Photomodulated Carbon Nanotubes Cold Cathode

Laurent Gangloff1, Stéphane Xavier1, Christophe Bourat2, Nicolas Martinez2, Jean-Paul Mazellier1, Jean-Philippe Schnell1, Kenneth Teo3, Pascal Ponard2 and

Pierre Legagneux1 1Thales Research and Technology, Thales, Palaiseau, France

2Thales Electron Devices, Thales, Thonon, France

3Aixtron, Cambridge, UK

Abstract: This paper presents the fabrication and

characterization of carbon nanotubes cold cathodes and

their improvement to photocathodes by adding p-i-n

photodiodes in the device structure. Integration and

operation of carbon nanotubes based photocathodes in

prototype of dual x-ray tube is presented.

Keywords: carbon nanotubes; photocathodes; field

emission ; x-ray ; microwave amplifiers.

Introduction Cold cathodes are promising electron sources to replace

thermoionic cathodes currently used in microwave tubes.

They present many advantages compared to thermoionics

cathode : compact, low weight, less power consumption

and low heat generation, easier modulation. The use of

carbon nanotubes (CNT) as field emitters for cold cathode

has generated lot of attractiveness around the world

because of their unusual properties. Growth of vertically

aligned CNT has been achieved by many groups and field

emission up to 100µA(1)

for individual CNTs has been

demonstrated opening the way to realize cold cathode with

very large current emission capabilities required for

travelling waves tubes (TWTs).

The carbon nanotube cathode

We have grown vertically aligned CNTs by plasma

enhanced chemical vapour deposition. The design of the

CNTs array is controlled by the position of Ni catalyst dots

previously deposited. Standard arrangement is network of

1µm diameter and 7nm thick Ni dots, hexagonally packed

in a circular array of 1mm diameter. These Ni dots are

deposited on a TiN layer which role is to prevent the

diffusion of the catalyst into the silicon substrate when it is

heated at high temperature. The vertical growth occurs on

these Ni dots when the substrate is heated and located in a

plasma phase containing carbon and carbon etching species (2)

(Figure 1). After growth, CNT cathodes are installed in a

high vacuum chamber and tested in triode configuration.

Current up to 4mA have been obtained in DC mode

corresponding to 0.5 A/cm² (Figure 2). CNT growth and

field emission results will be presented and discussed.

Figure 1 : SEM images of vertically aligned CNT

0,00E+00

5,00E+06

1,00E+07

1,50E+07

2,00E+07

2,50E+07

1,E-04 1,E-03 1,E-02 1,E-01 1,E+00

J tot max [A/cm^2]

Ch

am

p m

ax

[V

/m]

E (V/m) - L0504

E (V/m) - L0602

E (V/m) - K250611b

E (V/m) - I0301

E (V/m) - I0401

E (V/m) - I0101

E (V/m) - L0902

Figure 2 : Current density (x-axis) obtained for different cathodes (up to 0.5 A/cm²)

The carbon nanotube photocathode

By implementing p-i-n diodes acting as photoswitches in

the device structure we move from the cathode to the

photocathode. Photocathodes are especially interesting

because the field emitted electron beam can be modulated

by a laser beam instead of modulating the extraction

voltage as in conventional tubes. Basically, the CNT

photocathode consists of an array of vertically aligned

437978-1-4673-0369-9/12/$31.00 © 2012 IEEE

CNTs, as described previously, associated with individual

p-i-n photodiodes (3)

(Figure 3 and 4).

Figure 3 : Photocathode structure

CNT

Si substrate

Photodiode

area

Figure 4 : SEM picture of CNT grown on p-i-n photodiodes

Using such photocathodes we have fabricated and run the

first prototype of optically driven stereoscopic x-rays

device based on carbon nanotube (CNT) photocathodes.

The prototype is composed of 2 independently controlled

x-ray sources. As conventional x-ray system, high voltage

(up to 130kV) is applied between the photocathode and the

anode to obtain field emission current from the CNT array

and to generate x-rays on the tungsten coated window.

Current up to 1mA for a 1mm emission area has been

obtained with ON/OFF ratio of 5 to 10. Innovative idea of

this project is the use of photocathodes as electron source

AND for x-ray modulation allowing galvanic insulation,

fast switching and accurate control of the emission

current/x-ray dose. We will present fabrication of the

photocathodes with associated field emission

measurements and will show first images obtained using

such photocathodes based x-ray dual sources system.

X-ray ON

X-ray OFF

Figure 5 : Prototype of the dual x-ray tube with two satellites integrating photocathodes modulated by the laser beam

Figure 6 : X-ray images simultaneously obtained with the dual source at two different viewing angles

Acknowledgements

The work was partially funded by the E.C. and French

National Agency (ANR) respectively through the

TECHNOTUBES and SPIDERS projects.

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