Transcript Dia 1 - CARE 07

Real-time Ellipsometry on Cesium-Telluride
Photocathode Formation
Martijn Tesselaar & Peter van der Slot
CARE07
1/20
Contents
Introduction
• Electron Accelerator
• Photoelectric Effect
Ellipsometry on Cs2Te Photocathodes
• Photocathode preparation
• Rotating Compensator Ellipsometry
• RCE measurement results
Conclusions
2/20
Electron Accelerator Applications
External Beam Radiotherapy
Synchrotron radiation
Electron collider experiments
Free Electron Laser
3/20
Linear Accelerator
• Laser pulse on
photocathode =>
short electron bunch
• Radio Frequency
Electromagnetic
waves accelerate the
bunch
• Magnets are used
for confinement
4/20
Photoelectric Effect
Kinetic energy
hf I ph
Quantum Efficiency = Number of electrons emitted per photon QE 
e P
5/20
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
6/20
Quantum Efficiency
Start Cesium Deposition
• Photocathode
irradiated by UV
lamp during
deposition
• Photocurrent
measured using
picoamperemeter
• Photocathode
considered finished
at maximum QE
7/20
Ellipsometry on Cs2Te Photocathodes
Ellipsometer
• To study the deposition
process
• Optical method:
photocathode stays
inside, measurement
device outside
• Real-time measurements
register steps in the
deposition process
Preparation Chamber
8/20
Reflection from thin film structures
1 D
n1
• Path length difference:
OPL  n2  AB  BC   AD
 2n2
C
 2n2 d cos2
A
d
d
 2d tan  2   sin 1
cos  2
n2
2
• Resulting in phase difference:
 
B
OPL

 2 
4 n2 d cos  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
9/20
Sample & Polarization
• Sample optical
properties contained in
the ellipsometric
quantities  and :
Rs
 tan    e j
Rp
•  and  also depend on
film thickness, refractive
index and absorption
coefficient

~
N  n2  i

4
10/20
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
HeNe laser
Faraday
Isolator
HWP
Polarizer
QWP
Analyzer
BS
D1
Window
Sample
Copper
mirror
11/20
Quarter Waveplate Rotation
12/20
Stokes Vector
y
Beam characteristics:
2
2
1.
Intensity I  a1  a 2
2.
Polarization angle 
3.
Polarization ellipticity   a b
4.
Polarization rotation
direction (CW or CCW)
These 4 characteristics may be
represented in a 4 element
vector called the Stokes vector:
a2
a

a1 x
b
I
 S1  

  

  S 2   I cos2  cos2  
S  
S3
I sin 2  cos2   
  


S  


I
sin
2


 4 
13/20
Mueller Matrix
Each optical element may be
1
 cos 2
0
0


represented by a 4x4 matrix
 cos 2

1
0
0

called a Mueller matrix, for M S  
0
0
sin 2 cos  sin 2 sin  

example for a sample with


0
0
 sin 2 sin  sin 2 cos  
properties  and :

So that the exiting Stokes
vector is:
For a quarter wave plate
(with vertical fast axis) :
And a rotation matrix:
Sout  MSin
1
0
MC  
0

0
The total Mueller matrix of the
system with two reflections and
without window is found as: M T
0
1 0 0

0 0  1

0 1 0
0 0
0
1
0 cos 2
R    
0  sin 2

0
0
0
sin 2
cos 2
0
0
0

0

1
 R AM A R AM S M M M S R C M C RC R P M P RP 
14/20
Psi-Delta Calculation
I
• S1 in the outgoing stokes
vector is the intensity after
the Analyzer
• It is a Fourier series of the
compensator angle C
I C   A0  A2 cos2C   B2 sin 2C   A4 cos4C   B4 sin 4C 
• Fitting to measurement data
gives Fourier coefficients An
and Bn
•  and  are derived from An
and Bn by calculations
depending on the setup used
C (°)
Intensity after Analyzer as a
function of compensator angle C
15/20
Ellipsometry Measurements 1
Arrows indicate corresponding points in time
16/20
Ellipsometry Measurements 2
Arrows indicate corresponding points in time
17/20
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
18/20
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
19/20