By Morgan dixon Carleton College; - PowerPoint Presentation

Slide1

By Morgan dixonCarleton College; NorthfielD, MNMentors: Dr. Ivan Bazarov, Dr. Yulin Li, Dr. Xianghong LiuCornell university, summer 2011

Activation Studies of Gallium Arsenide

PhotocathodesSlide2

Figure 1: Design of the current ERLInjector at Cornell UniversityWhy 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)

photocathodeSlide3

Quantum Efficiency (QE) and LifetimeQE: 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 daysSlide4

The Physics of Photocathodes

Figure 2: Energy

band diagram of a

GaAs

crystal and a

GaAs

crystal activated to NEA.Slide5

QE Chamber

Figure 4: CAD drawing

of the UHV chamber used for all experiments.Slide6

CathodeStock 1. Remove old cathode by heating the base and melting the Indium 2. Clean old Indium from the Mo Base3. Place clean Indium on the base and attach the new cathode

Figure 5: CAD

drawing of the cathode stock

and the cathode heating elements.Slide7

LabVIEW program

Figure 6: Screenshots of the

labVIEW

program,

GUP.viSlide8

Cathode Preparation ProcedureIn 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 cathodeHeat cathode to 650C and hold for two hoursSlide9

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.Slide10

Yo-Yo Activation1. 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.

Initial

Cs Peak

First exposure to NF

3Slide11

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 N

2

, first and second trials.Slide12

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

usin

g N

2

, before and after cleaning out and recharging the gas manifold.Slide13

Comparison to First N2 ActivationComparison to Second N2 Activation

Activation with N

2 : scaled time axes

Figure 11: Activation

of cathode with N

2

before and after purification of gas.Slide14

Activation with NF3Wanted 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.Slide15

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.

NF

3

Cs peak

Figure 12: RGA scans from blank activation with NF

3

.Slide16

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 N

2

activated cathode.Slide17

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 NF

3

activated cathode and other peaks that desorbed at the same temperature

.Slide18

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 NF

3

activated cathode with twin peaks at masses 84 & 85 and possible fragment peaks at masses 49 & 64Slide19

ConclusionsThe equipment and computing resources are set up to begin work with the new QE chamber.Nitrogen will NOT activate GaAs photocathodesGaAs 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 NF

3.Slide20

AcknowledgementsThank 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.