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An accurate estimation of the experimental field-emission area remains a great challenge in vacuum electronics. The lack of convenient means, which can be used to measure this parameter, creates a critical knowledge gap, making it impossible to compare theory to experiment. In this work, a fast pattern recognition algorithm was developed to complement a field emission microscopy, together creating a methodology to obtain and analyze electron emission micrographs in order to quantitatively estimate the field emission area. The algorithm is easy to use and made available to the community as a freeware, and therefore is described in detail. Three examples of dc emission are given to demonstrate the applicability of this algorithm to determine spatial distribution of emitters, calculate emission area, and finally obtain experimental current density as a function of the electric field for two technologically important field emitter materials, namely an ultrananocrystalline diamond film and a carbon nanotube fiber. Unambiguous results, demonstrating the current density saturation and once again proving that conventional Fowler-Nordheim theory, its Murphy-Good extension, and the vacuum space-charge effect fail to describe such behaviour, are presented and discussed. We also show that the transit-time limited charge resupply captures the current density saturation behaviour observed in experiments and provides good quantitative agreement with experimental data for all cases studied in this work., Comment: arXiv admin note: substantial text overlap with arXiv:2012.03578

The anticorrelation between quantum efficiency (QE) and electron spin polarization (ESP) from a p-doped GaAs activated to negative electron affinity (NEA) is studied in detail using an ensemble Monte Carlo approach. The photoabsorption, momentum and spin relaxation during transport, and tunnelling of electrons through the surface potential barrier are modeled to identify fundamental mechanisms, which limit the efficiency of GaAs spin-polarized electron sources. In particular, we study the response of QE and ESP to various parameters such as the photoexcitation energy, doping density, and electron affinity level. Our modeling results for various transport and emission characteristics are in a good agreement with available experimental data. Our findings show that the behaviour of both QE and ESP at room temperature can be fully explained by the bulk relaxation mechanisms and the time which electrons spend in the material before being emitted.

In this work, a pattern recognition algorithm was developed to process and analyze electron emission micrographs. Various examples of dc and rf emission are given that demonstrate this algorithm applicability to determine emitters spatial location and distribution and calculate apparent emission area. The algorithm is fast and only takes $\sim$10 seconds to process and analyze one micrograph using resources of an Intel Core i5.

In the nitrogen-incorporated ultrananocrystalline diamond ((N)UNCD) films, representing an $n$-type highly conductive two-phase material comprised of $sp^3$ diamond grains and $sp^2$-rich graphitic grain boundaries, the current is carried by a high concentration of mobile electrons within the large-volume grain boundary networks. Fabricated in a simple thin-film planar form, (N)UNCD was found to be an efficient field emitter capable of emitting a significant amount of charge starting at the applied electric field as low as a few V/$\mu$m which makes it a promising material for designing electron sources. Despite the semimetallic conduction, field emission (FE) characteristics of this material demonstrate a strong deviation from the Fowler-Nordheim law in a high-current-density regime when (N)UNCD field emitters switch from a diode-like to resistor-like behavior. Such phenomenon resembles the current-density saturation effect in conventional semiconductors. In the present paper, we adapt the formalism developed for conventional semiconductors to study current-density saturation in (N)UNCD field emitters. We provide a comprehensive theoretical investigation of ($i$) the influence of partial penetration of the electric field into the material, ($ii$) transport effects (such as electric-field-dependent mobility), and ($iii$) features of a complex density-of-states structure (position and shape of $\pi-\pi^*$ bands, controlling the concentration of charge carriers) on the FE characteristics of (N)UNCD. We show that the formation of the current-density saturation plateau can be explained by the limited supply of electrons within the impurity $\pi-\pi^*$ bands and decreasing electron mobility in high electric field. Theoretical calculations are consistent with experiment., Comment: 15 pages, 14 figures

The complete design, fabrication, and performance evaluation of a compact, single cell, $X$-band ($\sim$9 GHz) electron injector based on a field emission cathode (FEC) are presented. A pulsed electron beam is generated by a 10's of kW radiofrequency (RF) magnetron signal from a plug-in thin film nitrogen-incorporated ultrananocrystalline diamond (N)UNCD FEC cartridge. Testing of the $X$-band injector with the (N)UNCD FEC was conducted in a beamline equipped with a solenoid, Faraday cup and imaging screen. The results show that typically the (N)UNCD FEC cartridge produces $\gtrsim$1 mA/cm$^2$ at a surface electric field of 28 MV/m. The diameter of the output beam generated from the 4.4 mm diameter (N)UNCD cartridge can be as small as 1 mm. In terms of its practical applications, the demonstrated $X$-band electron injector with the (N)UNCD plug-in FEC can serve as a source for X-ray generation, materials processing, travelling-wave tubes (including GHz and THz backward wave oscillators), or can be used to drive slow-wave accelerating structures. The results presented also suggest that this field emitter technology based on planar (N)UNCD thin films, which are simply grown on the surface of optically polished stainless steel, can enable a vast number of device configurations that are efficient, flexible in design, and can be packaged with ease., Comment: 5 pages, 6 figures

In this paper we study the effect of actual, locally resolved, field emission (FE) area on electron emission characteristics of uniform semimetallic nitrogen-incorporated ultrananocrystalline diamond ((N)UNCD) field emitters. To obtain the actual FE area, imaging experiments were carried out in a vacuum system in a parallel-plate configuration with a specialty anode phosphor screen. Electron emission micrographs were taken concurrently with $I$-$V$ characteristics measurements. It was found that in uniform (N)UNCD films the field emitting site distribution is not uniform across the surface, and that the actual FE area depends on the applied electric field. To quantify the actual FE area dependence on the applied electric field, a novel automated image processing algorithm was developed. The algorithm processes extensive imaging datasets and calculates emission area per image. By doing so, it was determined that the emitting area was always significantly smaller than the FE cathode surface area of 0.152 cm$^2$ available. Namely, the actual FE area would change from $5\times10^{-3}$ \% to 1.5 \% of the total cathode area with the applied electric field increased. Finally and most importantly, it was shown that when $I$-$E$ curves as measured in the experiment were normalized by the field-dependent emission area, the resulting $j$-$E$ curves demonstrated a strong kink and significant deviation from Fowler-Nordheim (FN) law, and eventually saturated at a current density of $\sim$100 mA/cm$^2$. This value was nearly identical for all studied (N)UNCD films, regardless of the substrate.

The letter introduces a diagram that rationalizes tunneling atomic force microscopy (TUNA) observations of electron emission from polycrystalline diamonds as described in recent publications. The direct observations of electron emission from grain boundary sites by TUNA could indeed be evidence of electrons originating from grain boundaries under external electric fields. At the same time, from the diagram it follows that TUNA and field emission schemes are complimentary rather than equivalent for results interpretation. It is further proposed that TUNA could provide better insights into emission mechanisms by measuring the detailed structure of the potential barrier on the surface of polycrystalline diamonds., Comment: 3 figure, 3 pages