CARBON NANO TIPS-BASED FIELD ELECTRON EMISSION CHARACTERIZATION FOR - PowerPoint PPT Presentation

CARBON NANO TIPS-BASED FIELD ELECTRON EMISSION CHARACTERIZATION FOR LOW- POWER HIGH-SPEED MULTIPLEXING APPLICATIONS BY SHADI SALEH ALNAWASREH

I NTRODUCTION  Field electron emission (FE) is emission of electrons induced by an electrostatic field from a solid surface into vacuum.  Field-induced emission of electrons was explained in 1928 by combining quantum tunneling theory with Fermi-Dirac statistical theory  Field electron emission has become a subject of considerable research activity in recent years, fueled in part by the ever-continuing improvements in a wide variety of applications, such as microwave amplifiers, electron microscopes, flash X-ray photography, flat panel plasma displays, and ion propulsion drives,

 The early experiments on electron emission did not just only lead to important technological developments, the results as well triggered new theoretical insight, which forms a vital part of the basis of today’s physics.  In contrast to the commonly used thermionic emission from hot filaments, field emission occurs at room temperature from unheated "cold" cathodes under the influence of an electric field.

 Field emission offers several attractive characteristics, including very fast response to field variation, resistance to temperature fluctuation and radiation, and exponential current-voltage relationship in which a small change in voltage can induce a large change of emission current.  The development of micro- and nano- fabrication technology has changed the situation dramatically. It has allowed for the fabrication of an electrode in the very small apex radius, thereby significantly lowering the operating voltage.

 The main purpose of this study is to examine the influence of sample conditioning treatment. This allows the preparation of sharp conical-carbon- fiber tips, in order to study means by which the emission current instability could be overcome, the aim being to develop an electron source with high emission current stability and increased brightness. Another aim is to study the electron emission mechanism from the carbon fiber microemitters.

M ECHANISMS OF E LECTRON E MISSION

 1- Photoemission  Electrons are emitted from atoms and from solids when they absorb energy from light. Electrons emitted in this way may be called photoelectrons. Photoemission results from the interaction between incident electromagnetic radiation and an electron near the surface of a conductive material.  The maximum kinetic energy of the electron leaving the surface will then be: Ekin = h ν – ϕ

 2- Secondary Electron Emission  Secondary electron emission results from the impact of a fast free (primary) electron on the surface of a material. The primary electron may be backscattered from the surface either elastically or inelastically. In the inelastic case, part of its energy is handed over to up to ≈ 30 other electrons by a cascade of collisions. Thus, some of these electrons can overcome the potential barrier ϕ .

 3- Thermionic Emission  In this type of emission the electron emission is achieved by heating the emitter. Due to heating, electrons get enough energy to escape from the surface of that material. An electron emitted from a hot conductor comes out with a velocity that represents the difference between the kinetic energy E kin possessed by electron inside the emitter and the local work function ϕ .  The emission current density J is usually given by:

 4- Cold Field Electron Emission  Cold field electron emission (CFE) is a statistical emission regime where  (i) the electrons in the emitting region are effectively in local thermodynamic equilibrium  (ii) most electrons escape by deep tunneling from states close to the emitter's Fermi level.  The applied field F causes the potential barrier between the metal and the vacuum to be of finite size by bending the electron potential energy down as the distance x from the surface increases. In terms of quantum mechanics, the electrons are able – with a certain probability – to tunnel through this often high but relatively narrow barrier.

 Cold field emission of electrons occurs under the influence of applied electric fields greater than about 3 V/nm.  The emission current density J is given by:

C ORE A SSUMPTIONS OF F OWLER – N ORDHEIM T HEORY  Has a free-electron band structure.  Has electrons obeying Fermi – Dirac statistics.  Is at zero temperature.  Has a smooth, flat planar surface (atomic structure is disregarded).  Has a work function that is uniform across the emitting surface and is independent of external field. It is also assumed that  There is a uniform electric field outside the metal surface.  The exchange-and-correlation interaction between the emitted electron and the surface can be represented by a classical image potential.  Barrier penetration coefficients may be evaluated using the JWKB approximation

D ATA I NTERPRETATION  As usual in the analysis of elementary electric and electronic components, a quite common data representation is the so called I – V plot. It presents the behavior of the current depending on the applied voltage. In fact, these quantities are the ones usually measured ( I ) respective controlled ( V ) within experimental analyzes of field emission. However, the theoretical approaches deal with the surface field F and the current density J . To link between I – V and J – F , there are two equations:

 Another method of data representation, which is particularly useful in field emission, is the Fowler – Nordheim (FN) plot. It acts on the same data set but presents it in a different way, namely as ( ) versus ( ). The benefit of this plot is to obtain a straight line.

D ESIGN AND M ETHODOLOGY  Ultra High Vacuum (UHV) Techniques  The word "vacuum" means "empty". However, there is no empty space in nature; there is no "ideal vacuum". Vacuum is only a partially empty space, where some of the air and other gases have been removed from a containing volume  Ultra high vacuum is the vacuum regime defined by pressure lower than about 10 − 9 mbar. UHV requires the use of unusual materials for equipment, and heating of the entire system to 180°C for several hours.

 The fundamental principle for UHV techniques states that the pressure of gas in the system should be reduced to the minimum. Thus, maximum speeds and non-stop pumping are used. The rotary mechanical pump, which is the base of our vacuum systems, would produce pressures of about 10 − 3 mbar. After this pump produces such pressure, a second vacuum pump that is technically connected to it, produces a higher vacuum. This setup involves, in addition to the rotary pump, either a diffusion pump or a turbo molecular pump.

P REPARATION OF E LECTRON E MITTERS  Carbon fiber emitters can be produced by electrolytic etching technique, where a 0.1 molar of sodium hydroxide (NaOH) solution is used. This etching process could be controlled by choosing a suitable etching current. After dipping the tip in the solution by about 2mm and increasing the voltage until a certain initial current of about 30 μA the etching process is started. The chosen etching currents produces sharp tips at the liquid surface, which are afterwards being ultrasonically cleaned and mounted in a standard field emission microscope (FEM) with a tip screen distance of 10mm. The anode is formed as a phosphored screen to allow for the recording of the emission images.

F IELD E MISSION M ICROSCOPE (FEM)

F INDINGS AND D ISCUSSIONS AND R ECOMMENDATIONS  The results obtained by using clean carbon fiber emitters, where several diameters have been analyzed. The current – voltage ( I - V ) characteristics, Fowler-Nordheim (FN) plots, and electron emission images have been recorded to study the emission characteristics and stability of the emission current, calculate the area efficiency of the carbon fiber emitters and compare the apex radii measured at the SEM images to those extracted from the FN plots.

a b Scanning electron micrographs of: (a) a sharp carbon fiber tip at 5 000 × (left) and (b) 60 000 × (right) magnification.

500 1200 1500 1000 Emission Current Emission Current I Emission Current I Emission Current I [nA] 1000 400 800 1000 800 300 [nA] 600 I [nA] [nA] 600 200 500 400 400 100 200 200 0 0 0 200 400 600 800 200 700 0 250 750 200 700 Applied Voltage U [V] Applied Voltage U [V] Applied Voltage U [V] Applied Voltage U [V] A B C D The I-V characteristics of a sharp tip after sample conditioning a) baking, b)follow up baking, c) thermal relaxation, and d) during cooling process, respectively. -10 -10 -10 -10 LOG(I/U^2[nA/V^2]) LOG(I/U^2[nA/V^2]) LOG(I/U^2[nA/V^2]) LOG(I/U^2[nA/V^2]) -12 -12 -12 -12 -14 -14 -14 -14 -16 -16 -16 -16 -18 -18 -18 -18 1 2 3 4 1 2 3 4 1 3 5 1 2 3 4 1000/U [1/V] 1000/U [1/V] 1000/U [1/V] 1000/U [1/V] A B C D Fowled-Nordheim plots of a sharp tip after sample conditioning a) baking, b)follow up baking, c) thermal relaxation, and d) during cooling process, respectively. 21