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

INTRODUCTION

 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

• ccurs 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.

MECHANISMS OF ELECTRON EMISSION

 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 = hν – 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

• f that material. An electron emitted from a hot

conductor comes out with a velocity that represents the difference between the kinetic energy Ekin 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:

CORE ASSUMPTIONS OF FOWLER– NORDHEIM THEORY

 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

DATA INTERPRETATION

 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

• n 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.

DESIGN AND METHODOLOGY

 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.

PREPARATION OF ELECTRON EMITTERS

 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.

FIELD EMISSION MICROSCOPE (FEM)

FINDINGS AND DISCUSSIONS AND RECOMMENDATIONS

 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

• f the carbon fiber emitters and compare the

apex radii measured at the SEM images to those extracted from the FN plots.

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

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. 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.

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100 200 300 400 500 200 700

Emission Current I [nA] Applied Voltage U [V]

200 400 600 800 1000 200 700

Emission Current I [nA] Applied Voltage U [V]

500 1000 1500 200 400 600 800

Emission Current I [nA]

Applied Voltage U [V]

200 400 600 800 1000 1200 250 750 Emission Current I [nA] Applied Voltage U [V]

• 18
• 16
• 14
• 12
• 10

1 3 5

LOG(I/U^2[nA/V^2]) 1000/U [1/V]

• 18
• 16
• 14
• 12
• 10

1 2 3 4

LOG(I/U^2[nA/V^2]) 1000/U [1/V]

• 18
• 16
• 14
• 12
• 10

1 2 3 4

LOG(I/U^2[nA/V^2]) 1000/U [1/V]

• 18
• 16
• 14
• 12
• 10

1 2 3 4 LOG(I/U^2[nA/V^2]) 1000/U [1/V]

Emission images obtained from a sharp carbon-fiber tip after sample conditioning by (a) baking (b) follow-up baking, (c) thermal relaxation, and (d) during cooling process at 1 μA, respectively. a b c d

CALCULATING THE RELATIVE EMISSION

AREA OF SEM RADIUS AND FEM RADIUS

 In this section we compare the apex radii

measured at the SEM images to those extracted from the FN plots, and discuss the values of the

• btained area efficiency.

Carbon fiber no r[SEM] [nm] r[FEM] [nm] Relative emission area α 1 59 57 2.17835E-05 2 66 64 3.11165E-05 3 90 68 3.69E-05 4 150 94 8.98E-05 5 58 56 1.92264E-05 6 70 67 3.30405E-05 7 85 78 4.87791E-05 8 110 100 8.825E-05

APPLICATIONS ON FIELD EMISSION OF

CARBON FIBER NANO SCALE

 A multiplexer is a device that selects one of

several analog or digital input signals and forwards the selected input into a single line.

 A multiplexer (MUX) circuit consists of:  (1) a number of data inputs.  (2) one (or more) select inputs.  (3) one output.

Schematic Diagram of the multiplexer device consists of two cathodes, one anode, a power supply, and an inverter.

Carbon fiber tips could be used as cathodes in multiplexer based

• application. When applying the high voltage, emission current

will occur at cathode number 1, else emission current will occur at cathode number 2. The nano–scale carbon cathodes will decrease the size of the multiplexer device and increase the power-uses efficiency, of the device.

SUMMARY AND RECOMMENDATIONS

 Carbon fiber emitters were prepared. The analytical

facility used enabled measuring various characteristics of the field electrons emission. The sample conditioning procedures that included initial baking, a follow up baking, thermal relaxation, and cooling process down to LN2 temperatures, produced improved stability in the emission current and higher

• brightness. The performance of these tips was found

to be dependant on the sample conditioning treatment.

 Two methods to determine the apex radii; graphically

measuring the values rSEM on the SEM images and extracting the values from the emission characteristics recorded in the FEM. Most values of these methods are generally in good agreement. The area efficiency of carbon fiber tips is within the expected range for this material.

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