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Real-time Ellipsometry on Cesium-Telluride Photocathode Formation

Martijn Tesselaar & Peter van der Slot CARE07

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ContentsIntroduction• Electron Accelerator• Photoelectric Effect

Ellipsometry on Cs2Te Photocathodes• Photocathode preparation• Rotating Compensator Ellipsometry• RCE measurement results Conclusions

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Electron Accelerator Applications

External Beam Radiotherapy

Electron collider experiments Free Electron Laser

Synchrotron radiation

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Linear Accelerator

• Laser pulse on photocathode => short electron bunch

• Radio Frequency Electromagnetic waves accelerate the bunch

• Magnets are used for confinement

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Photoelectric Effect

P

I

e

hfQE phQuantum Efficiency = Number of electrons emitted per photon

Kinetic energy

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Cs2Te Photocathode Preparation

1. Substrate at 120°C

2. Deposit Tellurium by Physical Vapor Deposition (PVD) for about 30 minutes

3. Deposit Cesium by PVD until cathode is completed

4. Cs and Te mixing produces multiple CsxTey layers

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Quantum Efficiency

• Photocathode irradiated by UV lamp during deposition

• Photocurrent measured using picoamperemeter

• Photocathode considered finished at maximum QEStart Cesium Deposition

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Ellipsometry on Cs2Te Photocathodes

• To study the deposition process

• Optical method: photocathode stays inside, measurement device outside

• Real-time measurements register steps in the deposition process

Preparation Chamber

Ellipsometer

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Reflection from thin film structures

2

1

A

B

C

D

n2

n1

d

22 cos2 dn

122

2 sintan2cos

2

dd

n

22 cos4

2dnOPL

• Path length difference:

• Resulting in phase difference:

ADBCABnOPL 2

Fresnel Reflection Coefficients give change in amplitude and phase determined by film thickness d, refractive index n and absorption coefficient of the thin film material

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Sample & Polarization

• Sample optical properties contained in the ellipsometric quantities and :

and also depend on film thickness, refractive index and absorption coefficient

j

p

s eR

Rtan

4~

2 inN

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Rotating Compensator Ellipsometry

• Compensator (QWP) rotates continuously

• Sample properties influence reflected beam characteristics

• Reflected beam characteristics influence intensity after analyzer

• Correlation between compensator angle and detector signal gives information about sample properties

Polarizer

Analyzer

HeNe laser

QWP

Faraday Isolator

BS D1

Sample

Copper mirror

HWP

Window

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Quarter Waveplate Rotation

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Stokes VectorBeam characteristics:

1. Intensity

2. Polarization angle

3. Polarization ellipticity

4. Polarization rotation direction (CW or CCW)

a

b

a2

a1 x

y

ba

22

21 aaI

These 4 characteristics may be represented in a 4 element vector called the Stokes vector:

2sin

2cos2sin

2cos2cos

4

3

2

1

I

I

I

I

S

S

S

S

S

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Mueller MatrixEach optical element may be represented by a 4x4 matrix called a Mueller matrix, for example for a sample with properties and :

So that the exiting Stokes vector is:

For a quarter wave plate (with vertical fast axis) :And a rotation matrix:

The total Mueller matrix of the system with two reflections and without window is found as:

cos2sinsin2sin00

sin2sincos2sin00

0012cos

002cos1

SM

0100

1000

0010

0001

CM

PRMPRCRMCRMMMARMARM PCSMSAT

inout MSS

1000

02cos2sin0

02sin2cos0

0001

R

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Psi-Delta Calculation• S1 in the outgoing stokes

vector is the intensity after the Analyzer

• It is a Fourier series of the compensator angle C

• Fitting to measurement data gives Fourier coefficients An and Bn

and are derived from An and Bn by calculations depending on the setup used

I

C (°)Intensity after Analyzer as a function of compensator angle C

CBCACBCAACI 4sin4cos2sin2cos 44220

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Arrows indicate corresponding points in time

Ellipsometry Measurements 1

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Ellipsometry Measurements 2

Arrows indicate corresponding points in time

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Ellipsometry Measurement 3• Calculation of , values for double reflection from sample (without

taking into account the window) results in complex values • As an illustration of what , values could be the graphs below are

calculated using an assumed single reflection from sample

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Conclusions

• Rotating Compensator Ellipsometry is a feasible method for studying photocathode growth

• Different preparation conditions result in different measured Fourier coefficients

• Ellipsometry results remain difficult to interpret