Forcing of self-excited round jet diffusion flames

In this experimental and numerical study, two types of round jet are examined under acoustic forcing. The first is a non-reacting low density jet (density ratio 0.14). The second is a buoyant jet diffusion flame at a Reynolds number of 1100 (density ratio of unburnt fluids 0.5). Both jets have regions of strong absolute instability at their base and this causes them to exhibit strong self-excited bulging oscillations at welldefined natural frequencies. This study particularly focuses on the heat release of the jet diffusion flame, which oscillates at the same natural frequency as the bulging mode, due to the absolutely unstable shear layer just outside the flame. The jets are forced at several amplitudes around their natural frequencies. In the non-reacting jet, the frequency of the bulging oscillation locks into the forcing frequency relatively easily. In the jet diffusion flame, however, very large forcing amplitudes are required to make the heat release lock into the forcing frequency. Even at these high forcing amplitudes, the natural mode takes over again from the forced mode in the downstream region of the flow, where the perturbation is beginning to saturate non-linearly and where the heat release is high. This raises the possibility that, in a flame with large regions of absolute instability, the strong natural mode could saturate before the forced mode, weakening the coupling between heat release and incident pressure perturbations, hence weakening the feedback loop that causes combustion instability. © 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Modelling 2DEG charges in AlGaN/GaN heterostructures

In this paper we compare different approaches to calculating the charge density in the 2DEG layer of AlGaN/GaN HEMTs. The methods used are (i) analytical theory implemented in MATLAB, (ii) finite-element analysis using semiconductor TCAD software that implements only the Poisson and continuity equations, and (iii) 1D software that solves the Poisson and Schrödinger equations self-consistently. By using the 1D Poisson-Schrödinger solver, we highlight the consequences of neglecting the Schrödinger equation. We conclude that the TCAD simulator predicts with a reasonable level of accuracy the electron density in the 2DEG layer for both a conventional HEMT structure and one featuring an extra GaN cap layer. In addition, while the sheet charge density is not significantly affected by including Schrödinger, its confinement in the channel is found to be modified. © 2012 IEEE.

Impact of donor traps on the 2DEG and electrical behavior of AlGaN/GaN MISFETs

As an important step in understanding trap-related mechanisms in AlGaN/GaN transistors, the physical properties of surface states have been analyzed through the study of the transfer characteristics of a MISFET. This letter focused initially on the relationship between donor parameters (concentration and energy level) and electron density in the channel in AlGaN/GaN heterostructures. This analysis was then correlated to dc and pulsed measurements of the transfer characteristics of a MISFET, where the gate bias was found to modulate either the channel density or the donor states. Traps-free and traps-frozen TCAD simulations were performed on an equivalent device to capture the donor behavior. A donor concentration of 1.14× 1013 ∼ cm-2 with an energy level located 0.2 eV below the conduction band edge gave the best fit to measurements. With the approach described here, we were able to analyze the region of the MISFET that corresponds to the drift region of a conventional HEMT. © 1980-2012 IEEE.

Deposition of carbon nitride films using an electron cyclotron wave resonance plasma source

Hydrogenated amorphous carbon nitride (a-C:N:H) has been synthesized using a high plasma density electron cyclotron wave resonance (ECWR) technique using N2 and C2H2 as source gases, at different ratios and a fixed ion energy (80 eV). The composition, structure and bonding state of the films were investigated and related to their optical and electrical properties. The nitrogen content in the film rises rapidly until the N2/C2H2 gas ratio reaches 2 and then increases more gradually, while the deposition rate decreases steeply, placing an upper limit for the nitrogen incorporation at 30 at%. For nitrogen contents above 20 at%, the band gap and sp3-bonded carbon fraction decrease from 1.7 to 1.1 eV and approximately 65 to 40%, respectively. Films with higher nitrogen content are less dense than the original hydrogenated tetrahedral amorphous carbon (ta-C:H) film but, because they have a relatively high band gap (1.1 eV), high resistivity (109 Ω cm) and moderate sp3-bonded carbon fraction (40%), they should be classed as polymeric in nature.

Carbon nanotube cathodes as electron sources for microwave amplifiers

Cold cathodes based on carbon nanotubes allow to produce a modulated electron beam. Using an array of vertically aligned CNs that exhibit an aspect ratio of about 200, we demonstrated the modulation of a high current density beam (∼ 1 A/cm2) at 1.5 and 32 GHz frequencies. Such CN cathodes are very promising for their use in a new generation of compact, highly efficient and low cost amplifiers that operate between 10 and 100 GHz. © 2007 IEEE.

Density, sp3 content and internal layering of DLC films by X-ray reflectivity and electron energy loss spectroscopy

A variety of hydrogenated and non-hydrogenated amorphous carbon thin films have been characterized by means of grazing-incidence X-ray reflectivity (XRR) to give information about their density, thickness, surface roughness and layering. We used XRR to validate the density of ta-C, ta-C:H and a-C:H films derived from the valence plasmon in electron energy loss spectroscopy measurements, up to 3.26 and 2.39 g/cm3 for ta-C and ta-C:H, respectively. By comparing XRR and electron energy loss spectroscopy (EELS) data, we have been able for the first time to fit a common electron effective mass of m*/me = 0.87 for all amorphous carbons and diamond, validating the `quasi-free' electron approach to density from valence plasmon energy. While hydrogenated films are found to be substantially uniform in density across the film, ta-C films grown by the filtered cathodic vacuum arc (FCVA) show a multilayer structure. However, ta-C films grown with an S-bend filter show a high uniformity and only a slight dependence on the substrate bias of both sp3 and layering.

Laser microstructuring of Si surfaces for low-threshold field-electron emission

Dense arrays of high aspect ratio Si micro-pyramids have been formed by cumulative high intensity laser irradiation of doped Si wafers in an SF6 environment. A comparative study using nanosecond (XeCl, 308 nm) and femtosecond (Ti: Sapphire, 800 nm and KrF, 248 nm) laser pulses has been performed in this work. The influence of pulse duration and ambient gas pressure (SF6) is also presented. Scanning electron microscopy has shown that upon laser irradiation conical features appear on the Si surface in a rather homogenous distribution and with a spontaneous self alignment into arrays. Their lowest tip diameter is 800 nm; while their height reaches up to 90 mum. Secondary tip decoration appears on the surface of the formed spikes. Areas of 2 X 2 mm(2) covered with Si cones have been tested as cold cathode field emitters. After several conditioning cycles, the field emission threshold for the studied Si tips is as low as 2 V/mum, with an emission current of 10(-3) A/cm(2) at 4 V/mum. Even though these structures have smaller aspect ratios than good quality carbon nanotubes, their field emission properties are similar. The simple and direct formation of field emission Si arrays over small pre-selected areas by laser irradiation could lead to a novel approach for the development of electron sources. (C) 2003 Elsevier B.V. All rights reserved.