ß-Ga2O3 Air-Channel Field-Emission Nanodiode with Ultrahigh Current Density and Low Turn-On Voltage.

Field-emission nanodiodes with air-gap channels based on single ß-Ga2O3 nanowires have been investigated in this work. With a gap of ∼50 nm and an asymmetric device structure, the proposed nanodiode achieves good diode characteristics through field emission in air at room temperature. Measurement results show that the nanodiode exhibits an ultrahigh emission current density, a high enhancement factor of >2300, and a low turn-on voltage of 0.46 V. More impressively, the emission current almost keeps constant over a wide range (8 orders of magnitude) of air pressures below 1 atm. Meanwhile, the fluctuation in field-emission current is below 8.7% during long-time monitoring, which is better than the best reported field-emission device based on ß-Ga2O3 nanostructures. All of these results indicate that ß-Ga2O3 air-gapped nanodiodes are promising candidates for vacuum electronics that can also operate in air.

Fast Photoresponse and Long Lifetime UV Photodetectors and Field Emitters Based on ZnO/Ultrananocrystalline Diamond Films.

We have designed photodetectors and UV field emitters based on a combination of ZnO nanowires/nanorods (ZNRs) and bilayer diamond films in a metal-semiconductor-metal (MSM) structure. The ZNRs were fabricated on different diamond films and systematic investigations showed an ultra-high photoconductive response from ZNRs prepared on ultrananocrystalline diamond (UNCD) operating at a lower voltage of 2 V. We found that the ZNRs/UNCD photodetector (PD) has improved field emission properties and a reduced turn-on field of 2.9 V µm(-1) with the highest electron field emission (EFE) by simply illuminating the sample with ultraviolet (UV) light. The photoresponse (Iphoto /Idark ) behavior of the ZNRs/UNCD PD exhibits a much higher photoresponse (912) than bare ZNRs (229), ZNRs/nanocrystalline diamond (NCD; 518), and ZNRs/microcrystalline diamond (MCD; 325) under illumination at λ=365 nm. A photodetector with UNCD films offers superior stability and a longer lifetime compared with carbon materials and bare ZNRs. The lifetime stability of the ZNRs/UNCD-based device is about 410 min, which is markedly superior to devices that use bare ZNRs (92 min). The ZNRs/UNCD PD possesses excellent photoresponse properties with improved lifetime and stability; in addition, ZNRs/UNCD-based UV emitters have great potential for applications such as cathodes in flat-panel displays and microplasma display devices.

Design of TiO_{2} nanotube based X-ray tube with single focusing electrode.

We studied in details the effect of various X-ray tube parameters (cathode size, anode size, anode - lens distance, etc.) on TiO_{2} nanotube field emission electron-beam focal spot size (FSS) and lens voltages for a single electrode lens system. The simulations were performed using commercially available SIMION 8.1 software. A wide range of parameters was simulated to determine conditions when FSS and lens focusing voltage had minimum values. In particular, the dependence of FSS and lens voltages on cathode size (d_{S}) was studied for different size lens apertures (d_{L}) and different anode voltages. The cathode size d_{S} was varied in the range from 0.1 mm to d_{L}; the d_{L} was changed from 4 to 24 mm in 4 mm increments. The simulations were performed for two different V_{A} values 60 kV and 120 kV. It was found that for 20 mm and 24 mm diameter lens apertures the maximum cathode size when the resulting FSS was not greater than 1600 µm (the focal spot size in clinical X-ray CT) were 17.5 mm and 16.2 mm, respectively. The lens voltages V_{f} corresponding to 17.5 mm and 16.2 mm cathode size in 20 mm and 24 mm aperture diameter x-ray tubes are 1850 V and -1550 V, respectively.

The effect of size distribution and degradation of carbon nanotubes on the Fowler-Nordheim plot behavior.

The behavior of the Folwer-Nordheim (FN) plot is investigated for large-area emitters arrays composed of perfectly aligned single-walled carbon nanotubes (CNTs). The field emission currents are calculated based on the standard FN model using the Murphy-Good equation. The aspect ratio of the CNTs is varied such that the height of the CNT has predefined uniform or Gaussian distributions. Continuous degradation of the high-aspect-ratio CNTs is assumed to take place at predefined values of the scaled barrier field. The effects, on the FN plot behavior, of the size distribution and the scenario of the degradation of the emitter are determined and discussed. These factors are found to produce the two behaviors of the experimental FN plots, i.e. the upward and the downward bending behaviors. This work shows the possibility to expand the application of the standard FN model for the investigation of the field emission characteristics from nanoscaled emitters such as CNTs.

The behavior of Fowler-Nordheim plot from carbon nanotubes-based large area field emitters arrays.

The Fowler - Nordheim (FN) plot behavior is investigated for field emitted electrons from virtual carbon nanotubes (CNTs)-based large area field emitters (LAFEs) arrays. The field emission currents are calculated using the Murphy-Good field emission equation assuming emission from two sets of geometrically-different CNTs. No screening effects are considered in the calculations. The FN plots nonlinear behavior, in the form of an upward bend, is observed, analyzed and attributed to the emitters' geometrical features and their numbers in the arrays. The calculations emphasize that the nonlinear characteristic depends, not only on the two-class geometries of the emitters but also on the statistical distribution of these emitters in the arrays. The calculations adopted in the present work allow fitting the experimental data of the LAFEs for any desirable range of applied voltages with minimal adjustable parameters. The present investigation is believed to help further development of the LAFEs for future applications.

Fabrication of Stable Carbon Nanotube Cold Cathode Electron Emitters with Post-Growth Electrical Aging.

We fabricated carbon nanotube (CNT) cold cathode emitters with enhanced and stable electron emission properties and long-time stability with electrical aging as a post-treatment. Our CNT field emitters showed improved electrical properties by electrical aging. We set the applied bias for effective electrical aging, with the bias voltage defined at the voltage where Joule heating appeared. At the initial stage of aging, the electron emission current started to increase and then was saturated within 3 h. We understood that 5 h aging time was enough at proper aging bias. If the aging bias is higher, excessive heating damages CNT emitters. With the electrical aging, we obtained improved electron emission current from 3 mA to 6 mA. The current of 6 mA was steadily driven for 9 h.

Superior B-Doped SiC Nanowire Flexible Field Emitters: Ultra-Low Turn-On Fields and Robust Stabilities against Harsh Environments.

Low turn-on fields together with boosted stabilities are recognized as two key factors for pushing forward the implementations of the field emitters in electronic units. In current work, we explored superior flexible field emitters based on single-crystalline 3C-SiC nanowires, which had numbers of sharp edges, as well as corners surrounding the wire body and B dopants. The as-constructed field emitters behaved exceptional field emission (FE) behaviors with ultralow turn-on fields (Eto) of 0.94-0.68 V/µm and current emission fluctuations of ±1.0-3.4%, when subjected to harsh working conditions under different bending cycles, various bending configurations, as well as elevated temperature environments. The sharp edges together with the edges were able to significantly increase the electron emission sites, and the incorporated B dopants could bring a more localized state close to the Fermi level, which rendered the SiC nanowire emitters with low Eto, large field enhancement factor as well as robust current emission stabilities. Current B-doped SiC nanowires could meet all essential requirements for an ideal flexible emitters, which exhibit their promising prospect to be applied in flexible electronic units.

Extremely Stable Current Emission of P-Doped SiC Flexible Field Emitters.

Novel P-doped SiC flexible field emitters are developed on carbon fabric substrates, having both low Eto of 1.03-0.73 Vµm-1 up to high temperatures of 673 K, and extremely high current emission stability when subjected to different bending states, bending circle times as well as high temperatures (current emission fluctuations are typically in the range ±2.1%-3.4%).

Theory of Carbon Nanotube (CNT)-Based Electron Field Emitters.

Theoretical problems arising in connection with development and operation of electron field emitters on the basis of carbon nanotubes are reviewed. The physical aspects of electron field emission that underlie the unique emission properties of carbon nanotubes (CNTs) are considered. Physical effects and phenomena affecting the emission characteristics of CNT cathodes are analyzed. Effects given particular attention include: the electric field amplification near a CNT tip with taking into account the shape of the tip, the deviation from the vertical orientation of nanotubes and electrical field-induced alignment of those; electric field screening by neighboring nanotubes; statistical spread of the parameters of the individual CNTs comprising the cathode; the thermal effects resulting in degradation of nanotubes during emission. Simultaneous consideration of the above-listed effects permitted the development of the optimization procedure for CNT array in terms of the maximum reachable emission current density. In accordance with this procedure, the optimum inter-tube distance in the array depends on the region of the external voltage applied. The phenomenon of self-misalignment of nanotubes in an array has been predicted and analyzed in terms of the recent experiments performed. A mechanism of degradation of CNT-based electron field emitters has been analyzed consisting of the bombardment of the emitters by ions formed as a result of electron impact ionization of the residual gas molecules.

Sci-Fri PM: Delivery - 06: A generalized solution to the wide field array calibration method.

Multi detector arrays are commonly used in radiation oncology for IMRT and Linac QA. The calibration of detector arrays is usually based on the wide field calibration technique. Unfortunately small beam changes between measurements will result in large error propagation. The present work introduces a generalized modified version of the wide field calibration method, robust against measurement to measurement variation. Our generalized framework uses an unlimited number of measurement pairs, n geometric positions providing n(n-1)/2 pairs. We solve this large over determined linear system using least squares with gradient method. Measurements were made on an Elekta synergy 6 MV beam with two IBA Matrixx detectors, each containing a 32 × 32 array (1024) of vented pixel ionization chambers. Data acquisition was by the IBA Omnipro Advance software, version 1.2 running in the "ONLINE" cine mode with a 10 sec integration time. Continuous beam sampling (10 seconds long) measured over 10 minutes demonstrated why consistent calibration using the conventional wide field calibration is a challenge. Overall signal changes of 1.6%, flatness changes of 0.3% and the beam symmetry changes of 0.2% over the full 10 minute beam-on time were observed. For the purpose of testing and demonstration of our method, we have chosen to make measurements in 5 geometric orientations relative to the beam, including 1 reference position, 2 rotations and 2 translations. With our method we were able to calibrate all 1024 detectors to better than 0.6% total uncertainty as demonstrated against inter and intra MatriXX comparison.