Activation Studies of Gallium Arsenide Photocathodes

1. Activation Studies of Gallium Arsenide Photocathodes By Morgan dixon Carleton College; NorthfielD, MN Mentors: Dr. Ivan Bazarov, Dr. Yulin Li, Dr. Xianghong Liu Cornell university, summer 2011

2. Why Study GaAsPhotocathodes? -GaAs is a semiconductor -Activate to negative electron affinity to create electron emission -Emitted electrons form the electron beam in applications such as Energy Recovery Linacs (ERL) photocathode Figure 1: Design of the current ERLInjector at Cornell University

3. Quantum Efficiency (QE) and Lifetime QE: measure of the number of emitted electrons per incident photon on the cathode Ip = photocurrent (aka the current of emitted electrons) Pl = laser power incident upon the cathode • Lifetime: span of time over which a photocathode produces a satisfactory QE • Time for QE to reduce to 1/e of it’s original value • Dark Lifetime vs. Operational Lifetime • Want to increase lifetime – currently a maximum of a few days

4. The Physics of Photocathodes Figure 2: Energy band diagram of a GaAs crystal and a GaAs crystal activated to NEA.

5. QE Chamber Figure 4: CAD drawing of the UHV chamber used for all experiments.

6. CathodeStock 1. Remove old cathode by heating the base and melting the Indium 2. Clean old Indium from the Mo Base 3. Place clean Indium on the base and attach the new cathode Figure 5: CAD drawing of the cathode stock and the cathode heating elements.

7. LabVIEW program Figure 6: Screenshots of the labVIEW program, GUP.vi

8. Cathode Preparation Procedure • In house diamond cutting process • Acid etching to remove surface oxides and other contaminants • Attach the clean cathode to the Mo base and place in the chamber • Pump down chamber and bake out to achieve UHV • Vacuum annealing to further remove oxide contaminants on the surface of the cathode • Heat cathode to 650C and hold for two hours

9. Co-Deposition Activation 1. Introduce Cs until peak in QE, then overdose. 2. Once QE drops 20% – 30% of peak value, introduce the oxidizing agent to the system. 3. Cease Cs and oxidizing agent deposition once QE peaks a second time. Figure 7: Typical activation curve using the co-deposition method.

10. Yo-Yo Activation 1. Introduce Cs until photocurrent peaks, then overdose. 2. Once photocurrent drops to half of the peak value, cease cesiaton and introduce oxidizing agent. 3. When photocurrent peaks again, shut leak valve and introduce Cs again. 4. Repeat “yo-yo’s” until total gain in photocurrent plateaus. Initial Cs Peak First exposure to NF3 Figure 8: Typical activation curve using the Yo-Yo method.

11. Activation with N2 : Before Gas Purification First two trials showed same activation process and same photocurrent. Both activations were very slow. Figure 9: Activation ofGaAs with N2, first and second trials.

12. Activation with N2 : After Gas Purification Previous activations due to contamination? Purified the N2 used for activation by pumping out the gas manifold. Reduced O2 content by nearly a factor of ten. Activation process slowed by nearly a factor of ten. Conclusion: N2 DOES NOT ACTIVATE CATHODES Figure 10: Activation using N2, before and after cleaning out and recharging the gas manifold.

13. Activation with N2 : scaled time axes Comparison to First N2 Activation Comparison to Second N2 Activation Figure 11: Activation of cathode with N2 before and after purification of gas.

14. Activation with NF3 Wanted to study the dark lifetime of cathodes activated using NF3 and compare to activation with O2. Accidently killed the cathode so lifetime studies could not be conducted. Figure 8: Typical activation curve using the Yo-Yo method.

15. Blank Activation with NF3 NF3 - “Activated” the cathode with Cs and NF3 even though no photocurrent was detected. - No guide to see when peaks, only have RGA scans to show when NF3 or Cs are in the chamber. Cs peak Figure 12: RGA scans from blank activation with NF3.

16. Thermal DesorptionSpectroscopy (TDS) -Linearly increase temperature of cathode at a rate of 5.25 C/min. -Adsorbatesdesorb from the cathode surface while RGA records composition of the desorbed materials. -Temperature of desorption gives information on how adsorbates are bonded to the surface. Figure 13: TDS graph of select masses from N2 activated cathode.

17. TDS of Cs on activated GaAs photocathode -Proper activation, Tdesorb = 394 C -Blank activation, Tdesorb = 354 C -Not enough data to make conclusions, however, the sharp peak suggests that the Cs is strongly bonded with the surface of the cathode. Figure 13: Cs desorbing from NF3 activated cathode and other peaks that desorbed at the same temperature.

18. TDS to identify unknown species -After the cathode died, the dominant species in the gas composition of the chamber changed and we had many unknown species in the chamber. -TDS revealed possible fragment peaks of an unknown species that we tried to use to identify the unknown composition of the mass 85 peak. -Never found a good fit for the species. Figure 14: TDS data of unknown species on NF3 activated cathode with twin peaks at masses 84 & 85 and possible fragment peaks at masses 49 & 64

19. Conclusions • The equipment and computing resources are set up to begin work with the new QE chamber. • Nitrogen will NOT activate GaAsphotocathodes • GaAsphotocathodes are very sensitive to contaminants in activating gasses, so gases used in activating processes should be very pure to be sure which species are activating the cathode. • Cs appears to strongly bind with the GaAs surface when the cathode is activated with NF3.

20. Acknowledgements Thank you very much to my mentors Ivan, Yulin and Xianghong, and all other members of Cornell’s photocathode project, for their wonderful support throughout the summer. This project was funded by the NSF.