L. D. Filip

Formation and characterization of carbon nanowires

This article reports on the formation and electronic characteristics of conducting carbon nanowires produced by swift heavy ion irradiation of a fullerene thin film. This study shows that it is possible to create arrays of carbon nanowires, which are perfectly parallel to each other and perpendicular to the substrate. As-deposited fullerene films exhibit poor field emission characteristics with breakdown fields as high as 51 V/μm, whereas low dose ion irradiated fullerene film produces a threshold field as low as 9 V/μm. The present approach of making conducting carbon nanowires by ion irradiation for potential field emitters and large area applications is also discussed.

Modeling of Optimized Field Emission Nanotriodes with Aligned Carbon Nanotubes of Variable Heights

Films of vertically aligned carbon nanotubes (CNTs) have recently being grown within patterned areas. Nanotriodes based on such CNT arrays as field emitters have the potential of improved performance if devising a way to subject the nanotubes to uniform extraction fields. In this paper, we propose and model an optimized field emission nanotriode with aligned CNTs of variable heights. The bunch of variable-height CNTs is centrally placed in the opening of the gate electrode, the CNTs being taller in the middle and shorter toward the bunch edge. A proof-of-concept analytical model to describe the electric field distribution is derived. Numerical computations of the electric field for three-dimensional structural configurations taking into account the CNT reciprocal field screening have been performed. The nanotriode characteristics have been calculated as a function of device geometry and its functional parameters. Optimal nanotriode configurations assuring quasi-uniform extraction fields and enhanced emission currents have been shown to exist.

The effect of the top electrode interface on the hysteretic behavior of epitaxial ferroelectric Pb(Zr,Ti)O3 thin films with bottom SrRuO3 electrode

The hysteretic behavior of the epitaxial Pb(Zr,Ti)O3 thin films with different top metal electrodes is studied, with emphasis on the influence of the leakage current and trap generation current on the shape of the loop as well as on the magnitude of the measured polarization. Cu, Pt, and SrRuO3 were used as top contacts and important differences were observed for measurements performed in both dynamic and static modes, although the contacts were deposited on the same epitaxial Pb(Zr,Ti)O3 film grown on SrRuO3/SrTiO3 substrate. A peculiar behavior was observed especially for the static hysteresis loops where, depending of the top contact, the loop is influenced mainly by the leakage current (Pt) or by the trap generation current (Cu and SrRuO3). The last one can contribute with an additive charge, having a linear dependence on the applied voltage, as suggested by the simple model developed to explain the abnormally high values of the dielectric constant extracted from the linear part of the static hysteres...

Ring-shaped images as a result of nonuniform field emission from capped carbon nanotubes

Ring-shaped images may appear under high field emission conditions for very thin carbon nanotubes (CNTs). Such image patterns cannot be explained by the corresponding field enhancement only. A model for electron field emission from the CNT is developed. The model refers to a capped nanotube (with cylindrical body and hemispherical cap). It is assumed that for high emission currents/high local temperatures, part of the electrons behave as quasi-free. As a result, the spatial confinement quantization of their states appears. The Schrodinger equation for the single electron can be solved separately on the cylindrical and spherical parts of the structure and the corresponding solutions can be connected smoothly at the circular intersection of the two regions. Many electronic states that are possible on the two regions separately turn out to be forbidden for the capped nanotube. The selection of the possible electronic states under the aforementioned complex conditions is determined by the geometric parameters of the tube, namely the ratio between its length and diameter. The occupation of the allowed one- electron states is considered as governed by the usual Fermi statistics. Together with the quantum probability of finding an electron in some specified area of the surface, this gives the electron distribution on the tube, which is one of the key factors determining the electron field emission from the CNT. Another key factor is the applied extraction field. The extraction field has been numerically computed using Simion nanotube-on-post diode configuration.

Electron transfer from a carbon nanotube into vacuum under high electric fields

The transfer of an electron from a carbon nanotube (CNT) tip into vacuum under a high electric field is considered beyond the usual one-dimensional semi-classical approach. A model of the potential energy outside the CNT cap is proposed in order to show the importance of the intrinsic CNT parameters such as radius, length and vacuum barrier height. This model also takes into account set-up parameters such as the shape of the anode and the anode-to-cathode distance, which are generically portable to any modelling study of electron emission from a tip emitter. Results obtained within our model compare well to experimental data. Moreover, in contrast to the usual one-dimensional Wentzel-Kramers-Brillouin description, our model retains the ability to explain non-standard features of the process of electron field emission from CNTs that arise as a result of the quantum behaviour of electrons on the surface of the CNT.

Exact equipotential profile mapping: A self-validating method

A general method for mapping the equipotential profile surrounding a conductive cylindrically symmetric high aspect ratio structure, such as a carbon nanotube or a Spindt tip, is devised. The surface of the object is replaced by a discrete set of charges located on the symmetry axis. The overall electrostatic potential must satisfy a set of boundary conditions imposed on the original surface. The optimum number of charges is determined through an iterative self-validating process such that the obtained equipotential mimics the surface of the object. The method is exemplified by calculating the electric field enhancement factor for rounded cones and cylinders resembling Spindt tips and carbon nanotubes, respectively.

Analytic transmission for field emission from coated cold cathodes

We provide a simple analytical formula for the transmission coefficient of electrons emitted into vacuum from a coated tip, which is valid both at resonance and far from resonance. This formula assumes emission through two triangular-shaped potential barriers and allows the calculation of the tunneling time of electrons, parameter of crucial importance for high-frequency devices. It is shown that the current density calculated with the simple analytical formula for the transmission coefficient successfully explains the previously obtained experimental results.

Modeling of Focused Carbon Nanotube Array Emitters for Field-Emission Displays

Vertically aligned carbon nanotubes (CNTs) grown in patterned areas are used as electron sources in field emission displays (FEDs), but detrimental electron beam spreading may occur in a vacuum space. In this paper, a novel emitter structure with two coaxial electrodes and vertically aligned CNTs is proposed and analyzed using three-dimensional (3D) computation of the electric field. One of the gate electrodes plays a role in electron extraction and the other one in electron beam focusing. Unlike the case of double-gated Spindt emitters, the focusing gate electrode is placed near the plane of the CNT tips while the extraction electrode is placed at some distance from it. An improved electric field uniformity within the CNT array and focusing of the electron beam are thus achieved. Electron beam confinement characteristics and field emission properties are calculated as functions of device geometry and its functional parameters.

Evidence of Tunneling in n-4H-SiC/SiO2 Capacitors at Low Temperatures

In this work, anomalous discontinuities observed in Capacitance-Voltage (C-V) characteristics on non-nitridated n-4H-SiC/SiO2 capacitors at low temperature are addressed. The appearance of abrupt capacitance minima, always at the same gate voltages (4V and 8V) and independent on probe frequency, led us to consider a resonant electron tunneling process from neutral donor states present at the SiC/SiO2 interface into two well defined energy levels in the oxide layer. Results of numerical simulations based on this model describe quantitatively the experimentally observed discontinuities at 4V and 8V and provide strong evidence for the presence resonant tunneling.

P1–32: Calculation of the enhancement factor for a high-aspect ratio structure

The field enhancement factor for a cylindrically symmetric structure is obtained using an electrostatic model based on a self-validating algorithm. The correct 0 V equipotential line is calculated such that it matches the surface of the given structure.