak which gradually decreases and ends with a peak at zero velocity. The zero energy fragments are attributed to completely caged H atoms. The angular dependence of the group (iii) is isotropic, while that of the other two is anisotropic similar to the monomers. At 193 nm only the fast and the slow part is observed without the peak at zero energy. Apparently the kinetic energy is too large to be completely dissipated in the cluster.

UV photolysis of (HBr) n clusters with known size distribution

The photodissociation of (HBr) n clusters is reported using polarized laser light at 243.1 nm. The cluster size is measured in a deflection experiment with He and Ne atoms. A time-of-flight mass spectrometer with low extraction fields is used to obtain the kinetic energy distributions of the hydrogen atoms. At averaged cluster sizes of n = 8.1 two distributions are observed with fast peaks of uncaged fragments and with a sharp peak at zero velocity which shows an isotropic angular distribution and which is attributed to complete caging.

Photodissociation and size analysis of (HBr)n clusters

AbstractHydrogen bromide clusters that are generated in an adiabatic expansion with Ar are size‐selected by scattering from a He or a Ne beam and a subsequent separation by a velocity selector. The complete fragmentation pattern for electron impact ionization of clusters n = 3,5,7,10, and 15 is obtained for electron energies of 70 eV. Up to cluster sizes of n = 15 strong fragmentation is observed. Based on these results the cluster sizes are measured as a function of different source conditions. For the same experimental conditions the (HBr)n clusters are photoionized at 243.1 nm. A time‐of‐flight mass spectrometer with low extraction fields is used to obtain the translational energy distributions of hydrogen atoms. Several distinct features are observed which sensitively depend on the cluster size. At small cluster sizes a tail appears towards lower energies in the fast hydrogen kinetic energy distribution which is caused by few collisions with the cluster molecules. Peaks at higher energies are attributed to internally excited HBr molecules which gain their energy in collisions with photofragmented H atoms. Small clusters exhibit pure rotational excitation with averaged J = 7, while the larger ones show vibrational excitations to v = 1 with J = 8 and v = 2. Finally, we observe a narrow H component peaking near zero velocity and exhibiting an isotropic angular distribution. This effect appears only at larger cluster sizes and is attributed to a complete caging of the photofragments.

A novel correction scheme for quantitative voltage contrast measurements using low extraction fields in the scanning electron microscope

A novel correction technique combined with a lock-in peak detection scheme for quantitative voltage contrast measurements using low extraction fields in the scanning electron microscope has been developed. This technique was found to be more accurate than the conventional feedback technique for quantitative voltage contrast measurements. Local field effects (of both types I and II) can be accounted for by this correction scheme without the use of very high extraction fields of 500-1000 V mm-1. The use of a low extraction field of 50 V mm-1 is especially advantageous for specimens that are sensitive to high fields and for measurements on specimens surrounded by regions of insulating material, which may charge up under high extraction fields. This scheme also allows the possibility of correction for minor charging due to a contaminant layer forming as a result of sample exposure to air or under electron beam bombardment.

Neutral and ion beam SIMS of non-conducting materials

Energetic neutral beams are increasingly being used as an alternative to ion beams for the analysis of low conductivity materials in secondary ion mass spectrometry, since their use greatly alleviates the problems caused by sample charging. After a brief review of the history of this development, this paper reviews various methods for producing inert gas atom beams of energies in the 0.5–15 keV range and a number of neutral gun designs based on these methods are discussed. They include the saddle-field atom gun, capillaritron, a conventional electron impact source based ion/atom gun and a novel gun producing a raster-scanned micro focused (⩾ 5 μm) beam of either atoms or ions. The effects of sample charging are discussed with special reference to a low extraction field configuration. A careful control of the surface potential, which is possible under either ion or atom bombardment in conjunction with charge compensating electron beams, can provide positive and negative spectra with a substantial molecular cluster ion contribution. Secondary ion mass spectra of insulating targets under ion (+electron) or atom bombardment are qualitatively similar in that the same characteristics molecular fragment ions occur. Differences in peak height distribution are at least partially explainable in terms of a less precise control of the surface potential in the case of ion and electron bombardment. On the other hand, essential differences between the two types of bombardment are observed in the damage rate in some polymer materials and the sputter rate in a range of low conductivity materials. These experiments, carried out after careful calibration procedures, provide evidence for the existence of a charge state induced damage mechanism and a sunstantial electronic sputtering contribution in these materials under ion bombardment.