Real-time Ellipsometry on Cesium-Telluride Photocathode Formation

1. Real-time Ellipsometry on Cesium-Telluride Photocathode Formation Martijn Tesselaar & Peter van der Slot CARE07

2. Contents Introduction • Electron Accelerator • Photoelectric Effect Ellipsometry on Cs2Te Photocathodes • Photocathode preparation • Rotating Compensator Ellipsometry • RCE measurement results Conclusions

3. Electron Accelerator Applications External Beam Radiotherapy Synchrotron radiation Electron collider experiments Free Electron Laser

4. Linear Accelerator • Laser pulse on photocathode => short electron bunch • Radio Frequency Electromagnetic waves accelerate the bunch • Magnets are used for confinement

5. Photoelectric Effect Kinetic energy Quantum Efficiency = Number of electrons emitted per photon

6. Cs2Te Photocathode Preparation • Substrate at 120°C • Deposit Tellurium by Physical Vapor Deposition (PVD) for about 30 minutes • Deposit Cesium by PVD until cathode is completed • Cs and Te mixing produces multiple CsxTey layers

7. Start Cesium Deposition Quantum Efficiency • Photocathode irradiated by UV lamp during deposition • Photocurrent measured using picoamperemeter • Photocathode considered finished at maximum QE

8. 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

9. n1 1 D C A d n2 2 B Reflection from thin film structures • Path length difference: • Resulting in phase difference: Fresnel Reflection Coefficients give change in amplitude and phasedetermined by film thickness d, refractive index n and absorption coefficient  of the thin film material

10. Sample & Polarization • Sample optical properties contained in the ellipsometric quantities  and : •  and  also depend on film thickness, refractive index and absorption coefficient

11. 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

12. Quarter Waveplate Rotation

13. y a2 a  a1 x b Stokes Vector Beam characteristics: • Intensity • Polarization angle • Polarization ellipticity • Polarization rotation direction (CW or CCW) These 4 characteristics may be represented in a 4 element vector called the Stokes vector:

14. Mueller Matrix Each 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:

15. 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 • 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

16. Ellipsometry Measurements 1 Arrows indicate corresponding points in time

17. Ellipsometry Measurements 2 Arrows indicate corresponding points in time

18. 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

19. 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