Pure Field Emission Research Articles

Thermal-field electron emission from three-dimensional Dirac and Weyl semimetals

A model is constructed to describe the thermal-field emission of electrons from a three-dimensional (3D) Dirac and Weyl semimetal hosting Dirac/Weyl node(s). The traditional thermal-field electron emission model is generalized to accommodate the 3D nonparabolic energy band structures in the Dirac/Weyl semimetals, such as cadmium arsenide $({\mathrm{Cd}}_{3}{\mathrm{As}}_{2})$, sodium bismuthide $({\mathrm{Na}}_{3}\mathrm{Bi})$, tantalum arsenide (TaAs), and tantalum phosphide (TaP). Due to the nontrivial energy decomposition of the energy dispersion and the vanishing transverse density of states, an unusual dual-peak feature is observed in the total energy distribution spectrum. This nontrivial dual-peak feature, absent from traditional materials, plays a critical role in manipulating the magnitude of the emission current through the variation of an applied field, temperature, and Fermi level. This feature suggests that a higher Fermi level will achieve a larger current density (apart from low work function). At zero temperature limit, a ${F}^{3}$ scaling law for pure field emission is derived and it is different from the well-known Fowler-Nordheim ${F}^{2}$ scaling law. Furthermore, these new behaviors have shown to exist beyond the Dirac cone approximated model. This model expands the recent understandings of electron emission studied for the Dirac two-dimensional (2D) materials into the 3D regime, and thus offers a theoretical foundation for the exploration in using Dirac semimetallic materials as novel electrodes.

Ni Schottky barrier on heavily doped phosphorous implanted 4H-SiC

The electrical behavior of Ni Schottky barrier formed onto heavily doped (N D > 1019 cm−3) n-type phosphorous implanted silicon carbide (4H-SiC) was investigated, with a focus on the current transport mechanisms in both forward and reverse bias. The forward current–voltage characterization of Schottky diodes showed that the predominant current transport is a thermionic-field emission mechanism. On the other hand, the reverse bias characteristics could not be described by a unique mechanism. In fact, under moderate reverse bias, implantation-induced damage is responsible for the temperature increase in the leakage current, while a pure field emission mechanism is approached with bias increasing. The potential application of metal/4H-SiC contacts on heavily doped layers in real devices is discussed.

Fiber tip-based electron source

The authors report on the first experimental characterization of a fiber tip-based electron source, where electron emission can be triggered by both electric field and optical excitation. Our approach consists of coating the open aperture of a commercial 100 nm apex size near-field scanning optical microscopy fiber tip with a 10 nm thick tungsten (W) layer, which is back-illuminated by a 405 nm continuous-wave laser beam in the presence of an extraction electric field. Despite the very low optical transmission of the fiber due to the subwavelength aperture size, measurements show a clearly enhanced emission when photoexciting the W layer with respect to pure field emission. The emission response time is slower than the optical trigger time, suggesting that thermal effects are predominant in the studied regime. To back up this hypothesis, the authors fabricated a nanometric thermocouple probe based on a Pt/Au junction and measured the temporal response of the tip temperature. The measured switch-on time for the tip temperature is consistent with the switch-on time of the optically enhanced electron emission.

Electrical characterization of all-epitaxial Fe/GaN(0001) Schottky tunnel contacts

We analyze the properties of Fe Schottky contacts prepared in situ on n-type GaN(0001) by molecular beam epitaxy. In particular, we investigate the suitability of these epitaxial Fe layers for electrical spin injection. Current-voltage-temperature measurements demonstrate pure field emission for Fe/GaN:Si Schottky diodes with [Si]=5×1018 cm−3. The Schottky barrier height of the clean, epitaxial Fe/GaN interface is determined by both current-voltage-temperature and capacitance-voltage techniques to be (1.47±0.09)eV.

Study on the Electron Emission Properties of ZnO Nanorod Arrays on Different Substrates

Large area well-aligned ZnO nanorod arrays on different substrates were synthesized by hydrothermal methods. The electron emission properties of the ZnO nanorod arrays on different substrates were investigated under both direct current (DC) and pulse electric fields. Owing to the excellent conductivity of substrates, the array on stainless steel substrate had better electron emission properties than that on silicon substrate. Under the DC and pulse electric fields, the electron emission of arrays had different production mechanisms which were pure field emission and plasma-induced emission respectively. During the plasma-induced emission, the plasma formed on the array surface, and the maximum emission current density of arrays on stainless steel was 118.87 A/cm2. The plasma-induced emission of ZnO nanorod arrays were always distributed uniformly. In this work, the results show that the ZnO nanorod arrays are expected to be applied to different electronic devices as electron beam sources under different electric fields.

Mechanisms of carriers transport in Ni/n-SiC, Ti/n-SiC ohmic contacts

A mechanism of carriers transport through metal-semiconductor interface created by nickel or titanium-based ohmic contacts on Si-face n-type 4H-SiC is presented herein. The mechanism was observed within the temperature range of 20 °C ÷ 300 °C which are typical for devices operating at high current density and at poor cooling conditions. It was found that carriers transport depends strongly on concentration of dopants in the epitaxial layer. The carriers transport has thermionic emission nature for low dopant concentration of 5×1016 cm−3. The thermionic emission was identified for moderate dopant concentration of 5×1017 cm−3 at temperatures higher than 200 °C. Below 200 °C, the field emission dominates (for the same doping level of 5×1017 cm−3). High dopant concentration of 5×1018 cm−3 leads to almost pure field emission transport within the whole investigated temperature range.

High-current electron emission characteristics of cathodes based on diamond films

Using different gas source, four types of diamond thin films were prepared on silicon substrate by microwave plasma chemical vapor deposition (MPCVD) technology, and characterized in detail through scanning electron microscopy (SEM), Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. High-current pulsed emission characteristics, tested with a 2 MeV line-inducing injector, showed that all of CVD diamond films had high emission current density (> 70 A/cm 2) and [100] textured B-doped microcrystalline diamond film possessed the largest emission current density of 115.1 A/cm 2. No obvious bright light and luminescent zones from side view CCD images indicated a possible pure field-emission mechanism of these diamond cathodes. Simultaneously, large decrease in the electron emission capability, above 15%, could be observed after several pulsed measurements, but this decrease could be completely recovered through the treatment of surface re-hydrogenation for emitted diamond cathodes, suggesting that emission performance of CVD diamond cathodes was closely relevant to hydrogen coverage ratio. The present data indicated that as-deposited CVD diamond films could be a potential candidate as cold cathode for the application in high-current electron emission field.

Comment on “Low level plasma formation in a carbon velvet cesium iodide coated cathode” [Phys. Plasmas 11, 1680 (2004)

First Page

Low level plasma formation in a carbon velvet cesium iodide coated cathode

Field emission cathodes have been a subject of research for many years. These cathodes hold the promise of effective electron emission in the absence of a heater. Such devices find application in the high power microwave (HPM) arena, as well as the conventional microwave industry and other areas such as flat panel displays. Over the past several years the Air Force Research Laboratory began to focus on cesium iodide cathodes as a field emission cathode of some interest. Previously reported results demonstrated a cesium iodide coated carbon velvet cathode capable of over one million pulses of operation with no degradation of emission. However, the exact emission mechanism remains somewhat unclear. This paper presents results showing that plasma formation on the cathode surface remains minimal at 1 μs pulse lengths. While ionized cesium and iodine lines exist in the light spectrum from the diode, these lines remain quite small, with the fluorescent emission from solid cesium iodide dominating the optical spectra in the diode. Hence, we propose that the cesium iodide coated carbon velvet operates in a space charge limited regime with pure field emission alone.

Temperature dependence of the anomalous leakage current in polysilicon-on-insulator MOSFET's

A new analytical model is presented for the temperature and bias dependence of the anomalous leakage current based on thermionic field emission via grain boundary traps in the gate-drain overlap region in polysilicon-on-insulator MOSFET's. The existing model based on pure field emission (tunneling) via grain boundary traps does not include a temperature dependence and therefore cannot explain the observed strong temperature dependence of leakage at low gate voltages, as well as the weaker temperature dependence at high gate voltages, which the new analytical model presented in this paper can. Below 150 K, we believe that impact ionization due to the increasing carrier mean free path leads to the observed increase in the leakage current with decreasing temperature. Since the analytical model does not include impact ionization, it cannot model the leakage current at low temperatures. >

Small-signal conductance vs temperature: A rapid means of determining the nature of current transport in high-doped n-GaAs Schottky barrier diodes

Measurements of small-signal forward conductance vs temperature at a constant bias current, and the reverse conductance vs temperature at a constant bias voltage have been carried out on ion-implanted n-GaAs Schottky barrier diodes fabricated on semi-insulated substrates, for the temperature range of 97–340 K. The results obtained have been interpreted in terms of diode ideality factors and compared with those calculated using the thermionic field emission model for the forward bias case and the pure field emission model for the reverse bias case.

Tunneling Currents in Zinc Oxide

An examination of the current-voltage characteristics of gold and palladium surface barriers on degenerate zinc oxide has been made. Both chemically prepared and cleaved surfaces were studied. The current conduction mode is shown to be thermionic-field emission at room temperature and to be pure field emission at liquid-nitrogen temperatures. The voltage dependence of the current is in good agreement with theory. The observed current magnitudes in both current modes were approximately one-tenth that calculated by simple theory.

Normalized thermionic-field (T-F) emission in metal-semiconductor (Schottky) barriers

Thermionic field (T-F) emission in uniformly doped metal-semiconductor (Schottky) barriers is analyzed to yield a normalized solution in closed form for the forward and reverse current (I)-voltage (V) relationship. A quasi one-dimensional approach and Maxwell-Boltzmann statistics are used. The formulation is expressed in terms of the ‘flat-band’ current density I m , the band bending E b in the semiconductor depletion region, the materials constant E 0 0 ( ln[ I I m ] = − E b E 0 0 at 0°K in the WKB approximation), and kT. The kinetic energy in units of E b at which the maximum injection of carriers occurs in the semiconductor is shown to be cosh -2( kT/ E 0 0). Current flow in the temperature range between pure thermionic emission ( kT/ E 0 0 ⪢ 1) and pure field emission ( kT/ E 0 0 ⪡ 1) is analyzed and criteria for the transition of T-F emission to thermionic and to field emission are given. Computer solutions for the energy distribution of the injected carriers and for the normalized I-V characteristic are presented in graphical form. The results permit a straightforward calculation of the barrier height and the impurity concentration in the semiconductor from measurements of current density and differential resistance at a single applied bias. Application of these results explains a reported discrepancy between barrier heights deduced from photothreshold, C-V and I-V characteristics of WGaAs and AuGaAs Schottky barriers. A relatively constant excess temperature T 0 (i.e., ln I ∝ ( T + T 0) when V ⪢ kT/ q) is predicted in the case of large E b / E 0 0 in the higher kT/ E 0 0 range where thermionic emission is nearly predominant. I ∝ [exp( qV/ kT) − 1] is shown to be a general expectation for all Schottky barriers near zero bias when the I-V characteristic is dominated by either thermionic or thermionic-field emission. The assumption of a Gaussian energy distribution of carriers leads to values for the slope of ln I vs. V in reasonable agreement with the results of the computer analysis, but the prediction of the absolute value of the current density deviates rapidly from the computed value when kT/ E 0 0 departs appreciably from unity. The Gaussian distribution also does not provide the smooth transition from T-F to thermionic emission characteristic of the computer solution.