BY MORGAN DIXONCARLETON COLLEGE; NORTHFIELD, MN

MENTORS: DR. IVAN BAZAROV, DR. YULIN LI , DR. XIANGHONG LIU

CORNELL UNIVERSITY, SUMMER 2011

Activation Studies of Gallium Arsenide

Photocathodes

Figure 1: Design of the current ERLInjector at Cornell University

Why Study GaAs Photocathodes?-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)

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photocathode

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 valueDark Lifetime vs.

Operational LifetimeWant to increase

lifetime – currently a maximum of a few days%100

lp

PI

ehcQE

The Physics of Photocathodes

Figure 2: Energy band diagram of a GaAs crystal and a GaAs crystal activated to NEA.

QE Chamber

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

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.

LabVIEW program

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

Cathode Preparation Procedure

In house diamond cutting processAcid etching to remove surface oxides and

other contaminantsAttach the clean cathode to the Mo base and

place in the chamberPump down chamber and bake out to achieve

UHVVacuum annealing to further remove oxide

contaminants on the surface of the cathode Heat cathode to 650C and hold for two hours

Co-Deposition Activation1. 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.

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. Figure 8: Typical activation curve using the Yo-Yo

method.

InitialCs Peak

First exposure to NF3

Activation with N2 : Before Gas PurificationFirst two trials showed same activation process and same photocurrent.

Both activations were very slow.

Figure 9: Activation of GaAs with N2, first and second trials.

Activation with N2 : After Gas PurificationPrevious 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.

Comparison to First N2 Activation

Comparison to Second N2 Activation

Activation with N2 : scaled time axes

Figure 11: Activation of cathode with N2 before and after purification of gas.

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.

Blank Activation with 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.

NF3

Cs peak

Figure 12: RGA scans from blank activation with NF3.

Thermal DesorptionSpectroscopy (TDS)

-Linearly increase temperature of cathode at a rate of 5.25 C/min.

-Adsorbates desorb 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.

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.

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

Conclusions

The equipment and computing resources are set up to begin work with the new QE chamber.

Nitrogen will NOT activate GaAs photocathodes GaAs photocathodes 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.

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.